Organic Chemistry

376 CHAPTER 10 Substitution Reactions of Alkyl Halides

bromide and water is an reaction, where “S”stands for substitution,“N”stands

for nucleophilic, and “ 1 ”stands for unimolecular. Unimolecularmeans that only one

molecule is involved in the rate-determining step. The mechanism of an reaction

is based on the following experimental evidence:

1. The rate law shows that the rate of the reaction depends only on the concentra-
   tion of the alkyl halide. This means that we must be observing a reaction whose
   rate-determining step involves only the alkyl halide.


2. When the methyl groups of tert-butyl bromide are successively replaced by hy-
   drogens, the rate of the reaction decreases progressively (Table 10.4). This
   is opposite to the order of reactivity exhibited by alkyl halides in reactions
   (Table 10.1).


3. The reaction of an alkyl halide in which the halogen is bonded to an asymmetric
   carbon forms two stereoisomers: one with the same relative configuration at the
   asymmetric carbon as the reacting alkyl halide, the other with the inverted
   configuration.

SN 2

SN 1

SN 1

SN 1

3-D Molecules:

t-Butyl bromide;

t-Butyl cation;

protonated t-Butyl alcohol;

t-Butyl alcohol

Table 10.4 Relative Rates of SN1 Reactions for Several Alkyl Bromides

(solvent is H 2 O, nucleophile is H 2 O)

tertiary

secondary

primary

methyl

Alkyl bromide Class of alkyl bromide Relative rate

CH 3 C

CH 3

CH

Br 1,200,000

CH 3 CH

CH 3

Br 11.6

CH 3 CH 2 Br 1.00*

CH 3 Br 1.05*

3

*Although the rate of the SN1 reaction of this compound with water is 0, a small rate is observed as a result of an SN 2

reaction.

Unlike an reaction, where the leaving group departs and the nucleophile ap-

proaches at the same time, the leaving group in an reaction departs beforethe nu-

cleophile approaches. In the first step of an reaction of an alkyl halide, the

carbon–halogen bond breaks heterolytically, with the halogen retaining the previously

shared pair of electrons, and a carbocation intermediate is formed. In the second step,

the nucleophile reacts rapidly with the carbocation to form a protonated alcohol.

Whether the alcohol product will exist in its acidic (protonated) form or basic (neutral)

form depends on the pH of the solution. At the alcohol will exist predomi-

nantly in its basic form.

Because the rate of an reaction depends only on the concentration of the alkyl

halide, the first step must be the slow and rate-determining step. The nucleophile,

therefore, is not involved in the rate-determining step, so its concentration has no

SN 1

pH=7,

SN 1

SN 1

SN 2

slow fast fast

mechanism of the SN1 reaction

CH 3 C

CH 3

CH 3

Br

+ Br−

CH 3 C+ + H 2 O

CH 3

CH 3

CH 3 C

CH 3

CH 3

CH 3 C + H 3 O

CH 3

CH 3

OH

H

OH

+ +

OH

H

C Br bond breaks proton transferproton transfer

heterolytically

proton transfernucleophile attacks the carbocation