Ethylene reaction with deuterium

As a consequence of the concerted mechanism, the Diels-Alder reaction is also stereoselective, implying that the relative configuration of the groups of the reactants is retained. Besides the numerous examples of heterosubstituted compounds,521,522 this was also proved by 1,3-butadiene and ethylene labeled with deuterium [Eqs. (6.88) and (6.89)] 531... 

Evaluation of the electrocatalytic isotopic reaction of alkenes was facilitated by using Kemball s statistical model to determine the origin of each species and the probability of each step (380,381). The model assumes olefin adsorption and reaction with deuterium or hydrogen to form ethyl radicals. These can revert to ethylene or they can add H or D to give ethane (i = 0, 1,...,4) ... 

Impurities in recoil systems present much the same problem as in any other chemical system. Their presence is perhaps a bit more serious in recoil work, particularly in gas-phase studies. This can be illustrated by a siiecific example. In studying the effect of perdeuteration on product yield from the reaction of carbon-11 with ethane, it was found that the ethylene- C yield tended to be erratic. A standard method of preparation of ethaiie-dg involves the reduction of ethylene-d4 with deuterium. Traces of ethylene remaining are removed by treatment with bromine, a most effective method. The removal of bromine is accomplished by the usual techniques and is continued until its presence can no longer be detected. Under the circumstance in question it meant that the mole fraction of bromine in the mixture was less than 10 . In the same system, ethylene- C was produced at a concentration of about 10 mole fraction. The trace of bromine remaining was sufficient to scavenge part of the labeled ethylene. The problem was readily solved by using another method of purification. 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

It should be noted that the correlations being discussed here are far from perfect and exceptions can be found in nearly each of the reaction series. (For the ethylene-hydrogen and deuterium-ammonia reactions, the correlation between catalytic activity and per cent d-character is nearly quantitative.) This is to be expected in view of the experimental difficulties involved in preparing clean and reproducible metal surfaces, particularly where different metals are being compared. In any attempt to correlate catalytic properties with work functions, it should also lie recognized that the work function is affected by adsorption, and therefore that the work functions of metals under catalytic conditions, or even their relative order, may be somewhat different than those of the clean metals. 

Results obtained from the reaction of ethylene with deuterium have been used to obtain information regarding the probabilities of the various changes which the adsorbed hydrocarbon species may undergo. The procedure, due originally to Kemball [102] and subsequently used by Bond et al. [103—105] and Wells and co-workers [106], is based upon a steady state analysis of the following general mechanism. 

Fig. 11. Distribution of (a) deuteroethylene yields and (b) deuteroethane yields as a function of conversion observed in the reaction of ethylene with deuterium over a nickel wire at 90°C [91],...

A systemmatic study of the reaction of ethylene with deuterium over alumina-supported Group VIII metals has been reported by Bond et al. [103—105], Table 7 shows a selection of typical results together with theoretical distributions calculated as discussed in Sect. 3.4. 

The reactions of the n -butenes with deuterium have been studied over alumina-supported platinum and iridium [103] and palladium [124]. In general, the results obtained are similar to those discussed above for ethylene—deuterium and propene—deuterium reactions. A comparison of the deuteroalkane distributions over platinum is shown in Fig. 17. 

Fig. 17. Deuteroalkane distributions observed in the reactions of deuterium with ethylene at 54°C (0)> propene at 73°C ( ) and but-l-ene at 67°C ( ) over platinum-alumina [103],...

The interaction of an alkyne or alkadiene with deuterium leads to the formation of deuteroalkenes whose isotopic composition yields valuable information regarding possible reaction mechanisms. In an attempt to interpret in detail the deuteroalkene distributions, two approaches have been used. The first, due to Bond [163], is a simplified version of the general theory proposed by Kemball for the hydrogenation of ethylene (see Sect. 3.4) and has been used to interpret the results of the reaction of acetylene with deuterium [163—165]. The method comprises a steady state analysis of the reaction scheme... 

