Elimination kinetics and mechanisms for several 2-alkoxyacetic acids in the gas phase

The kinetics of the gas-phase elimination of three 2-alkoxyacetic acids were investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor cyclohexene. The working temperature and pressure range were 350.4-410.8 degrees C and 57-261.5 Torr, respectively. The reactions proved to be homogeneous and unimolecular and to follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following equations: for 2-methoxyacetic acid, log k(1) (s(-1))=(12.10+/-0.22)-(19.3+/-2.8) kJ mol(-1) (2.303RT)(-1); for 2-ethoxyacetic acid, log k(1) (s(-1))=(12.76+/-0.29)-(199.6+/-3.7) kJ mol(-1) (2.303RT)(-1); and for 2-isopropoxyacetic acid, log k(1) (s(-1))=(12.40+/-0.32)-(193.7+/-3.9) kJ mol(-1) (2.303RT)(-1). The alkyl group R in ROCH(2)COOH does not seems to exert a significant effect on the rates. It is postulated that the predominant mechanism for the decomposition of the alkoxy acids involves a five-membered cyclic transition state, where the elimination of the RO substituents is assisted by the acidic proton of the COOH group.