Organic Chemistry

The carbon at which substitution occurs has inverted its configuration during the

course of the reaction, just as an umbrella has a tendency to invert in a windstorm. This

inversion of configurationis called a Walden inversion, after Paul Walden, who first

discovered that the configuration of a compound was inverted in an reaction.

Because an reaction takes place with inversion of configuration, only one substitu-

tion product is formed when an alkyl halide—whose halogen atom is bonded to an asym-

metric carbon—undergoes an reaction. The configuration of that product is inverted

relative to the configuration of the alkyl halide. For example, the substitution product of the

reaction of hydroxide ion with (R)-2-bromopentane is (S)-2-pentanol. Therefore, the

proposed mechanism accounts for the observed configuration of the product.

PROBLEM 1

Does increasing the energy barrier to an reaction increase or decrease the magnitude

of the rate constant for the reaction?

PROBLEM 2

Arrange the following alkyl bromides in order of decreasing reactivity in an

reaction: 1-bromo-2-methylbutane, 1-bromo-3-methylbutane, 2-bromo-2-methylbutane,

and 1-bromopentane.

PROBLEM 3 SOLVED

What product would be formed from the reaction of the following?

a. 2-bromobutane and hydroxide ion

b. (R)-2-bromobutane and hydroxide ion

c. (S)-3-chlorohexane and hydroxide ion

d. 3-iodopentane and hydroxide ion

SOLUTION TO 3a The product is 2-butanol. We know that because the reaction is an

reaction, the configuration of the product is inverted relative to the configuration of

the reactant. The configuration of the reactant is not specified, however, so we cannot spec-

ify the configuration of the product.

CH 3 CHCH 2 CH 3 + CH 3 CHCH 2 CH 3 + Br−

Br OH

the configuration

is not specified

SN 2

SN 2

SN 2

SN 2

the configuration of the product is inverted

relative to the configuration of the reactant

C ++HO− Br−

RR

R′

H H

Br HO

C

R′

RBr

R′

H

HO R

R′

H

SN 2

SN 2

SN 2

366 CHAPTER 10 Substitution Reactions of Alkyl Halides

Tutorial: SN 2

Paul Walden (1863–1957)was born
in Cesis, Latvia, the son of a farmer.
His parents died when he was a
child. He supported himself at Riga
University and St. Petersburg
University by working as a tutor. He
received a Ph.D. from the University
of Leipzig and returned to Latvia to
become a professor of chemistry at
Riga University. Following the
Russian Revolution, he went back to
Germany to be a professor at the
University of Rostock and, later, at
the University of Tübingen.

‡

+ +

three bonds are in the same plane

Figure 10.4

An SN 2 reaction between hydroxide ion and methyl bromide.

AU: OK as

changed?