Journal of Physical Chemistry p. 5363 - 5367 (1986)
Update date:2022-08-11
Topics:
Robaugh, D.
Tsang, W.
Phenyl ioide and o-iodotoluene have been decomposed in comparative-rate single-pulse shock-tube experiments using cyclopentane as a source of abstractable H atoms.The conditions for quantitative conversion of phenyl radical to benzene and o-methylphenyl radical to toluene are determined.On the basis of the reverse Diels-Alder reaction of 1,2-dimethylcyclohexene (1,2-DMC) as the internal standard, the rate expressions for phenyl iodide and o-iodotoluene decomposition have been found to be k(phenyl-I -> phenyl + I) = 8.6 x 1014 exp(-32 821/T) s-1 and k(o-iodotoluene-I -> o-methylphenyl + I) = 1.4 x 1015 exp(-33 346/T) s-1 at 1100 K and between 2 and 6 atm.Ortho methyl substitution has negligible effect on the rate of C-I bond cleavage under these conditions.The present results lead to a heat of formation for phenyl radical of 340.5 +/- 11 kJ/mol or a bond dissociation energy for the C-H bond in benzene of 475 +/- 11 kJ.At 1100 K, the rate constant for the combination of phenyl radical and iodine atom is near 1 x 1010 L/(mol*s), while the rate constant for phenyl attack on cyclopentane is =3 x 108 L/(mol*s).In addition, we call attention to the rate expression for 1,2-dimethylcyclohexene (1,2-DMC) decomposition, k(1,2-DMC) = 3.2 x 1015 exp(-35 228/T) s-1, which we determined from our comparative rate studies with cyclohexene and 1-methylcyclohexene as internal standards.It is suggested that the larger rate parameters may mark the onset of a diradical contribution to 1,2-dimethylcyclohexene decomposition.
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