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THE UNIVERSITY OF EDINBURGH

SCHOOL OF CHEMISTRY

CHEMISTRY 3

ORGANIC CHEMISTRY

LABORATORY MANUAL

2013-2014

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GENERAL ORGANISATION OF THE COURSE This manual contains just the information you need at the bench. You must read and use the information on LEARN in addition to this manual. On LEARN you will find: General safety information Course aims and outcomes Advice on how to write a report Pre-lab exercises for all the experiments Resources relevant to each experiment such as reference spectra etc. Information on the Unknowns exercise Also there are useful videos and helpful lab tips on www.kirsoplabs.co.uk

Organiser: Dr. Peter Kirsop (room 281, Tel. 50 4719, email p.kirsop@ed.ac.uk).

The lab work takes place over twenty-four laboratory sessions and consists of two types of exercise: Experiments: Each student is assigned experiments by bench:

BENCH A BENCH B BENCH C Experiments: 1,2,3,4,5,7,10 1,2,6,7,8,9,11 1,2,5,6,7,9,10 You must all start with Ex. 1. After that the order is up to you. Note that some experiments are shorter than others. There is a compulsory pre-lab quiz on LEARN for each experiment. This must be done before before staring the experiment and is worth 10% of the individual experiment mark. Unknowns: Each of you are allocated two unknowns to identify. These are listed in the lab and on LEARN. There are no set sessions for this, you complete these whenever convenient to you. Resources for this are in the computer room at the rear of the lab. All information about this, including the report sheet, a pre-printed form, is available on LEARN. Total lab mark is 7 x 20 for the experiments and 2 x 20 for the unknowns. Total 180. It is not necessary to complete all the experiments to pass the lab course. If you feel you are struggling then talk to a demonstrator or staff member about your progress as they will be able to give you guidance as to the best way to maximise your remaining time in the lab.

You will be issued with one laboratory notebook for recording experiments throughout all three (inorganic, physical and organic) labs. Your lab book must be inspected and initialled by a demonstrator or staff member at the end of every session.

The week following the six weeks of lab sessions is for writing up and no lab work may be done during that week. It is in your interests to hand in reports soon after you complete the experiment as this will allow you to take into account feedback from the marked reports before writing the next report. All reports are due in by: Lab / tutorial groups A, B and C: 5pm Friday of Semester 1 week 8 Lab / tutorial groups D, E and F: 5pm Friday of Semester 2 week 4 Without prior agreement by Dr. Peter Kirsop no reports will be accepted after this time and a mark of zero will be recorded for the missing reports.

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EXPERIMENT 1

Synthesis of Sulfanilamide from Aniline. Have you done the pre-lab? Sulfanilamide was the first substance to be used systematically and effectively as a chemotherapeutic agent for the prevention and cure of bacterial infection in humans. You will hear much more about these compounds in the third year medicinal chemistry course. This is a multi-step synthesis starting from aniline, and is almost identical to the method used industrially for sulfanilamide production.

RISK ASSESSMENT

Substances Used

Hazards Identified

Aniline Toxic. Possible carcinogen. Possible mutagen. Possible sensitizer. Readily absorbed through the skin

Acetic anhydride Flammable, causes burns on contact with skin, harmful by inhalation

Sodium acetate Irritant

Chlorosulfonic acid Causes severe burns, irritant, toxic. Fumes on contact with air

Ammonium hydroxide Corrosive, causes burns, irritant to skin, eyes and respiratory system

Conc. Hydrochloric acid Causes severe burns

Safety Procedures: Safety glasses, gloves and laboratory coats should be worn at all times.

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Emergency Procedures: Spillage: Wipe up wearing gloves. Fire: CO2 extinguisher Waste Disposal: Waste solvents should be placed in appropriate waste bottles located in fume cupboards First Aid: The nearest first aid workers are in the Technicians area.

Step 1 – Preparation of acetanilide from aniline

First, make up a solution of sodium acetate (8.5 g in 30 mL of water) and put to one side. Dissolve aniline (8.0 g) in a mixture of water (135 mL) and concentrated hydrochloric acid (7.5 mL) in a 250 mL round bottom flask. The solution should be colourless – if it is not add around 4 spatula’s of activated charcoal and stir for a few mins. Remove the charcoal by filtering through a celite layer. Add acetic anhydride (10 mL) to the solution whilst stirring with a stirrer bar and then add the sodium acetate solution. Stir for a few minutes then cool in an ice bath where a solid should form. Collect the solid (which should be colourless) by filtration. The product must be dry before moving on, so dissolve the solid in dichloromethane and add some anhydrous magnesium sulphate. Stir, then remove the magnesium sulphate by filtration. Recover the solid acetanilide by removing the solvent on a rotary evaporator. Record the yield, melting point and an IR spectrum. Confirm with a demonstrator that the step has been successful before starting the next step.

Step 2. Chlorosulfonation of Acetanilide Place acetanilide (5.0 g) in a dry 100 mL conical flask and heat, with occasional swirling, on a hotplate until it liquefies (Do not boil). Remove from the hotplate and allow to cool and solidify, then chill in an ice bath. Add chlorosulfonic acid (13 mL, in portions with a pipette CARE – reacts violently with water) whilst still in the ice bath and allow to warm slowly to room temperature, with occasional swirling. Most of the solid material should dissolve before proceeding. Heat the liquid on a boiling water bath for a 10 minutes with occasional swirling. Initially there will be effervescence as the reaction completes. Allow to cool to room temperature. Slowly poor the contents, with vigorous stirring, into a 250 mL beaker containing approx. 75 mL of ice water. This hydrolysis is exothermic and a milky white precipitate will form. Collect the solid by suction filtration. Press the solid with a spatula and, using vacuum, suck off as much of the water as possible. Transfer the wet solid to a weighed 100 mL conical flask. You should have around 8 – 10 g of crude wet solid. Do not stop at this stage – the product of this step will deteriorate over time. Go straight on to the next step.

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Step 3. Preparation of p-Acetamidobenzenesulfonamide Add concentrated ammonia solution (20 mL) and water (20 mL) to the conical flask with the product from the previous stage. Swirl the flask until the reaction mixture forms a solid mass – this should take about one minute. Warm the flask on a hotplate for 10 – 15 minutes but do not allow to boil. Allow to cool to room temperature and then chill in an ice bath. Collect the solid by suction filtration and wash the solid with approx 10 mL of water. Allow the solid to remain under suction for a few minutes to remove most of the water. Dry a sample of the product overnight in a desiccator to allow determination of the melting point and to obtain an IR. Use the rest to proceed onto the next step.

Step 4. Conversion to Sulfanilamide Place the crude product from the last step in a 100 mL conical flask and add a solution of hydrochloric acid made up of bench concentrated HCl (5 mL) in water (10 mL). Boil the mixture gently on a hotplate until all the solid has dissolved (5 – 10 min). If the solid does not dissolve after this time add another 10 mL of the acid / water mixture. Continue to heat at boiling point for a further 10 mins, but do not evaporate to dryness. Allow to cool to room temperature. Transfer the cooled liquid to a 250 mL beaker and slowly add saturated sodium bicarbonate solution with stirring until the solution is no longer acidic (litmus). Cool in an ice bath and collect the white precipitate of sulfanilamide using suction filtration. Recrystallise the product from ethanol or water. Record the yield (based on the quantity of acetanilide used at the start of stage 2) melting point and IR spectrum. Sample of the final product only is required.

Questions

1. In your report give mechanisms for all the reaction steps. (3 marks) 2. Why is an acetyl group added to aniline, and then later removed to regenerate the amine

in the final product? (1 mark)

3. What happens when the product of step 2, p-acetaminobenzenesulfonyl chloride, is allowed to stand for some time in contact with moisture from the atmosphere? (1 mark)

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EXPERIMENT 2

Stereospecific Reduction of Benzoin with Sodium Borohydride; Determination of the Stereochemistry by NMR spectroscopy

Have you done the pre-lab?

