Organic Chemistry

378 CHAPTER 10 Substitution Reactions of Alkyl Halides

If the nucleophile attacks the side of the carbon from which the leaving group depart-

ed, the product will have the same relative configuration as that of the reacting alkyl

halide. If, however, the nucleophile attacks the opposite side of the carbon, the product

will have the inverted configuration relative to the configuration of the alkyl halide.

We can now understand the third piece of experimental evidence for the mechanism of

an reaction: An reaction of an alkyl halide in which the leaving group is at-

tached to an asymmetric carbon forms two stereoisomers because attack of the nucleo-

phile on one side of the planar carbocation forms one stereoisomer and attack on the

other side produces the other stereoisomer.

PROBLEM 13

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

isopropyl bromide, propyl bromide,tert-butyl bromide, methyl bromide.

PROBLEM 14

Why are the rates of the reactions of ethyl bromide and methyl bromide given in

Table 10.4 so slow?

10.6 Factors Affecting Reactions

The Leaving Group

Because the rate-determining step of an reaction is the dissociation of the alkyl

halide to form a carbocation, two factors affect the rate of an reaction: the ease

with which the leaving group dissociates from the carbon and the stability of the car-

bocation that is formed. In the preceding section, we saw that tertiary alkyl halides are

more reactive than secondary alkyl halides, which are more reactive than primary alkyl

halides. This is because the more substituted the carbocation is, the more stable it is

and therefore the easier it is to form. But how do we rank the relative reactivity of a se-

ries of alkyl halides with different leaving groups that dissociate to form the same car-

bocation? As in the case of the reaction, there is a direct relationship between

basicity and leaving ability in the reaction: The weaker the base, the less tightly it

is bonded to the carbon and the easier it is to break the carbon–halogen bond. As a re-

sult, an alkyl iodide is the most reactive and an alkyl fluoride is the least reactive of the

alkyl halides in both and reactions.

relative reactivities of alkyl halides in an SN1 reaction

most reactive RI > RBr > RCl > RF least reactive

SN 1 SN 2

SN 1

SN 2

SN 1

SN 1

SN 1

SN 2

SN 1

if the leaving group in an SN1 reaction is attached to an asymmetric

carbon, a pair of enantiomers will be formed as products

++H 2 O HBr

+

RR

R′

H

Br HO

C

R′

RBr

R′

H

HO R R

R′

H

R′

H

C

R

R′

C H H

OH

+

OH

SN 1 SN 1