Right. In terms of nomenclature, we talked
about positive and negative inductive
effects. In the case of mesomeric effects,
the term given is M for mesomeric and positive
means putting electrons in. So what that means
is any group with a negative charge or a lone
pair of electrons.
Here we have some examples of those. I will
go into some of the lower examples a little
later on when we start looking at functional-group
chemistry. But, for the moment, content yourselves
with the awareness that chlorine, bromine
and also OH groups can contribute a lone pair
The nomenclature for mesomeric – which are
withdrawing electrons – is shown here, where
Y is more electronegative than X. So, in this
case, M for mesomeric, negative for pulling
electrons away again.
Here we have a few examples. I’m just going
to draw your attention to one of them which
is nitro or NO2, one of the more electronegative
species and also capable of stabilising the
Combinations of mesomeric and inductive effects:
sometimes you can get both, either acting
in concert or acting against each other. And
as we’ll see when we look electrophilic
aromatic substitution, you’ll be able to
observe that one tends to transcend the other.
Some functional groups have one type of inductive
effect but the opposite resonance or mesomeric
effect. And here we have an example.
If we take the aromatic compound phenol, which
is shown there as the benzene ring with an
OH group attached. We’ve already established
that, from an inductive perspective, oxygen
being more electronegative can pull electron
density away. This will give it a negative
inductive effect, remembering of course that
minus (-) means electrons being pulled away
and ‘I’ meaning inductive. On the other
hand, in terms of stabilising a positive charge
on the oxygen, it has a mesomeric effect.
It actually has a better resonance stability
effect than it has an inductive effect. And
this is one of the reasons why it’s so easy
to react phenols with electrophiles as we’ll
see a little later on.