Addition of Methyl Radicals to Unsaturated Hydrocarbons

Abstract

The rates of addition of methyl radicals, produced by the photolysis of acetone between 140" C. and 240" C., to u n s a t~~r a t e d hydrocarbons were determined by two independent methods. From the kinetics of the addition and the methyl radical combination reactions, addition of methyl radicals to ethylene, propylene, acetylene, and butadiene was found to proceed with activation energies of 7.0, 6.0, 5.5, and -2.5 kcal. respectively and stcric factors of the order of 10-". INTRODUCTIONThe kinetics of a simple free radical addition reaction such as CH3 + C213.1-4 C3H7 kd are generally difficult to investigate because of complications arising from the various other reactions in which the reactants also participate, and from the nature of the product, which is a free radical. The main difficulty obviously lies in the tendency for the unsaturated hydrocarbon to polymerize, which may obscure the fate of the free radicals in the system, and often pi-events quantitative study of any single process.In this investigation the unsaturated hydrocarbons were kept a t as low concentrations as possible to prevent them from participating in reactions other than addition by methyl radicals. Methods of Determining the Rate of AdditionIn a previous paper (5) it was shown that almost all the methyl radicals in the photolysis of acetone between 100" C. and 284" C. are accounted for by the following reactions:This conclusion was based on the fact that, within 3y0, where Rx is the rate of formation of the product X. I t follows that, if in the presence of an ~lnsaturated hydrocarbon the only other reaction of nlethyl radicals is addition, and Rco is unaffected, the rate of addition, designated as R', for this method; may be obtained from

The Photolysis of Azoethane

“…and log k21/k1z1/2 may be expected a t low teniperatures since the material balance approaches unity under these conditions. The present value of E?1-+El2 is in agreement with the activation energies for other reactioils of the addition of radicals to double boncls (10,14,15 …”

“…The results of the photolysis are shown in [17I Assuming ethylene, ethane, and butane to be formed only by reactions [12], [13], and [14], the following relations should be obeyed:…”

Kinetic modeling of the CH3 + C2H2 reaction data with sensitivity analyses

“…Mandelcorn and Steacie [7] were the first to report the relative rate constant for CH3 + CzHz to that for the CH3 recombination reaction. Their analysis was carried out on the basis of the measured C2Hz conversions as well as the yields of CO, CH4, and C2H6 from typical one-hour photolysis of CH3COCH3 and CzHz mixtures at wavelengths longer than 230 nm, with total pressures varying from 26 to 59 torr.…”

“…presented in Table I, were not used in Steacie's experiments and thus were set equal to zero in the modeling. According to our kinetic modeling, RL has contributions from reactions (41, (7), and (12); i.e.,…”

The stationary-state approximation in the photolysis of acetone

“…CD measured rates at two temperatures to give k, = 2.5 x 1016 exp (-70 700/RT) and k6 = 2.5 x 10" exp (-26 800/RT) and get a good fit to their data using reactions [1]- [8]. If the additional reactions of CH,CO are included, reactions [9], [12], [13], [14], [17], [18], [19], and [20], a best fit is obtained with k, = 1.8 x 1016 exp (-7 1 000/RT) and k6 = 1.25 x 10'' exp (-24 000/RT). However, the rate constants used by CD result in a sharp decrease in Qco with acetone pressure such,that calculation for the experimental conditions of Pearson give values of both Qco and QCzH6 that are 50% too low.…”