Organic Chemistry

210 CHAPTER 5 Stereochemistry

PROBLEM 34 SOLVED

(S)- can be converted to -2-methylbutanoic acid without

breaking any of the bonds to the asymmetric carbon. What is the configuration of

-2-methylbutanoic acid?

SOLUTION We know that -2-methylbutanoic acid has the relative configuration

shown because it was formed from (S)- -2-methyl-1-butanol without breaking any

bonds to the asymmetric carbon. Therefore, we know that -2-methylbutanoic acid has

the Sconfiguration. We can conclude then that -2-methylbutanoic acid has the R

configuration.

PROBLEM 35

The stereoisomer of 1-iodo-2-methylbutane with the Sconfiguration rotates the plane of po-

larized light counterclockwise. The following reaction results in an alcohol that rotates the

plane of polarized light clockwise. What is the configuration of -2-methyl-1-butanol?

5.13 The Absolute Configuration

of -Glyceraldehyde

Glyceraldehyde has one chirality center and, therefore, has two stereoisomers. The ab-

solute configuration of glyceraldehde was not known until 1951. Until then, chemists

did not know whether -glyceraldehyde had the Ror the Sconfiguration, although

they had arbitrarily decided that it had the Rconfiguration. They had a 50–50 chance

of being correct.

The configurations of many organic compounds were “determined”by synthesiz-

ing them from - or -glyceraldehyde or by converting them to - or -

glyceraldehyde, always using reactions that did not break any of the bonds to the

asymmetric carbon. For example, -lactic could be related to -glyceraldehyde

through the following reactions. Thus the configuration of -lactic was assumed to

be that shown below. Because it was assumed that -glyceraldehyde was the Renan-

tiomer, the configurations assigned to these molecules were relative configurations,

not absolute configurations. They were relative to -glyceraldehyde, and were based

on the assumptionthat (+)-glyceraldehyde had the Rconfiguration.

(+)

(+)

(-)

(-) (+)

(+) (-) (+) (-)

(R)-(+)-glyceraldehyde

C H

HC O

CH 2 OH

HO

(S)-(−)-glyceraldehyde

H C

HC O

HOCH 2

OH

(+)

()

C H + HO–

CH 2 I

CH 2 CH 3

CH 3

C H + I–

CH 2 OH

CH 2 CH 3

CH 3

(-)

(-)

(+)

(-)

(+)

C H

CH 2 OH

CH 3

C H

COOH

CH 3

CH 3 CH 2 CH 3 CH 2

(S)-(−)-2-methyl-1-butanol (+)-2-methylbutanoic acid

(-)

(-)-2-Methyl-1-butanol (+)

(+)-glyceraldehyde

C H

HC

HgO

O COOH COOH COOH

CH 2 OH

HO

(−)-glyceric acid

C H

COOH

CH 2 OH

HO

(+)-isoserine

C H

CH 2 NH 3

H 2 O HO

HNO 2

HBr

NaNO 2

H+

Zn

(−)-3-bromo-2-

hydroxypropanoic acid

C H

CH 2 Br

HO

(−)-lactic acid

C H

CH 3

HO