Equilibria and kinetics of the hydration and cyclisation of semiphorone in acidic media

Abstract

In acidic media 6-hydroxy-2,6-dimethylhept-2-en-4-one (semiphorone) undergoes three parallel transformations: (a) hydration to form 2,6-dihydroxy-2,6-dimethylheptan-4-one (triacetone dialcohol); (b) cyclisation to 2,2,6,6-tetramethyltetrahydro-γ-pyrone and, to a lesser extent, (c) dehydration to form 2,6-dimethylhepta-2,5-dien-4-one (phorone). At higher temperatures appreciable amounts of acetone, diacetone alcohol, and mesityl oxide are also formed, even with relatively short reaction times.

The equilibria and the kinetics of the cyclisation and hydration have been examined at different temperatures and catalyst concentrations, using polarographic, infrared, and nuclear magnetic resonance techniques. Indicative values of the thermodynamic data are given and reaction mechanisms are proposed.