Substitution, Nucleophilic, Unimolecular
Key feature: stepwise; formation of a
carbocation as intermediate; racemization of configuration at the
carbon being attacked; the reaction rate depends only on the substrate.
The reaction is favored by highly hindered
substrate and by a weak nucleophile such as H2O, CH3OH,
and other neutral solvent molecules. Shown below is an animation for
hydrolysis of t-butyl bromide, where the bromine atom leaves the carbon
in the rate-determining step. Note that no nucleophile (solvent) is
involved.
The result of this process (slow step, rate-limiting)
is the formation of a carbocation, in this case, a tertiary carbocation.
Recall that methyl group is electron-releasing; therefore the three
methyl groups in the above animation help to stabilize the cation
(positive charge) developed on the central carbon. The geometry of
the carbocation is such that the groups around the central carbon
are arranged in a trigonal planar fashion, with the central carbon
taking sp2 hybridization. (changed from sp3
in the starting material) The following shows the intermediate formed
(t-butyl carbocation) in the above reaction:
This geometry is important in determining the stereochemistry
of products derived from an SN1 reaction. Since the central
carbon has a trigonal planar geometry, the nucleophile (solvent molecules)
can now attack from either side of the plane. On the other hand, the
nucleophile in an SN2 reaction has to attack from only
one side -- the opposite side of the leaving group.
