Organic Chemistry

Section 10.7 More About the Stereochemistry of SN 2 and SN 1 Reactions 381

The Stereochemistry of Reactions
In contrast to the reaction, the reaction of (S)-2-bromobutane forms two
substitution products—one with the same relative configuration as the reactant and the
other with the inverted configuration. In an reaction, the leaving group leaves
before the nucleophile attacks. This means that the nucleophile is free to attack either
side of the planar carbocation. If it attacks the side from which the bromide ion left,
the product will have the same relative configuration as the reactant. If it attacks the
opposite side, the product will have the inverted configuration.

Although you might expect that equal amounts of both products should be formed
in an reaction, a greater amount of the product with the inverted configuration is
obtained in most cases. Typically, 50–70% of the product of an reaction is the in-
verted product. If the reaction leads to equal amounts of the two stereoisomers, the re-
action is said to take place with complete racemization. When more of one of the
products is formed, the reaction is said to take place with partial racemization.
Saul Winstein was the first to explain why extra inverted product generally is
formed in an reaction. He postulated that dissociation of the alkyl halide initially
results in the formation of an intimate ion pair. In an intimate ion pair, the bond be-
tween the carbon and the leaving group has broken, but the cation and anion remain
next to each other. This species then forms a solvent-separated ion pair—an ion pair in
which one or more solvent molecules have come between the cation and the anion.
Further separation between the two results in dissociated ions.

The nucleophile can attack any of these four species. If the nucleophile attacks only
the completely dissociated carbocation, the product will be completely racemized. If
the nucleophile attacks the carbocation of either the intimate ion pair or the solvent-
separated ion pair, the leaving group will be in position to partially block the approach
of the nucleophile to that side of the carbocation and more of the product with the in-
verted configuration will be obtained. (Notice that if the nucleophile attacks the undis-
sociated molecule, the reaction will be an reaction and all of the product will have
the inverted configuration.)

SN 2

solvent

undissociated

molecule

RX

intimate

ion pair

R+ X−

solvent-separated

ion pair

R+ X−

dissociated ions

R+ X−

SN 1

SN 1

SN 1

(S)-2-bromobutane (R)-2-butanol

C +++H 2 O HBr

CH 3 CH 3

CH 2 CH 3

H H

Br HO OH

C

CH 2 CH 3

(S)-2-butanol

product with

inverted

configuration

product with

retained

configuration

C

CH 3

CH 2 CH 3

H

SN1 conditions

SN 1

SN 2 SN 1

SN 1

(S)-2-bromobutane (R)-2-butanol

C ++HO− Br−

CH 3 CH 3

CH 2 CH 3

H H

Br HO

C

CH 2 CH 3

SN2 conditions

the configuration is inverted

relative to that of the reactant

An reaction takes place with

inversion of configuration.

SN 2

An reaction takes place with

racemization.

SN 1

Saul Winstein (1912–1969)was

born in Montreal, Canada. He

received a Ph.D. from the California

Institute of Technology and was a

professor of chemistry at the

University of California, Los Angeles,

from 1942 until his death.