The effects of 2′-substituents on the ionisation constants of biphenyl-4-carboxylic acid, 4-aminobiphenyl, and 4-biphenylylacetic acid

Abstract

The effects of a range of 2′-substituents on the ionisation constants of biphenyl-4-carboxylic acid and 4-aminobiphenyl are discussed and compared with the effects of the same substituents when present in the 4′-position. The almost general observation that a 2′-substituent weakens biphenyl-4-carboxylic acid cannot be rationalised in terms of normal substituent effects. The results are attributed to an acid-weakening effect having its origins in a displacement of electrons towards the 4-carboxyl group through steric pressure of the 2′-substituent on the π-electrons of the aromatic ring carrying the carboxyl group. Such an effect does not appear to dominate the situation for the limited series of 2′-substituted 4-aminobiphenyls and 4-biphenylylacetic acids studied, possibly because the 4-substituents in these cases (–NH₂ or–CH₂·CO₂H) less readily accept the displaced electron density.