The known relationships between the mechanism of aliphatic substitution and the structures of the compounds substituted form a starting point for the investigation of the laws governing the steric orientation of substitution. As a result of work reported in the preceding papers, certain general principles relating to the orientation of substitution are advanced. Detailed discussion is confined to the reciprocal replacements of *Hal and -OR, and within this field previously recorded stereochemical observations are to a large extent rationalised. A fuller statement of the conclusions is given on p. 1270.NOTWITHSTANDING the large number of investigations that have been carried out since Walden's discovery of the inversion known by his name, much uncertainty still attaches to the rules and conditions that control the steric course of substitution a t a saturated carbon atom. This is due partly to the difficulty of relating sign of rotation to molecular configuration. But a still more important cause is that the conditions controlling steric orientation are intimately dependent on the mechanism of substitution, and it is only within comparatively recent years that mechanism in aliphatic substitution has been fruitfully studied. In this field, mechanism has been elucidated principally through the examination of reaction kinetics, by means of which an intelligible relationship has been traced between mechanism, on the one hand, and the chemical structures of reactants, and the physical conditions of reaction, on the other. I t was an obvious sequel to apply kinetics in order to elucidate the factors controlling the steric course of substitution ; and the procedure has two advantages. First, we can often decide the steric orientation of a reaction by comparison on the basis of mechanism (which kinetic results help to establish) with some other reaction whose steric course is known. Secondly, it should be possible to establish an empirical connexion between steric orientation and kinetics; and thus, using our present knowledge of the relation of kinetics to mechanism, to link steric orientation with mechanism and its determining factors, chemical structure and physical conditions. We have therefore studied the bptical effect of substitution a t an asymmetric carbon atom under conditions, not chosen haphazard, but defined by parallel studies of kinetics. I t is our view that much of the previous work on the Walden inversion requires repetition with this form of control (cf. Cowdrey, Hughes, and Ingold, Nature, 1936, 138, 759).In the present paper we set forth conclusions based largely on the experimental studies recorded in the previous five papers. We deal here only with the reciprocal replacements of -Hal and *OR, and this only in a preliminary way : extensions of the study in this field, and likewise our investigations on the substitutions which involve *NR,, will be described in future groups of papers.On the theoretical side our starting point is the general survey given by Hughes and Ingold (J., 1935, 244) of aliphat...
