Gas-phase pyrolytic reactions. Rate data for pyrolysis of N-t-butylthioacetamide and N-acetylthioacetamide: role of polarity of transition state and γ-carbonyl group protophilicity

Abstract

In the gas phase, both N-t-butylthioacetamide and N-acetylthioacetamide undergo unimolecular first-order elimination reactions for which log A= 11.58 s^–1 and 10.64 s^–1, and E_a= 16.45 kJ mol^–1 and,117.15 kJ mol^–1, respectively. The results are in accord with a reaction pathway involving a cyclic six-membered transition state, and show each compound to be more reactive than its oxygencontaining analogue. At 600 K, the statistically corrected reactivity ratios: t-butyl thioacetate (1)/ t-butyl acetate (2); N-acetylthioacetamide (3)/diacetamide (4); and N-t-butylthioacetamide (5)/N-t-butylacetamide (6) are 83, 173, and 1 404, respectively. The above rate factors are consistent with the tenet that as C_α–X bond fission becomes less rate-contributing in these electrocyclic reactions, so attack by the CY bond upon the β-hydrogen atoms becomes more important. Thus, whereas t-butyl acetate at 600 K is some 68 700 times more reactive than N-t-butylacetamide, t-butyl thioacetate is only 4 060 times more reactive than N-t-butylthioacetamide.