Acetylene, deuterated

For a long time" , it has been known that there is rearrangement of 1-alkynes to 2-alkynes under basic conditions (for instance by alcoholic potassium hydroxide or powdered potassium hydroxide) at 175 C. It has been shown that in fact an equilibrium mixture is obtained . It contains isomeric compounds, mainly the starting 1-alkyne and isomeric 1,2-alkadiene and 2-aIkyne, but the latter, more stable from thermodynamic considerations, is predominant. Therefore the method could be used to prepare 2-alkynes from corresponding 1-alkynes. The reverse reaction is possible disubstituted acetylenes can be converted to sodium derivatives of 1-alkynes by sodium or sodamide . 1-Alkynes are recovered by hydrolysis. We have tried to apply both reactions to prepare acetylenes deuterated in defined positions with high isotopic purity. 

The elucidation of consistent mechanisms for acetylene hydrogenation is best achieved by considering first the results for acetylene deuteration, since these reveal information which influences the interpretation of the kinetics. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

It was found that only deuterated furanone was obtained if water had been replaced with deuterated water (D2O). This observation corroborated the fact that water was the hydrogen source in this reaction. Two possible mechanisms were proposed. The first one is described as follows (Scheme 18, path a) acetylene 119 can react with bimetallic species to afford /r, -acetylene complex 122, which reacts further with CO to give /tjTj -furanone... 

However, deuterated labelling experiments performed by Toste et al. in carbocy-clizations of acetylenic dicarbonyl compounds suggest the involvement of nucleophile for enol, making the reaction more straightforward [80-82]. 

An alternative explanation by Schrauzer and collaborators138,139 assumes mononuclear vanadium(II) as a two-electron reductant dinitrogen would be reduced in a first step to N2 . N2 and hydrazine would be obtained by disproportionation of N2H2 (diazene), inside the Mg(OH)2 lattice, which would protect the diazene from rapid decomposition to the elements. On reducing acetylene in deuterated media, the resulting ethylene was exclusively cis.139... 

It is generally agreed that the kinetics and the distributions of deuter-ated products from the reactions of alkynes or alkadienes with deuterium are satisfactorily interpreted in terms of the consecutive addition of two hydrogen atoms, of unspecified origin, to the adsorbed hydrocarbon to yield the monoolefin. The identity of the distributions of deuteroethyl-enes from the reaction of acetylene with equilibrated and non-equil-ibrated hydrogen—deuterium mixtures also provides strong evidence for such a mechanism [91]. 

The microwave spectrum of the complex between ethylene and ketene (and of deuter-ated derivatives) reveals56 a crossed structure (22), while the ketene/acetylene complex shows a planar geometry57. This difference in geometry is explained by the different quadrupole moments of two unsaturated hydrocarbons. 

Although not exhaustive, the above summary of experiments with hydrogen chemisorbed on transition-metals serves to illustrate how neutron vibrational spectroscopy is performed with catalytic substrates and the methods used to analyze the inelastic neutron spectra. In concluding this section we note that the technique can be extended to supported catalysts such as in recent experiments with hydrogen adsorbed on both MoS and alumina supported MoSp (38). Also, as another indication of the variety of systems which can be studied, we note earlier experiments with ethylene (39) and acetylene (40) adsorbed on silver exchanged 13X zeolites. "Tn this work, deuteration of the molecules was helpful in identifying the surface vibratory modes on these ionic substrates of greater complexity. 

Experiments with deuterated species show extensive H/D scrambling, which implies longer-lived intermediates in the reaction. Calculations at the B3LYP/6-311+G(2d,p) level suggest that the reaction pathway involves bond formation between the cycloheptatrienylidene dication and acetylene, followed by isomerization and hydrogen loss (Scheme 9). The authors of this study note the possibility of similar chemistry generating polycyclic aromatic compounds in interstellar space. 

Thus methane is released from the fines mainly as CD4 and CH4 in ratios (CD4 CH4) varying from 2.2 to 12.7 the most common ratio being in the range 3 1 to 5 1. Some is also released as CD3H but is considered to be an acid hydrolysis product and is included with CD4 for quantitative measurement. Other partly deuterated species (CH2D2,CH3D) are released in only trace concentrations. Similarly, ethane is released as both undeuterated and deuterated (and almost fully deuterated) species. Other species (ethylene, acetylene, propane, propene, butane and butene) appear to be mainly deuterocarbons. The Cj to C4 deuterocarbons are thought to arise from acid hydrolysis of carbides in the samples (Table 2). 

Hydrocarbon molecules are abundant constituents of planetary atmospheres and major compounds in combustible gas mixtures and in fusion edge plasmas [7-11]. Methane is the simplest of these hydrocarbon molecules. Acetylene, C2H2, is the simplest hydrocarbon molecule that contains 2 carbon atoms. Thus absolute total and partial photon [24-27] and electron [15,28-34] ionization cross-sections and nascent fragment ion energy distributions [19,20,28,36-40] have been studied extensively for these molecules. For the deuterated methane molecule electron impact ionization and dissociative ionization cross-sections were determined for the CD (x=l—4) molecule and radicals applying a fast neutral beam technique [41]. Electron impact total ionization cross-sections have been determined also theoretically applying the BEB (Binary-Encounter-Bethe) model [42], the DM (Deutsch-Mark) method [43] and the JK (Jain-Khare) method [44], Partial electron impact ionization cross-sections were calculated for methane [45,46] as well as total electron impact cross-sections for various CH radicals [47]. The dissocia-... 

It should be indicated that the methylacetylene and propylene " are more complex reactants than the nonsubstituted counterparts and depict nonequivalent hydrogen atoms at the acetylenic and methyl group (methylacetylene) and at the vinyl and methyl group (propylene). Therefore, even the detection of the atomic hydrogen loss makes it difficult to elucidate if the hydrogen atoms are lost from the methyl group, the acetylenic/vinyl units, or from both positions. In these cases, it is very useful to conduct experiments with partially deuterated reactants J3-... 

Here, the phenyl radical once again attacks the unsamrated bond. However, the steric effect and larger cone of acceptance (the methyl group screens the p carbon atom and makes it less accessible to addition) direct the addition process of the radical center of the phenyl radical to the a carbon atoms of methylacetylene and propylene (the carbon atom holding the acetylenic hydrogen atom). Consequently, crossed beam reactions with complex hydrocarbon molecules can be conducted and valuable information on the reaction pathways can be derived if (partially) deuterated reactions are utilized. 

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