The stereochemical course of ketone reductions can be influenced by the presence of hydroxyl groups close to the carbonyl function. This experiment illustrates the stereoselective reduction of benzoin using sodium borohydride as a reducing agent, followed by the conversion of the resulting 1,2-diol into its acetonide (isopropylidene) derivative catalysed by anhydrous iron (III) chloride, a reaction commonly used for the protection of 1,2-diols during a synthetic sequence. NMR spectroscopic analysis of the acetonide permits the determination of its relative stereochemistry and hence that of the diol.

Ph O

OHPh

NaBH4

EtOH

Ph OH

OHPh

OH

OHPh

Ph

Me2CO

FeCl3

Ph

Ph

O

O

O

O

Ph

Ph

Me

Me

Me

Me

RISK ASSESSMENT: 1. Preparation of 1,2-diphenylethane-1,2-diol Substances Used Hazards Identified Benzoin (FW 212.3) Irritant

Sodium borohydride (FW 37.3) Corrosive, flammable, toxic, reacts violently with water

Ethanol Flammable, toxic Light petroleum (bp 60-80oC) Flammable, harmful by inhalation Hydrochloric acid (6M) Corrosive

2. Preparation of acetonide derivative Substances Used Hazards Identified Acetone (pure) Flammable

Iron (III) chloride (anhydrous) Corrosive, hygroscopic Dichloromethane Irritant, toxic Light petroleum (bp 60-80oC) Flammable, harmful by inhalation Potassium carbonate solution (10%) Corrosive

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a hood.

Emergency procedures: Spillage: Wear gloves and wipe up with copious amounts of water. In the event of a sodium borohydride spillage, do not use water to clean up the area until you have removed the bulk of the material. Fire: CO2 extinguisher

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Waste Disposal Waste solvents to bottles in fume hoods.

First Aid: The nearest first aid workers are in the Technicians area.

PROCEDURES 1. Preparation of 1,2-diphenylethane-1,2-diol Dissolve benzoin (2.00 g, 9.4 mmol) in 20 mL of ethanol in a 100 mL conical flask (dissolution need not be complete). Stir the solution magnetically, and add sodium borohydride (0.40 g, 10.6 mmol) in small portions over 5min using a spatula (CARE!, exothermic reaction). If necessary, rinse in the last traces of sodium borohydride with 5mL of ethanol. Stir the mixture at room temperature for a further 20min, and then cool it in an ice bath whilst slowly adding 30 mL of water followed by 1 mL of 6M hydrochloric acid. Some foaming will occur at this stage. Add a further 10 mL of water, and stir the mixture for a further 20 min. Collect the product by suction filtration, and wash it thoroughly with 100 mL of water. Dry the product by suction for 30 min, and record the yield. This material is sufficiently pure to be used, so set aside 1.00 g of the product to be left drying until the next period for use in the next stage. Recrystallise the remainder (ca. 0.50 g) from hexane. Add acetone dropwise to the hot hexane until the solid just dissolves. On cooling crystals will form. Record the mp and IR spectrum of the product after one recrystallisation. Record an IR spectrum of benzoin for comparison. Make sure that this step has been successful (IR) before attempting the next stage. 2. Preparation of acetonide derivative (2,2-dimethyl-4,5-diphenyl-1,3-dioxolane) Dissolve 1.00 g of the diol in 30 mL of pure acetone, and add anhydrous iron (III) chloride (0.30 g). Transfer this reagent rapidly, as it is hygroscopic. Heat the mixture under reflux with a silica gel guard tube for 20 min, and then allow it to cool to room temperature. Pour the mixture into a 100 mL beaker containing 40 mL water, and add 10 mL potassium carbonate solution. Transfer the mixture to a 250mL separating funnel and extract with 3 x 20 mL portions of dichloromethane. Wash the combined organic extracts with 25 mL water, and then dry them over MgSO4. Evaporate the solvent on the rotary evaporator, and purify the crude acetonide by dissolving it in 15 mL boiling petroleum (bp 40-60 oC), and filtering whilst hot to then cool the solution in ice, whereupon the acetonide crystallised out. Collect the product by suction at the filter pump for 10 min. Record the yield, mp and IR spectrum of the product. Record the NMR spectrum (CDCl3) of your purified material for assignment of stereochemistry. Do not delay NMR analysis, as the product slowly decomposes at room temperature. Submit a sample of the final product only with the report.

Questions: 1. Assign your NMR spectrum of the acetonide derivative; including its stereochemistry,

and hence that of the diols. (2 marks) 2. Give the mechanism and stereochemistry of the reduction of benzoin; propose a

transition state for the reaction which accounts for the stereochemistry. (2 marks) 3. Give the mechanism of acetonide formation. What is the role of the FeCl3? (2 marks)

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EXPERIMENT 3

Catalytic Hydrogenation Have you done the pre-lab?

Catalytic hydrogenation is a valuable method for the reduction of various functional groups and is used widely in the synthesis and structure elucidation of organic molecules. It involves agitating the compound to be reduced, in an inert solvent, at or above atmospheric temperature and pressure, in an atmosphere of hydrogen in the presence of a suitable catalyst. Typical hydrogenation catalysts are transition metals (e.g. Pt, Pd, Ni) on different inert ‘supports’ (carbon, BaSO4, CaCO3), the nature of the support determining the activity of the catalyst. Catalytic hydrogenation has the advantages over other comparable types of reduction that it is simple in practice, often proceeds under mild conditions, and that the products are usually readily isolated. One of the most important applications of catalytic hydrogenation is in the reduction of carbon-carbon double bonds and this reaction is illustrated in the present experiment by the conversion of cinnamic acid (3-phenylpropenoic acid) into hydrocinnamic acid (3-phenylpropanoic acid). In the first part of this experiment the cinnamic acid is prepared using the Perkin reaction. This reaction has been known for many years and is a variation on the aldol reaction.

The second part is the hydrogenation of the cinnamic acid to hydrocinnamic acid:

1. Preparation of Cinnamic Acid. RISK ASSESSMENT

Substances Used

Hazards Identified

Benzaldehyde

Irritant, flammable

Acetic anhydride

Flammable, causes burns on contact with skin, harmful by inhalation

Potassium carbonate Irritant

PROCEDURE

Place benzaldehyde (5.0 mL) and anhydrous potassium carbonate (7.0 g) in a dry 100 mL round-bottomed flask with a stirrer bar. Then add acetic anhydride (8.2 mL) and fit a condenser with drying tube. Heat slowly to 180 oC whilst stirring in an oil bath. At approximately 100 oC foaming should be observed. Allow the mixture to reflux for 90 minutes.

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Allow the mixture to cool, then add 40 mL of water. Transfer the reaction mixture to a 500 mL beaker ensuring that no solid material remains in the flask. Add bench sodium hydroxide solution dropwise until alkaline (test with red litmus). Add 150 mL of diethyl ether to the beaker and stir rapidly to try and dissolve as much of the solid material as possible. Transfer the mixture, without any remaining solid material, to a 250 mL separating funnel. Shake well, then transfer the aqueous layer into a beaker. Acidify with bench concentrated HCl until acidic with a pH of between 2 to 3 (universal indicator paper). A precipitate of cinnamic acid will form; remove this using suction filtration. Wash the solid with water then dry the product in a desiccator overnight. Record the yield, melting point and IR of the dried product.

2. Hydrogenation of Cinnamic Acid.

RISK ASSESSMENT

Substances Used

Hazards Identified Cinnamic acid

Irritant

Ethanol Highly flammable

5% Palladium on charcoal

Flammable

Hydrogen

Extremely flammable

Celite Harmful by inhalation, irritating to eyes and respiratory system

Petrol (bp 60-80oC)

Extremely flammable, harmful

Equipment used and Hazards Identified: Hydrogen cylinder, water pump pressure, danger due to use of highly flammable gas. First Aid: The nearest first aid workers are in the Technicians area.

Safety Procedures: Instructions on use of hydrogen cylinder must be followed carefully; if unsure ask a technician, postgraduate demonstrator or a member of staff for assistance.

