Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

Abstract

A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k1 (s−1) = (11.01 ± 0.31) − (109.5 ± 2.8) kJ mol−1 (2.303 RT)−1 and for 3‐methyl‐1‐buten‐3‐ol log k1 (s−1) = (11.50 ± 0.18) − (116.5 ± 1.4) kJ mol−1 (2.303 RT)−1. Electron delocalization of the CH2CH and C6H5 appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd.

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

“…The OH of alcohols is a bad leaving group in thermal decomposition processes. However, a few of these substrates have shown to undergo a homogeneous, unimolecular elimination under maximum catalysis of HCl or HBr gas2–10. The acid‐catalyzed bimolecular dehydration of some alcohols may be carried out well below 100 °C compared with the uncatalyzed elimination of water, implying a reduction of the energy of activation by about 125 kJ/mol.…”

“…Steric acceleration was considered to be the probable factor in the rate enhancement of the gas phase elimination kinetics of acid‐catalyzed alkyl‐branched tertiary alcohols8, 9; however, the electronic effect was not ignored. A later work10 considered the electronic factor to be responsible for the rate increase. This conclusion was derived from kinetic data of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten3‐ol, where the allylic and benzylic substituents stabilize the positive carbon in the polarization of C δ+ ··· OH δ− in the TS through electron donation, increasing the rate.…”

Theoretical calculations of the kinetics and mechanisms of the gas phase elimination of primary, secondary, and tertiary 2‐hydroxyalkylbenzenes

“…However, some tertiary alcohols above 485°C undergo unimolecular decomposition. In addition to these facts, alcohols dehydrate in the gas phase when catalyzed by an acid in homogeneous, unimolecular elimination processes 1–8. Under acid catalysis, these alcohols are pyrolyzed well below 100°C, and the activation energy decreases to about 125 kJ/mol.…”