LEWIS REPRESENTATIONS - FisicoQuímica...5.- Bromination of 1-bromopropane with Br2 at 200ºC results in a mixture of 1,1-dibromopropane (90%), 1,2-dibromopropane (8,5%) and 1,3-dibromopropane

Organic Chemistry I 2018-19

LEWIS REPRESENTATIONS

1. Draw a Lewis structure for each of the following compounds:

a) C2H6 b) C2H4 c) C2H2 d) C3H8 e) C3H6 f) CH3OH

2. Borane (BH3) is very unstable and quite reactive. Draw a Lewis structure and explain the source of instability.

3. Identify any formal charges in the structures below:

4. Draw a structure for each of the following ions; in each case, indicate whichatom possesses the formal charge:

a) BH4- b) NH2

- c) C2H5+

ACID-BASE

5. Write equations for the following acid-base reactions. Predict whether the equilibrium will favor the reactants or the products.

(a) HCOOH + -CN (b) CH3COO- + CH3OH

(c) CH3OH + NaNH2 (d) NaOCH3 + HCN

(e) HCl + H2O (f) H3O+ + CH3O-

6. Write equations for the following acid-base reactions. Label the conjugate acids and bases, and show any resonance stabilization. Predict whether the equilibrium favors the reactants or products.

(a) CH3CH2OH + CH3NH- (b) CH3CH2COOH + CH3NHCH3


c) CH3OH + H2SO4 (d) NaOH + H2S

(e) CH3NH3+ + CH3O- (f) CH3O- + CH3COOH

(g) CH3SO3- + CH3COOH

7. Compound A has a pKa of 7 and compound B has a pKa of 10. How manytimes is compound A more acidic than compound B?

8. In each case, identify the more stable anion. Explain why it is more stable.

Nvs.

vs.

vsa) b)

c)

9. Propanolol is an antihypertensive agent and L-dopa is used in the treatment ofParkinson’s disease. In both cases identify the more acidic protons and arrange them in order of increasing acidity

O

OH

N

H HO

OH

OH

O

NH2

Propanolol L-dopa

10. What reaction will take place if water is added to a mixture of NaNH2/NH3?

11. Reason which is the strongest base that can be used when using water assolvent

12. Would ethanol be a suitable solvent in which to perform the following protontransfer? Explain your answer. (pKas of the acids, 25 and 38, respectively)

H NH2+ + NH3


ALKANES AND CYCLOALKANES(Structure)

13. In each case below, draw a Newman projection as viewed from the angleindicated:

14. Draw the expected potential-energy diagram for the rotation about theindicated bond in the following molecules:

a) 2,3-dimethylbutane, C2-C3 bond b) 2,2,4-trimethylpentane, C3-C4 bond

15. At room temperature, 2-methylbutane exists primarily as two alternatingconformations of rotation about the C2-C3 bond. About 90% of the moleculesexist in the more favorable conformation and 10% in the less favorable one. a)Calculate the free energy change G° between these conformations. b) Draw thepotential-energy diagram for the rotation.


16. Draw the lowest energy conformations for each of the following compounds.

Cl

Cl

a b c d e

17. Draw the most stable conformation for each of the following cyclohexanes;then, in each case, flip the ring and redraw the molecule in the higher energychair conformation.

a) Cyclohexanol b) trans-3-Methylciclohexanol c) cis-1-(1-Methylethyl)-2-methylcyclohexane d) trans-1-(1,1-Dimethylethyl)-4-clorocyclohexane

18. For each molecule in the previous problem, estimate the energy differencebetween the most stable and next best conformation. Calculate the approximateratio of the two at 300°K.

‘ Organic Chemistry I 2018-19

1.- A hypothetical alternative mechanism for the halogenation of methane has the following propagationsteps:

(1)

(2)

X + CH4 CH3X + H

H + X2 HX + X

a) Using DH° values from appropriate tables, calculate H° for these steps for Cl and Br. (problem 34,pg. 129).b) Compare the values with those for the accepted mechanism for the halogenation of methane. Doyou expect this alternative mechanism to compete successfully with the accepted one? DH°(kcal.mol-1): 58 (Cl2); 46 (Br2); 105 (CH4); 85 (CH3Cl); 71 (CH3Br); 103 (HCl); 87 (HBr).

