Elimination kinetics and mechanism of primary, secondary and tertiary α‐hydroxycarboxylic acids in the gas phase

Abstract

Abstract The rates of elimination of primary, secondary and tertiary α‐hydroxycarboxylic acids were determined in a seasoned, static reaction vessel over the temperature range 280–390°C and the pressure range 30–201 Torr. The reactions, in the presence of a free radical inhibitor, are homogeneous, unimolecular and follow a first‐order rate law. The rate coefficients are given by the following equations: for glycolic acid, log k 1 (s −1 ) = (14·03 ± 0·24) − (209·3 ± 1·5) kJ mol −1 (2·303 RT ) −1 ; for lactic acid, log k 1 (s −1 ) = (12·24 ± 0·11) − (182·8 ± 1·3) kJ mol −1 (2·303 RT ) −1 ; and for 2‐hydroxyisobutyric acid, log k 1 (s −1 ) = (12·91 ± 0·13) − (174·7 ± 1·5) kJ mol −1 (2·303 RT ) −1 . The basicity and the ease with which the hydroxy group is removed from primary to tertiary α‐hydroxycarboxylic acids are reflected in rate enhancement. The mechanism of these eliminations appears to proceed through a semi‐polar five‐membered cyclic transition state.