In continuation of the work (1946) of Dostrovsky, Hughes, and Ingold on the subject indicated in the title, the assumptions underlying their analysis are critically examined. One major modification of method is then introduced.On this basis, their calculations of the energetic effects produced by alkyl groups on the kinetics of the bromine-exchange reaction are revised. And they are extended to substitutions involving chlorine and iodine. They are also extended by a calculation of the associated entropic effects. The results are as follows.In these reactions, the steric energetic effects of the akyl groups are always retarding. But, contrary to earlier ideas of steric hindrance, such effects depend only slightly, and not always in the preconceived sense, on the sizes of the halogen atoms. They depend strongly on the structures of the alkyl groups, in which successive P-substitution starts by being less importantl but, again contrary to older preconceptions, ends by being much more important, than successive or-substitution. The polar energetic effects of the a k y l groups are retarding, and are associated almost exclusively with a-substitution.Three kinds of structural entropic effects, called ponderal,t steric, and polar, are recognised, which depend on the incidence and distribution of mass, bulk, and charge, respectively. The ponderal effect, which would be exerted by massive, but non-space-tilling, non-polar particles, such as neutrons added by isotopic substitution, is retarding i n general, though a sufficient accumulation of new masses, near the reaction centre w i l l weaken this effect, and could conceivably reverse it. Contrary, again, to elementary preconceptions, the steric entropic effect is accelerating if the steric thrust on the attacking reagent is directly outwards, but retarding if the main effect of it is to deflect the reagent sideways. The polar entropic effects of alkyl groups are neghgible.* Part XLVIIT, preceding paper. t Cf. footnote, p. 3216.[1955] Substitution at n Saturated Carboiz Atom. Part X L I X . 3201 1941, 37, 620 ; Day and Ingold, iM., p. 686), but also to make a beginning on the problem of quantitatively calculating expected steric effects on reaction rate.Such calculations have been reported for bimolecular nucleophilic substitutions in a series of simple alkyl halides (Hughes, Zoc. cit.; Dostrovsky, Hughes, and Ingold, J., 1946,173), but at a time when the experimental material available for comparison with the calculated results was comparatively slight. However, the calculations have since been extended; and additional experimental results have now become available, as recorded in the accompanying papers. In this paper we report the further calculations, and with their aid discuss the new experimental material. The previous calculations were developed in detail in the example of the bromineexchange reaction, Br-+ RBr + BrR + Brfor the seven alkyl groups, which compose the a-and the @-methylated series of branching homologues, Me, Et, Pri, But, and Et, Prn, Bui, neoPe (ne...
