The effects of 3′- and 4′-substituents on the ionisation constants of biphenyl-4-carboxylic acid and 4-aminobiphenyl

Abstract

Measurement of the ionisation constants for a range of 3′- and 4′-substituted biphenyl-4-carboxylic acids and 4-aminobiphenyls has confirmed the earlier conclusion that the electronic effects of substituents are transmitted to the 4-position, but in diminished magnitude compared with the analogous benzene systems. Inductive effects alone do not entirely account for the relative ionisation constants of the 3′-substituted acids, and this suggests that the mesomeric effect of the 3′-substituent should not be discounted. The relationship between the pK_a values for the 3′- and 4′-substituted acids and the Hammett substituent constants is discussed.