2.- The addition of certain materials called radical inhibitors to halogenation reactions causes the reactionsto come to a virtually complete stop. An example is the inhibition by I2 of the chlorination of methane.

Explain how this inhibition might come about. (problem 34, pg. 129 ).

3.- Write the major organic product(s) if any, of each of the following reactions. Calculate product ratiosusing data for relative reactivity at 150ºC. ((problem 22 and 23, pg. 128).a) Monochlorination of 2,2,4-trimethylpentaneb) Monobromination of 2,2,4-trimethylpentane

Additional Homework

4.- Propanal (CH3CH2CHO) and acetone (CH3COCH3) are isomers with the formula C3H6O. The heat ofcombustion of propanal is -434,1 kcal.mol-1, that of acetone -427.9 kcal.mol-1. a) Write the balancedequation for the combustión of either compound. b) What is the energy difference between propanaland acetone? Which has the lower energy content?. c) Which substance is more thermodynamicallystable, propanal or acetone? (problem 29, pg. 128).

5.- Bromination of 1-bromopropane with Br2 at 200ºC results in a mixture of 1,1-dibromopropane (90%),1,2-dibromopropane (8,5%) and 1,3-dibromopropane (1,5%). Calculate the relative reactivities of thehydrogens on each of the three carbons toward bromine atoms. Compare these results with those froma simple alkane such as propane, and suggest explanations for any differences. (problem 33, pg. 129).

6.- Cis-1-fluoro-4-methylciclohexane has two chair conformations in equilibrium (Gº values are 0,25for F and 1,70 for Me). Draw these conformations. Calculate Gº for these equilibrium and therespective equilibrium populations for each conformation. (pg 147).

1. For each of the following compounds, draw the important resonance forms. Indicate which structures are major and minor contributors or whether they would have the same energy.

aCH3OCH2

bCH2-CH=O

c H2C = CH-NO2

d H2CNN

2.

Determine which of the two protons identified below is more acidic and explain why:

3. Draw the chair conformations of cis and trans decalin

cis-decalin trans-decalin

4. Calculate the expected ratio of products for monochlorination and monobromination of 2,3-

dimethylbutane and suggest if these are synthetically useful methods.

Organic Chemistry I

PA- Stereochemistry

1. For each pair of structures shown, indicate whether the two species are constitutional isomers,

enantiomers, diastereomers of one another, or identical molecules. (prob. 39)

2. Identify the number of seteroisomers expected for each of the following:

3. Assign the appropriate designation of configuration (R or S) to the stereocenter in each of the

following enantiomers. (prob. 41)

4. Draw structural representations of each of the following molecules. Be sure that your structure

clearly shows the configuration of each stereocenter. a) (R)-3-bromo-3-methylhexane, b) (3R,5S)-

3,5-dimethylheptane, c) (2R,3S)-2-bromo-3-methylpentane; d) (S)-1,1,2-trimethylcyclopropane; e)

(1S,2S)-1-chloro-1-trifluoromethyl-1,2-methylcyclobutane; f) (1R,2R,3S)-1,2-dichloro-3-

ethylcyclohexane (prob. 45)

5. What are the stereochemical relations of the following molecules?

(e)

6. Identify all stereocenters in menthol. How many stereoisomers exist? Draw all the stereoisomers

and identify all pairs of enantiomers. (prob. 50)

7. Natural (-)-menthol, the essential oil primarily responsible for the flavor and aroma of peppermint,

is the 1R,2S,5R-stereoisomer. a) Identify (-)-menthol from the strucutres you have drawn for

problem 4. b) Another of the naturally occurring diastereomers of menthol is (+)-isomenthol, the

1S,2R,5R-stereoisomer. Identify it among your structures. c) A third is (+)-neomenthol, the 1S,2S,5R-

stereoisomer. Find it. d) Based on your understanding of the conformations of substituted

cyclohexanes, what is the stability order for the three diastereomers, menthol, isomenthol, and

neomenthol? (prob. 51)

8. For each of the following questions, assume that all measurements are made in a 10-cm

polarimeter sample containers. A) A 10 ml solution of 0.4 g of optically active 2-butanol in water

displays an optical rotation of -0.56º. What is its specific rotation? B) The specific rotation of

sucrose (common sugar) is +66.4º. What would be the observed optical rotation of such a solution

containing 3 g of sucrose in 10 ml of water. C) A solution of pure (S)-2-bromobutane in ethanol is

found to have an observed = 57.3º. If [] for (S)-2-bromobutane is 23.1º, what is the

concentration of the solution?

