Dehydration Reaction of 2-Methyl-3-(substituted phenyl)-2-propanols with Anhydrous Aluminum Bromide

Abstract

Dehydration reaction of 2-methyl-3-(substituted phenyl)-2-propanols (MPP) with aluminum bromide (AlBr₃) was carried out in a mixed solvent of decaline and 1-hexanol. A marked induction period was observed in the first stage of a dehydration. The lapse of induction time decreased with increasing electron releasing ability of the substituent. The dehydration rate was of the first-order with respect to MPP and of the second-order with respect to AlBr₃. The same was true the dehydration reaction of the compounds described below, regardless of the kinds of tertiary alcohols and Lewis acids used. The dehydration rate increased with increasing electron-releasing ability of substituent (ρ=-0.8). The negative value of the reaction constant suggests that the reaction preceeds under an E₁elimination mechanism. The activation energies of the dehydration reactions of these substituted tertiary alcohols with AlBr₃ were estimated to be 10-14 kcal/mol.In this dehydration reaction a good linear relation exists between activation energies and activation entropies ; a sign of each slope is positive. The mechanism of this dehydration reaction was considered to be two reaction stages : first, initial formation of alkoxyaluminum dibromide by the dehydrobrornination reaction of a 1 : 1 addition complex produced from alcohol and catalyst, and second, subsequent dehydration of alkoxyaluminum dibromide upon further addition of hydrogen bromide-catalyst complex.