Substantial formation of hydrates and hemiacetals from pyridinium ketones.

Pyridinium ketones have been found to exist as hydrates and hemiacetals in considerable amount in aqueous and alcoholic solutions, respectively. The relative position of the pyridinium positive charge has a large effect on the equilibrium constants. The polar substituent constants, sigma*, of the pyridinium group substituted at different positions can be estimated from the hydration constants.