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Richards (1936) found from sound dispersion measurement that at ordinary temperatures, in exciting the lower vibrational states of ethylene, collisions between hydrogen and ethylene molecules were ten times as effective, and collisions between deuterium and ethylene molecules 1∙27 times as effective as collisions between ethylene molecules themselves. This meas that hydrogen is about eight times as effective as deuterium. The Specific effects of foreign gases in facilitating the interconversion of translational and vibrational energy as determined from measurements of the velocity of sound in the mixtures, have been explained by Franck and Eucken (1933) in terms of the chemical affinity between the colliding partners. This can however scarcely explain the results of Richards. The difference which he observed between hydrogen and deuterium seems to exclude any explanation for the superiority of hydrogen based on chemical affinity. The specific effect of hydrogen in activating molecules in unimolecular decompositions has been described in previous communications from this laboratory (Hinshelwood 1927; Hinshelwood and Askey 1927; Hinshelwood and Staveley 1936). The comparison of hydrogen with helium seemed to suggest that the difference could not be explained by differences of mass and velocity but rather by differing chemical properties. It seemed that the comparison of the effect of hydrogen with that of deuterium would provide significant information, and that the comparison with the results of Richards would be of interest.

The rates of cleavage of 14 symmetrically substituted diaryl disulphides by cyanide ion have been measured in 60% aqueous t-butyl alcohol at pH 9.2. A plot of log k against σ� shows that while the reaction rate is accelerated by inductive electron withdrawal from the benzene rings, substituents capable of conjugative interaction are not correlated by their σ� parameters. +R substituents cause reaction to occur much faster than predicted on the basis of their σ� values, while -R substituents react more slowly than predicted. Measurement of rates of cleavage of three series of unsymmetrically substituted disulphides by cyanide or hydroxide shows that these unusual substituent effects arise from substituents in the thiocyanate-forming aryl ring. This behaviour is explained in terms of a change in the electronic behaviour of the thio- cyanate-forming sulphur atom from -I, + R in the disulphide to -I,-R in the rate-determining transition state for the reaction. The study does not show whether the cleavage involves an SN2 process or rapid equilibrium formation of a pentacovalent intermediate.

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