NAME:

1

CEM 850 Exam-1 September 20, 2003

This exam consists of 16 pages including the Table at the end. Be sure to write your name on allpages of the exam. Please write legibly and draw all structures clearly. Good luck.

1. (10 pts) Consider the following eight representations of brominated hydrocarbons:

CH3CHBrCH=C(CH3)2

(CH3)2CHCH=CHCH2Br

H

CH3H3C

H

H

Br

1-Bromo-3-methylcyclopentane

2-Bromo-2,2-dimethylpropane 2-Bromo-4-methyl-4-pentene

A B

DC

F

H

E

G

Br

Br

In this collection, identify…

a) Two compounds that could have cis and trans isomers: ______ and ______.

b) A chemically meaningful but IUPAC incorrect name: ______; write the correct name

here: _________________________________________________.

c) A chemically impossible structure (violates octet rule): _______.

d) Two representations of the same compound: ______ and ______.

e) All but one of the chemically reasonable structures [i.e. not the answer to question (c)]

have the same formula, which is: ___________. Which is the one with a different

formula (hint: think of degrees of unsaturation): ______.

NAME:

2

2. (10 pts)

(a) (1 pt each) Form acceptable Lewis structures for the following uncharged compoundsby adding the necessary nonbonding electrons and multiple bonds:

N

H

NC

O

H

HH H C N

(b) (1 pt each) Assign charges (if any) to the atoms in the following structures:

B H

H

HH

O N O

(c) (2 pts) Among the three proposed resonance structures below, one is not acceptable.Circle the bad structure and briefly explain what's wrong with it.

H2C NO

OH2C N

O

OH2C N

O

O

Explanation:

NAME:

3

(c) (4 pts) Ions H2+ and He2

+ have similar bond strengths (ca. 60 kcal/mol) but neutralizationof their positive charges by adding an electron results in bond strengthening for one anddissociation for the other. Label the simple MO interaction pictures below, and use them tohelp explain the similarity of the cations and the difference between the neutral systems.

Explanation:

3. (10 pts) Consider the four Newman projections below.

H

CH3

H

H CH3

H3C

HH

CH3

H CH3

H3CH

H

CH3

H CH3

H3C

H3CH

H

H CH3

H3C

A B C D

(a) (2 pts) These diagrams depict a compound of formula _____________ whose IUPAC

name is _________________________________.

(b) (2 pts) Which two structures represent eclipsed conformations? _____ and _____.

NAME:

4

(c) (2 pts) Which two structures represent energy minima (low points) on the torsional potential

energy surface? _____ and _____.

(d) (2 pts) For two of the structures there are additional conformations of equal energy. Which

ones are they? _____ and _____. Draw their same-energy conformations by completing

the Newman projections below.

H CH3

H3C

H CH3

H3C

(e) (2 pts) Place structures A-D on the potential energy diagram shown below.

-180˚ 180˚0˚-60˚ 60˚ 120˚-120˚

ENERGY

4. (10 pts)

(a) (3 pts) Fill in > or < symbols to rank the following pairs of groups according to the Cahn-Ingold-Prelog priority rules:

Example: Br _>_ H

C(CH3)3 ___ C≡CH

CH2Br ___ CH=O

CH2CH=CH2 ___ CH2CH2CH3

NAME:

5

(b) (4 pts) In the following molecules, (i) identify (circle) all stereogenic centers; (ii) use theCahn-Ingold-Prelog convention to prioritize the groups on the stereogenic centers (labelthem 1,2,3,4); (iii) assign these centers' configurations (R/S); and (iv) name one of thecompounds (you choose) according to IUPAC.

Cl

Cl

Cl

Cl

(a) (b)

(c) (3 pts) Which of the following pairs represent identical molecules (I), diastereomers (D),enantiomers (E), or structural isomers (S)? Please circle your answer:

ClCl

Cl

Cland compound (a) above

and compound (b) above

OH

O

H

HBr

BrHO

O

H

BrH

Br

(a)

(b)

(c)

I D E S

I D E S

I D E S

NAME:

6

5. (4 pts) Gibbs free energy differences between axial-substituted and equatorial-substitutedchair conformations of cyclohexene are given in the attached Table. Calculate the ratio ofequatorial to axial vinylcyclohexane at 25 oC.

