Now, let’s have a quick look at elimination
We mentioned before that it is possible for
us to protonate the OH group on an alcohol,
but it isn’t just a question of it being
a prelude to nucleophilic substitution. What
can happen, in this particular case, is dehydration.
And the example reaction shown here is where
we have our alcohol in the presence of sulphuric
acid which is essentially dehydrated, note
because we’re generating water, to give
rise to a carbon-carbon double bond.
In terms of elimination via the E1, that is
elimination, unimolecular reaction, this tends
to be preferred in the order of tertiary,
it’s better than secondary, it’s better
The difference between alkyl halides and alcohols
is their elimination reactions don’t proceed
with a base, as a deprotonation of the hydrogen
from the hydroxyl group would happen instead.
An interesting experiment that you may want
to do, if you happen to have access to concentrated
sulphuric acid, is actually to react it with
a bowl of sugar. Conc. sulphuric acid will
actually remove the water from the carbohydrate
sugar leaving only carbon. And therefore,
you can char your sugar in the presence of
sulphuric acid and generate carbon. This shows
how strong sulphuric acid is as a dehydrating
So, let’s have a quick look at the mechanism
So, we said before, it’s possible to protonate
the oxygen. This is by virtue of the fact
that the oxygen in an alcohol has lone pairs,
two of them, in fact. They can be moved onto
the H+ and you generate a positive charge
which formally we recognise here on the oxygen.
So, the first step is OH protonation.
The second step is elimination of water. This
is shown as a unimolecular mechanism.
And then, finally, we see that we’ve got
a possibility of reproducing or releasing
H+ via the formation of a double bond by the
loss of electrons from a carbon-hydrogen sigma
bond on the beta carbon to what was our alcohol.
This constitutes the third step.
Finally, we have, as we saw before in the
case of the haloalkanes, two possible regioisomers
of the alkenes we wish to generate. That is
where the double bond forms within the centre
and that’s where the double bond forms at
Vicinal and geminal hydrogens.
This a term that you’ll come across when
you look at nomenclature, particularly common
names, such as vinyl chloride, for example,
in polymer chemistry.
Vicinal hydrogens are co-hydrogens which are
on carbons which are neighbouring to each
other whereas geminal hydrogens are hydrogens
which are on the same carbon. And this, from
a nomenclature perspective, is important to
Now, I want to briefly touch upon ester formation.