Kinetics of C6H5 radical reactions with 2-methylpropane, 2,3-dimethylbutane and 2,3,4-trimethylpentane

Abstract

The absolute bimolecular rate constants for the reactions of C 6 H 5 with 2-methylpropane, 2,3-dimethylbutane and 2,3,4-trimethylpentane have been measured by cavity ringdown spectrometry at temperatures between 290 and 500 K. For 2-methylpropane, additional measurements were performed with the pulsed laser photolysis/mass spectrometry, extending the temperature range to 972 K. The reactions were found to be dominated by the abstraction of a tertiary C-H bond from the molecular reactant, resulting in the production of a tertiary alkyl radical:

Kinetics of phenyl radical reactions with ethane and neopentane: Reactivity of C 6 H 5 toward the primary C‐H bond of alkanes

“…The PLP/MS technique has been described in detail in a recent review [11]; the technique has been successfully employed for measurement of the rate constants of C 6 H 5 reactions with H 2 [12], CH 4 [13], CH 2 O [14], and iso-C 4 H 10 [3]. For the H 2 and CH 4 reactions, the PLP/MS results agree closely with those obtained by P/FTIRS and shock-tube studies [15], whereas for the CH 2 O and i-C 4 H 10 reactions, PLP/MS rate constants agree quantitatively with the values determined by CRDS.…”

“…From a series of studies carried out at 333 K, they determined the relative reactivities of C 6 H 5 toward the primary, secondary, and tertiary C-H bonds (0.12 Ϯ 0.01):1.01:4.8 for aliphatic hydrocarbons larger than C 4 . Further discussion on this and other relative rate data will be made later on the basis of our new results for the primary C-H reaction and those for secondary and tertiary C-H reactions reported previously [3,4].…”

“…Sensitivity analyses for C 6 H 6 (a) and C 6 H 5 CH 3 (b) in the C 6 H 5 ϩ C 2 H 6 reaction at 900 K. Reaction conditions are given in Table II Significantly, as shown in Table II, the absolute yields of toluene can be quantitatively predicted with the rate constant k 3 ϭ 1.38 ϫ 10 13 exp(Ϫ23/T) cm 3 mol Ϫ1 s Ϫ1 determined previously from three independent sets of experiments on C 6 H 5 ϩ H 2 [12], CH 4 [13], and i-C 4 H 10 [3]. This is reassuring and important because the concentration of C 6 H 5 CH 3 depends strongly on those of CH 3 and C 6 H 5 .…”

Kinetics of phenyl radical reactions with propane, n‐butane, n‐hexane, and n‐octane: Reactivity of C6H5 toward the secondary CH bond of alkanes

“…Another interesting aspect of C 6 H 5 reactions with alkanes is that the rate constant per CH bond for C 2 H 6 and neo ‐C 5 H 12 is approximately the same over the temperature range studied, 565–1000 K, whereas the rate constant per t ‐CH for the reactions with isobutane, 2,3‐dimethylbutane and 2,3,4‐trimethyl pentane increases significantly with the molecular size of the alkane 8. The observation was attributed in part to the greater accessibility of the H‐atoms as the carbon chain length lengthens.…”

“…The reactivity of C 6 H 5 radicals toward CH bonds in hydrocarbons is not only fundamentally relevant, but also practically important for quantitative understanding of fossil fuel combustion. In our two separate series of studies on the kinetics and mechanisms of C 6 H 5 reactions employing the cavity ringdown spectrometric (CRDS) technique 1–8 and the pulsed laser photolysis/mass spectrometric (PLP/MS) methods 9–15, with some of the studies aided by quantum chemical calculations 4, 6, 13, 14, we have quantitatively established the reactivity of the phenyl radical toward a variety of chemical bonds and functional groups.…”