Advices For Studying Organic Chemistry

2) Both of the 2,3-dichloropentane diastereomers are chiral ⇒ each exhibits optical

activity.

i) The two diastereomers have different physical properties (e.g., m.p. & b.p.)

and are separable by conventional means (by GC, LC, or by careful fractional

distillation).

A Mechanism for the Reaction

The Stereochemistry of Chlorination at C3 of (S)-2-Chloropentane

++C

C

HCH 2

(2S,3S)-2,3-Dichloropentane Trigonal planar radical

Diastereomers

Cl

Cl Cl

Cl 2 Cl 2

Cl Cl

(chiral) (chiral) (chiral)

CH 3

H Cl

CH 3

C

C

H

CH 2

CH 3

H Cl

CH 3

CH 2

C

CH 2

CH 3

H Cl

CH 3

C

C

CH 2

CH 3

H Cl

H

CH 3

(2S,3R)-2,3-Dichloropentane

Abstraction of a hydrogen atom from C3 of (S)-2-chloropentane produces a radical

that is chiral (it contains a stereocenter at C2). This chiral radical can then react

with chlorine at one face [path (a)] to produce (2S,3S)-2,3-dichloropentane and at

the other face [path (b)] to yield (2S,3R)-2,3-dichloropentane. These two

compounds are diastereomers, and they are not produced in equal amounts. Each

product is chiral, and each alone would be optically active.

(^)