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

Abstract

The gas‐phase elimination kinetics of the title compounds were examined over the temperature range 220.1 –349.0 °C and pressure range 19–120 Torr. 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, logk1 (s−1) = (12.03 ± 0.28) − (170.8 ± 3.2) kJ mol−1 (2.303RT)−1; for 2‐acetoxypropionic acid, logk1 (s−1) = (13.16 ± 0.24) − (174.2 ± 2.6) kJ mol−1 (2.303RT)−1 ; for 2‐acetoxy‐2‐methylpropionic acid, logk1 (s−1) = (13.40 ± 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 parallel elimination to give methacryclic acid and acetic acid. The rates of elimination are found to increase from primary to tertiary carbon bearing the acetoxy group. The mechanism appears to proceed through a discrete polar five‐membered cyclic transition state, where the acidic hydrogen of the COOH assists the leaving acetoxy group, followed by the participation of the carbonyl oxygen for α‐lactone formation. The unstable α‐lactone intermediate decomposes rapidly into the corresponding carbonyl compound and CO 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‐acetoxypropionate. This substrate was studied in the ranges 370.0 –430.0 °C and 36–125 Torr. This reaction is homogeneous, unimolecular and follows a first‐order rate law. The products are methyl acrylate and acetic acid. The rate coefficients is given by the equation logk1 (s−1) = (12.63 ± 0.35) − (201.7 ± 4.4) kJ mol−1 (2.303RT)−1. Copyright © 2000 John Wiley & Sons, Ltd.

Experimental and theoretical study of the homogeneous, unimolecular gas‐phase elimination kinetics of 2‐furoic acid

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