Section 10.6 Factors Affecting SN 1 Reactions 379The Nucleophile
The nucleophile reacts with the carbocation that is formed in the rate-determining step
of an reaction. Because the nucleophile comes into play afterthe rate-determining
step, the reactivity of the nucleophile has no effect on the rate of an reaction
(Figure 10.6).
In most reactions, the solvent is the nucleophile. For example, the relative rates
given in Table 10.4 are for the reactions of alkyl halides with water in water. Water
serves as both the nucleophile and the solvent. Reaction with a solvent is called
solvolysis. Thus, each rate in Table 10.4 is for the solvolysis of the indicated alkyl bro-
mide in water.
Carbocation Rearrangements
A carbocation intermediate is formed in an reaction. In Section 4.6, we saw that a
carbocation will rearrange if it becomes more stable in the process. If the carbocation
formed in an reaction can rearrange, and reactions of the same alkyl
halide can produce different constitutional isomers as products, since a carbocation is
not formed in an reaction and therefore the carbon skeleton cannot rearrange. For
example, the product obtained when is substituted for in 2-bromo-3-
methylbutane by an reaction is different from the product obtained by an re-
action. When the reaction is carried out under conditions that favor an reaction,
the initially formed secondary carbocation undergoes a 1,2-hydride shift to form a
more stable tertiary carbocation.
The product obtained from the reaction of 3-bromo-2,2-dimethylbutane with a nu-
cleophile also depends on the conditions under which the reaction is carried out. The
carbocation formed under conditions that favor an reaction will undergo a
1,2-methyl shift. Because a carbocation is not formed under conditions that favor an
reaction, the carbon skeleton does not rearrange. In Sections 10.9 and 10.10, we
will see that we can exercise some control over whether an or an reaction
takes place, by selecting appropriate reaction conditions.
SN 1 SN 2SN 2
SN 1SN 1SN 1 SN 2HO- Br-SN 2SN 1 SN 1 SN 2SN 1SN 1SN 1SN 1SN 1 1,2-hydride shiftSN 2
HO−secondary
carbocation3-methyl-2-butanol2-bromo-
3-methylbutaneCH 3 CHCHCH 3CH 3CH 3 CHCHCH 3CH 3
H 2 O
H 2 OH 2 OH 3 O+tertiary
carbocationCH 3 CCH 2 CH 3CH 3CH 3 CHCHCH 3CH 3OHBr+ + CH^3 CCH^2 CH^3CH 3OH
H++Br−+Br−2-methyl-2-butanolCH 3 CCH 2 CH 3CH 3OHSN 1 1,2-methyl shiftSN 2
HO−secondary
carbocation3,3-dimethyl-2-butanol3-bromo-2,2-
dimethylbutaneCH 3 C CHCH 3CH 3 C CHCH 3 CH 3 C CHCH 3 CH 3 C CHCH 3CH 3 C CHCH 3 CH 3 C CHCH 3CH 3CH 3
H 2 O
H 2 OCH 3CH 3OHCH 3CH 3
tertiary
carbocationCH 3 CH 3Br+ +CH 3OH
H+CH 3
2,3-dimethyl-2-butanolCH 3OH CH 3H 2 O+H 3 O++Br−+Br−When a reaction forms a carbocation
intermediate, always check for the
possibility of a carbocation
rearrangement.