Gas-phase pyrolysis of thiopheneacetic acids, thienylethanols, and related compounds — protophilicity of ring π-electrons and relative acidities of hydrogen-bond donors of hydroxyl groups

Abstract

Based on kinetic data of thermal gas-phase elimination reactions, the following Arrhenius log A (s–1) and Ea (kJ mol–1) values, respectively, are obtained: 10.76 and 153.5 for 3-thiopheneacetic acid (1), 10.08 and 149.4 for 2-thiopheneacetic acid (2), 12.04 and 207.1 for 2-(3-thienyl)ethanol (3), 11.55 and 203.3 for 2-(2-thienyl)ethanol (4), 10.91 and 123.4 for 2-thiopheneglyoxylic acid (5), 11.05 and 223.8 for 1-(2-thienyl)propan-1-one (6), and 10.33 and 149.8 for 3-thiophenemalonic acid (7). The products of these pyrolytic reactions were either carbon dioxide or formaldehyde in addition to methylthiophene or thiophenecarboxaldehyde. Both positional and molecular reactivities of the substrates and related compounds are compared, and the results are rationalized on the basis of a reaction pathway involving a concerted six-membered transition state.Key words: thiophenes, gas-phase, pyrolysis, kinetics, mechanism.