The Reaction Between Methyl Radicals and Isobutane

Abstract

A B S T R A C TI t has been confirmed that the apparent rate constant for the reaction between methyl radicals, produced by the photolysis of deuterated acetone, and isobutane increases with decreasing isobutane pressure. A n explanation is proposed to account for this observation suggesting that the production of methane by disproportionation between methyl and t-butyl radicals was not negligible as has been assumed previously. I N T R O D U C T I O NObservations made recently in this laboratory indicated that values of the apparent rate constants for the abstraction of hydrogen by methyl radicals, calculated using conventional methods, depended on the pressure of the added hydrogen donor. The reaction of methyl radicals with isobutane using the pyrolysis of di-t-butyl peroxide a s a source of methyl radicals is an exanlple of such a system (I). When mixtures of deuterated acetone and hydrogen were photolyzed, the rate constant for the reaction of methyl radicals with hydrogen was found to be a function of the hydrogen pressure (2). In view of the apparent similarities between the two systems, an investigation of the photolysis of deuterated acetone -isobutane ~llixtures was made. E X P E R I M E N T A LThe apparatus and methods of analysis were similar to those employed in the photolysis of the deuterated acetone -hydrogen mixtures (2, 3). The deuterated acetone consisted of 98.2% acetone-ds and 1.8% acetone-d5. Phillips Research Grade isobutane was outgassed by bulb-to-bulb distillations a t -160" C. The temperature of the reaction cell was 471% 0.5" K for all experiments. R E S U L T S A N D DISCUSSIONIn the photolysis of mixtures of deuterated acetone and isobutane, it is usually assumed that methane and ethane are formed b y the following reactions:When the deuterated acetone was photolyzed alone ( 2 ) , small amounts of CD3H were formed due to the presence of acetone-d5 in the sample, and its yield was given bywhere [ A ] is the acetone concentration in molecules/cc. Allowance for this CDjH was made when calculating values of k,/kalt2.

Computational study on the recombination reaction between benzyl and propargyl radicals

“…5. In the calculations, the CASPT2 potential energy curves were used after shifting them by +3 kJ mol −1 to match the rate constant estimated by the geometric mean rule 48–50, ∼8 × 10 −11 cm 3 molecule −1 s −1 at 300 K, using the literature rate constants for propargyl + propargyl 9 and benzyl + benzyl 51 reactions. The geometric mean rule has been widely validated for cross recombination reactions of hydrocarbon radicals 52 including resonance‐stabilized radicals 10,53 though there is neither clear theoretical rationalization nor justification.…”

Reaction of ethyl with isobutyl and t-butyl free radicals

“…and Kutschke (4), is the correct statistical formulation. Derivation of this ratio assumes that each radical-radical collision results in either disproportionation or combination, and the steric factor for the suin of mutual disproportionation plus combination reactions is near unity.…”

Analytical resolution of a parallel second‐order reaction mechanism

“…(3) in Ref. [17]) and, for the sake of the conservation of (5), meaningless conclusions might have been drawn from the use of φ = 2 to estimate k AB when only the self-reaction rate coefficients are known [1b].…”

“…In addition, several attempts have been made to widen the applicability of a rule analogous to (5), for the cases in which disproportionation competes with recombination [17,18], and for the sake of conservation of (5), meaningless conclusions have been drawn [17].…”