Emergency Procedures: Fire: IF THERE IS A FIRE NEAR THE CYLINDER EVACUATE THE AREA IMMEDIATELY, SOUND THE ALARM, DO NOT ATTEMPT TO EXTINGUISH THE FIRE. PROCEDURE 1. Dissolve cinnamic acid (1.0 g) in ethanol (30 ml), add water (6 ml) and place the solution, together with a magnetic stirrer bar, in the 100 ml RB reaction flask. This reaction is very sensitive to any trace contaminants, so you will need to use the specially clean flask and stirrer bar provided by the technician, which should be returned at the end of the session. Add 5% palladium on charcoal catalyst (0.1 g). Seal the flask with the provided B24 septum. Clamp the flask above a magnetic stirrer in the fume cupboard. 2. Remove the plunger from a 5 mL plastic syringe and attach a balloon to the open end. With the assistance of a demonstrator fill the balloon with hydrogen from the cylinder at the far end of the laboratory. Place your finger over the end of the syringe to prevent the gas from escaping.

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3. Back at your fume cupboard remove your finger and quickly fit a needle. Push the needle through the septum and into the flask. 4. Start the magnetic stirrer and set it to the highest speed. Position the flask slightly to one side of the stirrer to cause the solution to “splash” whilst stirring. This helps the uptake of the hydrogen. 5. Push a second needle into the septum for ten seconds or so to allow the hydrogen to displace some of the air in the flask. 6. The balloon will slowly deflate as the hydrogen is taken up. After approximately one hour the balloon should have deflated by a significant amount. Remove the balloon and allow it to completely deflate in the fume cupboard. Switch off the stirrer and remove the septum. 7. Filter off the catalyst through a Buchner funnel in which the filter paper has been covered with a layer (ca. 5 mm) of Celite. Deposit the layer by filtering an ethanol-Celite slurry. Evaporate the filtrate on a rotary evaporator until the oily residue of hydrocinnamic acid is clear and free from droplets of water. Dissolve the oil, while still hot, in light petroleum (bp 60-80oC) (ca. 2 ml), pour the solution into a conical flask, and allow it to cool. Scratching with a spatula or glass rod may be necessary to initiate crystallisation of the acid. When crystallisation is complete, filter off the acid, wash it with a small volume of pentane and allow it to dry in air. Record the yield, m.p, IR and 1H NMR spectrum of the acid (consult a demonstrator on the procedure to be used for recording the 1H NMR spectrum). Obtain a sample 1H NMR spectrum of the starting cinnamic acid from the laboratory technician and compare it to the 1H NMR spectrum of your product, noting significant differences in absorption. Assign your NMR spectrum. Submit a sample of your product with the report.

EXERCISES AND QUESTIONS

1. Using the proton NMR (available on LEARN) determine if the cinnamic acid produced in the first step is cis or trans. Justify your decision. (1 mark)

2. Calculate the theoretical requirement, and the volume, of hydrogen for complete

hydrogenation of your sample of cinnamic acid to hydrocinnamic acid. (1 mark) 3. Predict the product of the catalytic deuteration of your cinnamic acid, paying particular

attention to its stereochemistry. (1 mark) 4. What would be the expected product from the reduction of your cinnamic acid with

lithium aluminium hydride? (1 mark)

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EXPERIMENT 4

Preparation of 7-Trichloromethyl-8-bromo-1-p-menthane by Radical Addition of

Bromotrichloromethane to -Pinene

Have you done the pre-lab?

Free radical additions to alkenes are examples of chain reactions, with each cycle of addition generating more radical species. Although the reaction should be self-sustaining once initiated, continuous production of radicals is necessary to maintain the reaction due to quenching processes taking place, in which two radicals combine and are removed from the reaction sequence. Benzoyl peroxide is used in the following experiment as the initiator of the reaction, but radical species may also be generated conveniently in the laboratory using UV light.

Me

Me

CH2

CCl3

Me MeBr

BrCCl3

(PhCO2)2 cat.

RISK ASSESSMENT: Substances Used Hazards Identified

-Pinene (FW 136.2) Flammable, irritant Bromotrichloromethane (FW 198.3) Toxic, irritant NOTE: bromotrichloromethane is highly toxic. Always handle in the hood Benzoyl peroxide (FW 242.2) Explosive NOTE: benzoyl peroxide is an oxidising agent and liable to explode if heated or ground as the dry solid. Handle with extreme caution. Cyclohexane Flammable Methanol Flammable, toxic

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a hood. Bromotrichloromethane must be handled in a hood at all times. Any waste residues should be transferred to the chlorinated solvent waste, rinsing with dichloromethane if necessary.

Emergency procedures: Spillage: wear gloves and wipe up with copious amounts of water Fire: CO2 extinguisher

Waste Disposal Waste solvents to bottles in fume hoods. First Aid: The nearest first aid workers are in the Technicians area.

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PROCEDURE: Place the -pinene (1.2 mL = 1.02 g, 7.5 mmol), bromotrichloromethane (0.85 mL, 8.2 mmol) and benzoyl peroxide (CARE!) (approx. 5 mg, catalytic quantity) in a 100 mL three-necked flask equipped with a reflux condenser with nitrogen bubbler and a nitrogen inlet. The third neck is stoppered. Add 30 mL cyclohexane, taking care to wash down all the material that may be adhering to the walls of the flask. Heat the mixture under a nitrogen atmosphere under reflux for 40 min, taking care not to apply heat above the surface of the liquid. Add 5 mL of water to the mixture and remove the solvent and excess bromotrichloromethane on the rotary evaporator in a fume hood. It will be necessary to heat the water bath (ca. 40 oC) to remove the solvents, if bumping is a problem transfer the mixture to a larger flask using a further 5 mL of water. Cool the aqueous residue in an ice bath until the oil solidifies (ca. 15 min), break up the solid with a spatula and filter it with suction. Collect the solid and wash it with 10 mL of water followed by three 5 mL portions of ice-cold methanol. Dry the residue with suction and record the yield of the crude product. The material is

fairly pure, and should be analysed at this stage by TLC (referencing to the -pinene starting material) and 1H NMR (CDCl3). Try the TLC in a light petroleum 40/60 : ethyl acetate mixture and vary the ratio, if required, to get good separation of the product and starting material. Assign the NMR spectrum. The product may then be purified further by recrystallisation from methanol (avoid prolonged heating as this can lead to decomposition). Record a melting-point of your recrystallised compound. Submit a sample of your product with the report.

Questions: 1. Draw out the mechanism which occurs in the radical chain reaction that you have just

carried out. (2 marks) 2. Explain which structural features of benzoyl peroxide make it a useful means of

generating radicals thermally in the laboratory (1 mark) 3. Predict the major products (if any) of the following reactions and explain your

reasoning: (4 marks)

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EXPERIMENT 5

Asymmetric Synthesis: Sharpless Epoxidation of Geraniol

Have you done the pre-lab?

The term asymmetric synthesis is applied to a reaction which produces two enantiomers or diasteromers in unequal amounts. Since the reaction of a non-chiral substrate with a non-chiral reagent must give an optically inactive product [either a meso-compound or an equal mixture of (+) and (-) enantiomers], it is necessary to have a chiral influence somewhere - either in the substrate or the reagent. In recent times there has been an active search for new chiral reagents which can accomplish asymmetric syntheses using a wide variety of substrates with high degrees of optical purity. One of the most successful has been the chiral epoxidation of allylic alcohols developed by Professor K.B. Sharpless at M.I.T. The actual oxidising agent is t-butylhydroperoxide (ButOOH), but the reaction is catalysed by a mixture of titanium tetraisopropoxide [Ti(OiPr)4] and diethyl tartrate. The latter compound is conveniently available in optically active form, and acts as a chiral catalyst in transferring its asymmetry to the final product.

RISK ASSESSMENT

Substances Used

Hazards Identified Geraniol

Irritant

Molecular sieves Irritant

Dichloromethane

Possible risk of irreversible effects

L-(+)-Diethyl tartrate

-

Titanium (IV) isopropoxide Flammable, irritant

t-Butylhydroperoxide

Highly flammable, causes burns, harmful, possible risk of irreversible effects

Acetone Highly flammable

Solid CO2

causes burns

30% Aqueous sodium hydroxide

Causes burns, harmful

Sodium chloride Irritant

Magnesium sulphate

Harmful

Equipment used and Hazards Identified: Assembly of hot apparatus; hazards due to high temperatures; use of solid CO2-acetone bath; hazards due to low temperatures; addition of harmful materials using syringes.

