The Reaction of Methyl Radicals With Isobutane

Abstract

An investigation is reported of the reaction of methyl radicals, produced in the photochemical decomposition of azomethane, with isobutane. 'l'lle energy of activation of this process was found to be 6.7 f 0.8 kcal./mole, assuming that the combination of methyl radicals has a n activation energy of zerci. From some experiments with n-butane, a value of 9 f 1 kcal./mole was obtained. INTRODUCTION.-\ recent study of the photolysis of azomethane (5) has shown that it may be used as a source of meth>.l radicals for an investigation of reactions of the type C H s + R H -+ C H I + R where RH is an); hydrogen containing compound. The choice of suitable compounds is, however, limited as the activation energy of hydrogen abstraction from azomethane itself is low. Since the method of investigation involves the determination of rate differences, the velocities of the two competitive reactions must be of roughly the same order of magnitude if quantitative results are required. Isobutane was chosen in the first instance as a simple hydrocarbon containing a tertiary carbon-hydrogen bond, where there is greater ease of abstraction (6, 7) than from a corresponding primary or secondary bond. E X P E R I M E N T A LThe apparatus and analytical procedure have been described elsewhere (5). The nitrogen-methane and ethane fractions were analyzed by the mass spectrometer and there was no evidence that the latter were contaminated with isobutane.The light source was a I-Ianovia S-500 medium pressure mercury arc and the collin~ated light beam had a volume within the quartz reaction cell of 110 cc. The cell volume was 170 cc. Two filters were used to cut out the short ultraviolet wave lengths. Filter A , a Corning filter No. 2-37 or 586, was opaque to wave lengths below 3150 A and had a transmission such that 96% of the absorbed radiation was the 3660 lines. Filter B was a Corning filter No. 0-53 or 774 and limited the incident radiation to wave lengths greater than 2800 a.The azomethane was a sample that had been prepared previously ( 5 ) . I t was kept as a liquid a t -78O C., vapor pressure 6.6 rnm., in a trap darkened to exclude daylight. Research Grade isobutane, obtained from the Phillips Petroleutn Company, was found t o contain traces of methane and ethane ( N 0.05 mole yo) which were sufficient to cause errors of several per cent in the analysis of the reactiorr products. The samples used were, therefore, introhfanzrscript received Febrzrary 10, 1953.

Evaluated Site-Specific Rate Constants for Reaction of Isobutane with H and CH3: Shock Tube Experiments Combined with Bayesian Model Optimization

“…Many early experiments measured the total rate constants for the reaction of CH 3 with isobutane (reactions R656 + R657). Steacie and co-workers − published a series of four papers in the 1950s measuring the total rate constant relative to the CH 3 self-reaction (reaction ), from 298 to 506 K. By dividing the measured rate of reaction with isobutane by the square root of reaction , they report relative values that are independent of the CH 3 concentration. CH 3 radicals were created by photolyzing a variety of precursors, including acetone, dimethyldiazene, and dimethylmercury, and typically had a 5–30% correction for the reaction of CH 3 with the precursor.…”

“…Isobutane is the simplest hydrocarbon with a tertiary carbon and its reaction with H and CH 3 has been studied by several researchers. − Much of this work was performed in the 1950s and 1960s with older experimental methods that provided little or no site-specific rate information. Most of these older studies focused on methyl radicals − and determined rate constants relative to self-recombination of CH 3 . Less experimental work has isolated the reaction of H with isobutane, , but H chemistry plays a significant role in hydrocabon combustion and pyrolysis. ,,, Although some of the more recent studies have derived the relative rate of primary and tertiary H abstraction ( k p / k t ) from isobutane (e.g., the 2001 work of Goos et al), most have measured only the total rate constant ( k p + k t ). − ,, …”

“…Most of these older studies focused on methyl radicals − and determined rate constants relative to self-recombination of CH 3 . Less experimental work has isolated the reaction of H with isobutane, , but H chemistry plays a significant role in hydrocabon combustion and pyrolysis. ,,, Although some of the more recent studies have derived the relative rate of primary and tertiary H abstraction ( k p / k t ) from isobutane (e.g., the 2001 work of Goos et al), most have measured only the total rate constant ( k p + k t ). − ,, …”

“…The [propene]/[isobutene] ratio gives experimental values of relative rate constants for H abstraction from primary and tertiary sites. To account for small amounts of secondary chemistry, we use a kinetics model based on JetSurF 2.0 and then use MUM-PCE to constrain the kinetics to both the present shock tube data and previous literature data, ,− ,− ,− resulting in evaluated rate constants for the abstraction of H from primary and tertiary carbons by hydrogen atoms and methyl radicals. In the Supporting Information, we provide an explanation of MUM-PCE intended for other kineticists who might want to use the program.…”

The pyrolysis of mercury dimethyl. Part 2.—The reaction mechanism

“…Thc absence of measurable quantities of propane and butane among the products of pyrolysis of mercury dimethyl 1 is no argument against mechanism 11, since hydrocarbons containing secondary hydrogen atoms react much more rapidly with methyl radicals than does ethane. The activation energy for the step has been variously given as 5.5,24 8*4,45 8-3,?8 9 5 l, 46 and 8.6 kcal.47 Although these figures are not in perfect agreement, the consensus of opinion is that the activation energy for the removal of a secondary hydrogen atom by a methyl radical is 2-3 kcal less than for the removal of a primary hydrogen atom, a conclusion supported by Allen's work on the action of methyl radicals on propane.48 It follows, therefore, that any propane, butane and higher paraffins formed will b2 consumed by methyl radicals very much more rapidly than is ethane ; and since the resultant ethane at 303" C amounts to less than 3 % of all the gaseous products,l the final concentrations of any higher paraffins would be too low to measure with the small amount of material available. Hydrogen abstraction and radical recombination could thus from energetic considerations proceed very rapidly, leading ultimately to a mixture of hydrocarbons of high molecular weight and corresponding closely to the formula (CH2),, demanded by the carbon-hydrogen balance.…”