Very low-pressure pyrolysis (VLPP) of alkyl cyanides. I. Thermal unimolecular reactions of isopropyl cyanide

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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.

Section: Discussionmentioning

confidence: 64%

Very low-pressure pyrolysis of alkyl cyanides. III. tert-Butyl cyanide. Effect of the cyano group on bond dissociation energies and reactivity

King¹,

Goddard²

1976

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“…These studies have yielded values for the heats of formation and stabilization energies of -cyanoalkyl radicals. The results have been used to predict that the Arrhenius expressions for the decomposition of ethyl cyanide (EtCN) via the unimolecular pathways c2h5cn -* ch3 + ch2cn (1) C2H6CN -C2H4 + HCN (2) are given by2 log (k1/s"1) = 15.4 -338.9 kJ mol~1/2.3RT and log (k2/s"1) = 13.6 -343.0 kJ mol1/2.3RT.…”

Section: Introductionmentioning

confidence: 99%

Kinetics and mechanism of the thermal decomposition of ethyl cyanide

King¹,

Goddard²

1978

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The pyrolysis of ethyl cyanide in the presence and absence of radical-chain inhibitors has been investigated in the temperature range 896-1020 K using a stirred-flow reactor at atmospheric pressure with nitrogen as the carrier gas. In the absence of additives the decomposition proceeds by a homogeneous free-radical chain process to yield HCN, C2H4, H2, and C2H3CN as major products. On the assumption of first-order kinetics rate constants for the overall reactions C2H6CN -* HCN + C2H4 (3) and C2H5CN --H2 + C2H3CN (4) are given by the Arrhenius expressions log (k3/s_1) = (14.3 ± 0.4) -(283.7 ± 6.7) kJ mol_1/2.3RT and log (k4/s_1) = (12.1 ± 0.3) -(247.3 ± 5.9) kJ moT1/2.3ñT. Some aspects of a radical-chain mechanism are discussed. Addition of large amounts of PhNH2 or PhCH3 relative to ethyl cyanide completely suppresses the overall reactions 3 and 4 and the mechanism of ethyl cyanide decomposition becomes C2H5CN -*• CH3 + CH2CN (1), CH3 + RH -* CH4 + R, CH2CN + RH -CH3CN + R where RH is PhNH2 or PhCH3. In the presence of excess PhNH2 the first-order rate constants for CH3CN formation, which are indicative of the unimolecular C-C bond fission reaction 1, are reasonably consistent with the Arrhenius expression log (fei/s-1) = 15.25 -338.9 kJ mol~1/2.3RT. The results are in good agreement with predictions based on the very low-pressure pyrolysis (VLPP) of alkyl cyanides and provide further confirmation that the stabilization energy of the cyanomethyl radical is =¿21 kJ mol'1.

“…On this basis, a value of ~5 kcal/mol may be obtained from the kinetics of pyrolysis of several cyanosubstituted small ring compounds. 3,11,13 This is strongly supported by a substantial body of recent data on the pyrolysis of alkyl cyanides.12,13 Thus the agreement between stabilization energies derived from cis-trans isomerizations and values from other sources appears not to be as good as suggested by Marley and Jeffers.…”

mentioning

confidence: 91%

Resonance stabilization energies from cis-trans isomerization studies. Comment

King¹

1977

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In a recent publication,1 Marley and Jeffers reported single pulse shock tube relative rate measurements for the cis-trans isomerization of crotononitrile, 1,3-pentadiene, and 3-methyl-l,3-pentadiene. The results are log k (crotononitrile) = 13.2 -58.1/9, log k (1,3-pentadiene) = 13.6 -53.0/9, and log k (3-methyl-1,3-pentadiene) = 14.0 -55.0/9, where 9 = 2.303RT kcal/mol. On the assumption of a biradical mechanism for cis-trans isomerization the authors compared these findings with their previously methyl-1,3-pentadiene almost certainly proceeds via the intermediate formation of 1,2,3-trlmethylcyclobutane and not via a biradical mechanism. These workers also studied the cis-trans Isomerization of 1,3-pentadiene. The situation was less clear but again intermediate formation of a cyclobutane seemed more likely than a biradical mechanism. A similar pathway for 3-methyl-1,3-pentadiene seems likely. This paper was overlooked by Marley and Jeffers.