UNIVERSIDAD QUÍMICA ORGÁNICA I DE OVIEDO

PA-2/ 2015-16 1.- Formulate the potential product of each of the following reactions (write “no reaction” where

appropriate). (problem 40, pg. 246):

a) CH3CH2CH2Br + KOHCH3CH2OH

b) CH3CH2CH2Br + CsICH3OH

c) CH3CH2CH2Br + KSCNCH3CH2OH

d) CH3CH2F + LiClCH3OH

e) CH3CH2OCH2CH3 + NaOHH2O

f) CH3CH2I + CH3CO2KDMF

2.- Show how each of the following transformations might be achieved. (problem 41, pg. 247).

a) b)

c) d)

NCH3

NCH3H3C

OH

(R)-CH3CHCH2CH3

OH

(S)-CH3CHCH2CH3

CH3HHOHH3CO

CH3

CH3CNHHH3CO

CH3

H

H

OH

H

H

Br

3.- Rank the members of each of the following groups of species in the order of basicity, nucleophilicity,

and leaving-group ability. Briefly explain your answers. (problem 42, pg. 247). a) H2O, HO-, CH3CO2- b) -NH2, NH3, -PH2 c) -OCN, -SCN 4.- Explain the outcome of the following transformations mechanistically. (problem 52, pg. 248).

a) HSCH2CH2Br + NaOHEtOH

S

b) BrCH2CH2CH2CH2CH2Br + NaOHMeOH

O 5.- Propose three synthesis of trans-1-methyl-2-(metyllthio)ciclohexane, beginning with the reagent

indicated. (problem 48, pg. 248).

a) cis-1-Chloro-2-methylcyclohexane b) trans-1-Chloro-2-methylcyclohexane c) cis-2-Methylcyclohexanol 6.- The following table presents rate data for the reactions of CH3I with three different nucleophiles in two

different solvents. What is the significance of these results regarding relative reactivity of nucleophiles under different conditions? (problem 51, pg. 248).

Nucleophile krel, CH3OH krel, DMF Cl- 1 1,2 x 106 Br- 20 6 x 105 NCSe- 4 x 103 6 x 105

UNIVERSIDAD ORGANIC CHEMISTRY I DE OVIEDO

PA-3/ 2016-17

1- What is the major substitution product of each of the following solvolysis reactions?

(Chapter 7, problem 25, page 278)

2- Rank the compounds in each of the following groups in order of decreasing rate of

solvolysis in aqueous acetone (Chapter 7, problem 31, page 279)

3- Write the products of the following elimination reactions. Specify the predominat

mechanism (E1 or E2) and formulate it in detail. (Chapter 7, problem 40, page 280)

a) b)

c) d)

e) f)

BrCH3CH2OH Br CF3CH2OH

Et

Cl

CH3OHBr

H3C

O

OH

ClD2O

Cl

H

OD

a)

b)

c)

ClCl

Cl

O

O

H3C

Cl OH

CH3

Cl

Br Cl

a) c)

d)

(CH3)3C-OHH2SO4 conc.,

CH3CH2CH2CH2ClLDA, [(CH 3)2CH] 2NH

C

H

Br

KOH (exceso), EtOH

Cl

CH3

NaOCH3, CH3OHb)


4- Give the major product(s) of the following reactions. Indicate which of the following

mechanism(s) is in operation: SN1, SN2, E1, or E2. If no reaction takes place, write

“no reaction.” (Chapter 7, problem 46, page 280)

CH2Cl

H

F

H

CH2CH3

BrH3C

a) b)

c)

KOC(CH 3)3, (CH 3)3COH KBr, acetona

H2O INaNH2, NH3 líquido

d)

e) f)Br

NaOEt, EtOH

Br

CH2CH2CH3H3CH2C

H3CNaI, acetona

OH

H

g) h)KOH, EtOH

Cl

Br KCN (exceso)

MeOH

OTs

(R)-CH 3CH2CHCH 3

i) j)NaSH, EtOH

(CH3CH2)3COHHCl conc., H2O

k)l)

m) n)

(CH 3)3CClNaNH2, NH3 líquido

CH2CH3

I

MeOH

Br

(CH 3)3CCHCH 3

KOH, EtOH CH3CH2ClCH3CO2H


PA-3/ 2016-17

1- What is the major substitution product of each of the following solvolysis reactions?

