DMSO and dimethylformamide.
So, let’s have a look at these parameters
and how they influence what is the most likely
mechanism? What are the steric chemical implications
of these reactions? The parameter of the haloalkane
is in order of reactivity in SN2 - primary,
secondary, tertiary whereas the reverse is
true in the case of SN1.
In the case of SN2, to facilitate this, a
strong nucleophile is highly desirable. However,
in the case of SN1, this is largely irrelevant.
A good leaving group is something which is
better solvated, iodide or bromide is usually
a reasonable example, is preferred for the
unimolecular nucleophilic substitution reaction.
And finally, the solvent. Polar aprotic for
SN2 and good ionising and stabilisation of
the cation, in the case of SN1.
Right. Now, that was substitution reactions.
And as you can see, there are many different
ways that you can carry out substitution with
a wide variety of different nucleophiles.
But, now, what I want to talk about is elimination
reactions. These are other competing reactions
that can take place in the presence of nucleophiles
that behave as bases and can often frustrate
somebody when they’re trying to actually carry
out a substitution reaction.
You may recall that we have this general model
of a haloalkane which is shown up on the screen
right now. And again, instead of looking at
nucleophilic substitution in the alpha position,
what we’re looking at now is proton or hydrogen
abstraction in the beta position, okay, which
is the carbon along. So, let’s talk a little
bit more about that.