Ethene, reaction with deuterium mechanism

Scheme 9.2 Mechanism of the reaction of ethene with deuterium over a metal catalyst.

The purpose of this section is to provide an overview of the principal kinetic features of the hydrogenation of ethene and of propene, as providing a framework (or at least part of one) within which discussion of mechanisms must be conducted. Their reactions with hydrogen (and with deuterium) are quite comparable the addition of the methyl group leads to somewhat higher reactivity, due to weaker chemisorption as might be predicted from its lower heat of hydrogenation (Table 7.1). Relative rates for other alkenes will be considered later. The problem of deactivation by carbon deposition has already been mentioned, but quantitative... 

This section is concerned with the reactions of ethene and of propene with deuterium on forms of metal catalyst other than single crystals, which are covered in the next section. A fuller discussion of reaction mechanisms is reserved to Section 7.2.6. 

Rapid exchange of ethene and propene on iron and nickel films appears to proceed in this way. Very detailed studies by Japanese scientists using microwave spectroscopy have identified the structure of propene-di formed in reaction of propene with deuterium over metals of Groups 10 and 11, either supported on silica or as powders. Interpretation of the results is somewhat difficult because although addition and exchange show very similar kinetics, and are therefore thought to have the same intermediates, the locations of the deuterium atom in the propene-di are not entirely as expected by the alkyl reversal mechanism. Except on palladium and platinum, the major initial product was propene-2-di " this could arise if... 

Naito et al. studied hydrogenation with use of adsorption measurements, mass spectrometry, and microwave spectroscopy for product analysis. In the room temperature deuteriation of propene, butene, and 1,3-butadiene, the main products were [ H2]-propane, [ H2]-butane, and l,2-[ H2]-but-l-ene, respectively. They showed, using mixtures of H2 and D2, that deuterium was added in the molecular form and at a rate proportional to the partial pressure of D2, as opposed to D surface coverage the reaction rates were zero order in hydrocarbon. They proposed, therefore, in contrast to the model of Dent and Kokes for ethene (but note in this case that reaction rate was 0.5 order in hydrogen pressure and proportional to ethene surface coverage), that hydrogenation proceeded by interaction of adsorbed hydrocarbon with gas-phase D2, that is by an Eley-Rideal mechanism. 

The emphasis in this Chapter will be on the factors that determine the degree of selectivity with which the intermediate alkene is formed whereas in the previous Chapter, it lay more on the nature of the products formed, this is of lesser importance here, because under most circumstances the major product is that which arises from the addition of two hydrogen (or deuterium) atoms to the same side of the chemisorbed alkyne, i.e. Z-addition. Minor amounts of the products of -addition do occur, and the mechanism by which they are formed merits discussion. However in the case of ethyne itself, this consideration is irrelevant unless deuterium is used, so the overwhelming thrust of the work on its hydrogenation has been towards understanding the mechanism by which ethene is so selectively formed. Further significant aspects of the reaction are best discussed after a brief description of the industrial problems. 

As was the case with ethene and other alkenes, replacing hydrogen by deuterium and analysis of reaction products reveals hitherto hidden aspects of reaction mechanism. Early work with platinum and nickel indicated that neither... 

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