Safety Procedures: Procedure must be carried out in a fume hood. Gloves must be worn when using extremes of temperature and when handling dangerous materials (titanium isopropoxide, tert-butylhydroperoxide). Wear safety glasses and a laboratory coat.

Me Me

OH

MeCO2Et

CO2Et

HO

HO Me

Me

Me

OH

O

Ti(OiPr)4

tBuOOH

+

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Emergency Procedures: Spillage: Wear gloves; if dangerous materials are involved (dichloromethane, tert-butylhydroperoxide) wear suitable protection to guard against inhalation; wipe up using plenty of water. Fire: CO2 extinguisher. First Aid: The nearest first aid workers are in the Technicians area.

Waste Disposal: Give syringe used for titanium isopropoxide to technician for cleaning immediately after use, place waste solvents in appropriate bottles in fume hoods.

PROCEDURES

(2S,3S)-EPOXYGERANIOL

The following preparations should be carried out in the laboratory period BEFORE the experiments: (a) Dry, overnight in an oven at 125 °C:

100 ml 3-necked RB flask Magnetic stirrer bar Screw cap thermometer holder (glass portion only) 25 ml measuring cylinder 50 ml conical flask

Powdered molecular sieves (b) Make sure that the following reagents are available:

dry dichloromethane solution of t-butylhydroperoxide (fridge)

First, remove the sieves from the oven and allow to cool in a desiccator. Then assemble the hot apparatus. To avoid condensation of moisture on apparatus on cooling, do not remove from the oven too soon before assembly. Place the magnetic stirrer bar in the flask, and attach a low-temperature thermometer (available from the technician) in the screw-cap adapter, a nitrogen bubbler (see a demonstrator), and a rubber septum to the necks, and allow the apparatus to cool down to room temperature under an atmosphere of nitrogen (be sure to use pressure tubing to connect the nitrogen line). Then add the powdered molecular sieves (0.28 g) and dry dichloromethane (15 ml) to the flask, start the magnetic stirrer, and cool the flask in a dry ice-acetone bath to -10°C. Temperature control

is VERY important in this experiment – put the acetone in the bath first and slowly add the dry ice until the required temperature is reached. Using plastic syringes, add L-(+)-diethyl tartrate (0.13 ml) and titanium (IV) isopropoxide (0.15 ml) to the flask through the septum, followed by the solution of t-butylhydroperoxide in dichloromethane (6M, ca 2.5 ml) using a 5 ml plastic syringe. (Plastic syringes can be obtained from the technician and should be rinsed with ethanol and returned immediately after use). The mixture is stirred at -10 °C for 10 min and then cooled to -20 °C by the addition of more dry

ice to the acetone bath. Dissolve geraniol (1.54 g) in dry dichloromethane (1 ml) in the 50 ml conical flask and add this solution dropwise via a syringe to the vigorously stirred mixture, so that the temperature does not rise above -15 °C. After the addition is complete, the mixture is stirred for a further 60 min at -15 to -20 °C. It is important to ensure that the temperatures stated in the procedure are closely adhered to.

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Next, allow the mixture to warm up to 0 °C, and add water (3ml). Allow the mixture to

warm to room temperature - two phases may be visible - and add a solution of sodium hydroxide (30%, 0.7 ml) saturated with sodium chloride to hydrolyse the tartrates. Continue to stir the mixture vigorously for another 10 min. Allow to settle, separate the organic (lower) layer, and extract the aqueous layer twice with bench dichloromethane (2 x 10 ml). Dry the combined organic layers (MgSO4) and evaporate the solvent (rotary evaporator). The crude product is purified by flash chromatography on silica gel. Record a tlc of the product using ethyl acetate/hexane mixtures as eluent, referenced to geraniol. Vary the eluent until the product epoxide has an RF of 0.3; this will be the eluent you will use for chromatography. It may be necessary to try a number of different solvent systems before an acceptable separation is obtained on the tlc plate. You need to use a permanganate dip to visualise the spots. Set up the flash chromatography equipment (see video on LEARN or kirsoplabs.co.uk), and purify the crude epoxygeraniol. Record an NMR spectrum (CDCl3), yield and IR spectrum of the final product. Analyse the NMR spectrum to assess the purity of the epoxygeraniol. Sample 1H NMR of the starting geraniol are available for comparison purposes and are fixed to the glass windows of the staff area. OPTICAL ROTATION OF (2S,3S)-EPOXYGERANIOL Optical rotation is a useful method for determining the enantiomeric purity of a compound and hence allows you to determine the extent to which you have successfully transferred asymmetry from the diethyl tartrate to the epoxide in the course of this reaction. Record the optical rotation

[]D of your epoxygeraniol using an accurately known concentration of around 0.3 g in chloroform made up to 10 ml in a volumetric flask.

Calculate the []D according to the following equation:

[]D = 100 x = measured rotation l x c l = path length (i.e. length of cell) in dm c = concentration in g/100ml

The optical rotation should be presented in the form []D = 22.4º

(c 4.3 g/100 ml in CHCl3), where you have substituted your values for []D and c. Do not submit a sample of your product. EXERCISES AND QUESTIONS 1. Assign your NMR spectrum and comment on the purity of your product. (4 marks) 2. Draw the correct stereochemical structure for (2S,3S)-epoxygeraniol. (2 marks) 3. Suggest a method of making the (2R,3R) isomer. (1 mark)

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EXPERIMENT 6

A Knoevenagel Initiated Annelation Reaction Have you done the pre-lab? The Hagemann ester (1) has been used extensively in recent years as a building block for a wide variety of important natural products. It is commonly made by allowing ethyl acetoacetate and formaldehyde to react in the presence of piperidine catalyst followed by dehydration and partial deethoxycarboxylation with sodium ethoxide in ethanol.

O

O O

+H H

O

O

COOEt

1. piperidine / EtOH

2. EtOH / EtO-1

The reaction initially involves a Knoevenagel condensation, a subset of the general aldol condensation. Typically an amine such as piperidine is used as the catalyst in this condensation rather than hydroxide or ethoxide. The three stages are summarised below:

The reaction you will perform is similar to the formation of the Hagemann ester with an additional stereochemical component. It involves the reaction of two moles of methyl acetoacetate and one mole of acetaldehyde with a piperidine catalyst from which two stereoisomers are produced, namely, the trans isomer (2) and the cis isomer (3).

O

O O

H

O

O

COOMe

1. piperidine

+H3O+

O

COOMe

+

2 3

It is possible to use the proton NMR spectrum of the mixture of products to identify the two isomers. The Karplus curve shows the relationship between the coupling constant and the angle between two adjacent carbon-hydrogen bonds, and the value of J, the coupling constant, is different for the hydrogen atoms on carbons 4 and 5 in the cis and trans products as the angle between the two hydrogen atoms will be different.

17

O

COOMe

12

3

45

6

O

COOMe

12

3

45

6

The Karplus curve, showing how the coupling constant varies with the dihedral angle:

15

10

5

45 90 180

Angle / deg

J / Hz

In this experiment the proton NMR spectrum clearly shows two different signals for the hydrogens on carbon 4 for the two isomers.

RISK ASSESSMENT

Substances Used

Hazards Identified

Acetaldehyde Highly flammable, harmful Methyl acetoacetate Irritating to the eyes Piperidine Harmful by ingestion, inhalation or through skin contact. Methanol Highly flammable, toxic Hydrochloric acid Causes burns Ether Volatile, highly flammable

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a hood. First Aid: The nearest first aid workers are in the Technicians area.

Emergency procedures: Spillage: Wear gloves and wipe up with copious amounts of water. Fire: CO2 extinguisher

18

Waste Disposal: Place waste solvents in appropriate bottles in fume hoods.