NED: NOTE THE TYPO ABOVE (CYCLOHEXENE VS CYCLOHEXANE) WASBROUGHT TO THE STUDENT’S ATTENTION DURING THE EXAM

6. (3 pts) Calculate the difference in Gibbs free energy in kilocalories per mole between thealternative chair conformations of the following:

a) trans-4-methylcyclohexanol

b) cis-4-methylcyclohexanol

c) trans-1,4-dicyanocyclohexane

NAME:

7

7. (5 pts) There are four cis, trans isomers of 2-isopropyl-5-methylcyclohexanol.

a) Using a planar hexagon representation for the cyclohexene ring, draw structural formulas forthe four cis, trans isomers.

b) Draw the more stable chair conformation for each of your answers in part a.

c) Of the four cis, trans isomers, which is the most stable?

NAME:

8

8. (4 pts) Select the member of each pair that shows the greater rate of SN2 reaction with KI inacetone.

9. (4 pts) Which compound in each set undergoes more rapid solvolysis when refluxed inethanol? Show the major product formed from the more reactive compound.

(a) orCl Cl (b) Cl Bror

(c) orCl Cl (d) BrorBr

(a)Br Br

or Cl Br(b) or

BrBr

or(c)Cl Cl(d) or

NAME:

9

10. (4 pts) The following nucleophilic substitution occurs with rearrangement. Suggest amechanism for formation of the observed product. If the starting material has the S configuration,what is the configuration of the stereocenter in the product?

NCl

NaOH

H2ON OH

NAME:

10

11. (5 pts) Solvolysis of the following bicyclic compound in acetic acid gives a mixture ofproducts, two of which are shown. The leaving group is the anion of a sulfonic acid, ArSO3H. Asulfonic acid is a strong acid, and its anion, ArSO3

–, is a weak base and a good leaving group.Propose a mechanism for this reaction.

OSO2Ar

H

HH3C

O

OH

H

H

+

O CH3

O

NAME:

11

12. (5 pts) A comparison of the rates of SN1 solvolysis of the bicyclic compounds 1 and 2 inacetic acid shows that compound 1 reacts 1011 times faster that compound 2. Furthermore,solvolysis of 1 occurs with complete retention of configuration: the nucleophile occupies the sameposition on the one-carbon bridge, as did the leaving OSO2Ar group.

(a) Draw the structural formulas for the products of solvolysis of each compound. (b) Account for the difference in rate of solvolysis of 1 and 2. (c) Account for complete retention of configuration in the solvolysis of 1.

OSO2ArOSO2Ar

1 2

NAME:

12

13. (6 p ts) When cis-4-chlorocyclohexanol is treated with sodiumhydroxide in ethanol, it gives only thesubs t i tu t ion produc t trans-1,4-cyclohexanediol 1. Under the sameconditions, trans-4-chlorocyclohexanolgives 3-cyclohexenol 2 and the bicyclicether 3.(a) Propose a mechanism for formation ofproduct 1, and account for its configuration.(b) Propose a mechanism for formation ofproduct 2.(c) Account for the fact that the bicyclicether 3 is formed from the trans isomerbut not from the cis isomer.

OH

Cl

OH

OH

NaOH

CH3CH2OH

cis-4-chlororcyclohexanol 1

OH

Cl2

NaOH

CH3CH2OH

OH

+ O

3trans-4-chlororcyclohexanol

NAME:

13

14. (8 pts) Using correct arrow notation, provide mechanisms for the following transformations.

(a)CH3

CH3

BrHBr

(b)

OH O1. Hg(OAc)2

2. NaBH4

NAME:

14

15. (8 pts) Provide the reagents/conditions that would enable the following transformations.

(a)Br OCH3

(b) Br DOH

NAME:

15

16. (4 pts) Consider the two reactions shown below and answer the following questions:

O

CH3CH2OH

HCl (catalytic)

CH3CH2OH

HCl (catalytic)

(a) What are the expected products from the reaction of the cycloalkenes?(b) Which cycloalkene reacts faster? Use chemical structures to explain your choice.

Bonus Question: (2 pts) The hydroboration of alkenes is presented in Chapter 8 of Jones. Apioneer of organoborane chemistry is Professor H.C. Brown of Purdue University. In 1979 Brownwas awarded the Nobel Prize for his work in the area. Brown’s decision to study organoboranescame many years before when shortly before beginning graduate school he read an early bookon the subject. His girlfriend as a graduation present gave the book to him. Why did she decideto give H.C. a book about organoborane chemistry?a. She was an inorganic chemist herself and found the subject facinating.b. It was the cheapest book in the storec. The author of the book was her Uncle.d. Her favorite color was red and the book had a red cover.e. She hoped that after reading the section on sodium borate Brown would quit graduate schooland take over the family Borax concession.

NAME:

16