In principle both mfcnnolecular reactions between 8.3 and external water, and intramolecular reactions between coordinated ethylene and coordinated water could lead to the formation of 8.4. On the basis of experiments with deuterium-labeled ethylene followed by stereochemical analysis of the product, it appears that the /nfmnolccular reaction with solvent water molecule is the pathway that is followed. This is shown in Fig. 8.3. Note, however, that the D-labeling results are valid only for a specific set of conditions. These conditions are not the same as in the industrial process. Intramolecular attack by... 

Deuterium labelling combined with deuterium decoupling is well known as a method in spectral analysis and in stereochemical and conformational studies. The deuterium-decoupled proton spectra of XCHD.CHDY compounds produced during mechanistic studies are useful for determining the stereochemical course of reactions such as the methoxymercuration of ethylene, (478) the conversion of tyramine into tyrosol, (479) the non-catalytic addition of deuterium molecules to cyclopentadiene, (480) and alkyl transfer and olefin elimination in 2-phenyl-l,2-dideuterioethyl transition metal compounds. (481) The proton spectra of [ H ]- and [ Hg]-t-butylcyclohexanes were... 

The probability of reductive adsorption of hydrogen at relatively low temperatures is sufficiently problematic that it would be desirable to reinvestigate reactions between deuterium and ethylene on chromias of differing temperatures of activation and with a mass spectrometer of higher sensitivity than that employed in the earlier work. For the moment, the exact mechanism of formation of alkane-do in hydrogenation of olefins with deuterium on chromias activated at lower temperatures must remain uncertain. 

A procedure of great utility in unraveling reaction mechanisms is that presented by Kemball 42). This has the advantage that the minimum of mechanistic detail is imposed on the model, but it suffers from the disadvantage that it is only applicable at the moment to the reaction of ethylene with deuterium, for reasons stated below. The assumptions of the model are that there exist adsorbed C2X4 and CjXg species which react with the following probabilities ... 

The reaction of ethylene with deuterium has been studied using nickel wire 30, 43-45), nickel-kieselguhr 27, 32, 46) and evaporated nickel films 42, 47) as catalysts. The use of differing temperatures and differing partial pressure ratios makes a comparison between them difficult. 

Fio. 5. The reaction of ethylono with deuterium over nickel wire at 90° pressures of deuterated ethylenes as a function of conversion (4S). 

Intermediate and Final Deuteroethane Distributions from the Reaction of Ethylene with Deuterium over Nickel Catalysts... 

A remarkable effect was observed when sintered iron films were used. The multiple exchange process disappears and a normal distribution is obtained (see Pig. 7b). The use of sintered films also enabled a course of reaction to be followed (cyclohexene at 0°, cyclopentene at —35°) stepwise olefin exchange was observed, slightly more marked with cyclopentene than with cyclohexene, and the results bear a marked resemblance to those shown for the reactions of ethylene and of 1-butene with deuterium over nickel in Figs. 5 and 6. Sintering also removes the ability of iron films to catalyze the disproportionation of cyclohexene to cyclohexene and benzene, and for this reason it was... 

Table XIX presents a selection of the results obtained in a study of the reaction of ethylene with deuterium over rhodium-alumina (31), together with some calculated distributions obtained by the method previously employed. The proportion of deuterated ethylenes in the initial products rises from 30% at —18° to 75% at 110°. In contrast to the behavior of palladium, ethane-dj is the major ethane throughout and hydrogen exchange is significant at all but the lowest temperature studied. The parameters used in the calculations attribute the greatest effect of temperature to the variation of the chance of ethylene desorption, which rises from 25% at —18° to 62% at 110°. The effect of temperature on the chance of alkyl reversal is relatively small. Another resjject in which the reaction over rhodium differs from that over palladium is that the chance of acquisition of deuterium in the hydrogenation steps is higher, and indeed it appears that, as with iridium, molecular deuterium may be substantially responsible for the conversion of ethyl radicals to ethane. E — E, is 3 kcal mole and E, — E, is 4.5 kcal mole. The reaction is first-order in hydrogen and zero in ethylene.

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