(Chapter 7, problem 25, page 278)

2- Rank the compounds in each of the following groups in order of decreasing rate of

solvolysis in aqueous acetone (Chapter 7, problem 31, page 279)

3- Write the products of the following elimination reactions. Specify the predominat

mechanism (E1 or E2) and formulate it in detail. (Chapter 7, problem 40, page 280)

a) b)

c) d)

e) f)

BrCH3CH2OH Br CF3CH2OH

Et

Cl

CH3OHBr

H3C

O

OH

ClD2O

Cl

H

OD

a)

b)

c)

ClCl

Cl

O

O

H3C

Cl OH

CH3

Cl

Br Cl

a) c)

d)

(CH3)3C-OHH2SO4 conc.,

CH3CH2CH2CH2ClLDA, [(CH 3)2CH] 2NH

C

H

Br

KOH (exceso), EtOH

Cl

CH3

NaOCH3, CH3OHb)


4- Give the major product(s) of the following reactions. Indicate which of the following

mechanism(s) is in operation: SN1, SN2, E1, or E2. If no reaction takes place, write

“no reaction.” (Chapter 7, problem 46, page 280)

CH2Cl

H

F

H

CH2CH3

BrH3C

a) b)

c)

KOC(CH 3)3, (CH 3)3COH KBr, acetona

H2O INaNH2, NH3 líquido

d)

e) f)Br

NaOEt, EtOH

Br

CH2CH2CH3H3CH2C

H3CNaI, acetona

OH

H

g) h)KOH, EtOH

Cl

Br KCN (exceso)

MeOH

OTs

(R)-CH 3CH2CHCH 3

i) j)NaSH, EtOH

(CH3CH2)3COHHCl conc., H2O

k)l)

m) n)

(CH 3)3CClNaNH2, NH3 líquido

CH2CH3

I

MeOH

Br

(CH 3)3CCHCH 3

KOH, EtOH CH3CH2ClCH3CO2H

ORGANIC CHEMISTRY I 2015-2016

Quiz 1 (06-11-2015) Maximun Score 40

(ANSWER ONLY four out of the following five questions; please be aware that 1,2 and 3 are mandatory). (You have to select one additional question between 4 or 5)

Student Name: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DNI: . . . . . . . . . . . . . . . . . . . . . . . . 1- Answer the following questions and tasks in relation to the given species. (5 boxes x 2 POINTS) I) Draw a Lewis structure for each compound. Assign charges were appropriate:

a) [(CH3)3C]+ (connectivity H-C-C) b) (H3COO)- (connectivity H-C-O-O) II) Define the hybridization of each carbon atom in carbocation a. III) Depict the detailed structure behind carbocation a: IV) Are any of these charged compounds useful to illustrate the notion of resonance? (Tick the appropriate box)

Yes No


2- For the following molecules, give the name according to the IUPAC system of nomenclature or convert the given names into the molecular structure, as required. (4 boxes x 2.5 POINTS)

3- Providing additional concise and clear justifications, answer the following two questions concerning the structure and chemistry of trans-1-bromo-3-(1-methylethyl)cyclohexane. (2 boxes x 5 POINTS) I) Draw its most stable conformation. Data: |ΔG˚| (kcal.mol-1); (0.55 for Br, 1.75 for 1-methylethyl).

II) Draw the structures of all alkenes that can be prepared from its reaction with a base, under E2 conditions.


4. Show how the following transformation might be accomplished. Briefly account for the key mechanistic notion behind the proposed strategy. (10 POINTS)

5- The reaction of (R)-3-chloro-2,2,3,5,5-pentamethylhexane with bromine and light furnishes two major compounds. (5 boxes x 2 POINTS) I) Draw their structures:


II) Define the specific stereochemical relationship they display: III) Indicate whether or not it is reasonable to attempt their separation by distillation. Briefly account for your answer. IV) Provide the key first reaction involved in the propagation chain. V) Is the resulting intermediate chiral? Provide a short rationale for your answer.

