Organic Chemistry

The difference between the products obtained from an reaction and from an

reaction is a little easier to visualize in the case of cyclic compounds. For exam-

ple, when cis-1-bromo-4-methylcyclohexane undergoes an reaction, only the

trans product is obtained because the carbon bonded to the leaving group is attacked

by the nucleophile only on its back side.

However, when cis-1-bromo-4-methylcyclohexane undergoes an reaction, both

the cis and the trans products are formed because the nucleophile can approach the

carbocation intermediate from either side.

PROBLEM 18

If the products of the preceding reaction are not obtained in equal amounts, which

stereoisomer will be present in excess?

PROBLEM 19

Give the products that will be obtained from the following reactions if

a. the reaction is carried out under conditions that favor an reaction

b. the reaction is carried out under conditions that favor an reaction

1.

2.

PROBLEM 20

Which of the following reactions will go faster if the concentration of the nucleophile is

increased?

Br SCH 3

++CH 3 S−

CH 3 O−

Br

Br−

Br−

Br−

H

++

H OCH 3

O

O

Br OCCH 3

++CH 3 CO−

a.

b.

c.

cis-1-chloro-2-methylcyclobutane+sodium hydroxide>water

trans-1-bromo-4-methylcyclohexane+sodium methoxide>methanol

SN 1

SN 2

SN 1

SN 2

SN 2

SN 1

Br−^ has diffused away, giving H 2 O

equal access to both sides of the

carbocation

C+ H 2 O

CH 3

CH 2 CH 3

H

Br−^ has not diffused away, so it blocks

the approach of H 2 O to one side of the

carbocation

H 2 O C+ Br−

CH 3

CH 2 CH 3

H

H 2 O H 2 O

382 CHAPTER 10 Substitution Reactions of Alkyl Halides

Movies:

SN 1 inversion; retentionSN 1

HO−

SN2 conditions

+ + Br−

cis-1-bromo-4-methylcyclohexane trans-4-methylcyclohexanol

H

Br

H

CH 3

OH

Br

H

CH 3

H 2 O

SN1 conditions

+ + + HBr

cis-1-bromo-4-methylcyclohexane trans-4-methylcyclohexanol cis-4-methylcyclohexanol

OH

H

H

CH 3

H

Br

H

CH 3

H

OH

H

CH 3