386 CHAPTER 10 Substitution Reactions of Alkyl Halides
Table 10.6 Summary of the Reactivity of Alkyl Halides in Nucleophilic
Substitution ReactionsMethyl and 1°alkyl halides only
Vinylic and aryl halides Neither nor
2 °alkyl halides and
1 °and 2°benzylic and 1°and 2°allylic halides and
3 °alkyl halides only
3 °benzylic and 3°allylic halides SN 1 onlySN 1SN 1 SN 2SN 1 SN 2SN 1 SN 2SN 2When an alkyl halide can undergo both an reaction andan reaction, both
reactions take place simultaneously. The conditions under which the reaction is carried
out determine which of the reactions predominates. Therefore, we have some experi-
mental control over which reaction takes place.
When an alkyl halide can undergo substitution by both mechanisms, what condi-
tions favor an reaction? What conditions favor an reaction? These are
important questions to synthetic chemists because an reaction forms a single sub-
stitution product, whereas an reaction can form two substitution products if the
leaving group is bonded to an asymmetric carbon. An reaction is further compli-
cated by carbocation rearrangements. In other words, an reaction is a synthetic
chemist’s friend, but an reaction can be a synthetic chemist’s nightmare.
When the structureof the alkyl halide allows it to undergo both and reactions,
three conditions determine which reaction will predominate: (1) the concentrationof the
nucleophile, (2) the reactivityof the nucleophile, and (3) the solventin which the reaction
is carried out. To understand how the concentration and the reactivity of the nucleophile
affect whether an or an reaction predominates, we must examine the rate laws
for the two reactions. The rate constants have been given subscripts that indicate the
reaction order.The rate law for the reaction of an alkyl halide that can undergo both and re-
actions simultaneously is the sum of the individual rate laws.From the rate law, you can see that increasing the concentrationof the nucleophile
increases the rate of an SN 2 reaction but has no effect on the rate of an SN 1 reaction.contribution to the rate
by an SN2 reactionrate =+k 2 [alkyl halide][nucleophile] k 1 [alkyl halide]contribution to the rate
by an SN1 reactionSN 2 SN 1Rate law for an SN2 reaction = k 2 [alkyl halide] [nucleophile]Rate law for SN1 reaction = k 1 [alkyl halide]SN 2 SN 1SN 2 SN 1SN 1SN 2SN 1SN 1SN 2SN 1 SN 2SN 1 SN 2unimolecular. These numbers do not refer to the number of steps in the mechanism. In
fact, just the opposite is true: An reaction proceeds by a one-step concerted mech-
anism, and an reaction proceeds by a two-step mechanism with a carbocation
intermediate.
We have seen that methyl halides and primary alkyl halides undergo only reac-
tions because methyl cations and primary carbocations, which would be formed in an
reaction, are too unstable to be formed in an reaction. Tertiary alkyl halides
undergo only reactions because steric hindrance makes them unreactive in an
reaction. Secondary alkyl halides as well as benzylic and allylic halides (unless they
are tertiary) can undergo both and reactions because they form relatively
stable carbocations and the steric hindrance associated with these alkyl halides is
generally not very great. Vinylic and aryl halides do not undergo either or
reactions. These results are summarized in Table 10.6.SN 1 SN 2SN 1 SN 2SN 1 SN 2SN 1 SN 2SN 2SN 1SN 2