The mechanism of thermal elimination. Part 14. Pyrolysis of diacetamide, 2-acetoxypyridine, diacetyl sulphide, and thioacetic acid: possible involvement of enol forms in gas-phase eliminations

Abstract

In the gas phase, diacetamide undergoes unimolecular first-order elimination to give keten and acetamide with log A 12.42 s^–1 and E_a 158.23 kJ mol^–1 so that the reaction proceeds via a cyclic six-membered transition state. At 600 K the reaction is 6.7-fold slower than the comparable pyrolysis of acetic anhydride, but 4.4-fold faster than pyrolysis of diacetyl sulphide. This shows that in contrast to ester pyrolysis, heterolysis of the C_α–X bond in these compounds is relatively unimportant, the major rate-determining step being nucleophilic attack of the carbonyl group upon the β-hydrogen; the nucleophilicity of this carbonyl group is probably enhanced by resonance with the lone pair of the group X. The very high reactivity of the diacetyl compounds relative to their ester equivalents suggests that they pyrolyse via the enol forms. Pyrolysis of diacetyl sulphide is accompanied by first-order decomposition of thioacetic acid (one of the primary reaction products) by a number of pathways which give rise to keten, hydrogen sulphide, carbon oxysulphide, methanethiol, and methyl thioacetate, the latter probably arising from a combination of methanethiol with unchanged thioacetic acid which also accounts for the low stoicheiometry (1.8) of the reaction. The rate of elimination of thioacetic acid is governed by the parameters log A 12.5 s^–1 and E_a 175.7 kJ mol^–1; slightly different values are obtained from the acid produced by decomposition of diacetyl sulphide viz. 12.5 and 173.1, respectively, probably due to the differing initial proportions of the thiono- and thiolo-forms. Decomposition is much faster than the corresponding reactions of acetic acid and diacetamide, probably because the nucleophilicity of the thiol group is not lowered by resonance to the extent that operates for the corresponding nucleophiles in acetic acid and acetamide. 2-Acetoxypyridine also undergoes thermal elimination to 2-pyridone and keten, but satisfactory kinetics could not be obtained, due possibly to surface effects and an equilibrium between the N-acetyl and O-acetyl tautomers. The bulk of the compound is much more stable than diacetamide because it exists largely in the O-acetyl form.