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

The 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, C 2 C 3 bond fission and C 3 C 4 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 C 2 C 3 fission are consistent with the high‐pressure Arrhenius parameters given by\documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_1 (\sec ^{ - 1}) = (15.4 \pm 0.3) - (76.7 \pm 1.7)/\theta $$\end{document}where θ=2.303 RT kcal/mole. The activation energy leads to DH 298 0 [C 2 H 5 CH 2 CN]=76.9±1.7 kcal/mole and Δ H   ƒ,298 0 (ĊH 2 CN, 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   298 0 [CH 2 (CN)H] ∼ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for Δ H ƒ 0 (ĊH 2 CN) should be usable for prediction of the activation energy for C 2 C 3 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 C 2 C 3 fission are compatible with the Arrhenius expression\documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_5 (\sec ^{ - 1}) = (15.4 \pm 0.3) - (73.1 \pm 1.7)/\theta $$\end{document}A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH 3 CHCH 2 CN, generated by C 3 C 4 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.