UNIT 4

A2 ORGANIC CHEMISTRY

MECHANISMS AND REAGENTS.

ALDEHYDES & KETONESIdentified by • Tollens reagent • Fehling’s Solution

Aldehydes are oxidised to carboxylic acids. Therefore, the Tollens’ and Fehling’s must be reduced.

Tollens’ Reagent

Sodium hydroxide, aqueous silver nitrate this produces a precipitate. Ammonia is then added to make aqueous [Ag(NH3)2]+

This is reduced to silver when an aldehyde is added.

Fehling’s Solution

This is a blue solution containing Cu2+ ions. When aldehydes are added and heated, these ions are reduced to cooper (I) oxide Cu2O, producing a brick-red ppt.

AQA A2 text book, p64

ALCOHOLS FROM ALDEHYDES & KETONES

ALDEHYDE

KETONE

PRIMARY ALCOHOL

SECONDARY ALCOHOL

Lithium tetrahydridoaluminate LiAlH4

Sodium tetrahydridoborate NaBH4

[H]

Mechanism• Nucleophilic addition• The reducing agents produce the H- ion (the nucleophile).• The carbonyl bond is polar, due to oxygen being more electronegative.• First Step is the hydride ion being attracted to the ƍ+ carbon.• Second step, the product of first step gains a proton.

AQA A2 text book, p66

HYDROXYNITRILES FROM ALDEHYDES & KETONES

Mechanism• Similar to making alcohols!• Nucleophilic addition• The reducing agents produce the CN- ion (the nucleophile).• The carbonyl bond is polar, due to oxygen being more electronegative.• First Step is the cyanide ion being attracted to the ƍ+ carbon.• Second step, the product of first step gains a proton from the solvent or

hydrogen cyanide.

ALDEHYDE

KETONEHYDROXYNITRILE

HCN

KCN/H2SO4

AQA A2 text book, p68

HYDROLYSIS OF THE NITRILE GROUP

AQA A2 text book, p69

The nitrile is hydrolysed in acidic conditions, producing a carboxylic group.

HYDROXYNITRILEH2O/ H+

CARBOXYLIC ACID

AMMONIUM

ESTERS FROM CARBOXYLIC ACIDS & ALCOHOLS

CARBOXYLIC ACID

ALCOHOLS

ESTER

WATER

H2SO4 or HCl cat, reflux

AQA A2 text book, p71

No need for Mechanism• But you do need to be able to draw and name ester.• OH from carboxylic acid and H from OH in alcohol bond to produce water.• First part of name comes from the alcohol, second from the carboxylic acid.

ESTER HYDROLYSIS

AQA A2 text book, p74

ESTER

WATER

CARBOXYLIC ACID

ALCOHOLS

Acid Catalysed Hydrolysis• Reverse of esterification.• Dilute sulfuric acid or hydrochloric acid.• Heat under reflux.

Base-Catalysed Hydrolysis• Form a salt• Dilute strong base, sodium hydroxide.• Heat under reflux.• This method is quicker than acid catalysed.

ESTER

SODIUM HYDROXIDE ALCOHOLS

SODIUM CARBOXYLATE

Heat with excess dilute acid to produce carboxylic acid

VEGETABLE OILS & ANIMAL FATSAlkyl chain in Vegetable oils are unsaturated. Alkyl chain in Animal fats are saturated.

TRIACYLGLYCEROLS or TRIGLYCERIDES (three “esters”) TRIESTERS

AQA A2 text book, p74-75

TRIACYGLYCEROL

No need for MechanismHydrolysed by heating with aqueous sodium hydroxide or potassium hydroxide.Produces glycerol (propane-1,2,3-triol) and sodium/potassium salt.

SODIUM HYDROXIDE

PROPANE-1,2,3-TRIOL

SODIUM CARBOXYLATE

Three moles Three moles produced

BIODIESELNo need for Mechanism• Base catalysed Transesterification.• Heating vegetable oil with methanol and sodium hydroxide.• Produces methyl esters with long chains and propane-1,2,3-triol.

TRIACYGLYCEROL

SODIUM HYDROXIDE

PROPANE-1,2,3-TRIOL

METHYL ESTER

AQA A2 text book, p75

METHANOL

Three moles

Three moles

ACYL CHLORIDESMechanism• Nucleophilic Substitution reactions. (two steps addition + elimination)• Greater ƍ+ on carbon due to Cl and O bond.• 1st Step (addition) nucleophile attacks carbon ƍ+ . Pair of electrons in C=O is

transferred to oxygen.• 2nd Step (elimination) lone pair of electrons on oxygen forms C=O, chlorine is

eliminated. Hydrogen attached to oxygen atom breaks.

AQA A2 text book, p76-81

ACYL CHLORIDE

ESTER

N-SUBSTITUTED AMIDE

CARBOXYLIC ACID

AMIDE

water alcohol

Primary amine

ammonia

Hydrogen chloride, HCl is the secondary product.

ACID ANHYDRIDE

Mechanism• Same as acyl chlorides.• Secondary product is a carboxylic acid.

Preferred in synthesis, cheaper, less violent reactions, no fumes of HCl

ACID ANYDRIDE

ESTER

N-SUBSTITUTED AMIDE

CARBOXYLIC ACID

AMIDE

water alcohol

Primary amine

ammonia

SynthesisDehydrate a carboxylic acid (P4O10 cat)

AQA A2 text book, p76-81

NITRO COMPOUNDS FROM BENZENE

Mechanism• Generate the nitronium ion. Nitric acid and sulfuric acid.• Protonation of nitric acid, then protonated nitric acid breaks down.• Electrophilic substitution(addition then elimination).• Electrophile is attracted to the delocalized electron.• C-H bond breaks and restores the stable delocalized electron structure.