ORGANIC CHEMISTRY I

NMR (will be discussed at PA and TG)

1. How many signals would be present in the 1H NMR spectrum of the cyclopropane derivatives shown

below? (problem 35)

2. The 1H NMR spectrum of CH3COCH2C(CH3)3, 4,4-dimethyl-2-pentanone, taken at 300 MHz, shows

signals at the following positions: 307, 617, and 683 Hz downfield from tetramethylsilane. (a) What are

the chemical shifts () of these signals? (b) What would their positions be in hertz relative to

tetramethylsilane if the spectrum were recorded at 90 MHz? at 500 MHz?(c) Assign each signal to a set

of hydrogens in the molecule (problem 32)

3. Order the 1H NMR signals of the following compounds by chemical shift position (lowest to highest).

Which one is the most upfield? The most downfield? (problem 33)

4. The 1H NMR spectra for two haloalkanes are shown below. Propose structures for these compounds

that are consistent with the spectra. (a) C5H11Cl, spectrum A; (b) C4H8Br2, spectrum B. (problem 38)

5. Below are shown three C4H8Cl2 isomers on the left and three sets of 1H NMR data that one would

expect on application of the simple N + 1 rule on the right. Match the structures to the proper spectral

data. (problem 43)

6. The 1H NMR chemical shifts are given for each of the following compounds. As best you can, assign

each signal to the proper group of hydrogens in the molecule and sketch a spectrum for each

compound, incorporating spin-spin splitting whenever appropriate. (a) Cl2CHCH2Cl, 4.0 and 5.8 ppm;

(b) CH3CHBrCH2CH3, 1.0, 1.7, 1.8 and 4.1 ppm ; (c) CH3CH2CH2COOCH3 1.0, 1.7, 2.3, and 3.6 ppm; (d)

ClCH2CHOHCH3 1.2, 3.0, 3.4, and 3.9 ppm (problem 46)

7. One of the following compounds will have one peak in its 1H NMR spectrum and two peaks in its 13C

NMR spectrum. Which one? (problem 66)

8. 1H NMR spectra C through F correspond to four isomeric alcohols with the molecular formula C5H12O.

Try to assign their structures (problem 47)

9. Treatment of the alcohol corresponding to NMR spectrum D in the problem above with hot

concentrated HBr yields a substance with the formula C5H11Br. Its 1H NMR spectrum exhibits signals at

1.0 (t, 3H), 1.2 (s, 6H), and 1.6 (q, 2H) ppm. Explain (problem 50)

10. Propose a reasonable structure for each of the following molecules on the basis of the given molecular

formula and the 1H and proton-decoupled 13C NMR data. (a) C7H16O, spectra J and K (* = CH2 by DEPT);

(b) C7H16O2, spectra L and M (the assignation in M were made by DEPT) (problem 58)

11. A compound with molecular formula C7H14O exhibits the following 13C-NMR spectra:

Several structures are consistent with these spectra. To determine which structure is correct, a 1H NMR spectrum was acquired which exhibits six signals. One of those signals is a singlet at 1.9 ppm with an integration of 3, and another of the signals is a doublet at 0.9 ppm with an integration of 6. Using this information, identify the correct structure of the compound.


PA‐5/ 2015‐2016 1‐ Name the following alcohols according to the IUPAC nomenclature system. Indicate stereochemistry (if any) and label the hydroxy groups as primary, secondary, or tertiary. (Textbook, problem 24, p 326)

2‐ Draw the structures of the following alcohols. (a) 2‐(Trimethylsilyl)ethanol; (b) 1‐methylcyclopropanol; (c) 3‐(1‐methylethyl)‐2‐hexanol; (d) (R)‐2‐pentanol; (e) 3,3‐dibromocyclohexanol. (Textbook, problem 25, p 326) 3‐ Rank each group of compounds in order of increasing boiling point : (Textbook, problem 26, p 326) (a) Cyclohexane, cyclohexanol, Chlorocyclohexane; (b) 2,3‐dimethyl‐2‐pentanol, 2‐methyl‐2‐hexanol, 2‐heptanol.

