By A. Zlatkis
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Extra info for Electron Capture Theory and Practice in Chromatography
11. These results showed that ethylene exchange was rapid and the deuteroethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. l l ( a ) it can also be seen that the major initial products are ethaned, and ethanedl. D + H*C=CH2 +- H,C=CHD + H*C--CH3 I I I I * * * * 33 followed by a disproportionation reaction of the type * * * As the reaction proceeds, the ethyIene becomes progressively more deuterated [Fig.
With iridium, increasing deuterium pressure has a pronounced effect on the deuteropropane distributions, whereas with ethylene no such effect is apparent. This has been attributed to differing temperature dependencies of the alkyl disproportionation reaction for the two olefins , although the same effect could be achieved by considering that the relative surface coverage of deuterium is greater with propene than with ethylene. 7 REACTIONS OF THE n-BUTENES Whilst the use of deuterium allows a deeper insight into the mechanism of catalytic reactions than was previously possible, it nevertheless does not allow an absolutely rigorous analysis to be made.
This gives rise t o selectivity in catalytic hydrogenation, as noted in Sect. 3, and an understanding of the factors which govern the selectivity in any system is of prime importance t o the specification of detailed mechanisms. A further problem arises when one considers the hydrogenation of alkynes and alkadienes containing four or more carbon atoms. In such cases, it is possible that the intermediate olefin may be formed in more than one isomeric form. Hence, the stereospecificity of the hydrogenation must be considered and attempts made t o explain the observed stereospecificity in terms of the mechanism (see Sect.