Ethene, hydrogenation activation energy

In the absence of reactant hydrogen, these two steps describe self-hydro-genation. To the best of our knowledge, only one kinetic parameter has been reported in the literature for the overall rate expression of ethene self-hydrogenation (381). This is the activation energy, which has been estimated from TPD data to be 18 kcal mol-1. 

The reaction of OH radicals with alkenes differs from that with alkanes in that OH has a choice of attaching to the C=C double bond versus abstracting a hydrogen atom. Measurements of the temperature dependence of rate coefficients in addition to the observed product distributions have provided ample evidence for the preponderance of OH addition, at least for the smaller alkenes and alkadienes. The preference for addition is due to the fact that hydrogen abstraction reactions require an activation energy in contrast to the process of OH addition. Atkinson et al. (1979) have reviewed reactions of OH radicals with a number of alkenes. The large rate coefficients associated with addition reactions leave little doubt that longer side chains are necessary before H-atom abstraction can become competitive. An exception may be the more weakly bonded allylic H atom adjacent to the C=C double bond, as Atkinson et al. (1977) have pointed out. The present discussion will concentrate on ethene and propene as suitable examples for the oxidation of alkenes. 

Fig. 7. Effects of rare-earth addition on activation energies for the hydrogenation of ethene.

Perhaps the simplest conceivable route to diaminocarbenes is the thermolyis of dimers (derivatives of tetrakis(dialkylamino)ethene, 12 in Scheme 6). This reaction, and the controversy surrounding it, will be discussed in section 2.4, but no examples of successful preparation of diaminocarbenes by this process are known. However diaminocarbenes, and especially imidazol-2-ylidenes, are relatively stable thermally and so pyrolytic methods might be an attractive way to generate these carbenes tree of metal ion complexation. The 1,2-shift to give an amidine is certainly exothermic, but has been predicted to have an activation energy of around 170 kJ/mol even for hydrogen migration, and one example of a 1,2-silyl shift has been shown to be intermolecular. [38]... 

Activation energies are very frequently between 30 and 50 kJ mol and for different metals in comparable physical form it is sometimes said that the same value suffices for all. Orders in hydrogen are always positive and very often are unity, while orders in ethene are usually either zero or sometimes negative. So general is this behaviour that attention is naturally drawn to the few exceptions. With platinum, orders of reaction are essentially independent of the physical form and the support, but are temperature-dependent (Table 7.4), the order in... 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

Finally, as concerns Pt(lll), the reaction of co-adsorbed ethene-d4 and hydrogen has been studied using a combination of laser-induced thermal desorption, mass-spectrometry and RAIRS exchange occurred above 215 K with an activation energy of 46 kJ mol this being below the point at which conversion to... 

Ag/Si02 and Ag/Ti02 after activation by oxidation and reduction were active for ethyne hydrogenation at 353-443 K (Table 9.1) both showed 100% selectivity to ethene and no oligomer formation at the lower temperatures, but selectivity fell and more oligomers were made as temperature increased. Rates were slower than for butadiene the activation energy on Ag/Si02 was 39 kJ mol". ... 

For example, alkyl carbenes readily rearrange by a 1,2-hydrogen shift to produce alkenes (Scheme 5.64). Evanseck and Houk determined from ab initio calculations that the activation energy for the rearrangement of methylcarbene to ethene was O.Gkcal mol". ... 

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