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

Abstract

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.

Factors Influencing the C−O−Bond Homolysis of Trialkylhydroxylamines

“…This suggests that a plot of E a vs BDE(C−H) should show a reasonable correlation shows a plot constructed from literature or estimated bond dissociation energies BDE(C−H) . It includes data reported by Mulder et al for TEMPO−methyl, − c- hexyl, and − c- hexadienyl and by Howard et al .…”

Determination of Organo−Cobalt Bond Dissociation Energetics and Thermodynamic Properties of Organic Radicals through Equilibrium Studies

“…The enthalpy of formation for • C(CH 3 ) 2 CN in CDCl 3 evaluated through Scheme 6 (Δ H f ( • C(CH 3 ) 2 CN) = 40.8 kcal mol -1 ) compares favorably with the value of 39.8 kcal mol -1 determined in the gas phase from low pressure pyrolysis experiments . The accuracy of Δ H f ( • C(CH 3 ) 2 CN) determined by this procedure is limited primarily by the quality of thermodynamic data available for the styryl radical.…”

“…Δ H f ( • C(CH 3 ) 2 CN) = 39.8 kcal mol -1 ; Δ H f ( • CH(CH 3 )C 6 H 5 ) = 33 kcal mol -1 ; , Δ H f (H • (g)) = 52.1 kcal mol -1 ; Δ H f (CH 2 (CH 3 )CN) = 38.2 kcal mol -1 ; Δ H f (CH 2 CH(C 6 H 5 )) = 35.2 kcal mol -1 …”

CryptoCope Rearrangement of 1,3-Dicyano-5-phenyl-4,4-d2-hexa-2,5-diene. Chameleonic or Centauric?

“…Reevaluation of the stabilization energy afforded by cyano vis-à-vis hydron to a secondary alkyl radical is based without reservation on the bond dissociation energy, DH°[(CH 3 ) 2 C(CN)−CH 3 ] = 74.7 ± 1.6 kcal mol -1 , so elegantly determined by King and Goddard in pyrolyses of alkyl nitriles at very low pressures. Recast in eq 1 using currently accepted values of enthalpies of formation of tert -butylnitrile and methyl radical lead to an enthalpy of formation of the dimethylcyano radical of +39.2 kcal mol -1 . A corresponding stabilization energy (SE) is defined by the replacement of hydron by cyano and evaluated by the isodesmic manipulation of eq 2: (SE CN/H ) = −12.9 kcal mol -1 . , The stabilization energy resulting from replacement of a methyl group by cyano is derived by eq 3: SE CN/CH3 = −10.8 kcal mol -1 .…”

“…Similar recalculations of the heats of formation of H 2 C(CN) • [+55.6 kcal mol -1 , +57.8 kcal mol -1 ] 33 and CH 3 CH(CN) • [+49.5 kcal mol -1 ] 31 lead to stabilization energies of −15.0 and −12.7, and −11.8 kcal mol -1 , respectively. A value calculated for H 2 C(CN) • at the G2 level of theory is SE CN/H = −6.9 kcal mol -1 …”