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

Article abstract of DOI:10.1002/(SICI)1099-1395(199612)9:12<787::AID-POC850>3.0.CO;2-X

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°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 k₁, (s^-1) = (12·10±0·22) - (193·3±2·8) kJ mol^-1 (2·303 RT)^-1; for 2-ethoxyacetic acid, log k₁, (s^-1) = (12·76±0·29) - (199·6±3·7) kJ mol^-1 (2·303 RT)^-1; and for 2-isopropoxyacetic acid, log k₁, (s^-1) = (12·40±0·32) - (193·7±3·9) kJ mol^-1 (2·303 RT)^-1. The alkyl group R in ROCH₂COOH 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 substituants is assisted by the acidic proton of the COOH group.