A Guidebook to Mechanism in Organic Chemistry

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: