Steric effects—III : Composition of the E' s parameter. Variation of alkyl steric effects with substitution. Role of conformation in determining sterically active and inactive sites

Abstract

The steric effect of alkyl groups as parameterized by E' s covers a range of over seven powers of ten. In this article an attempt is made to portray in mechanism terms the manifold nature of the steric effect as a function of substitution, i.e. the composition of the steric effect. One most consider the contribution of the topological carbon sites as a function of the true coordinates of these atoms. To this end, we evaluate the geometry of these alkyl groups in carboxylic acid indirectly by a search for the probable conformations of these acids. The starting point in this study is the determination of the minimum energy conformations in carboxylic acids using the empirical force field method based on analogous alkylmethyl ketone models. This approach to the ground state behaviour of these acids leads directly to a division of the complete set of alkyl groups into two subsets within which a Conformational sequence or filitation is associated with the substitution process which generates the successive alkyl groups of that subset. It is shown that eclipsed conformations are preferred in the case of secondary and tertiary groups bearing Et and Me sub-groups, giving way to preferred bisected conformations when t-Bu and i-Pr sub-groups are present. For further elucidation of steric perturbation terms these two subsets must be considered independently. These important observations lead to an overall coherent interpretation of steric effects. Three regions of distinct behaviour as a function of substitution have been identified. In Region I (the so-called “mormal” region, where the contribution of the introduction of successive methyl groups to the overall E' s increases monotonically), besides excellent topological site correlations, an excellent hybrid correlation is proposed. For a conformationally homogeneous (i.e. eclipsed) set of alkyl groups the overall E' s of groups is well expressed by a linear combination of the weighted contribution of sub-groups and the Newman six-number. This correlation leads to an interpretation of the composition of the overall steric effect of an alkyl group, as measured by the E' s parameter, in terms of conformationally defined sites and stresses the steric importance of certain privileged sites. This result is consistent with the molecular mechanics viewpoint of preferred conformations but at variance with previous assumptions. The levelling effect (Region II), in the light of conformational information, is understood in terms of sterically active and inactive sites, while the inversion effect (Region III) is considered to be the result of molecular distortion brought about by relief of local internal steric strain within the alkyl groups. The X-ray structure of i-Pr 3CCO 2H illustrates this point.