PROCEDURE A mixture of 0.88 g (0.02 mol) of acetaldehyde, 4.65 g (0.04 mol) of methyl acetoacetate and 1.7 g (0.02 mol) of piperidine in 25 mL of 50% aqueous methanol is allowed to stand stirring at room temperature in a 250 mL conical flask covered with parafilm for at least 48 hours. At the end of this time the reaction 10 mL of 6M hydrochloric acid (fume hood) is slowly and carefully added. The reaction mixture bubbles at this point. The solution is then extracted twice with 30 mL portions of ether, Use TLC to confirm that the reaction has gone to completion, by comparing your product with a sample of methyl acetoacetate in solvent. In this case a good solvent system to try is 4 parts light petroleum 40-60 to 1 part ethyl acetate. If TLC indicates that all or most of the starting material has been consumed then a flash chromatography column should be used to separate the products from remaining starting materials. Note the RF values of the product(s) and starting material. First dry the combined ether extracts over anhydrous magnesium sulfate for 10 minutes. The solution is then gravity filtered to remove the magnesium sulfate and the ether removed using a rotary evaporator leaving a yellow oil. Record the weight of the oil for calculating the crude yield of the reaction. Then set up the flash chromatography equipment (see video on LEARN or kirsoplabs.co.uk), and purify the crude product. Record an NMR spectrum (CDCl3), yield and IR spectrum of the final product. Analyse the NMR spectrum to assess the purity of the product. Do not submit a sample of your product. EXERCISES AND QUESTIONS 1. Draw Newman projections for the cis and trans isomers 2 and 3. Use the Karplus relationship to predict the J coupling constants for the two isomers. (2 marks) 2. You should be able to see two doublets on your NMR between 3 and 3.5 ppm. These doublets correspond to the hydrogen on carbon 4 on the product. One doublet is for the trans- isomer, the other for the cis- isomer. Use your answer to question 1 to determine which doublet corresponds to which isomer, and so work out the ratio of the two isomers in the products. Label the cis and trans isomers on your NMR. (2 marks) 3. Draw a mechanism for the reaction, including the intermediates. (4 marks)

19

EXPERIMENT 7

Diazotisation of Aminocyclohexanol

Have you done the pre-lab? This is a two-step synthesis. In the first stage an epoxide used as a precursor to the amino alcohol. Upon diazotisation the two possible products are shown below. It is not trivial to isolate the product so in this experiment it is derivatised with Brady’s reagent. Brady’s reagent contains 2,4 dinitrophenylhydrazine and, as you should remember from the second year lab, it reacts with aldehydes and ketones to give a precipitate which is easily recovered. Measurement of the melting point of this derivative and examination of NMR spectra give evidence for the products.

1. Preparation of 2-Amino-cyclohexanol

RISK ASSESSMENT

Substances Used

Hazards Identified

Cyclohexene oxide Flammable, irritant, harmful by inhalation.

Concentrated ammonia soln. Corrosive, causes burns, irritant to skin, eyes and respiratory system

Methanol Flammable, harmful by ingestion

2-Aminocyclohexanol Irritant, harmful by ingestion and inhalation

Equipment used and Hazards Identified: Standard glassware/equipment used; hazards associated minimal provided good laboratory practice is observed

Safety Procedures: Carry out procedure in a fume hood Wear safety glasses and a laboratory coat First Aid: The nearest first aid workers are in the Technicians area.

Waste Disposal: Place any waste solvents in the appropriate labelled bottles in the fume hood

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PROCEDURE Place of cyclohexene oxide (10 mL) and of concentrated ammonia solution (25 mL) into a 100 mL round bottomed flask with stirrer bar. Add methanol (25 mL) and stir the mixture at room temperature for at least 24 hours. Weigh the empty flask before you start. Remove the water, methanol and remaining ammonia using a rotary evaporator with the water bath set to at least 60 oC and the pressure set initially to 150 mbar then down to 20 mbar. Leave it on the evaporator until a viscous clear liquid remains. After removing from the evaporator and allowing to cool a solid should form. If a solid does not form add ethanol and then dilute with light petroleum (60 – 80). Poke the mixture with a glass rod and a solid should result. If in doubt consult a demonstrator. Record IR, melting point and yield. Check the IR with a demonstrator before moving on to the next stage.

2. Diazotisation and derivitisation of aminocyclohexyl alcohol

RISK ASSESSMENT

Substances Used

Hazards Identified

Sodium nitrite Toxic by ingestion, irritant, oxidising agent

Sulphuric acid Causes severe burns

Urea No significant hazard

Brady’s reagent Toxic by ingestion, causes burns, flammable.

Glacial acetic acid Causes severe burns

Equipment used and Hazards Identified: Use of standard glassware; hazards associated with this are minimal providing good laboratory practice is used Heating under reflux; hazards associated with the use of running water and electricity

Safety Procedures: Carry out in fume hood; Wear safety glasses and a laboratory coat at all times

Emergency Procedures: Spillage: Wear gloves, wipe up using plenty of water Fire: CO2 extinguisher

Waste Disposal: Place waste solvents in bottles provided in the fume cupboard First Aid: The nearest first aid workers are in the Technicians area.

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PROCEDURE Add 2-aminocyclohexanol (1.2 g) to glacial acetic acid (10 mL) in a 100 mL round bottomed flask with a stirrer bar and cool in an ice bath. Dissolve sodium nitrite (3.5 g) in water (10 mL) and add dropwise to the acetic acid / aminocyclohexanol solution whilst stirring in the ice bath. Brown fumes should be seen. Allow to stir vigorously for 30 mins, the add urea (1.3 g) and allow to warm to room temperature. When the foaming has stopped (approx 1 hour) add Brady’s reagent (25 mL). A yellow precipitate should form. Warm the mixture to 70 oC and keep at this temperature for 10 minutes Suction filter the mixture and transfer the yellow solid to a conical flask. Add acetone (50 mL) to the flask. This will dissolve much of the solid and leave a residue of white powder. Remove the powder from the solution by filtration. Put the orange solution on a rotary evaporator to recover a bright orange powder. Record the melting point, yield and an IR spectrum. Submit a sample of your final product only with the report.

EXERCISES AND QUESTIONS

1. Consult the 1H NMR spectrum of the aminocyclohexanol (lab or LEARN) and measure

the coupling constants for the protons at H 2.5 and 3.2 ppm. These are the protons adjacent to the NH and OH respectively. Use these coupling constants to provide evidence that the that the product is in the conformation shown below:

NH2

OH

This is the trans di-equatorial product. Typical J values are:

Jaxial – axial = 8 to 13 Hz Jequatorial – equatorial = 2 to 6 Hz

Explain, with a mechanism, how this product is formed. (4 marks)

2. Examine the carbon DEPT and 1D NMR of the DNPH derivative (LEARN). Use this information to determine which product you have obtained after diazotisation. Provide a mechanism for the formation of the product but no mechanism is needed for the derivitisation step. (4 marks)

HNNH2

NO2

NO2

2,4 dinitrophenylhydrazine

(DNPH)

22

EXPERIMENT 8

The Sharpless Asymmetric Dihydroxylation of Stilbene

Have you done the pre-lab?

The cis-dihydroxylation of an olefin with osmium tetroxide is a racemic reaction, but if the same reaction is carried out in the presence of a chiral catalyst the production of one enantiomer can be favoured over the other. Such catalysts have been developed in the laboratories of Nobel Laureate Professor K. Barry Sharpless at MIT in the USA. In this experiment the Sharpless asymmetric dihydroxylation (abbreviated to AD) uses the enantiomerically pure chiral ligand (DHQD)2PHAL (1) which is derived from the natural product quinine; DHQD stands for dihydroquinidine (2) and PHAL which is an abbreviation for phthalazine (3).

N

OHH

NEt

MeO

N

O H

NEt

OMe

N

OH

NEt

MeO

NNNN

2 31

The catalyst is available as part of a commercially available mixture which contains the necessary osmium, the chiral ligand and potassium ferricyanide as a co-oxidant, to re-oxidise the osmium after each catalytic cycle. The catalyst with the ligand shown above is sold as AD-

mix Dihydroxylation of the other face of an olefin can be performed using AD-mix which uses a different chiral ligand. This is discussed in detail in the asymmetric synthesis lectures. In this experiment the dihydroxylation reaction is to be carried out using trans-stilbene as the olefin. Determination of the enantiopurity of the product, 1,2 diphenyl-ethane 1,2 diol, will be by measurement of optical rotation using a polarimiter.

