reagent characteristic as polarizability.I0 n u t we have not considered the physical nature of the special interaction of reagent with substrate. Some of the most important forces of attractiori between molecules not undergoing cheiiiical reaction are those variously known as van der SS'aals or London or dispersion forces. l1 These forces are proportional to the product of the polarizabilities of the atoms concerned and inversely proportional tr: the seventh power of the distance between the atoms. London forces have an important influence on many physical properties. They are, for exaniple, largely responsible for the greater solubility of iodine ( 1 2 ) and of stannic iodide in broirioalkane solvents than in corresponding chloroalkanes.12,'3 One would expect London forces to contribute to the lowering of transitioii state energy m i d thus tci ( I O ) For many years, pnlorizability i n a nucleophilic re:Lgent has been Considered t o contribute subst:tntially to nucleopliilic reactivity and polarizability in a groui] being (1ispl:icetl h contribute substantially t o displaccability. Cf, Faruday SOL.,34, 195 (1938); G . E.E;. Hrnnch and lf.Calvin " T h e Theory of Organic Chemistry," Prentice-Hall, Inc., New York, X Y., 1941, p . 422; C. G. Swain and C. I? Scott, Tmxs J n m x A L . , 75. 1-11 (1953). These views appear t o be sound, b u t they should be clearly differentiated from the proposition t h a t has Iieen nrri\,ed a t in this paper, namely, t h a t t h e special i n c ~ e a s cin reactivity which attends the reaction of a reagent such as the iodide ion with a substrate having a large and heavy halogen atom a t or nenr the site of substitution is due t o t h e high polarizabilities of both reagent and halogen substituent. T h e general contribution of polarizability t o nucleophilic reactivity is allowed for, and t h e special reagent characteristic is revealed. by comIiarison of substrate ratios such as t h e k R I / k R R , ratios in Table 57. (11) 0. IC. Rice. "Electronic Structure and Chemical Binding," McGram-Hill Book Co., Inc., New York. h-.Y,, 1940, p. 354; J. R . Partington, "An Advanced Treatise on Physical Chemistry," Val, I , Longmans, Green and C o . , London, 1949. p . 741. (12) J. H. Hildebrand and R. L. Scott, "The Solubility of Nonelectrolytes," 3rd ed., Reinhold Publishing Corp., New York, N. Y., 1050. pp. 373 and 37G. (13) S. A. Shchukarev, L. S. Lilich and A. B. Sheinin, Iloklmf,' Aknd. X a u k S.S.S.IZ, 85, 1333 (1982); C. A , . 47, 383 ( 1 0 5 X i .
[ CONTRInUTION
an increase in reaction rate, when the transition state structure is such as to bring atonis of high polariiability close to one another. This would appear to be the nature of the effect with which this paper is concerned.'* It should be etnph,irized that the accelerative effect arising from the mutual interaction of substrate and reagent atoms of high polarizability does not mean that change from a substituent or reagent of low to one of high polarizability necessarily brings about an increase in reaction rate. The new substituent, for example, may have steric, inductive and/or mesomeric effects which depress the reaction rate. Examples can be found in Tables I, I1 and 111. What it does mean is that after allowaiices have been made for these other factors, as we have done by coilsidering suitable rate ratios, the change to a substituent o f higher polarizability will have caused more acceleration, or less deceleration, the greater the effective polarizability of the nucleophilic reagent Assuming that the special characteristics in reagent arid substrate are correctly identified as polarizabilities and that their special interaction is in the nature of London forces, we can expect that this factor exerts a significant influence in yet other classes of chemical reactions. It will be of interest to discover other manifestations of this effect. It is a pleasure to acknorvledge discussions with Drs. Herbert C. Brown and \Vi11 D. Merritt, Jr., which have been of great help in formulating the ideas set forth in this paper. (14) There ii one troublesorne point concerning thls Interprctatlon If the special interiction I S Indeed of t h e nature of I ondon forces, I t seems strange t h i t bromine atoms in in- ancl p bromobenzyl chloridci qhould still eyert a slgnifcant Influence o n t h e iodlde hyilroxlde ratio London forces fall off extremely r-ipiill\ \rlth (1l-t lxire ( T tble I V ) 1his point requires further consi
