Aldehydes, as we’ve seen before, are readily
oxidised and ketones are oxidised with considerable
difficulty. If we go back to when we were
looking at alcohols, we showed that it was
possible to oxidise the alcohol first up to
an aldehyde, if it was primary, and then all
the way up to a carboxylic acid. If on the
other hand we had a secondary alcohol, we
could get it as far as the ketone, but we
couldn’t oxidise it any further.
If we look at aldehydes, there are a number
of different ways in which you can convert
it to a carboxylic acid. Exhaustive oxidation
with potassium permanganate, shown on the
top, potassium dichromate and also, in this
particular case, a silver diamine nitrate
which is otherwise known as Tollen’s reagent
which is a characteristic test for the presence
of aldehydes since the silver+ becomes reduced
and then coats the inside of the tube in the
presence of an aldehyde.
The oxidation can only occur in the case of
ketones under severe conditions to give a
mixture of acids which are of little synthetic
use since you effectively split the entire
Let’s have a look at reduction. We’ve
already seen chemical reduction using sodium
borohydride and lithium aluminium hydride.
However, it is also possible to reduce ketones
and aldehydes via catalytic amounts of nickel,
platinum or palladium shown here as Ni, Pt
or Pd in the presence of hydrogen gas. And
these are able to reduce ketones and aldehydes
down to alcohols.
This is of particular importance when you
don’t necessarily want to use an aggressive
reducing agent which may perhaps reduce another
part of your molecule which you want to keep
and hydrogenation can be the best choice when
you have other groups that would react with
a more aggressive hydride-based reducing agent
like lithium aluminium hydride or sodium borohydride.
Carbonyl compounds are reduced to alcohols
and in this particular case, ketones would
be reduced to secondary alcohols and aldehydes
would be reduced to primary alcohols.