A Guidebook to Mechanism in Organic Chemistry

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