BENZENE

UNSTABLE POSITIVELY CHARGED

INTERMEDIATE

NITROBENZENE

Temperature should be kept at 50oC. Further substitution occurs at 100oC (1,3-dinitrobenzene).

AQA A2 text book, p90-91

ICE DYES & DIAZONIUM SALTS

PHENYLAMINE DIAZONIUM SALT

Sodium nitrate(III), HClIce bath (chilled)

Ice DyesNo need for mechanism but need to be able to draw compounds.Sodium chloride and water is also formed.

AQA A2 text book, p92-93

Coupling Reaction (azo dyes)

No need for mechanism but you need to be able to draw compounds.Diazonium salt reacts with phenylamine or other aromatic compounds to produce azo dyes. HCl is also produced.

WATER

SODIUM CHLORIDE

TRINITROTOLUENE, TNTNo need for mechanism but need to be able to draw compounds.

METHYLBENZENE 2-METHYL-1,3,5-TRINITROBENZENE

conc. HNO3

conc. H2SO4

AQA A2 text book, p93

FIEDEL-CRAFTS ACYLATIONMechanism• Generate the electrophile. Aluminium chloride and acyl chloride.• Electrophilic substitution(addition then elimination).• Electrophile is attracted to the delocalized electron.• C-H bond breaks and restores the stable delocalized electron structure.• Aluminium chloride is regenerated whren [AlCl4]- reacts with H+

.

BENZENE

UNSTABLE POSITIVELY CHARGED

INTERMEDIATE

PHENYLETHANONE

HYDROGEN CHLORIDE

AQA A2 text book, p94

MAKING ALIPHATIC AMINES

Mechanism• If you can do this you can describe• Amine acting as a nucleophile• Amine acting as a base• How to make secondary, tertiary amines and quarternary ammonium salts

From HaloalkanesExcess ammonia reacting with primary haloalkanes.

AMMONIA

HALOALKANE

AMMONIUM SALT

HALIDE ION

AMINE

AMMONIUM HALIDE

nucleophile NH3

base NH3

From NitrileAmine can be made by reducing nitrile

NITRILE AMINE LiAlH4

H2/NiAQA A2 text book, p100-107

4[H]

MAKING AROMATIC AMINES

Reduction of aromatic nitro compounds

NITROBENZENE AMINE

conc.HCl/Snor

conc. HCl/Fe

6[H] WATER

AQA A2 text book, p101

NH3+

INTERMEDIATE

NaOH

ZWITTERIONSActing as a BaseAccepting protons.

Acting as an AcidDonating protons.

AMINO ACID H+

AMINO ACID OH-

AQA A2 text book, p108-109

HCl SALT

WATERNaOH SALT

NITROGEN IS PROTONATED

NEGATIVE CHARGE ON OXYGEN

With Acid

With Base

PROTEINS FROM AMINO ACIDS

AQA A2 text book, p110

Amino group reacting with carboxylic group of another amino acid. PEPTIDE LINKEach link produces one water molecule. Condensation reaction.Can form ploymers (amino acids)

AMINO ACID

AMINO ACIDDIPEPTIDE WATER

AMINO ACIDS FROM PROTEINS

Hydrolysis of peptide link.

DIPEPTIDE

AMINO ACID

AMINO ACID Heat Water

6 moldm-3 HCl for 24 hours

AQA A2 text book, p112

ADDITION POLYMERS

Mechanism• Addition polymerisation• Involves one functional groups.• Carbon, carbon double bond.• Need to be able to identify repeating units

Taught in AS Chemistry Unit 2.

AQA A2 text book, p114-115

AQA AS text book, p218-219

ALKENE POLY(ALKENE)

Monomer Polymer

POLYESTERS

Mechanism• Condensation polymerisation• Involves two functional groups.• Polyesters are formed from a dicarboxylic acid and a diol.• Carboxyl group forms an ester link with the hydroxy group.• Need to identify ester link.• Identify the repeating units.

AQA A2 text book, p116-117

DICARBOXYLIC ACID

DIOL

POLYESTER

WATER

POLYAMIDES

Mechanism• Condensation polymerisation• Involves two functional groups.• Polyamide are formed from a dicarboxylic acid and a diamines.• Hydroxyl group from dicarboxylic acid reacts with hydrogen attached to nitrogen of

polyamide.• Forms an amide link.• Need to identify amide link.• Identify the repeating units.

DICARBOXYLIC ACID

DIAMINE

POLYAMIDE

WATER

AQA A2 text book, p118-119

Questions on Nylon often appear on exams.

BIODEGRADABLE POLYMERS

Esters and amides can be hydrolysed by water, acids, alkalis and enzymes.This is the same for polyesters and polyamides.Break ester link or amide link!

AQA A2 text book, p120-121

POLYAMIDE

DICARBOXYLIC ACID DIAMINE

DICARBOXYLIC ACID DIAMINE

SODIUM SALT OF DICARBOXYLIC ACID DIAMINE

Water

NaOH

ACID

With polyesters the secondary product is a diol.

You need to be able to identify repeating units.