Ph

Ph OH

Ph

H

OH

Ph

H

H

H

AD-mix

23

RISK ASSESSMENT

Substances Used

Hazards Identified

AD-mix Hygroscopic, toxic trans-Stilbene Irritating to the eyes tert-Butanol Flammable Sodium sulfite Irritant Ethyl acetate Highly flammable

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a hood.

Emergency procedures: Spillage: Wear gloves and wipe up with copious amounts of water. Fire: CO2 extinguisher First Aid: The nearest first aid workers are in the Technicians area.

Waste Disposal: Place waste solvents in appropriate bottles in fume hoods.

PROCEDURE 5 mL of tert-butanol, 5mL of water and 1.4 g of AD-mix ** are added to a 50 mL round bottom flask. It is capped with a septum and put under a nitrogen atmosphere using a balloon. The mixture is cooled to 0O C in an ice-water bath and 0.18 g (1mmol) of trans-stilbene is added. The reaction is left to stir for at least 24 hours. Sodium sulfite (1.5 g) is then added and the mixture is stirred for 40 min at room temperature. The mixture is extracted 4 times with ethyl acetate (10 mL the first time, then 3 × 5 mL). The combined organic extracts are dried over MgSO4 and the solvent removed using a rotary evaporator to give the diol.

** NOTE: 1.4 g of AD–mix is composed of 0.980 g of K3Fe(CN)6, 0.410 g of K2CO3, 0.0078 g of (DHQD)2PHAL and 0.00074 g of K2OsO2(OH)4. In your report show the relative ratios of these ingredients. Using thin-layer chromatography to check that the reaction has gone to completion by running a sample of the product against stilbene (both need to be in solvent for this). A good solvent for running the TLC for these compounds is light petroleum 40-60. If the TLC of your product shows the product to be impure you will need to perform flash-column chromatography to purify the products. Note the quantity of product and calculate the yield. Submit a sample for 1H NMR analysis. Run an IR spectrum and obtain the melting point.

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OPTICAL ROTATION Optical rotation is a useful method for determining the enantiomeric purity of a compound and hence allows you to determine the extent to which you have successfully transferred asymmetry from the chiral catalyst to the product in the course of this reaction. Record the optical rotation

[]D of your 1,2 diphenyl-ethane 1,2 diol, using an accurately known amount of your product in ethanol made up to 10 ml in a volumetric flask.

Calculate the []D according to the following equation:

[]D = 100 x = measured rotation l x c l = path length (i.e. length of cell) in dm c = concentration in g/100ml

The optical rotation should be presented in the form []D = xx.xº

(c x.xx g/100 ml in CHCl3), where you have substituted your values for []D and c. Do not submit a sample of your product. Questions 1. Find the literature value for the optical rotation of your product. Does this agree with your observed value? If not, can you think of any reasons for the discrepancy. State where you found your literature value. (4 marks) 2. Draw the correct stereochemical structure for your 1,2 diphenyl-ethane 1,2, diol, stating if the chiral centres are R or S. (2 marks) 3. Assign the NMR and IR spectra of your product and comment on purity (2 marks)

25

EXPERIMENT 9

Heterocyclic Ring Expansion – Reaction of a Dimethylpyrrole with Dichlorocarbene.

Have you done the pre-lab?

Carbenes are a very reactive species, and a typical reaction of a carbene is to insert itself into a pi or sigma bond. This is driven by the extreme electrophilicity of the carbene – a carbon atom with only six electrons will do almost anything to get another two. Carbenes will undergo insertion into cyclic compounds and this can be used to increase the number of carbon atoms in the ring; this is known as a ring expansion and an example is shown below:

EtOOC NN

heatCOOEt

This is a very useful reaction as it is an example of forming carbon-carbon bonds as well as enabling the formation of rings that would be difficult to synthesise by any other means. In this experiment a carbene is generated from sodium trichloroacetate and reacted with a five membered heterocycle, 2, 5 dimethylpyrrole, to give a six membered heterocycle. Heterocycles are a very important class of organic compounds, particularly in biological systems, and their synthesis is very important in the drug industry. The starting material for this ring expansion, 2,5-dimethylpyrrole, is a reactive compound and has a short shelf-life. Fortunately synthesis is quite straightforward from inexpensive starting materials. Here it is synthesised it from 2,5-hexanedione (acetonylacetone):

OO

MeMe(NH4)2CO3

heat

HNMe Me

The 2,5-dimethylpyrrole is isolated by distillation and can be stored for up to few days in the refrigerator in readiness for the expansion reaction, shown overleaf.

26

RISK ASSESSMENT

Substances Used

Hazards Identified

Acetonylacetone Irritating to the eyes, do not inhale vapour Ammonium carbonate Eye, skin and respiratory irritant. May be Harmful if inhaled Sodium trichloroacetate Irritant 1,2-Dimethoxyethane Toxic, flammable Dilute Hydrochloric acid Causes burns Diethyl ether Extremely flammable, may form explosive peroxides Sodium hydroxide Causes burns

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a hood.

Emergency procedures: Spillage: Wear gloves and wipe up with copious amounts of water. Fire: CO2 extinguisher First Aid: The nearest first aid workers are in the Technicians area.

Waste Disposal: Place waste solvents in appropriate bottles in fume hoods.

1. Preparation of 2,5-dimethylpyrrole Place ammonium carbonate (10 g) and acetonylacetone (5 g) in a 100 mL B24 round bottom flask. Fit a B24 water jacket condenser and reflux gently for one hour in an oil bath at 115 OC. After this time all the solid material should have disappeared. Transfer the oil formed to a 50 mL round bottom flask and distil at a reduced pressure of max 25 mbar. If the fume cupboard pump will not go down this low consult a demonstrator. The dimethylpyrrole should distil at a temperature of approx 75 OC at 25mbar, but will be lower at lower pressure. Make a note of the exact temperature and pressure used in your distillation and include this in your report. The distillate should be a clear oil. Take an IR spectrum of the oil and then transfer it in a labelled stoppered flask into the refrigerator until you are ready to use it in the next stage. Confirm with a demonstrator that your IR spectrum is the target compound before proceeding with the next step.

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2. Preparation of 3-chloro-2,6-dimethylpyridine Sodium trichloroacetate ( 11.0 g, 0.06 mol) and dimethoxyethane (25 mL) are added to a 100 mL round bottom flask, along with a magnetic stirrer follower bar. A reflux condenser and nitrogen bubbler are fitted and the apparatus is flushed through with nitrogen. The condenser is briefly removed and 2,5-dimethylpyrrole (1.9 g, 0.02 mol) added. This addition should be done as quickly as possible to minimise contact with the atmosphere. The reaction mixture is refluxed gently in an oil bath at around 100 OC, whilst stirring, for 3 to 4 hours. This is best done by setting up the reaction and starting the reflux in a morning session and leaving the reaction refluxing over lunch. If you are not returning for the afternoon session you should notify a demonstrator or staff member so that they can have the stirrer hotplate switched off at the appropriate time and you can move onto the next stage at your next session. The reaction mixture should be a dark brown colour at this stage. Using thin layer chromatography, in a 4:1 mixture of light petroleum to ethyl acetate, run a spot of the reaction mixture against a solution of 2,5-dimethylpyrrole to confirm that all the starting material has been consumed. Note the RF values and record them in your report. Remove any solid material using suction filtration and transfer the filtrate to a 250 mL round bottom flask. Remove the volatile components of the mixture on a rotary evaporator. After cooling add 20 mL of dilute hydrochloric acid, shake well and then thoroughly extract the mixture with diethyl ether (3 × 30 mL). Discard the ethereal extract. Add sufficient bench (2 molar) sodium hydroxide solution to the aqueous component of the above procedure to make the pH alkaline, using red litmus or universal indicator paper to monitor this. Once again extract the mixture with diethyl ether (3 × 30 mL). Combine the ethereal extracts and dry them over magnesium sulfate. Remove the magnesium sulfate using filtration, and then remove the diethyl ether on a rotary evaporator. You should be left with a quantity of yellow oil. Note the yield. Record an IR spectrum and submit a sample for 1H NMR spectroscopy. Assign your NMR spectrum Do not submit a sample of your product.

