MECHANISM AND TEMPERATURE-DEPENDENT KINETICS OF THE DEHYDRATION OF TERT-BUTYL ALCOHOL IN HOT COMPRESSED LIQUID WATER

In the presence of acid or base, or under neutral conditions, isobutyl ene is the only observed product of the reactions of tert-butyl alcoho l in compressed liquid water at 225, 250, and 320 degrees C. On the ba sis of a kinetic analysis of limited results at 250 degrees C, an earl ier paper concluded that tert-butyl alcohol dissociates as an Arrheniu s acid at 250 degrees C and thereby catalyzes its own dehydration to i sobutylene. Kinetic analyses of the data sets contained in this paper do not corroborate the alleged ability of tert-butyl alcohol to proton ate water. Instead, we find that hydronium ions formed by the ordinary dissociation of water are the primary catalytic agents for the dehydr ation reaction. In agreement with the earlier work, all three data set s are consistent with a heterolytic reaction mechanism involving proto nated alcohol, carbocation, di-tert-butyl ether, and protonated ether as intermediates. The kinetics still suggest that tert-butyl alcohol w eakly dissociates as a Bronsted acid at these conditions. Values of K- w determined by the kinetic model enjoy good agreement with electroche mical values available in the literature. Values of the other paramete rs (rate constants) which compose the kinetic model are evaluated with less precision, and a novel method is proposed to estimate the uncert ainty associated with each parameter.