Advices For Studying Organic Chemistry

A Mechanism for the Reaction

The Stereochemistry of Chlorination at C2 of Pentane

+Cl Cl

Cl

Cl Cl

Cl 2 Cl 2

C

CH 3

CH 2 CH 2 CH 3

H C +

H 3 C

H 3 CH 2 CH 2 C

H

C

CH 3

HCH 2 CH 2 CH 3

CH 3 CH 2 CH 2 CH 2 CH 3

C2

(S)-2-Chloropentane Trigonal planar radical

Enantiomers

(50%)

(R)-2-Chloropentane

(achiral) (50%)

Abstraction of a hydrogen atom from C2 produces a trigonal planar radical that is

achiral. This radical is achiral then reacts with chlorine at either face [by path (a)

or path (b)]. Because the radical is achiral the probability of reaction by either

path is the same; therefore, the two enantiomers are produced in equal amounts,

and a racemic form of 2-chloropentane results.

(^)

10.8A GENERATION OF A SECOND STEREOCENTER IN A RADICAL

HALOGENATION:

1. When a chiral molecule reacts to yield a product with a second stereocenter:
   
   
   1. The products of the reactions are diastereomeric (2S,3S)-2,3-dichloropentane
      and (2S,3R)-2,3-dichloropentane.
      i) The two diastereomers are not produced in equal amounts.
      ii) The intermediate radical itself is chiral ⇒ reactions at the two faces are not
      equally likely.
      iii) The presence of a stereocenter in the radical (at C2) influences the reaction that
      introduces the new stereocenter (at C3).