A very low-pressure pyrolysis (VLPP) apparatus has been constructed and shown to yield kinetic data consistent with other VLPP systems. The technique has been applied to the pyrolysis of cyclobutyl cyanide over the temperature range of 833-1203'K.The reaction was found to proceed via a single pathway to yield ethylene and vinyl cyanide. If A , is based on previous high-pressure data for this reaction and for cyclobutane pyrolysis, then RRKM theory calculations show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by The cyano group reduces the activation energy for cyclobutane pyrolysis by 6 f 1 kcall mole, and on the basis of a biradical mechanism this value may be attributed to the cyano stabilization energy.

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.

The unimolecular decomposition of tert-butyl cyanide has been investigated over the temperature range 1023-1254 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competing pathways of C-C bond fission and HCN elimination, with the former accounting for >95% of the overall decomposition. Taking into account the mutual interaction of the two pathways in the falloff region, application of unimolecular reaction rate theory shows that the experimental rate constants are consistent with high-pressure Arrhenius parameters given by log (k\, s_1) = (15.9 ± 0.3) -(74.9 ± 1.6)/0 for bond fission, and log (fe2, s_1) = (14.1 ± 0.3) -(74.1 ± 1.6)/0 for HCN elimination, where = 2.303RT kcal/mol. The activation energy for C-C fission leads to DH°[(CH3)2C(CN)-CH3] = 74.7 ± 1.6, AHf°[(CH3)2CCN,g] = 39.8 ± 2.0, and DH°[(CH3)2C(CN)-H] = 86.5 ± 2.0, all in kcal/mol at 298 K. The stabilization energy of the -cyanoisopropyl radical has been found to be 5.5 ± 2.2 kcal/mol. This is in excellent agreement with values which we have determined previously for the cyanomethyl and a-cyanoethyl radicals. The results of this work and previous studies are included in a discussion on the effect of the CN group on bond dissociation energies and reactivity. Revised values of the contributions of the [C-(H)2(CN)], [C-(H)(C)(CN)], and [C-(C)2(CN)] groups to the heat of formation of free radicals are calculated.

The kinetics of the thermal decomposition of trans-1,2-dicyanocyclobutane, which yields only vinyl cyanide, have been studied in the temperature range of 570"-660"K using a stirred-flow reactor. The reaction was found to be first order and homogeneous with rate constants represented by the Arrhenius equation

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