Very low‐pressure pyrolysis (VLPP) of alkyl cyanides. II. n‐propyl cyanide and isobutyl cyanide. The heat of formation and stabilization energy of the cyanomethyl radical

Abstract

AbstractThe very low‐pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n‐propyl cyanide over the temperature range of 1090–1250°K. Decomposition proceeds via two pathways, C2C3 bond fission and C3C4 bond fission, with the former accounting for >90% of the overall decomposition. Application of unimolecular reaction rate theory shows that the experimental unimolecular rate constants for C2C3 fission are consistent with the high‐pressure Arrhenius parameters given by where θ=2.303RT kcal/mole. The activation energy leads to DH2980[C2H5CH2CN]=76.9±1.7 kcal/mole and ΔH(ĊH2CN, g)=58.5±2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1±2.6 kcal/mole, which is the same as the value for the α‐cyanoethyl radical. This result suggests that DH[CH2(CN)H] ∼ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for ΔHƒ0(ĊH2CN) should be usable for prediction of the activation energy for C2C3 fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 1011–1123°K. The decomposition reactions parallel those for n‐propyl cyanide, and the experimental data for C2C3 fission are compatible with the Arrhenius expression A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH3CHCH2CN, generated by C3C4 fission in isobutyl cyanide, yields vinyl cyanide and not the expected product, crotonitrile. This may be explained by a radical isomerization involving either a 1,2‐CN shift or a 1,2‐H shift.