Butene-deuterium exchange

A comparable study of the Ni-catalyzed reactions of isobutene, 1-butene, and cis-butene-2 in the presence of H2 and D2 has revealed a very complex series of chemical reactions that take place, including induced isomerization of 1-butene to 2-butene, deuterium exchange, induced cis-trans isomerization of butene-2, and finally, addition to the double bond. Below 200 mm Hg pressure, the rates of exchange, addition, and isomerization are about equal for 1-butene and are about % order in olefin and order in H2. With increasing excess of H2 this approaches zero order in olefin and % order in H2, while for large excess of 1-butene, all reactions become inhibited (30 to 150°C). Although the authors have attempted to discuss mechanisms in connection with the data, the lack of information on the isotherms makes that of dubious value. 

Other systems for which the results cannot be explained by energy transfer include the ketone sensitized isomerization of / -methylstyrene, 1,2-dichloro-ethylene and 2-butene studied by Caldwell,147,148 deuterium exchange between acetone-d8 and tetramethylethylene reported by Japar, Pomerantz, and Abrahamson,149 and the acetone sensitized isomerization of 1-methoxy-l-butene examined by Turro and Wriede.71... 

The amides of alkali and alkaline-earth metals catalyse hydrogen exchange in hydrocarbons even in the absence of liquid ammonia. For example, the heterogeneous deuterium exchange of benzene and 2-methylbutene-l occurs with a considerable velocity on solid KND2 and Ca(ND2)2 at 70°. This gives rise to the isomerization of 2-methyl-butene-1 to 2-methylbutene-2 (Shatenshtein et al., 1958a). 

The explanation given above recalls earlier speculations presented by Ledoux et al. (22, 23). The authors assigned a different role to various surface centers on a palladium film in the reactions of isomerization, hydrogenation, and deuterium exchange of butenes. The authors defined... 

J. Turkevich I was in error in the statement that hydrogen-deuterium exchange reaction is the only reaction that takes place with the help of acid sites, for we have found that butene-1 to butene-2 transformation is catalyzed by both Bronsted and Lewis acid sites. The Bronsted sites, however, give a marked stereospecificity in producing nonequilibrium mixtures of cis and trans, while the Lewis acid sites give an equilibrium mixture of the geometric isomers. 

Alcohol (a) can be used to indicate the extent of syn- and anti-elimination (see Scheme 8) and (b), through a study of H/ H exchange can indicate whether the mechanism is El, E2, or ElcB. Thus for an El mechanism, deuterium exchange can occur in all positions in the butenes and loss of a- H from [2- Hi ]butan-2-ol provides strong evidence of this mechanism. 

The distribution of deuterium-exchanged thiophens over a sulphided C0-M0/AI2O3 catalyst was also consistent with thiophen lying flat on the catalyst surface before hds. The hds activity (and also the hydrogenation activity towards butenes and benzene) correlated with activity for multiple deuterium exchange. 

Recent extensions of this chemistry to include cyclic olefins (30), in fact, makes the intermediacy of acetylenes highly questionable. This mechanism is further doubted in light of deuterium studies carried out in a similar, homogeneous system (31). 2-Butene-ds and 2-butene disproportionation gave C4H4D4 (mass 60) as the only new product, consistent with Eq. (7). The intermediacy of cyclobutadienes would have opened a path to vinylic deuterium exchange which was not observed. 

The low rates of isomerization are not unexpected in view of results given in the last section. Evidence that the mechanisms of olefin exchange and isomerization are closely related is provided by a brief study of the isomerization of the butenes in the presence of deuterium (31). The rates of entry of deuterium into the reactant olefin were small, and in the case of cis-2-butene its deuterium number increased linearly with the fraction which had isomerized. Olefin exchange was, as expected, particularly slow in the case of irans-2-butene. Hydrogen exchange was also slight, especially with 1-butene. 

Evidence concerning the acidity of the carbon atoms of the tetramethylammonium ion has been discussed by Doering and Hoffman 43> and by Cram 35>. Deuterium exchange in the tetramethylammonium ion in deuterated water at 83 °C proceeds with a rate constant of 9.4 X IO-10 sec-1. In t-butyl alcohol-O-d at 50 °C, the rate constant is about 10-7 sec-1. Thus the exchange rate is about 10 2 slower than the exchange rate in 3-phenyl-l-butene, which indicates that the acidity of the methyl protons in the quaternary salt is comparable to triphenylmethane... 

M. Salmerdn and G.A. Somoijai. Desorption, Decomposition, and Deuterium Exchange Reactions of Unsaturated Hydrocarbons (Ethylene, Acetylene, Propylene, and Butenes) on the Pt(lll) Crystal Face. J. Phys. Chem. 86 341 (1982). 

It was found that by treating either n-butane or isobutane with 10 mole % deuterium bromide-aluminum bromide catalyst for 20 hours at 25°, no isomerization of the butanes occurred and only 6 and 9.5% of the deuterium exchanged with n-butane and isobutane, respectively. When, however, 0.1 mole % butenes was added to n-butane and the isomerization reaction was carried out under the same experimental conditions, over 40% of the butane isomerized to isobutane and 92% of the deuterium underwent an exchange reaction. These results indicate clearly that olefins take an active part in isomerization. The results obtained are in agreement with the proposed mechanism of isomerization. 

Copper-nickel alloy films similarly deposited at high substrate temperatures and annealed in either hydrogen or deuterium were used to study the hydrogenation of buta-1,3-diene (119) and the exchange of cyclopentane with deuterium (120). Rates of buta-1,3-diene hydrogenation as a function of alloy composition resemble the pattern for butene-1 hy-... 

Early studies with deuterium-labeled57 58 and l4C-labeled compounds59 60 established that cleavage of the double bonds occurs. For example, the degenerate metathesis of 2-butene and perdeutero-2-butene57 results in the formation of [l,l,l,2-2H4]-2-butene as the only new labeled compound [Eq. (12.11)] which is consistent with alkylidene exchange [Eq. (12.10)] ... 

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