Right. So, let us move on to hydrolysis reactions.
So, this is how you break them apart. So,
we talked about hydro… reactivity with nucleophiles,
water, to all intents and purposes, also serves
as a nucleophile. And so, in this particular
scenario, we have to consider the reactivity
with water is much the same as reacting with
any other. So, in this particular scenario,
when it comes to hydrolysis as we have already
said, amides, which are almost impossible
to form without a coupling reagent, cannot
easily be hydrolysed in the presence of water.
In fairness, it is possible refluxing conditions
in concentrated sulfuric acid to achieve this,
but again, this is not gentle and can disrupt
other parts of the molecule that you may wish
Carboxylic esters are relatively easy to hydrolyse
and indeed, there are a number of different
mechanisms, acid catalysed and base catalysed,
for their hydrolysis and indeed, that is how
fats are broken down into glycerol and also,
fatty acids used in soap manufacture in a
process known as saponification.
Anhydrides are relatively easy to hydrolyse.
However, they are not nearly as reactive with
water as acid chlorides and sometimes, acetylation
reactions with acid anhydrides can actually
proceed in the presence of water where there
is a better nucleophile such as an amine group.
In both cases, acid chlorides and anhydrides
can be substituted in the carbonyl position
to generate carboxylic acids.
In the case shown in green here, acid catalysts
or base catalysts, in the case of carboxylic
acid, result either in the formation of the
conjugate base, carboxylate or indeed, in
the acid itself, the carboxylic acid. But,
these things are, especially in the case of
the amides, relatively difficult to achieve.
Right. Now, I’d like to talk to you about
the way in which esters can be formed using
acid chlorides. You can see on the top left
hand side that we have got an acetyl chloride
here and the alcohol we are reacting it with
is a cyclohexanol. The reaction works in the
same way that all reactions with these carboxylic
acid derivatives does, which is to say addition
elimination. Lone pair on the oxygen on the
alcohol attacks the carbonyl carbon, opens
up the carbon-oxygen double bond which then
reforms and kicks off the chloride. This generates,
in this case, a cyclohexylethanoate.
You can achieve the same effect by reacting
a cyclohexanol with an acid anhydride. Note
on the right hand side, we are using acetic
anhydride. In this case, the reaction proceeds
in the same way: attack by the lone pair on
the oxygens on the one of the carbonyl carbons,
shown here in black, or that could be the
other one, doesn’t matter, opens up that
carbon-oxygen double bond which then reforms
and kicks off an acetic acid group, shown
in red there beneath the arrow. In both cases,
the same compound, the same ester is formed,
which is shown in the green box here.
Now, let’s also look at amide formation,
much the same way you can use acid chlorides
and acid anhydrides, shown left and right
respectively, and they will react to give
you an amide. The way in which this functions
is that you got your amine which is one of
the reactants in this particular… in this
particular reaction and the lone pair on the
nitrogen attacks the carbonyl carbon as before,
opens up the carbon-oxygen double bond which
then reforms and kicks off a chloride ion
and that is shown underneath the arrow forming
that amide in the center.
The same applies if you choose the correct
acid anhydride which is shown in the bottom
right. Same thing applies: lone pair on the
nitrogen attacks the carbonyl carbon, in this
case shown in black, but it doesn’t matter
because if you look both sides have similar
substitution patterns, opens up that carbonyl
carbon which then reforms and kicks off the
carboxylate and that’s how we form amides.