SN1 Substitution Reactions - Arrow Pushing in Organic Chemistry - page 84

Scheme 5.1 Initial phase of an S N 1 reaction involves dissociation of a leaving group from the starting
molecule, generating a carbocation.
Scheme 5.2 Second phase of an S N 1 reaction involves reaction of a carbocation with a nucleophile
generating a new product.
with a nucleophile as shown in Scheme 5.2. Because the initial step in this reaction involves
a single molecule dissociating from its leaving group, the initial stage of this process is con-
sidered a unimolecular reaction. The involvement of only 1 species in the initial phase of
the reaction enhances the mechanistic designation to S N 1.
Because, with S N 1 reactions, a reactive carbocation is formed before incorporation of a
nucleophile, other products may form in addition to the simple substituted materials antici-
pated. These additional products arise from the specific properties of carbocations. The
properties of carbocations and their related mechanistic outcomes are presented in the
following sections.
5.2 HOW ARE S N 1 REACTIONS INITIATED?
In order for an S N 1 reaction to proceed, initial formation of a carbocation is required. A
primary method for the formation of carbocations occurs during solvolysis reactions.
Scheme 5.3 Solvolysis of tert-butylbromide in methanol produces MTBE via an S N 1 mechanism.