A Guidebook to Mechanism in Organic Chemistry

Dicarboxylic Acids

salicyclic acid (III) by delocalising its charge, an advantage not

shared by the m- and /^-isomers nor by o-methoxybenzoic acid:

The effect is even more pronounced where hydrogen bonding can

occur with a hydroxyl group in both o-positions and 2,6-dihydroxy-

benzoic acid has a pK„ of 1 - 30.

j^viii) Dicarboxylic acids

As the carboxyl group itself has an electron-withdrawing inductive

effect, the presence of a second such group in an acid would be

expected to make it stronger, as shown by the following pK„ values:

HCO,H HOJCCOJH

3-77 1-23

CH 3 C0 2 H H0 2 CCH,COaH

.4-76 2-83

CHsCH 2 C0 2 H HOaC CH 2 CH 2 C0 2 H

4-88 4-19

C,H 5 C0 2 H H0 2 C C 6 H. COjH

4-17 o- 2-98

/ m- 3-46

p- 3-51

The effect is very pronounced but falls off sharply as soon as the

carboxyl .groups are separated by more than one saturated carbon

atom. Maleic acid (V) is a much stronger acid than fumaric (VI)

(pK.^1 is 1 -92 compared with 3-02) due to the hydrogen bonding that

can take place with the former, but not the latter, stabilising the anion

(cf. salicylic acid):