Pyrolysis of ?-hydroxyketones and ?-ketoesters: Gas-phase elimination kinetics of 3-hydroxy-3-methyl-2-butanone and methyl benzoylformate

Abstract

The rates of gas-phase elimination reactions of methyl benzoylformate (1) and 3-hydroxy-3-methyl-2-butanone (2) were obtained at T ϭ 600 K. The two substrates undergo unimolecular first-order elimination for which the Arrhenius equations are, respectively, log k ϭ 13.2 Ϫ 53270/(4.574 ϫ 600) for (1) and log k ϭ 12.4 Ϫ 53060/(4.574 ϫ 600) for (2). The products of pyrolysis of (1) are benzaldehyde, formaldehyde and CO, while those of (2) are acetaldehyde and acetone. The kinetics of the elimination reactions show benzoylformic acid to be 10 6 -fold more reactive than (1), and pyruvic acid ca. 10 5 -fold more reactive relative to (2); an indication of the rate-controlling part played by the acidity of the hydrogen atom involved in the elimination process of the present compounds in this particular type of reaction.

Kinetics and mechanism of thermal gas-phase elimination of ?-substituted carboxylic acids: role of relative basicity of ?-substituents and acidity of incipient proton

“…A relative rate factor of 2 Â 10 3 is obtained when the rates of pyrolysis of 2-phenoxypropanoic acid (1) and 2-phenoxy-1-propanol (10) are compared. The rate factor is much greater (3.3 Â 10 7 ) when the comparison involves the more basic 2-(N-phenylamino) moiety of the acid (7) and that of the corresponding alcohol (9). It should be noted that a maximum factor of only 100 has been reported involving the rates of 2-bromopropanoic acid and its methyl ester.…”

“…7,13 Dilute (ppm) standard solutions of the substrates in a suitable solvent (acetonitrile) were prepared, and a suitable internal standard (chlorobenzene) was added to each solution. Care was taken to ensure that both the solvent and the internal standard are not pyrolysable under the conditions of the reaction.…”

“…The configuration and model of both the pyrolysis and HPLC units have been detailed in a previous paper. 7 The experimental and analytical techniques for evaluation of rates have been described in detail. 13 Rates were reproduced in presence and absence of a radical scavenger (cyclohexene); homogeneity and any possible adverse surface effects were also tested following earlier procedures.…”

“…The on-line GC-MS pyroprobe and the flow techniques for the identification of the constituents of the products of pyrolysis have been described in previous reports. 7,15 The products analysed were for pyrolyses conducted at temperatures commensurate with those of the kinetic measurements. No secondary decomposition was observed for any of the compounds under study up to 98% pyrolysis.…”

“…[5][6][7] In this paper, we report on the thermal gas-phase elimination of a-substituted propanoic acid and b-substituted 1-propanol, the substituent in both systems being an aryl moiety. Relative rate factors of 10 2 -10 7 have been observed, indicative of the major contribution to reactivity by the mesomerically electron-accommodating phenoxy group and its thio and amino analogues.…”