Right. Now, we talked about SN2 reactions,
but there is the so called SN1 reaction. This
is substitution nucleophilic unimolecular.
This takes place in the absence that I said
of actually an attacking nucleophile and we’ll
go through that. But, before I do, I just
want to talk about the steric chemistry. If
we said, or if we look at the previous SN2
reaction, you can see we get inversion of
the steric chemistry for SN2. An S, or sinister,
stereochemistry under Cahn-Ingold-Prelog rule
is inverted to an R or recto stereochemistry.
However, when an SN1 reaction takes place,
you can actually convert an enantiomer into
a racemic mixture, where you have approximately
equal amounts of S and R enantiomers. And
that is the basis of an SN1 reaction in terms
of the stereochemistry.
So, why is that? Let’s have a quick look.
In the SN1 mechanism, the reaction coordinate
is actually not dependent upon the concentration
of your nucleophile. In an SN1 reaction, the
loss of halide takes place first, thus generating
a carbocation. As you can see, in the center
of this particular reaction series, you’ve
got here a plain old mirror that show because
what you have, as I showed you before in the
case of the alkanes, I am sorry, alkenes,
is a p-orbital, which is perpendicular to
that trigonal planar arrangement. This means,
after that carbocation is formed, which is
an intermediate, is that the nucleophile,
in this case our hydroxide, can attack from
By being able to attack from either side,
which is a fast step in this particular reaction,
you can convert an enantiomerically pure compound
into its racemate, which, in fairness, is
often not particularly desirable. The reality,
when it comes to SN1 reactions, is that you
tend to see more of one enantiomer than the
other. This is due to the ion pair effect.
But, that goes beyond our terms of reference.
The reaction is said to be unimolecular, that
is, the number 1 in SN1 stands for unimolecular.
Only the halogeno-alkane is participating
in the slow step of the reaction and therefore,
the rate is dependent solely on the concentration
of the haloalkane. Therefore, the reaction
kinetics are thus where the reaction rate,
k, is proportional to the concentration of
the haloalkane. Bear in mind, this is just
to do with kinetics; nothing to do with thermodynamics.
Now, let’s have a look at some of the reaction
parameters relating to SN1 reactions, specifically,
the nucleophilicity of the nucleophile, the
solvent, the alkyl group and the leaving group.
Let’s have a look at the stability of the
carbocations first because as we indicated,
the slow step is actually the formation of
the carbocation. The nucleophilicity of the
nucleophile is to a large extent irrelevant
in this case, since it is the fast step of
the reaction that it participates in. Unlike
the SN2 reaction, the order of stability or
the order of reactivity for this particular
mechanism is reversed.
As you can appreciate from having looked at
the way in which addition reactions take place
over alkenes, you can see that a positive
charge surrounded by electron donating alkyl
groups, shown as R in this particular case,
can better stabilise that carbocation and
therefore, the conversion of a haloalkane
to a carbocation... carbocation by the loss
of its halide is more likely to occur for
a tertiary haloalkane.
The next, and most stable, is the secondary,
shown there as 2, primary - 1 and then finally
methyl. The methyl group is a very unstable
carbocation, due to a slight inductive effect
of the alkyl groups that partially stabilise
All that was said previously regarding the
stability of the nucleophiles is true for
SN1 reactions. However, also, to facilitate
the charge separation in the first step, a
good ionising solvent will be needed. So,
this effectively alters our parameter set
for the SN1 reaction. So, ideally, what we
want is something that stabilises the carbocation
in the first case. So, unlike with the SN2,
solvents such as water and formic acid and
alcohols are generally considered better ionising
solvents than some organic solvents such as
DMSO and dimethylformamide.