Factors affecting Electron-availability in Bonds
Delocalisation takes place (XX), so that an electron-deficient atom
results at C 8 , as well as at C, as in a simple carbonyl compound. The
difference between this transmission via a conjugated system and the
inductive effect in a saturated system is that here the effect suffers
much less diminution by its transmission; C 3 is almost as positive
as Cx was in (XIX).
The stabilisation that can result by delocalisation of a positive or
negative charge in an ion via its n orbitals can be a potent feature in
making the formation of the ion possible in the first place (c/. p. 40).
It is, for instance, the stabilisation of the phenoxide ion (XXI) by
delocalisation* of its charge \jja the delocalised w orbitals of the
nucleus that is largely responsible for the acidity of phenol, i.e. the
ease with which it will lose a proton in the first place (cf. p. 41):
H
(XXI)
An apparently similar delocalisation can take place in undime-
ciated phenol itself involving an unshared electron pair on the
oxygen atom I
but this involves separation of charge and will thus be correspond
ingly less effective than the stabilisation of the phenoxide ion which
does not.
Similar stabilisation of the anion with respect to the neutral mole
cule is not shared by benzyl alcohol, which is thus no more acidic than
aliphatic alcohols, for the intervening saturated carbon atom pre
vents interaction with the n orbitals of the nucleus: