A Guidebook to Mechanism in Organic Chemistry

Aromatic Carboxylic Acids

Thus picric acid is a very strong acid. The effect of introducing

electron-donating alkyl groups is small:

C,H 6 OH 9-95

o-MeQH 4 OH 10-28

m-Me-C«H 4 -OH 1008

P-Me-C.H.-OH 10-19

The resulting substituted phenols are very slightly weaker acids, but

the effect is marginal and irregular, indicating that the effect of such

substituents»in destabilising ^fle phenoxide ion by disturbing the

interaction of its negative charge with the delocalised w orbitals of the

aromatic nucleus is small, as might have been expected.

(vii) Aromatic carboxylic acids
Benzoic acid, with a pK„ of 4-20, is a stronger acid than its saturated
analogue cyclohexane carboxylic acid (pKa =4-87) indicating that a
«phenyl group, like a double bond, is here exerting an electron-with­
drawing effect—compared with a saturated carbon atom—on the
carboxyl group, due to the stf hybridised carbon atom to which
the carboxyl group is attached (p. 42). Benzoic acid is nevertheless
weaker than formic acid (pKa = 3 • 77), showing that the phenyl group
exerts an overall electron-donating effect compared with hydrtgen.
The difficulty in forecasting the relative effects a phenyl group will
have on electron-density in the undissociated acid and in its anion
arises from the fact that its n electron system can act as an electron
source or an electron sink at will, depending on the nature of the
group to,which it is attached. This is well illustrated by its equally
effective stabilisation of the phenoxide anion, PhO® (p. 17) and the
®
benzyl carbonium ion Ph • CH 2 (p. 63).
The introduction of alkyl groups has vejy little effect on the strength
of benzoic acid (cf. similar introduction in aliphatic acids)

C,H 5 CO,H 4-20

m-Me-C,H 4 COaH 4-24

p-MeC 6 H 4 C0 8 H 4-34

but electron-withdrawing groups increase its strength, the effect, as

with the phenols, being most pronounced when they are in the o-

and ^-positions: