Effect of Conjugation, Hyperconjugation, and Steric Hindrance on Methyl Affinities

Abstract

The mechanism of the addition of radicals to aromatic and olefinic compounds is discussed in terms of a potential energy diagram which explains the linear relation between localization energy and the logarithm of methyl affinity per reactive center. It is pointed out that such a linear relation is expected for series of compounds, each having the same type of reaction center, e.g., C–C=C–Hin a series of aromatic hydrocarbons or C=C–H–Hin a series of substrates like ethylene, styrene, butadiene, etc. The relation between the slope of such a line and the nature of the reactive center is discussed. The foregoing treatment is extended to explain the effects caused by hyperconjugation, and an excellent agreement is found between theory and experiment. It is shown that compounds having a methyl substituent on the reactive center are less reactive. This inhibitory action of the methyl substituent is explained in terms of steric hindrance, and it is shown that for a particular center the inhibitory effect amounts to a constant factor independent of the original reactivity of the center. Still greater steric effects are caused by phenyl groups and by two methyl substituents located on a reactive center. The utility of this ``blocking'' technique for determining the most reactive centers is pointed out.