Special note on washing glassware for this experiment. The second part of this experiment produces a brown residue that can be tricky to remove from your glassware. The best way to do this is by using lots of the green washing up liquid and plenty of hot water along with a brush. The washing-up liquid is stored in a large glass bottle on the sink near the window.

Questions 1. Draw the mechanisms for both parts of the experiment. (4 marks) 2. Deduce the product you would obtain on using 2,3-dimethyl indole (below) rather than the pyrrole in part 2. (1 mark)

N

Me

Me

28

EXPERIMENT 10

Synthesis of 6-Methylquinoline

Have you done the pre-lab? This experiment benefits from being run over two laboratory sessions on the same day. Quinoline 1 was first isolated from coal tar in 1834 and its name is derived from quinine, which is extracted from quina, the South American name for the bark of quinine containing cinchona species. Substituted quinolines have interesting biological properties, quinoline motif is present in several therapeutical compounds i.e. quinidine. The well known Skraup synthesis of quinoline from aniline and glycerol is shown in Scheme 1.

Substituted anilines can be employed in the Skraup synthesis to give substituted quinolines; for instance methyl anilines produce methylquinolines with a methyl substituent in the benzene ring of quinoline. In this experiment you will carry out a Skraup synthesis of a substituted quinoline. It is a well-known method for synthesizing simple heterocyclic compounds in one pot.

RISK ASSESSMENT: Substances Used Hazards Identified 4-Toluidine Toxic

Glycerol Irritating to eyes Iodine Harmful, don’t inhale vapour Acetic Anhydride Flammable, causes burns Celite Irritating to eyes and lungs Conc. Sulfuric acid Corrosive, causes severe burns 5 M Sodium Hydroxide Corrosive, causes burns Diethyl Ether Extremely flammable, may form explosive 1 M Hydrochloric Acid Corrosive Ethyl Acetate Highly flammable, irritant

Safety Procedures: Safety glasses, lab coats and gloves must be worn for all chemical manipulations. Reaction and work-up must be carried out in a fume hood.

Emergency procedures: Spillage: Wipe up wearing gloves. Fire: CO2 extinguisher First Aid: The nearest first aid workers are in the Technicians area.

29

Waste Disposal Place waste solvents in bottles provided in the fume hood.

PROCEDURES Add 4-toluidine (750 mg, 5.8 mmol), glycerol (2 mL, 27 mmol) and iodine (3-4 crystals) to a 25 mL round bottomed flask containing a stirrer bar. Attach a reflux condenser and cool the mixture in an ice bath, with stirring, before adding concentrated sulfuric acid (2 mL) dropwise. The resulting mixture is heated to 100-110 °C in an oil bath, stirring for 1 hour. Allow the reaction mixture to cool and then add ice water (10 mL). Transfer the reaction mixture to a 100 mL conical flask and cool it in an ice bath. Wash the reaction vessel with ice water (5 mL), add the washings to the cold conical flask and make the reaction mixture basic with 5 M sodium hydroxide. Add ether (25 mL) to the flask and vacuum filter the biphasic mixture with the aid of a Celite pad. Wash the conical flask with ether (2 x 20 mL) and pour the organic solution through the filter pad. Separate the biphasic filtrate using a separating funnel, wash the aqueous layer with ether (20 mL) and then combine all the organic fractions. Using TLC analysis (ethyl acetate/pet. ether 60-80 (1:4) of the ethereal solution, check for the presence of any starting materials. Dry the ethereal solution over magnesium sulphate, filter under suction and concentrate on the rotary evaporator. Add acetic anhydride (1 mL) and leave the resulting solution to react for 30 minutes. Add 1 M hydrochloric acid (20 mL) dropwise and stir for a further 10 minutes. Transfer the solution to a separating funnel and wash the reaction flask with ethyl acetate (20 mL), before adding the solvent to the separating funnel. Separate the two layers, wash the aqueous layer with more ethyl acetate (20 mL) and neutralise the aqueous layer with 2 M NaOH. Finally, extract the aqueous solution with ether (2 X 20 mL) and keep the ethereal solution. Dry the ethereal solution over magnesium sulphate, filter under suction. Fill a small sinter funnel with an ethereal silica gel slurry. Carefully pour the ethereal solution of the product onto the silica and apply suction until level of liquid is just above the top of the silica. Add more ether (20 mL) to the top of the silica and pass it through the filter using suction. Pour the filtrate into a pre-weighed RBF and then concentrate it to give the product. Record the yield and IR. Submit a sample for 1H NMR spectroscopy. Do not submit a sample of your product.

Questions: 1. Assign 1H and 13C NMR spectra of the 6-methylquinoline (LEARN). You are advised to

refer to cosy spectrum of 6-methylquinoline (LEARN) for correct analysis. (4 marks) 2. Compare your 1H NMR spectrum with the reference spectrum (LEARN). Comment on

the purity of your product. (4 marks)

3. Give the mechanism for the Skraup synthesis of 6-methylquinoline. (2 marks)

4. What is the acetic anhydride for? How does it aid in the purification of the product? (2 marks)

30

EXPERIMENT 11

A Diels- Alder Reaction

Have you done the pre-lab? The Diels-Alder reaction is a cycloaddition of an electron rich diene and an electron poor dienophile (alkene or alkyne). In this experiment, diene is 1,3-cyclohexadiene, which is locked in its “s-cis” conformation. The dienophile is N-phenylmaleimide which possesses a strong electron withdrawing carboximido group also held in a cis geometry.

RISK ASSESSMENT Substances Used

Hazards Identified

1,3-Cyclohexadiene Flammable

N-Phenylmaleimide Toxic, irritant

Ethanol Flammable, irritant

Equipment used and Hazards Identified: Microwave oven, the biggest concern is the proper assembly of the microwave reaction vials. Students need to assemble the microwave reaction vials with extreme care specifically the final tightening of the cap on the reaction vials before these are placed into the slots. Standard glassware/equipment used; hazards associated minimal provided good laboratory practice is observed.

Safety Procedures: Carry out procedure in a fume hood. Wear safety glasses and a laboratory coat.

Waste Disposal: Place any waste solvents in the appropriate labelled bottles in the fume hood.

First Aid: The nearest first aid workers are in the Technicians area.

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PROCEDURES

1. Synthesis of endo-N-phenylbicyclo[2.2.2]-oct-5-ene-2R, 3S-dicarboximide (a Diels-Alder adduct) using microwave oven

Ask demonstrator about the use of microwave and related equipment i.e microwave reaction vials, caps and magnetic stirrers (microwave magnetic stirrers are different than the normal magnetic stirrers) Weigh N-phenylmaleimide (0.20 g) directly into the microwave vial (2-5 mL vial). Add 1,3-cyclohexadiene (0.12 mL) to the vial. Add absolute ethanol (5 mL) and the magnetic stirrer, swirl the vial to dissolve the N-phenylmaleimide. Complete the assembly for microwave heating At this stage ask a demonstrator to check it. Place your sealed vial into a slot on the turntable (record your slot number). Microwave reaction parameters are as follow; Time: 10 minutes Temperature: 130 °C Prestirring: 2 minutes Absorption: High Vial type: 2-5 mL At the end of the heating period, recover you vial and vent it in your fume hood, then cool the reaction mixture first to RT and then place the vial in an ice bath. Suction filter the crystals, wash them with chilled ethanol (3 mL). Place the sample in the desiccator for 1 h. Determine the yield, melting point of the product. Record IR of you sample. Submit a sample for 1H, 13C NMR spectroscopy.

2. Synthesis of endo-N-phenylbicyclo[2.2.2]-oct-5-ene-2R, 3S-dicarboximide (a Diels-Alder adduct) using traditional chemistry

Weigh N-phenylmaleimide (0.20 g) directly into the 10 mL RBF. Add ethanol (5 mL) and a stirbar. Swirl to dissolve the maleimide. Add 1,3-cyclohexadiene (0.12 mL) to the reaction mixture and gently swirl. Complete a reflux set-up and heat for 1.5 hours. The product will precipitate out of solution as a white solid. At the end of the reflux period, cool the reaction mixture first to RT and then in an ice bath. Suction filter the crystals and wash them with chilled ethanol (3 mL). Place the sample in the desiccator for 1 h. Determine the yield, melting point of the product. Record IR of you sample. Submit a sample for 1H NMR spectroscopy. Do not submit a sample of your product.

