Let’s have a look at the reaction parameters.
Nucleophiles - atoms, ions or molecules
that have an electron pair that may be donated
in forming a covalent bond to an electrophile
(or a Lewis acid). They have a negative charge,
partial negative charge, an electron pair
or pi electrons available.
Let’s have a look at some examples of nucleophiles
used in SN reactions. Negative ones include
our friend hydroxide, methoxide, it’s the
CH3O-, cyanide (CN-), NH2- and dicarbide.
Neutral nucleophiles include water, methanol
and ammonia, but are not restricted; there
are a great many others.
It’s possible to classify a species in order
of its nucleophilicity. That is to say how
willing it is to donate it’s electrons in
the formation of a new bond. The most reactive
nucleophiles are therefore said to be more
nucleophilic than those less reactive ones.
In general, for a given element, negatively
charged species are more nucleophilic - hydroxide,
cyanide, even sometimes, other halides.
Nucleophilicity decreases on moving from left
to right in a period of the periodic table
and nucleophilicity increases from top to
bottom i.e. with increasing size, along a
group of the periodic table (basicity varies,
however, in the opposite manner).
Thus, if you look at the bottom of the board,
you’ll see the order of the nucleophilicity
- nitrile or cyanide being the highest, followed
by iodide, methoxide, hydroxide, bromide,
chloride, ammonia, fluoride, acetate, alcohol
and water; the alcohol, of course, being able
to beat methanol in this... in this case.
Let’s also have a look at the solvent effects.
Polar protic solvents solvate the anion that
is produced, thus decreasing its nucleophilicity.
Polar aprotic solvents such as dimethylsulphoxide
or dimethylformamide solvate the counter-ion,
thus leaving the anion more naked and therefore,
more nucleophilic. So, in the case of an SN2
reaction, polar aprotic solvents are the best
to achieve the highest degree of nucleophilicity
for your given nucleophile. In different
solvents, the order of... the order of nucleophilicity
can indeed change slightly... slightly as
a direct consequence of this.
Let’s have a look at the alkyl group. The
degree of substitution, as we saw in the case
of the alkanes, in the previous lecture, can
have a marked influence on the reactivity
of a haloalkane and the degree of substitution
on the carbon has a great effect on whether
it’s SN2 or as we will see, SN1.
The order of reactivity is thus in terms of
an SN2 reaction and let me talk you through
it. In the case of an SN2 reaction, what you’re
dependent upon is a nucleophile directly attacking
something which has a leaving group, in this
case, a halide that wishes to leave. When
you have something like a methyl or primary
alkyl group as part of your haloalkane, these
are sterically less congested. With that we’re
talking about just the idea that hydrogen
is a small atom and it doesn’t get in the
way of the reaction. If, on the other hand,
we have three alkyl groups such as in the
case of our tertiary, there is insufficient
space for a nucleophile to be able to attack
effectively and therefore, carry out its bimolecular
SN2 exchange of electrons.
Now, let’s quickly have a look in the context
of an SN2 reaction of the leaving group. Bearing
in mind that the leaving group, in this case
our halide, is also technically a nucleophile.
It has a negative charge and it may actually
want to react again. Conjugate bases of a
strong acid are usually good leaving groups,
so things like iodide, chloride and bromide,
although fluoride, less so.