ELIMINATION KINETICS AND MECHANISM OF PRIMARY, SECONDARY AND TERTIARY ALPHA-HYDROXYCARBOXYLIC ACIDS IN THE GAS-PHASE

The rates of elimination of primary, secondary and tertiary alpha-hydroxycarboxylic acids were determined in a seasoned, static reaction vessel over the temperature range 280-390-degrees-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 k1 (s-1) = (14.03 +/- 0.24) - (209.3 +/- 1.5) kJ mol-1 (2.303RT)-1; for lactic acid, k1 (s-1) = (12.24 +/- 0.11) - (182.8 +/- 1.3) kJ mol-1 (2.303RT)-1; and for 2-hydroxyisobutyric acid, log k1 (s-1) = (12.91 +/- 0.13) - (174.7 +/- 1.5) kJ mol-1 (2.303RT)-1. The basicity and the ease with which the hydroxy group is removed from primary to tertiary alpha-hydroxycarboxylic acids are reflected in rate enhancement. The mechanism of these eliminations appears to proceed through a semi-polar five-membered cyclic transition state.