Questions 1. Draw the mechanism for Diels-Alder cycloaddition reaction to afford endo-N-

phenylbicyclo[2.2.2]-oct-5-ene-2R, 3S-dicarboximide. (2 marks) 2. Assign 1H and 13C spectra of the endo-N-phenylbicyclo[2.2.2]-oct-5-ene-2R, 3S-

dicarboximide. (4 marks)

32

THE IDENTIFICATION OF UNKNOWN ORGANIC COMPOUNDS

In this exercise you have been assigned two unknown compounds to identify on the basis of their spectra. Check the laboratory notice board to find your allocated unknowns. All the spectra are on the PCs in the instrument room. There is a card for each compound which are in boxes in the instrument room. The cards have elemental analysis data, some notes and the DEPT spectra, but all this information is also loaded onto the computers. The reports for the two unknowns is completed by filling in the form available on LEARN. To access the spectra go to Computer, select C(OS) and open the folder “Unknowns”, then “Third Year Unknowns”. Open the folder for your compound and you can view the infra red, mass spec and DEPT NMR spectra. The FID’s are also here, for use in the ACD/NMR program. The first job is to find the molecular formula. Use the elemental analysis data to find the empirical formula. How to do this is described on the report sheet.

You can use the molecular formula to find out more about the compound by calculating the number of “double bond equivalents”.

For CaHbOc dbe = [(2a + 2) – b]/2

For CaHbOcNd dbe = [(2a + 2) – (b – d)]/2

Halogens count as hydrogen for this purpose and you will see that the number of oxygen atoms

is immaterial. The dbe value is the sum of the number of multiple bonds of all kinds (C=C, CC,

C=O, C=N, CN, N=O, etc) and rings (each ring counts as 1 dbe) in the molecule, and must always have an integral value. Note that for triple bonds the dbe = 2, and that for an aromatic ring the dbe = 4 (3 double bonds and one ring). Obviously, if you can identify securely by spectroscopic or other means the number of multiply bonded functional groups, e.g. carbonyl, present, the remainder will be rings. It is therefore possible to decide if the compound is acyclic, monocyclic, bicyclic, etc. and this is extremely helpful in determining a structure.

Example Calculation of dbe:

for C4H8O2 a = 4, b = 8, c = 2; dbe = [(2 x 4 + 2) - 8]/2 = 1

for C4H6NCl a = 4, b = 7 (6H + 1Cl), c = 0, d = 1; dbe = [(2 x 4 + 2) - (7 - 1)]/2 = 2

The 1H and 13C NMR spectrum of each compound has been recorded and should be processed using the ACD/NMR software available on the lab computers (see below). The spectra have been run at 250 MHz for proton, and 62.5MHz for carbon. Enough information is provided for complete identification. Remember that there will be a signal in the carbon NMR due to the deuterated solvent. CDCl3 appears as three peaks at around 77 ppm, so ignore these.

33

1. OPENING A FILE using ACDLABS NMR processor (on lab PC’s ) Go to File > Import > From 1D NMR Directory. In the ‘Look in’ box go to Computer > C(OS) > unknowns > chem3 and locate in this folder the folder bearing the allocated spectra number. Open this folder and click on ‘fid’. Go to the bottom of the box and click ‘Open’. The FID will now appear in the window of the program. If the program has just been opened there may be a window on the program starting up asking to open an existing file or create a new one. If this dialog box does appear click ‘Cancel’ and continue as above.

2. OBTAINING THE NMR SPECTRA FROM THE FID (Preforming a Fourier Transform) Go to ‘Process’ and then to ‘Fourier Transform’ and finally click on ‘Default Transform’. The ‘Default Transform’ function provides the most appropriate Fourier transform for the input data. A recognisable NMR spectrum should now have appeared in the window, replacing the FID. Now click on the ‘Phase’ button that lies on the third from the top command line and click on ‘Auto Simple’ then the green tick to correct the phase of the NMR spectra.

3. ANALYSING THE SPECTRA

To zoom into areas of interest:

Click on the image of a magnifying glass with a + in the middle of it and a double headed

arrow above it on the second from the top command line. Now click the mouse at one

side of the area of interest and drag the magnifying glass to the other side of this area,

then release.

To zoom out:

To zoom out and show the whole spectrum, click on the image of a magnifying glass

with a – in the middle of it. However, to undo a zoom, click on the magnifying glass with

a green arrow on it.

To obtain the chemical shift of the peaks – listing the peaks:

Click on the ‘Peak Picking’ button that can be found on the same command line as

‘Phase’ and then on ‘Auto’. There should now be numbers above each peak indicating

their chemical shift in ppm. It may be helpful to zoom into areas with the peaks to make

sure that all of the peaks have their chemical shift denoted. If the ‘Auto’ function has

missed some peaks out, these can be picked out manually by clicking on ‘Peak by Peak’

and then on the individual peaks in the spectra. If no numbers appear above the peaks

in the spectra or to check that all of the peaks have been listed, then click on the image

of an NMR spectrum with a box above it that can be found on the same command line

as the zooming functions. Once all of the peaks have their chemical shift denoted click

on the green tick.

To view the peak list:

Click on the image of a table with a small NMR spectrum in front of it that can be located

on the same command line as the zooming functions. A small dialog box will then open

which will show the number of peak and the chemical shift in ppm and Hz. This box can

be closed as it is included with the spectrum when it is printed.

To integrate the spectrum:

Click on ‘Integrate’ and then on ‘Manual’. Now click on one side of a singlet or multiplet

and drag the cursor to the other side of the singlet or multiplet, then release. Do this for

each singlet/multiplet in the spectra. Once all peaks have been integrated, click on the

green tick.

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4. PRINTING THE SPECTRUM AND PEAK LIST It is often helpful to zoom into the area of interest if all peaks lie in one portion of the spectrum before printing as this can make analysing the printed off spectrum much easier. If the peaks in the spectrum are such that they lie quite far apart from each other, then zooming in is not particularly useful. Once zoomed into the area of interest if this is applicable, go to File > Print > Standard. On the window that opens make sure that under the ‘Spectrum’ tab the following are ticked:

1. ‘Always Landscape Orientation’

2. Units: ppm

3. Display mode: Normalised Intensity

4. Spectrum Region: Zoomed Region (or for whole spectrum NMR, select ‘Whole

Spectrum’ instead)

Under the ‘Tables’ tab, make sure that the Table of peaks as well as the corresponding ppm AND Hz are ticked but leave ALL other options un-ticked. Under the ‘View’ tab, makes sure that only the following are ticked:

1. Integrals

2. Horizontal and Vertical Scale as well as the Vertical Scale Factor

Under the ‘Text’ Tab, tick the option for ‘Page Title’ and give the spectrum and meaningful title. The option for ‘Current Date’ may also be helpful. Click on the ‘OK’ button to print the spectrum off and close the programme by clicking File > Exit and do not save the transformed spectrum.

Having obtained 1H and 13C NMR spectra of your three unknowns, in each case record your results on the data sheets provided. First look at the IR spectrum to see if you can pick out functional groups (OH, C=O, etc). Then consider the NMR spectra, molecular formula, mass spec and double bond equivalents. Information on interpretation of NMR, DEPT, Mass and IR spectroscopy are given on LEARN. Use the information to identify the unknown compound. Ensure that you describe fully the logical deduction processes needed to identify the compound. The majority of the marks are for the correct interpretation of the spectra, and not for just identifying the compound.

Doing this from home You can, if you wish, do this exercise from home. You will need a USB flash memory stick to transfer the files from the lab computers. The folders with each unknown are in C:OS >unknowns > third year unknowns. Copy across the folders with your unknowns. The folders contain the information on the cards in the lab as well as NMR and mass spec data. You will also need to download and install the ACD software which is free for students. Go to: http://www.acdlabs.com/resources/freeware/ and follow the instructions to set up the NMR processor on your PC.

MARKS for each unknown

Molecular formula / dbe 2

Proton NMR interpretation 5

Carbon / DEPT NMR interpretation 5

IR interpretation 1

Mass spec. interpretation 4

Structure 3