4‐ Explain the order of water solubilities for the compounds in each of the following groups: (a) Ethanol> chloro‐ethane>ethane; (b) methanol>ethanol>1-propanol. (Textbook, problem 27, p 326) 5‐ 1,2‐Ethanediol exists to a much greater extent in the gauche conformation than does 1,2‐dichloroethane. Provide an explanation. Would you expect the gauche: anti conformational ratio of 2‐chloroethanol to be similar to that of 1,2‐dichloroethane or more like that of 1,2‐ethanediol? (Textbook, problem 28, p 326)

6‐ The most stable conformation of trans‐1,2‐cyclohexanediol is the chair in which both hydroxyl groups are equatorial. (a) Draw the structure of the compound in this conformation. (b) Reaction of this diol with the chlorosilane R3SiCl, R = (CH3)2CH (isopropyl), gives the corresponding disilyl ether. Remarkably, this transformation causes the chair to flip, giving a conformation where both silyl ether groups are in axial positions. Explain this observation by means of structural drawings. (Textbook, problem 29, p 326) 7‐ Rank the compounds in each group in order of decreasing acidity. (a) CH3CHClCH2OH, CH3CHBrCH2OH, BrCH2CH2CH2OH; (b) CH3CCl2CH2OH, CCl3CH2OH, (CH3)2CClCH2OH; (c) (CH3)2CHOH, (CF3)2CHOH, (CCl3)2CHO (Textbook, problem 30, p 327) 8‐ Do you expect hyperconjugation to be important in the stabilization of alkyloxonium ions (e.g., RO+H2, R2O

+H)? Explain your answer. (Textbook, problem 33, p 327) 9‐ Give the major product(s) of each of the following reactions. Aqueous work‐up steps (when necessary) have been omitted. (Textbook, problem 36, p 327)

10‐ Formulate the product of each of the following reactions. The solvent in each case is (CH3CH2)2O. (a) CH3CHO: 1. LiAlD4, 2. H

+, H2O; (b) CH3CHO: 1. LiAlH4, 2. D+, D2O; (c) CH3CH2I and LiAlD4. (Textbook, problem 38, p 328)

11‐ Give the major product(s) of each of the following reactions [after work‐up with aqueous acid in (d), (f), and (h)].

(Textbook, problem 40, p 328)


12‐ Which of the following halogenated compounds can be used successfully to prepare a Grignard reagent for alcohol synthesis by subsequent reaction with an aldehyde or ketone? Which ones cannot and why? (pKa for terminal alkyne, see case c, is 25). (Textbook, problem 42, p 328)

Br

a)H3C CH3

Cl

b)H OH c)

H Br d)

Cl

O

13‐ Give the major product(s) of each of the following reactions (after aqueous work‐up). The solvent in each case is ethoxyethane (diethyl ether). For each reaction presented in Problem 43, write out the complete, step‐by‐step mechanism using curved‐arrow notation. Include the aqueous acid work‐up. (Textbook, problems 43 and 44, p 328)

14‐ Give the expected major product of each of the following reaction sequences. PCC refers to pyridinium chlorochromate. (Textbook, problem 48, p 329)

15‐ Suggest the best synthetic route to each of the following simple alcohols, using in each case a simple alkane as your initial starting molecule. What are some disadvantages of beginning syntheses with alkanes? (a) 2‐propanol; (b) 2‐methyl‐2‐propanol; (c) 1‐(bicyclo[4.4.0]‐decan‐1‐yl)ethanol. (Textbook, problems 51 and 56, p 330) 16‐ On which side of the equation do you expect each of the following equilibria to lie (left or right)? (Textbook, problem 28, pg. 377)

17‐ What are the most likely product(s) of each of the following reactions? (Textbook, problem 36, pg. 379)

18‐ Propose efficient syntheses for each of the following ethers, using haloalkanes or alcohols as starting materials. (Textbook, problem 48, pg. 380)

19‐ Propose a synthesis of 1‐(3‐hydroxypropyl)cyclohexanol beginning with cyclohexanone and 3‐bromopropan‐1‐ol. (Textbook, problem 51, pg. 380) 20‐ Give the major product(s) expected from each of the reactions shown below. Watch stereochemistry. (Textbook, problem 56, pg. 381)


PA-6/ 2015-2016 1- Name each of the following molecules in accord with the IUPAC system of nomenclature.


