Organic Chemistry

362 CHAPTER 10 Substitution Reactions of Alkyl Halides

2. The carbon–halogen bond breaks heterolytically without any assistance from the
   nucleophile, forming a carbocation. The carbocation—an electrophile—then re-
   acts with the nucleophile to form the substitution product.

Regardless of the mechanism by which a substitution reaction occurs, it is called a

nucleophilic substitution reactionbecause a nucleophile substitutes for the halogen.

We will see that the mechanism that predominatesdepends on the following factors:

• the structure of the alkyl halide


• the reactivity of the nucleophile


• the concentration of the nucleophile


• the solvent in which the reaction is carried out

10.2 The Mechanism of an Reaction

How is the mechanism of a reaction determined? We can learn a great deal about the

mechanism of a reaction by studying its kinetics—the factors that affect the rate of the

reaction.

The rate of a nucleophilic substitution reaction such as the reaction of methyl bro-

mide with hydroxide ion depends on the concentrations of both reagents. If the con-

centration of methyl bromide in the reaction mixture is doubled, the rate of the

nucleophilic substitution reaction doubles. If the concentration of hydroxide ion is

doubled, the rate of the reaction also doubles. If the concentrations of both reactants

are doubled, the rate of the reaction quadruples.

When you know the relationship between the rate of a reaction and the concentration

of the reactants, you can write a rate lawfor the reaction. Because the rate of the

reaction of methyl bromide with hydroxide ion is dependent on the concentration of

both reactants, the rate law for the reaction is

As we saw in Section 3.7, a proportionality sign can be replaced by an equals sign

and a proportionality constant. The proportionality constant, in this case k, is called the

rate constant. The rate constant describes how difficult it is to overcome the energy

barrier of the reaction—how hard it is to reach the transition state. The larger the rate

constant, the easier it is to reach the transition state (see Figure 10.3 on p. 365).

Because the rate of this reaction depends on the concentration of two reactants, the re-

action is a second-order reaction(Section 3.7).

ratek[alkyl halide][nucleophile]

1 r 2

rate ̃[alkyl halide][nucleophile]

methyl bromide methyl alcohol

CH 3 Br + HO− CH 3 OH + Br−

SN 2

C C+

C+ C

X + X−

+ Nu Nu

δ+δ−

−

substitution

product