Pyrolysis of propionitrile and the resonance stabilisation energy of the cyanomethyl radical

Abstract

The pyrolysis of propionitrile has been studied at seven temperatures over the range 789–850 K and pressures between 10 and 100 Torr.; Under these conditions the principal reaction products which are formed by essentially homogeneous processes are hydrogen, hydrogen cyanide, methane, ethane, ethene, acetonitrile and acrylonitrile. For short reaction times (< 3% conversion) the products are formed at rates which do not vary noticeably with reaction time.

A free radical chain mechanism has been proposed which accounts for all the above products. The chain initiating step is the reaction C₂H₅CH →·CH₃+·CH₂CN. (1). Measurements of the rate of formation of methane in the subsequent reaction ·CH₃+ C₂H₅CN → CH₄+ CH₃ĊHCN (2) yield the rate expression log₁₀(k₁/s^–1)=(15.5 ± 0.3)–(78 700 ± 950)/θ where θ= 2.303 RT/cal mol^–1. The activation energy leads to D(H₃C—CH₂CN)= 80.4 ± 1 kcal mol^–1 and a resonance energy of 5.4 ± 1.4 kcal mol^–1 for the cyanomethyl radical.