Chemical Reactions In vapour state or in aprotic solvent Hydrogen bonding of RCOOH with H O where K eq , is equilibrium constant and K a is the acid dissociation constant. For convenience, the strength of an acid is generally indicated by its p K a value rather than its K a value. p K a = – log K a The p K a of hydrochloric acid is – . , where as p K a of trifluoroacetic acid (the strongest carboxylic acid), benzoic acid and acetic acid are .
, . and . , respectively. Smaller the p K a , the stronger the acid ( the better it is as a proton donor).
Strong acids have p K a values < , the acids with p K a values between and are considered to be moderately strong acids, weak acids have p K a values between and , and extremely weak acids have p K a values > . Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (p K a is ~ for ethanol and for phenol). In fact, carboxylic acids are amongst the most acidic organic compounds you have studied so far. You already know why phenols are more acidic than alcohols.
The higher acidity of carboxylic acids as compared to phenols can be understood similarly. The conjugate base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom. The conjugate base of phenol, a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom. Therefore, resonance in phenoxide ion is not as important as it is in carboxylate ion.
Further, the negative charge is delocalised over two electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalised over one oxygen atom and less electronegative carbon atoms in phenoxide ion (Unit , Class XII). Thus, the carboxylate ion is more stabilised than phenoxide ion, so carboxylic acids are more acidic