THE GAS-PHASE ELIMINATION KINETICS OF 2-HYDROXY-2-METHYLBUTYRIC ACID AND 2-ETHYL-2-HYDROXYBUTYRIC ACID

The elimination kinetics of the title compounds have been examined ove r the temperature range of 270-320 degrees C and pressure range of 19- 117 torr. The reactions, carried out in seasoned vessels, with the fre e-radical suppressor toluene always present, are homogeneous, unimolec ular, and follow a first-order rate law. The products of 2-hydroxy-2-m ethylbutyric acid are 2-butanone, CO, and H2O; while of 2-ethyl-2-hydr oxybutyric acid are 3-pentanone, CO, ana H2O. The rate coefficient is expressed by the following Arrhenius equation: for 2-hydroxy-2-methylb utyric acid, log k(1)(s(-1)) = (12.87 +/- 0.19) - (171.2 +/- 2.1) kJ m ol(-1) (2.303 RT)(-1); and for 2-ethyl 2-hydroxybutyric acid, log k(1) (s(-1)) = (12.13 +/- 0.34) - (159.4 +/- 3.7) kJ mol(-1) (2.303 RT)(-1) . Augmentation of alkyl bulkiness at the a-position of the 2-hydroxyca rboxylic acids showed an increase in the rate of dehydration. The elec tron release of alkyl groups, rather than steric acceleration, appears to enhance the pyrolysis decomposition of these substrates. These rea ctions are believed to proceed through a semi-polar five-membered cycl ic transition type of mechanism. (C) 1995 John Wiley & Sons, Inc.