00:01
So, let’s have a quick look at what the
significance is of the carbonyl functional
group. Here, if you can recall, we were looking
at hybridisation back in Module I, at the
beginning of this course, I talked about sp2,
sp3 and sp-hybridisation and it isn’t just
restricted to carbons being bound to other
carbons.
In this particular case, as you can hopefully
see, we have here a single unhybridised p
orbital on the carbon and also one on the
oxygen. This enables a pi bond to be formed
over the existing sigma bond. Hence, the reason
we have a double bond in place. This also
means that that carbon is sp2-hybridised and
therefore, it must be planar. As you can see
here, it is on the plane. Both oxygen and
carbon are, in this case, sp2-hybridised.
So, what are the origins of the reactivity
of carbonyls? Well, let’s have a look again
at dipoles. We talked about electronegativity.
01:04
As well as having a larger mass in the case
of oxygen, it also has greater electronegativity.
01:09
Thus, it pulls electron density within a double
bond system from carbon towards oxygen.
01:15
The dipole moment is shown here in the...
with the red arrow where the direction of
pull of electrons is from the carbon to the
oxygen. The electronegativity of the oxygen
is 3.44 in the Pauling scale and that of carbon
is 2.55. So, the bond is pretty polarised,
as you can see here: delta negative forming
on the oxygen, delta positive forming on the
carbon.
And so, therefore, it has technically two
routes of action. It can either act as an
electrophile on the delta positive side, and
therefore, react with nucleophiles, or it
can react with electrophiles on the oxygen;
of course, electrophiles often requiring electrons
from being positively charged themselves.
02:05
So, this leads to two possible attacks: electrophilic
attack or nucleophilic attack. These properties
will affect the chemistry of all compounds
having a carbonyl, not least because the resonance
form that I’ve shown at the bottom of this
board is equally possible where we formalise
the charge and the positive charge can reside
formally on the carbon and a negative charge
can reside formally on the oxygen by virtue
of the electronegativity difference between
the two.
So, the ability of the carbonyl oxygen to
accommodate a negative charge is the main
chemistry determinant of the group by providing
a site for nucleophilic attack at the carbon
and the main reactions of aldehydes and ketones
when it comes to adding alkyl or aryl groups
to them is nucleophilic addition where the
carbon itself behaves as an electrophile.
So, let’s have a look at what we mean. Well,
let’s say, for example, we had a nucleophile.
We’ll call it Nu-. If we wanted to react
that with an aldehyde, what we would find
is two things. A: It is more reactive by virtue
of the fact we don’t have as much electron
donation from the R groups and B: We also
have less steric hindrance. And so, aldehydes
are more reactive than ketones and we will
revisit this a little later on in this particular
slide lecture.
03:35
Aldehydes are more reactive and indeed, the
carbon in ketones is less electrophilic due
to the presence of two R groups having a positive
inductive effect. So, if you go back to what
I was saying a few lectures ago about induction
and so forth, hopefully, you can see here
what the effects are on the strength of that
dipole.
03:55
If we, for example, have 2 electron-donating
alkyl groups pumping electrons into that carbon,
we decrease the charge separation across that
carbonyl. If, on the other hand, we only have
a single alkyl group pushing donor electrons
into that carbonyl, we have a better charge
separation when compared to the corresponding
ketone.
04:15
It should also be mould in mind that there
is a steric effect as well and those steric
effects relate to the size of the R groups
and the R groups almost always being larger
than hydrogen, otherwise, of course, they
would merely be hydrogen groups.
04:29
Steric hindrance and positive inductive effect
basically mean that aldehydes are more reactive
than ketones.
So, let’s have a look at some of these addition
reactions. This is a general scheme showing
what I meant. You may recall some similarities
between the addition reactions over alkenes
which we talked about right at the beginning
of this module and addition reactions, in
this case, of the carbonyl.
04:53
In this particular scenario, we have actually
expanded our understanding of carbonyls, so
it isn’t necessarily just alkenes and carbonyl
compounds, but also where we have carbon double
bound to nitrogen. These are known as imines
and are usually very important intermediates
in the synthesis of furthermore complicated
chemicals and what we are actually doing here
is we are adding over that double bond in
the same way that we added bromine or bromine
water over our alkene. We are effectively
doing exactly the same thing. The only difference
is that this is nucleophilic addition rather
than an electrophilic attack by that species
containing the double bond.