Kinetics and mechanism of the gas-phase elimination of primary, secondary and tertiary 2-acetoxycarboxylic acids

The gas-phase elimination kinetics of the title compounds were examined ove r the temperature range 220.1 - 349.0 degreesC and pressure range 19-120 To rr. These reactions proved to be homogeneous and unimolecular and to follow a first-order rate law. The overall rate coefficients are expressed by the following Arrhenius equations: for 2-acetoxyacetic acid, logk(1) (s(-1))=( 12.03 +/- 0.28) - (170.8 +/- 3.2) kJ mol(-1) (2.303RT)(-1); for 2-acetoxypr opionic acid, logk(1) (s(-1))= (13.16 +/- 0.24) - (174.2 +/- 2.6) kJ mol(-1 ) (2.303RT)(-1); for 2-acetoxy-2-methylpropionic acid, logk(1) (s(-1)) = (1 3.40 +/- 0.72) - (160.9 +/- 5.03) kJ mol(-1) (2.303RT)(-1). The products of the acetoxyacids are acetic acid, the corresponding carbonyl compound and CO gas, except for 2-acetoxy-2-methylpropionic acid, which undergoes a para llel elimination to give methacryclic acid and acetic acid. The rates of el imination are found to increase from primary to tertiary carbon bearing the acetoxy group. The mechanism appears to proceed through a discrete polar f ive-membered cyclic transition state, where the acidic hydrogen of the COOH assists the leaving acetoxy group, followed by the participation of the ca rbonyl oxygen for alpha -lactone formation. The unstable alpha -lactone int ermediate decomposes rapidly into the corresponding carbonyl compound and C O gas. The importance of the acidic H of the COOH assistance in the acetoxy acid mechanisms may be revealed in the elimination kinetics of methyl 2-ac etoxypropionate. This substrate was studied in the ranges 370.0 - 430.0 deg reesC and 36-125 Torr. This reaction is homogeneous, unimolecular and follo ws a first-order rate law. The products are methyl acrylate and acetic acid . The rate coefficients is given by the equation logk(1) (s(-1)) = (12.63 /- 0.35) - (201.7 +/- 4.4) kJ mol(-1) (2.303RT)(-1) Copyright (C) 2000 John Wiley & Sons, Ltd.