DFT studies of homogeneous catalysis in the gas phase: Dehydration kinetics of several tertiary alcohols with hydrogen chloride

The mechanisms for the acid-catalyzed gas-phase dehydration of the tertiary alcohols 2-methyl-2-propanol, 2-methyl-2-butanol, and 2-methyl-2-pentanol were examined at B3LYP/6-31G(d), B3LYP/6-31G(d,p), B3LYP/6-31G(2d,p), B3LYP/6-31G(2d,2p), B3PW1/6-31G(d), B3PW1/6-31G(d,p), B3PW1/6-31G(2d,p), B3PW1/6-31G(2d,2p), MPW91PW91/6-31G(d), MPW91PW91/6-31G(d,p), MPW91PW91/6-31G(2d,p), and MPW91PW91/6-31G(2d,2p) levels of theory. Calculation results suggest that the dehydration processes catalyzed by hydrogen chloride to give the corresponding olefin and water occur with the formation of Van der Waals complexes between the alcohol and hydrogen chloride. The transition states are six-membered cyclic structures involving one molecule of HCl and one of the alcohol. These reactions appear to be molecular in nature. Analysis of the progress along the reaction coordinate, in terms of bond orders, NBO charges, and geometrical parameters suggest these reactions to be moderately polar and nonsynchronous and are dominated by the breaking of the H-Cl bond, together with an important cleavage of C-O bond in the transition state.