STERIC EFFECTS OF ALKYL-GROUPS - A CONE ANGLE APPROACH

A 'cone angle', theta-R, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The theta-R values for some 57 groups have been calculated mathematically by constructing the corresponding Corey-Pauling-Koltun molecular models. The variation in theta-R with the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between theta-R and the two existing steric parameters (Taft's E(s) scale and Dubois' E(s)' scale) have been established. For some 23 alkyl groups theta-R has been used to explain the variation in the rate constant of the base-catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft-Ingold hypothesis that, while the base-catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid-catalysed one is controlled almost solely be the steric effect.