Kinetic study of the cyclohexadienyl radical. Part 3.—Mutual interaction and thermal decomposition

Abstract

The unusual pattern of the reactions shown by the cyclohexadienyl radical below 200' is consistent with the high stabilization energy and complex pattern of spin density in the radical, andwith itsclose relationship to benzene. We have investigated the scope of the reactions of the cyclohexadienyl radical in the gas phase, in the absence of significant concentrations of other radicals and in the presence of an excess of cyclohexadiene-1,4. Metathesis between these species would be revealed by a proces; of chain isomerization of cyclohexadiene-l,4 to cyclohexadiene-1,3, but this did not occur at a significant rate below 200". Mutual interaction was the only significant process of consumption of cyclohexadienyl radicals below lOO", and conformed to the pattern :-+C12H14 isomers (69 2,) between 63 and 101". At 136" cyclohexene was formed in this system, and the relative abundance of both cyclohexene and benzene increased rapidly as the reaction temperature was raised, until at 186" the rate of formation of each exceed the rate of the primary generation of cyclohexadienyl radicals. The results are consistent with the thermal dissociation of the cyclohexadienyl radical :CYClo-C6H7*+C6H6+H. ; Ea = 31 kcal/mole, followed by the formation of cyclohexene and the regeneration of the cyclohexadienyl radical in a chain sequence. The enthalpy of formation of the cyclohexadienyl radical wzs estimated as 44 kcall mole, and the stabilization energy as 25 kcal/mole.

Kinetics and mechanism for the H‐for‐X exchange process in the H + C6H5X reactions: A computational study

“…• (C 6 H 7 ) = 44 kcal/mol) [15]. If the latter quantity is revised to that accepted in this study f H 298 • (C 6 H 7 ) = 49.5 ± 2 kcal/mol, the estimate of Amano et al would approach our recommended f H 298…”

“…• (0 K), somewhat low values of H (14) • (0 K), and somewhat too high values of H (15) • (0 K). These errors then propagate to f H 0 • (C 6 H 7 ).…”

“…C 6 H 7 + C 7 H 8 = 1,4-C 6 H 8 + C 7 H 7 (15) were combined with the experimental heats of formation f H 0 • of ethylene (14.6 ± 0.1 kcal/mol) [19], toluene (17.5 kcal/mol) [20], benzene (24.0 ± 0.2 kcal/mol), and allene (47.4 ± 0.3 kcal/mol) from Ref. [21], allyl radical (42.7 ± 0.5 kcal/mol) [22], benzyl radical (54.7 ± 1.0 kcal/mol) [23], ethyl radical (30.9 ± 0.7 kcal/mol) * [24], and 1,4-cyclohexadiene * The value f H 298 • (C 2 H 5 ) = 28.44 ± 0.5 kcal/mol is taken from Ref.…”

“…From the difference in the activation energies of the forward and reverse reactions, James and Suart [15] derived H (6) = −28 ± 5 kcal/mol (without a reference to any particular temperature). Nicovich and Ravishankara [4] analyzed the thermal dependence of ln K 6 and found that H (6) (542 K) = −26.7 ± 2.8 kcal/mol.…”

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7with O2

“…Cyclohexadienyl type radicals, c-C 6 H 6 -X, are important intermediates in reacting systems involving aromatic compounds, whereas their reactivity in the gas phase has been poorly characterised. In particular, reactions of the simplest radical, the cyclohexadienyl c-C 6 H 7 (C 6 H 6 -H adduct), have only been the subject of a few investigations: recombination with other radicals, [1][2][3][4] self reaction, 5 reaction with NO and O 2 6,7 or decomposition reaction. 5 Most studies have concerned the hydroxycyclohexadienyl radicals, in particular the OH-benzene and the OH-toluene adducts, because of their role in atmospheric chemistry.…”