Kinetics of the gas-phase iodine-catalyzed isomerization of methyl acetylene and the propargyl stabilization energy

Abstract

The gas-phase iodine-catalyzed isomerization of methylacetylene to allene has heen investigated in the temperature range 280–330°C. The occurrence of side and secondary reactions precludes the observation of a simple rate expression. Arguments are presented favouring a mechanism involving M + I₂⇌ M + 2I, I + CH₃CCH ⇌C₃H₃+ HI (1,–1), C₃H₃+ HI →CH₂CCH₂+ I (2). The data yield the following rate constant expression log{k₁/(1 +k_–1/k₂) l. mol^–1 s^–1}=(10.99±0.19)–(101.4±2.1 kJ mol^–1)/2.303 RT from which is derived the bond dissociation energy, DH°(H—CH₂CCH)= 393 ± 5 kJ mol^–1(93.9 kcal mol^–1) at 298 K. The propargyl radical C₃H₃ is stabilized by 17±5 kJ mol^–1(4.1 kcal mol^–1).