Organic Chemistry

a. Which of the products contained the label?

b. What product would have contained the label if the alkyl bond had broken?

PROBLEM 25 SOLVED

Early chemists could envision several possible mechanisms for hydroxide-ion-promoted

ester hydrolysis:

1. a nucleophilic acyl substitution reaction


2. an reaction


3. an reaction

Devise an experiment that would distinguish among these three reactions.

SOLUTION Start with a single stereoisomer of an alcohol whose OH group is bonded to

an asymmetric carbon, and determine the specific rotation of the alcohol. Then convert the

alcohol into an ester, using a method that does not break any bonds to the asymmetric car-

bon. Next, hydrolyze the ester, isolate the alcohol obtained from hydrolysis, and determine

its specific rotation.

If the reaction is a nucleophilic acyl substitution reaction, the product alcohol will have the

same specific rotation as the reactant alcohol because no bonds to the asymmetric carbon

are broken (Section 5.12).

If the reaction is an reaction, the product alcohol and the reactant alcohol will have

opposite specific rotations because the mechanism requires back-side attack on the asym-

metric carbon (Section 10.2).

If the reaction is an reaction, the product alcohol will have a small (or zero) specific

rotation because the mechanism requires carbocation formation, which leads to racemiza-

tion of the alcohol (Section 10.7).

SN 1

SN 2

CH 3 H

C

CH 2 CH 3

HO−

H 2 O

CH 3 CCl

O

OH

CH 3 H

C

CH 2 CH 3

O CCH 3

O

CH 3 CHCH 2 CH 3 + CH 3 CO−

O

OH

(S)-2-butanol

(S)-2-butyl acetate

2-butanol

R′

+ R′ R′OH

+ HO−

R O O−

C

O

R

C

O

SN 1

R′

++H O− R′OH

R O O−

C

O

R

C

O

SN 2

OR′

++H −

O

RC

OH

O R′ R′OH

O−

O

−

R

C

O

R

C

O

(^18) O C¬O
(^18) O
alkyl C O bond
18
acyl C O bond
CH 3 CH 2 O CH 2 CH 3
C
O
Section 17.12 Hydroxide-Ion-Promoted Ester Hydrolysis 699