Determination of the Hammett substituent constants for the 2-, 3-, and 4-pyridyl and -pyridinium groups

Hammett σ-values for the 2-, 3-, and 4-pyridyl and -pyridinium groups have been determined for some reactions of pyridine derivatives; the mean values are 0·71, 0·55, 0·94, 3·11, 2·10, and 2·57, respectively. The σ-values for the 2-pyridyl group for the ionisation of the pyridine-2-carboxylic, -2-acetic, and -2-hydroxamic acids, and 2-hydroxypyridine, are lower than the corresponding σ- values for other reactions. This is probably caused by intramolecular hydrogen-bonding, which will stabilise the neutral form of the acid and thus decrease the ionisation constants.