The mechanism of gas-phase dehydration of cyclohexanol and the methylcyclohexanols catalysed by zirconium phosphate and zirconium phosphite

By means of deuterium labelling experiments, the mechanism of the gas-phase dehydration of cyclohexanol over a solid zirconium phosphate catalyst has been shown to involve a carbocation with a long lifetime. Product distribut ion studies have been used to show that the corresponding dehydrations of t he 4-methylcyclohexanols also proceed through a carbocation mechanism, but have shown that the 2-methylcyclohexanols react partly by a carbocation mec hanism and partly by a synchronous mechanism. Reactions catalysed by zircon ium phosphite are slower, but give qualitatively similar products. Our resu lts, and those of earlier workers, may be explained by consideration of the stereochemistry of the substrate absorbed on the catalyst surface; variati ons from the carbocation reaction result from orientations of the substrate adopted to relieve steric compression at the catalyst surface. (C) 1999 El sevier Science B.V. All rights reserved.