The relative reactivities of a series of hydrogen donors toward the methyl radical were measured in solution at 110°. Methyl radicals were generated by the thermal decomposition of ferf-butyl peracetate in mixtures of the hydrogen donor RH and the standard substrate tert-butyl mercaptan-S-t/, and the ratio [CHJ [CH3D] was determined by mass spectrometry: CH3• + RH CH4 + R• (¿HH) and CH3• + tm-BuSD -CH3D + tert-BuS• (¿sd)• A wide range of substrates was studied, including compounds containing alkyl, allylic, benzylic, and cycloalkyl hydrogens, and the data are combined to give a scale of rate constants for reaction of methyl radicals with any given type of hydrogen. Our scale of reactivities for hydrogen-abstraction reactions and Szwarc's scale3 for addition reactions can be put on a common basis using the reactivity of isooctane as the bridge: CH3• + (olefinic or aromatic compound) -» CH3-C-C • (¿add) and CH3• + C8H18 -CH4 -\-C8H17• (¿¡so). This permits us to tabulate the relative reactivity toward methyl radicals of virtually every type of hydrocarbon-saturated, olefinic, or aromatic; from these data the relative reactivity of any hydrocarbon of interest can be predicted. The selectivity of the methyl radical toward various types of hydrogens is compared with that of a number of other radicals.The methyl radical is found to be quite similar to the phenyl radical in selectivity. Calculations show the relative reactivity of a series of donors toward the methyl radical is the same in the gas phase and in solution.(1) Reactions of Radicals. Part 41. This work was supported in part by NIH Grant GM-11908. (2) (a)

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filtered Bausch and Lomb SP 200-W super pressure mercurylamp for periods of 10 sec to 2 min. Spectra were measured at room temperature with a Varían V-4500 epr spectrometer equipped with a -in. magnet. Sweep rates were calibrated by the spectrum of p-benzosemiquinone in aqueous ethanol.

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