containing the double bond.
Right. Okay. So, how do carbonyl compounds
react? What I have shown here on the board
is an idealised ketone where we have two different
alkyl or aryl groups shown as R and R’ attached
to a carbonyl, that’s the carbon double
bond to the oxygen.
Now, I said to you that nucleophilic addition
is the typical reaction and indeed, the typical
reagent or reactant in this particular case
is usually of the form NuH or HNu where, for
example, the nucelophile could indeed be,
let’s say, water or indeed, it could be
cyanide. Therefore, for example, we could
react hydrogen cyanide.
Bear that all in mind, we’ve got a nucleophile
here which is idealised. We’ve called it
Nu-. What happens in this case is that the
nucleophile attacks the carbonyl at the delta
positive carbon, opens up the double bond
between the carbon and the oxygen and places
a formal negative charge on the oxygen. This
is then capped by the counterride, which in
this case of NuH would be H+ and this generates
here our nucleophile directly added on to
R carbon-oxygen double bond, as you can
Now, the actual name of the group that’s
formed as a consequence depends very much
on the substituent of the nucleophile itself.
First step, as I said, nucleophilic attack
on the carbonyl. Second step, protonation
of the resulting anion and that’s all you
really need to know for the nucleophilic addition
to carbonyl compounds.
Bear in mind one important geometric factor.
We said before that carbonyl compounds start
off as trigonal, trigonal planar. This is
by virtue of the fact that the carbon and,
indeed, the oxygen are sp2-hybridised. By
the time the reaction has finished, however,
both the intermediate and the final compound
are tetrahedral; tetrahedral, in a not too
dissimilar fashion, to the molecule methane,
So, what else? Reduction to an alcohol.
So, here we have another idealised ketone
in a sort of similar fashion to when we were
talking about alcohols. Whereas it’s possible
to oxidise an alcohol up to a ketone, it’s
also possible, when desirable, to reduce a
ketone down to an alcohol. In this particular
case, I’ve shown only a ketone. This is
reduced down to, in this particular case,
a secondary or, in this case, primary alcohol
where R or R’ can either be hydrogen or
indeed, it can be alkyl. So, we’ve made
no distinction in this case.
Lithium aluminium hydride is quite flammable
and reacts violently with water and so, should
actually be treated with a fair amount of
caution. However, it will reduce most ketones
and indeed, it reduces mostly any of the carbonyl
containing compounds. The things which are
slightly more reactive such as aldehydes,
sodium borohydride is a useful way of achieving
So, let me have a look and let me show you
a particular example of a nucleophilic addition
reaction and this is the addition of cyanide
over a carbon-oxygen double bond. In this
particular case, what we see is a formation
of a cyanohydrin compound. This is where we’ve
got the nucleophilic attack of the cyanide
on the carbonyl carbon to give and then kept
by the H+ form from it in effectively one
Once you’ve done that, what you can then
do is hydrolyse that nitrile group which is
then formed with acid and water to give you
a carboxylic acid, but look at what we’ve
actually done. What we’ve effectively done
is we’ve added a carbon unit to something
which is already has a carbon unit on it,
the carbonyl carbon.
So, what this allows us to do is effectively
extend the chain, in this case, the aliphatic
chain and to incorporate a carboxylic acid
group at the end of it. This itself can always
be reduced down and a number of other reactions
can be done with this, but this is an important
synthetic step is that we can progressively
add to successive number of carbons in this
Okay. Addition of water.
Remember what I said is that the nature of
the compounds that are formed, that is to
say, their names and nomenclature depend very
much on the nucleophile which is added. In
this particular scenario, water reacts with
acetone to give something called a gem diol,
gem diol being short for geminal diol. If
we go back to nomenclature, right back at
the very beginning and we were talking about
germinal; geminal meaning that we actually
have two substituents on the same carbon.
So, therefore, this is two OH groups or two
old groups, therefore, diol groups attached
to the same carbon and therefore, this is
a geminal diol.
Note that it is also tetrahedral in structure
and indeed this forms very quickly and so,
for example, in aldehydes, like for example,
formaldehyde, the geminal diol is formed pretty
much on contact with water and is the origins
of formalin which is a 40 percent solution
used to preserve specimens.
Now, there’s something else that can happen.
Bear in mind what I said. We’ve got water
which consists of the nucleophile OH- and
also has the H+ needed to cap the oxygen or
the O- that’s formed in the first step of
nucleophilic addition. Hemiacetal formation
is the result of the reaction of a carbonyl
compound with an alcohol and this is also
a very important reaction from a biological
perspective as well, as we’ll see a little
later on when we start looking at sugars.
Let’s have a look again at the structure
of the alcohol which is shown at the bottom
of the board. We’ve shown an aliphatic or
aromatic group as R’, also showed that we’ve
got oxygen lone pairs. And remember what we
said, anything which contains electrons like
that, which is rich in electrons, can act
as a nucleophile.
And what happens in this particular case,
is that the lone pairs on the oxygen attack
the delta positive of the carbonyl carbon.
The freed H+ from the alcohol then caps the
O- that’s formed as part of the nucleophilic
addition. As you can see here, what we’ve
now done is substitute it or add it over R’O
onto the carbonyl carbon. And whenever you
see this structure where you have a alkyl,
oxygen, carbon and then an OH group, irrespective
of what circumstance, it is usually considered
a hemiacetal.... hemiacetal.