Now I’d like to introduce the concept of
Within the bond between the carbon and the
chlorine atom that I showed you a little earlier
on, I showed you the arrow that denoted the
pulling of electrons away from one atom to
the other, i.e. polarising that sigma bond
– that sigma molecular orbital.
However, mesomeric effects are independent
of that and they are resonance effects. The
idea of this is that pairs of electrons can
move within molecules where there is space
to do so. Not just at random. These are carefully
controlled. And one of the most important
things in the context of not just organic
chemistry but in understanding from a structural
perspective of how things like beta-lactam
antibiotics work, for example, and how, for
example, HIV-1 protease inhibitors work is
to understand the concept of electron movement.
I’ve introduced the idea of atoms and so
forth but what’s very important about this
is the understanding of arrows. The green
arrow which is in the lower centre part of
the board that you can see there shows the
movement of a pair of electrons – two electrons,
not one electron: two. This is absolutely
crucial because electrons are what makes chemistry
work. If you ever see a positive charge, you
would never consider moving a pair of electrons
from something which is already positive.
Electrons move towards the positive i.e. the
head of the arrow would move towards the positive,
not the other way around. So this is sometimes
conceptually difficult to appreciate so I’d
advise you to take some time to get your head
around it. The idea here is that the electrons
move from one end of the arrow to the head
of the arrow. And it involves two of them.
So, as you can see here, the end shows where
the pairs begin from and the other end, where
the head of the arrow is, shows where they
end up. And that’s very important to note.
What does mesomeric actually mean and what’s
the significance of the double-headed arrows
I was talking about?
Within molecules, such as this hypothetical
molecule A bound to B bound to C, you can
see an example of a resonance or mesomeric
effect. In this hypothetical molecule, A has
a double bond to B. That is to say it has
a sigma and a pi bond. B is bound to C via
a sigma bond. And on the other side you can
see much the same thing: one double bond,
one single bond.
However, the placement of the double bond
is shown to depend upon the presence of the
electrons. In this scenario where we have
C, it’s possible to move the pair of electrons
from C to form a double bond B-to-C and to
place the negative charge on A. This can go
back and forth and this type of arrangement
is known as resonance or a mesomeric effect.
And typically speaking the more resonance
structures that can form, the more stable
the molecule. Note we’re not changing the
overall charge of the molecule. It remains
the same. Okay? That’s something to bear
in mind. The two dots above C does not correlate
to a negative charge. It correlates to a lone
pair of electrons.
So resonance structures, which are where mesomeric
stability is derived from, only differ in
their electron distribution. The atoms do
not move nor would you ever, for example,
draw the movement of a double-headed arrow
from a positive charge on, say, C for example
in one of the resonant structures to B. A
double-headed arrow is only used between resonance
structures or, as we’ll see later on, when
you’re talking about nucleophilic attack.
So A is now negative because it has accepted
an electron pair from the p bond and C is
now positive because it has donated an electron
pair to form a pi bond. B remains uncharged
throughout because it has the same number
of bonds before and after.
This can be extended beyond the simple molecule
such as that allylic system I showed earlier.
Here we have an example molecule where we
have 1, 2, 3, 4, 5, 6, 7 individual atoms
and we have a number of bonds in between them.
What we see here is the lone pair on A being
donated to form a pi bond between A and B,
the pi bond being C opening up to form one
between C and D and so on and so forth. And
the existence of these individual resonance
structures imparts a degree of stability.
In reality, the electrons are constantly moving
backwards and forwards along the molecule
so we would have to draw an average structure.
And that average structure is shown at the
bottom there. And it’s shown by a single
solid line and then a dashed line just above
it. This implies that we don’t have formal
double bonds but rather a bond and a half.
And this is shown to represent the resonance
that we see in this molecule. A resonance
structure you’d be perhaps familiar with
is the resonance that’s found in benzene.
And we’ll come onto that a little later