Steric effects on the formation of alkyl radicals and alkyl carbenium ions
In order to determine the variation of the steric effect with branching on the formation of alkyl radicals and alkyl carbenium ions, logarithms of rate constants for formation were correlated with the branching equation. Significant correlations were obtained for all the sets studied. The results of the correlations are (1) the composition of the alkyl steric effect as a function of branching is highly variable both for alkyl radical and for alkyl carbenium ion formation. No single steric substituent constant could possibly account for all the data. (2) The relative importance of branching at C2 to that at C1 is a linear function of the amount of branching in the leaving group. This is true for both alkyl radical and alkyl carbenium ion formation. (3) Whereas branching at C1 and C2 causes steric acceleration in the formation of both alkyl radicals and alkyl carbenium ions, branching at C3 results in steric deceleration. The results demonstrate the utility of the branching equation in the study of the steric effects of alkyl groups. It is capable of representing the complete range of alkyl steric effects, and can detect a change from steric acceleration to steric deceleration due to branching at different carbon atoms of the same alkyl group. The expanded branching equation has been shown to be useful in the determination of the variation of alkyl steric effect with the order of branching at a given carbon atom. Radical formation is much more sensitive to a second branch at C1 than is carbenium ion formation.