2- Draw the structures of the following alcohols: a) 4,4-dichloro-trans-2-octene; b) (Z)-4-bromo-2-iodo-2-pentene; c) 5-methyl-cis-3-hexen-1-ol; d) (R)-1,3-dichlorocycloheptene; e) (E)-3-methoxy-2-methyl-2-buten-1-ol. (Textbook, problem 29, p 488) 3- Write the most likely major products of each of the following haloalkanes with sodium ethoxide in ethanol or potassium tert-butoxide in 2-methyl-2-propanol (tert-butyl alcohol): a) 1-chloro-1-methylcyclohexane; b) (2R, 3R)-2-chloro-3-ethylhexane; c) (2R, 3S)-2-chloro-3-ethylhexane; (2S, 3R)-2-chloro-3-ethylhexane. (Textbook, problem 41, p 492)

4- Explain in detail the differences between the mechanisms giving rise to the following two experimental results.

(Textbook, problem 50, p 493) 5- Match each of the following structures with the IR data that correspond best. Abbreviations: w, weak; m, medium; s, strong; br, broad. a) 905 (s), 995 (m), 1040 (m), 1640 (m), 2850-2980 (s), 3090 (m), 3400 (s, br) cm-1; b) 2840 (s), 2930 (s) cm-1; c) 1665 (m), 2890-2990 (s), 3030 (m) cm-1, d) 1040 (m), 2810-2930 (s), 3300 (s, br) cm-1.


6- A hydrocarbon with an exact molecular mass of 96.0940 exhibits the following spectroscopic data: 1H NMR = 1.3

(m, 2H), 1.7 (m, 4 H), 2.2 (m, 4H), and 4.8 (quin = 3 Hz, 2 H) ppm; 13C NMR = 26.8, 28.7, 35.7, 106.9, and 149.7 ppm. The IR spectrum is shown below. Hydrogenation furnishes a product with an exact molecular mass of 98.1096. Suggest a structure for the compound consistent with these data.



7- The following mass spectrum belongs to one of the following compounds: hexane, 2-methylpentane or 3-methylpentane. Indicate the one that best matches by analyzing the observed fragmentation pattern.


8- The bicyclic alkene car-3-ene, a constituent of turpentine, undergoes catalytic hydrogenation to give only one of the two possible stereoisomeric products. The product has the common name of cis-carene, indicating that the methyl group and the cyclopropane are on the same face of the cyclohexane ring. Suggest an explanation for this selectivity and predict the expected major product of hydrogenation for the given exocyclic alkene (1).

(Textbook, problems 34 and 35, p 557-558)

9- Show synthetic pathways to selectively prepare each epoxide from the given alkene.

(Textbook, problems 43, p 558-559) 10- Propose efficient methods for accomplishing each of the following transformations. Most will require more than one step: a) 2-bromobutane into 1-iodobutane; b) 2-butanol into (2R*,3S*)-2,3-butanediol; c) 2-butanol into (2R*,3R*)-2,3-butanediol; d) 6-methylhepta-1,5-diene into 4-(3,3-dimethyloxiran-2-yl)butanal.

(Textbook, problems 44, p 559)

11- The following 1H NMR spectrum corresponds to a molecule with the formula C3H5Cl. The compound shows significant IR bands at 730, 930, 980, 1630, and 3090 cm-1. a) Deduce its structure; b) Assign each NMR signal to a

hydrogen or group of hydrogens; c) The “doublet” at = 4.05 ppm has J = 6 Hz. Is this in accord with your assignment in section b; d) This “doublet”, upon fivefold expansion, becomes a doublet of triplets (inset, spectrum A), with J in the range of 1 Hz, for the triplet splitting?. Is it reasonable in light of your assignment in section b.



12- The following 1H NMR spectrum corresponds to a molecule with the formula C4H8O. Its IR spectrum has important bands at 945, 1015, 1665, 3095 and 3360 cm-1. a) Determine the structure of the unknown; b) Assign each

NMR signal and add a comment on the more relevant IR signals; c) Explain the splitting patterns for the signals at = 1.3, 4.3 and 5.9 ppm (see inset for 10-fol expansion); d) Comment on the mass spectrum and the expected peaks for the likely fragmentation modes.


13- The reaction of compound in problem 10 with SOCl2 produces a chloroalkene, C4H7Cl, whose NMR spectrum is

almost identical to the previous one but for the broad signal at = 1.5 ppm that is absent. Its IR spectrum shows bands at 700, 925, 985, 1640, and 3090 cm-1. Treatment with H2 over PtO2 results in C4H9Cl (spectrum is given below). Its IR spectrum reveals the absence of all the bands quoted for the its precursor, except for the signal at 700 cm-1. Identify the two molecules.


