The Mechanism of Dehydration of Alcohols over Alumina Catalysts

Abstract

Publisher Summary The mechanism of dehydration of alcohols over “acidic” and “nonacidic” alumina is the same. In the presence of the “acidic” alumina, the readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. The dehydration of tertiary alcohols over aluminas can be interpreted by a carbonium ion mechanism. Experimental evidence demonstrates that secondary and primary alcohols are dehydrated by a concerted mechanism, whereby both the intrinsic acid and base sites of the alumina participate. The steric course of the reaction proves that the dehydration of alkylcyclohexanols proceeds via a trans elimination. There is a strong parallel between elimination reactions in solution and the dehydration of alcohols over alumina. The trans elimination reactions and the anchimeric assistance of alcohols over aluminas suggest that the dehydration must occur within either the submicroscopical pores, or crevices, or channels of the aluminas. The aluminas therefore, must surround the alcohol molecules providing acid sites to act as proton donors or electron acceptors and basic sites to act as proton acceptors or electron donors.