Organic Chemistry

Section 10.2 The Mechanism of an SN 2 Reaction 365

>Figure 10.2

The approach of to a methyl

halide, a primary alkyl halide, a

secondary alkyl halide, and a

tertiary alkyl halide. Increasing the

bulk of the substituents bonded to

the carbon that is undergoing

nucleophilic attack decreases access

to the back side of the carbon,

thereby decreasing the rate of an

SN 2 reaction.

HO-

Progress of the reaction

Free energy

Progress of the reaction

Free energy

∆G‡

∆G‡

a. b.

C

H

HO

CH 3 Br + HO− CH 3 OH + Br− R CHBr + HO− R CHOH + Br−

R R

Br

R R

δ− δ−

C

H

HO Br

H H

δ− δ−

Figure 10.3
Reaction coordinate diagrams for (a) the reaction of methyl bromide with hydroxide ion;
(b) an SN 2 reaction of a sterically hindered alkyl bromide with hydroxide ion.

SN 2

have the following relative reactivities in an reaction because,generally, primary
alkyl halides are less hindered than secondary alkyl halides, which, in turn, are less
hindered than tertiary alkyl halides:

The three alkyl groups of a tertiary alkyl halide make it impossible for the nucleophile
to come within bonding distance of the tertiary carbon, so tertiary alkyl halides are un-
able to undergo reactions. The reaction coordinate diagrams for the reactions
of unhinderedmethyl bromide and a sterically hinderedalkyl bromide show that steric
hindrance raises the energy of the transition state, slowing the reaction (Figure 10.3).
The rate of an reaction depends not only on the numberof alkyl groups at-
tached to the carbon that is undergoing nucleophilic attack, but also on their size. For
example, while ethyl bromide and propyl bromide are both primary alkyl halides,
ethyl bromide is more than twice as reactive in an reaction because the bulkier
propyl group provides more steric hindrance to back-side attack. Also, although
neopentyl bromide is a primary alkyl halide, it undergoes reactions very slowly
because its single alkyl group is unusually bulky.

SN 2

SN 2

SN 2

SN 2 SN 2

relative reactivities of alkyl halides in an SN2 reaction

most methyl halide > 1 ° alkyl halide > 2 ° alkyl halide > 3 ° alkyl halide

reactive

least

reactive

SN 2

Tertiary alkyl halides cannot undergo

SN 2 reactions.

3-D Molecules:

Methyl chloride;

t-Butyl chloride

Figure 10.4 shows that as the nucleophile approaches the back side of the carbon of
methyl bromide, the bonds begin to move away from the nucleophile and its
attacking electrons. By the time the transition state is reached, the bonds are all
in the same plane and the carbon is pentacoordinate (fully bonded to three atoms and
partially bonded to two) rather than tetrahedral. As the nucleophile gets closer to the
carbon and the bromine moves farther away from it, the bonds continue to
move in the same direction. Eventually, the bond between the carbon and the nucle-
ophile is fully formed, and the bond between the carbon and bromine is completely
broken, so that the carbon is once again tetrahedral.

C¬H

C¬H

C¬H

CH 3 CCH 2 Br

CH 3

CH 3

neopentyl bromide