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

✍ Scribed by Keith D. King; Richard D. Goddard

• Publisher: John Wiley and Sons
• Year: 1975
• Tongue: English
• Weight: 830 KB
• Volume: 7
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550070606

✦ Synopsis

The very low-pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n-propyl cyanide over the temperature range of 1090-1 250°K. Decomposition proceeds via two pathways, CZ-Ca bond fission and C3-C4 bond fission, with the former accounting for >goy, 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 log k, (sec-l) = (15.4 f 0.3) -(76.7 f 1.7)/0 where 0 = 2.303RTkcal/mole. The activation energy leads to DH!98[C&-CHzCN] = 76.9 f 1.7 kcal/mole and AH/P298(CH&N,g) = 58.5 f 2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1 f 2.6 kcal/mole, which is the same as the value for the a-cyanoethyl radical. This result suggests that DH&8[CHt(CN)-H] -93 kcal/mole, which is considerably higher than previously reported. The value obtained for aH,*(CH,CN) should be usable for prediction of the activation energy for Cz-Ca fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 101 1-1 123OK.

The decomposition reactions parallel those for n-propyl cyanide, and the experimental data for Cz-C3 fission are compatible with the Arrhenius expression log ks (sec-') = (15.4 f 0.3) -(73.1 =k 1.7)/S A significant finding of this work is that HCN elimination from either compound is practically ponexistent under the experimental conditions. Decomposition of the radical, CH &HCH&N, 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.