14- Give the expected product of reaction of 2-methyl-1-pentene with each of the following reagents: a)HCl; b) ICl; c) D2, Pd-C, CH3CH2OH; d) H2SO4 (catalytic) and water; e) BH3 in THF and then NaOH+H2O2; f) Br2 in ethanol; g) O3 and then Me2S.


ORGANIC CHEMISTRY I (Midterm Exam) 08-01-2016

Student Name: DNI:

1 2 3 4 5 6 7 total MARK

1) Fill the boxes providing the missing compounds (starting materials, reagents or products) for the following reactions: (10 x 10 point) (100 points)

a)

b)

c)

d)

e)

f)

g)

h)

i)

j)

2) The reaction of 2-bromo-3-methylbutane with potassium tert-butoxide in t-BuOH gives A (major) and B (minor), two regioisomers with the molecular formula C5H10. (5 x 6 points) (30 points)

a) Provide the structure and the name for compounds A and B.

b) Indicate the reaction that takes place and provide a detailed energy diagram, depicting the structures of each specific transition sate accounting for the synthesis of both compounds.

c) Give eliminations conditions favouring the formation of B over A. Explain the origin of the change in selectivity. Depict the energy profile for the reaction coordinate under the proposed condition.

d) A mixture of A and B can be equilibrated in favour of compound B by exposure to acid. Show the mechanism for this process, and give the reasons behind the noticed equilibrium shifting.

e) Depict a reasonable 1H NMR spectrum accounting quantitatively for the structure of A. Briefly outline the most significant features for the chemical shift values and relevant coupling constants.

3) A compound (C7H16O) has been synthesized by treatment of 2,4-dimethyl-2-pentene with borane/THF

followed by NaOH/H2O2. Answer the following questions concerning its structure.

(3 x 10 points) (30 points)

I) NMR data:

13C = 81.8, 31.6 and 17.0 ppm.

1H = 3.0 (t, 1H); 1.7 (m, 2H); 1.5 (bs, 1H); 0.9 (d, 12H).

Draw the structure and make the assignments for each signal.

II) MS spectrum for the compound:

a) Inside the spectrum, mark the base peak as B+

b) Draw in the box below the structure of fragment 73

c) A very small peak at 98 can be detected. Explain it.

III) IR spectrum

Confirm the presence/absence of the following bonds (scratch off the wrong answer):

(In the boxes at the right, describe the approximate frequency for each bond)

Csp2-H Presence Absence cm-1

C=C Presence Absence cm-1

Csp3-H Presence Absence cm-1

O-H Presence Absence cm-1

4) For the following molecules, give the name according to the IUPAC system of nomenclature or convert

the given names into the molecular structure, as required. (4 x 5 points) (20 points)

cis-1-bromo-2-(2-ethoxyethyl)cyclohexane

(2S,3S)-butane-2,3-diol

5) Write detailed mechanism for the following transformations. Make use of proper arrows to indicate the electron flow. Describe the structure of the key intermediate, in each case. Do not use additional reagents to carry out those transformations. (2 x 20 points) (40 points)

a)

b)

6) Describe efficient synthetic routes to convert the given starting materials into the target products. Mechanistic details are not requested. (2 x 20 points) (40 points) a)

b)

7) Mark with an X the checkbox that contains the right answer. (4 x 10 points) (40 points) a) The following reaction gives:

b) The more thermodynamically stable alkene is:

c) The order of increasing stability for the given conformations of trans-1-bromo-3-methylcyclohexane is:

(Absolute values for G˚ (equatorial/axial) = 1.75 (CH2CH3); 0.55 (Br)

d) The conformational analysis of 3-methylhexane around the C3-C4 bond shows:

three different maximum and two minimum two different minimum and one maximum

three different minimum and three maximum one minimum and two maximum

Documents

LEWIS REPRESENTATIONS - FisicoQuímica...5.- Bromination of 1-bromopropane with Br2 at 200ºC results in a mixture of 1,1-dibromopropane (90%), 1,2-dibromopropane (8,5%) and 1,3-dibromopropane