The Effect of a Substituent already Present
But this is not the only way in which a substituent can affect electron-
availability in the nucleus.
(ii) Mesomeric effect of substituents
A number of common substituents have unshared electron pairs
on the atom attached to the nucleus and these can interact with its
delocalised n orbitals
MeO:-, MeO® MeO®
and the same consideration clearly applies to OH, SH, NH 2 , halo
gens, etc.
It will be noticed that electron-availability over the nucleus is there
by increased. An effect in the opposite direction can take place if the
substituent atom attached to the nucleus itself carries a more elec
tronegative atom to which it is multiply bonded, i.e. this atom is then
conjugated with the nucleus and can interact with its delocalised w
orbitals:
H H H*
I n I I
c±o C—o° c—o®
The same consideration clearly applies to CO • R, C0 2 H, SOaH, NO,,
CN, etc. Here it will be seen that electron-availability over the
nucleus is thereby decreased.
(iii) The overall effect
Clearly any group that, overall, is electron-donating is going to
lead to more rapid substitution by an electrophilic reagent than in
benzene itself, for the electron-density on the ring carbon atoms is
now higher; correspondingly, any group that is, overall, electron-
withdrawing is going to lead to less rapid substitution. This is re
flected in the relative ease of attack of oxidising agents, which are, of