Nucleophilic Substitution ReactionsTheSN1 mechanism is preferred for tertiary alkyl halides and, depending on the solvent,
may be preferred in secondary alkyl halides. TheSN1 mechanism does not operate on
primary alkyl halides or methyl halides. To understand why this is so, let’s take a look at
how theSN1 mechanism works.
Figure 159 SN1 nucleophilic substitution of a generic halide with a water molecule to
produce an alcohol.
At the top of the diagram, the first step is the spontaneous dissociation of the halide from
the alkyl halide. Unlike theSN2 mechanism, where the attacking nucleophile causes the
halide to leave, theSN1 mechanism depends on the ability of the halide to leave on its
own. This requires certain conditions. In particular, the stability of the carbocation^8 is
crucial to the ability of the halide to leave. Since we know tertiary carbocations are the
most stable, they are the best candidates for theSN1 mechanism. And with appropriate
conditions, secondary carbocations will also operate by theSN1 mechanism. Primary and
methyl carbocations however, are not stable enough to allow this mechanism to happen.
Once the halide has dissociated, the water acts as a nucleophile to bond to the carbocation.
In theSN2 reactions, there is an inversion caused by the nucleophile attacking from the
opposite side while the halide is still bonded to the carbon. In theSN1 mechanism, since
the halide has left, and the bonds off of the α-carbon have become planar, the water molecule
is free to attack from either side. This results in, primarily, a racemic^9 mixture. In the final
step, one of the hydrogens of the bonded water molecule is attacked by another water
molecule, leaving an alcohol.
Note: Racemic mixtures imply entirely equal amounts of mixture, however this is rarely the
case inSN 1. There is a slight tendency towards attack from the opposite side of the halide.
This is the result some steric hinderence from the leaving halide which is sometimes close
enough to the leaving side to block the nucleophile’s approach from that side.
Solvent
Like theSN2 mechanism, theSN1 is affected by solvent as well. As with structure, however,
the reasons differ. In theSN1 mechanism, a polar, protic solvent is used. The polarity of
the solvent is associated with the dielectric constant of the solvent and solutions with high
dielectric constants are better able to support separated ions in solution. InSN2 reactions,
we were concerned about polar hydrogen atoms ”caging” our nucleophile. This still happens
with a polar protic solvent inSN1 reactions, so why don’t we worry about it? You have
to keep in mind the mechanism of the reaction. The first step, and more importantly,
the rate-limiting step, of theSN1 reaction, is the ability to create a stable carbocation by
getting the halide anion to leave. With a polar protic solvent, just as with a polar aprotic
solvent,we’re creating a stable cation, however it’s the polar hydrogens that stabilize the
halide anion and make it better able to leave. Improving the rate-limiting step is always
the goal. The ”caging” of the nucleophile is unrelated to the rate-limiting step and even in
8 Chapter 118 on page 381
9 https://en.wikipedia.org/wiki/Racemic