Co-operative hydration of the carbonyl group — "Bifunctional catalysis" by 2-hydroxypyridine is achieved via a dihydrate of 2-pyridone

Abstract

Although 2-hydroxypyridine is one thousand times less acidic than its isomer 3-hydroxypyridine and ten million times less acidic than acetic acid, in the aqueous hydration of 1 mol/L acetone at 298 K the catalytic effect of added 2-hydroxypyridine is 600 000 times greater than that of 3-hydroxypyridine and 20 times greater than that of acetic acid. This experimental result is successfully reproduced by a theoretical treatment that assumes that solvent molecules participate in a cyclic cooperative mechanism that avoids the formation of charged intermediates. However, the theoretical treatment does not support the seemingly obvious conclusion that the origin of the enhanced catalytic activity of 2-hydroxypyridine lies in its bifunctional nature. Rather, this enhanced catalytic activity can be traced to the exceptional thermodynamic stability of the principal reactant complex, which contains a dihydrate of 2-pyridone.Key words: hydration, isotope shift, bifunctional, cooperativity, cyclic mechanism, ab initio, SCRF