Organic Chemistry

Section 10.5 The Mechanism of an SN 1 Reaction 377

effect on the rate of the reaction. If you look at the reaction coordinate diagram in
Figure 10.6, you will see why increasing the rate of the second step will not make an
reaction go any faster.
How does the mechanism for an reaction account for the three pieces of exper-
imental evidence? First, because the alkyl halide is the only species that participates in
the rate-determining step, the mechanism agrees with the observation that the rate of
the reaction depends on the concentration of the alkyl halide and does not depend on
the concentration of the nucleophile.
Second, the mechanism shows that a carbocation is formed in the rate-determining
step of an reaction. We know that a tertiary carbocation is more stable, and is
therefore easier to form, than a secondary carbocation, which in turn is more stable
and easier to form than a primary carbocation (Section 4.2). Tertiary alkyl halides,
therefore, are more reactive than secondary alkyl halides, which are more reactive than
primary alkyl halides in an reaction. This relative order of reactivity agrees with
the observation that the rate of an reaction decreases as the methyl groups of tert-
butyl bromide are successively replaced by hydrogens (Table 10.4).

Actually, primary carbocations and methyl cations are so unstable that primary alkyl
halides and methyl halides do not undergo reactions. (The very slow reactions
reported for ethyl bromide and methyl bromide in Table 10.4 are really reactions.)
The positively charged carbon of the carbocation intermediate is hybridized,
and the three bonds connected to an hybridized carbon are in the same plane. In
the second step of the reaction, the nucleophile can approach the carbocation from
either side of the plane.

SN 1

sp^2

sp^2

SN 2

SN 1

relative reactivities of alkyl halides in an SN1 reaction

most reactive 3 ° alkyl halide > 2 ° alkyl halide > 1 ° alkyl halide least reactive

SN 1

SN 1

SN 1

SN 1

SN 1

X

R+^ +^ X−

+^ H 2 O

R OH+^ H 3 O+

R OH

R

Free energy

∆G‡

carbocation

intermediate

rate-determining

step

Progress of the reaction

H

+

>Figure 10.6

Reaction coordinate diagram for an

SN 1 reaction.

H+ H+

Br

Br−

C

Br−

C

H 2 O OH 2

CHO

H

HO C COH

H

C OH

+

++

a b

a b

inverted configuration

relative to the configuration

of the alkyl halide

same configuration

as the alkyl halide

+ +

Primary alkyl halides and methyl halides

cannot undergo SN 1 reactions.

Tutorial:

SN 1