The mercury (3P1) photosensitized hydrogenation of ethylene has been studied a t room temperature as a function of cthylene concentration, mercury concentration, and light intensity. In addition to combination and disproportionation, ethyl radicals have been shown to take part in the reactionThe conditions favoring this reaction have been established and anomalous values previously found for the ratio of ethane to butane have been explained. The \~alue obtained for thc ratio of the rate constants for the disproportionation and combination of ethyl radicals, O.15f . O l , is in excellent agreement with thc values obtained by other methods. Hcsane formation is of some importa~lce a t low light intensities and hie11 ethylenc conccntrations, and is adeoaatelv accounted for by the reactions C?H;+CzH, = C4H9 CdH9+C?ISj = ceH1.1.
INTRODUCTIONI t is now generally agreed that in systems containing ethyl radicals both combination and disproportiollation will take place. From the variety of data discussed by Steacie (13) it is clear that in some instances it is not justifiable I I to equate the ratio of the rate constants for disproportionation and combination, k4/k3, to ( C~H G ) / ( C~H~O ) , (CPH~)/(C~HIO), or to i((C?H,) S (C&S)/ 1 (C4H10) because of the occurrence of secondary reactions which consume ethylene or because the products may be formed in some other way. When these factors are taken into account both reactions appear to have small or negligible activation energies and steric factors not greatly different from unity (6). Nevertheless, in the case of the mercury photosensitized hydrogenation of ethylene there are differences in the apparent value of k4/k3 a t room temperature which are greater than the experimental errors.There is little doubt that the basic mechanism of the reaction a t room temperature is the following:However, from the amounts of ethane and butane formed Moore and Taylor (11) found k4/k3 to be 0.17, while Le Roy and I
The aldol condensation of acetaldehyde has been carried out in borate buffers in deuterium oxide solution, and the amount of deuterium in the unchanged aldehyde has been studied as a function of aldehyde concentration and extent of reaction. The results are interpreted in terms of successive ionisation and condensation reactions, and values are obtained for the ratio of velocity constants and for the magnitude of the kinetic isotope effect.IT is generally agreed 1 that the aldol condensation of an aldehyde or ketone RCOMe, catalysed by hydroxyl ions, takes place according to the following scheme : ki k-i ka k-1 Fast RCOMe + OH-RCOCH,-+ H,O ( a ) (6) (c) R-CO*CH,CRMe.O' + H,O RCO.CH,-+ R-COMe g--RCOCH,CRMe.O-RCOCH,.CRMe*OH + OH-
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