Cis-But-2-ene

Where we have reason to suspect the involvement of a particular species as a labile intermediate in the course of a reaction, it may be possible to confirm our suspicions by introducing into the reaction mixture, with malice aforethought, a reactive species which we should expect our postulated intermediate to react with particularly readily. It may then be possible to divert the labile intermediate from the main reaction pathway—to trap it—and to isolate a stable species into which it has been unequivocally incorporated. Thus in the hydrolysis of trichloromethane with strong bases cf. p. 46), the highly electron-deficient dichlorocarbene, CClj, which has been suggested as a labile intermediate (p. 267), was trapped by introducing into the reaction mixture the electron-rich species cis but-2-ene (11), and then isolating the resultant stable cyclopropane derivative (12), whose formation can hardly be accounted for in any other way ... 

Fig. 14. Distribution of the n-butenes as a function of extent of hydrogenation of but-l-ene over palladium—alumina at 37°C [124]. o, But-l-ene , trans-but-2-ene , cis-but-2-ene. Dotted lines indicate thermodynamic equilibrium yields.

The second method, derived by Hamilton and Burwell [127] for the hydroisomerisation of cis-but-2-ene over palladium—alumina, requires the assumption that the isomerisation and hydrogenation reactions have identical kinetic form and that the rates of hydrogenation of the butenes are identical. It was shown that, under these conditions... 

With palladium—alumina, the products of the reaction of but-l-yne with deuterium [189] were but-l-ene, 99.1% frans-but-2-ene, 0.2% cis-but-2-ene, 0.2% n-butane, 0.5%, until at least 75% of the but-l-yne had reacted. But-l-ene hydrogenation and hydroisomerisation were observed to occur when all the but-l-yne had reacted. The formation of but-2-ene as an initial product was postulated as being the result of a slow isomerisation of but-l-yne to absorbed buta-1 2-diene... 

Metal Temp. (°C) Initial PH2/pC2H2 cis-But -2-ene frans-But -2-ene But-l-ene Selec- tivity... 

The hydrogenation of buta-1 2-diene appears to have received relatively little attention. Over palladium—alumina at room temperature, the products of the gas phase hydrogenation were c/s-but-2-ene, 52% but-l-ene, 40% frans-but-2-ene, 7% and n-butane, 1% [189]. Some isomerisation of the buta-1 2-diene to but-2-yne (10%) together with traces of but-l-yne and buta-1 3-diene was also observed. A similar butene distribution (namely, cis-but-2-ene 52%, but-l-ene 45% and frans-but-2-ene 3%) was observed in the liquid phase hydrogenation over palladium [186]. 

In the nickel- and cobalt-catalysed reactions [166,207] it was observed that the butene distribution depended upon the temperature of reduction of the catalyst. For both powders and alumina-supported catalysts prepared by reduction of the oxides, reduction at temperatures below ca. 330° C gave catalysts which exhibited so-called Type A behaviour where but-2-ene was the major product and the frans-but-2-ene/cis-but-2-ene ratio was around unity. Reduction above 360° C (Ni) or 440° C (Co) yielded catalysts which gave frans-but-2-ene as the major product (Type B behaviour). It is of interest to note that the yield of cis-but-2-ene was not significantly dependent upon the catalyst reduction temperature with either metal. 

The observed deuterobutene distributions together with the calculated AAprofiles, for those metals where unique A/-profiles could be obtained, the surface D/H ratio and the calculated deuterobutene distributions are shown in Table 29. One of the major features of these results is that, over all the metals studied, the trans- and cis-but-2-ene profiles show pronounced maxima at -N2 This clearly shows that the predominant route to the formation of but-2-ene was direct 1 4-addition of two hydrogen atoms to adsorbed buta-1 3-diene. 1 2-Addition of hydrogen to yield but-l-ene-A/j followed by isomerisation would have led to a zero value for but-2-ene-A/j and a maximum at but-2-ene-AA3 or higher depending upon the number of butene—butyl interconversions before desorption of the but-2-ene. The detailed interpretation of the A/-profiles has been discussed fully by Wells and co-workers [166,167] who have proposed the two mechanisms shown in Fig. 37. 

Mechanism A is a generalised mechanism which was proposed for those metals where the frans-but-2-ene cis-but-2-ene ratio was around unity. This mechanism contains a variety of reversible steps which permit the conformational interconversion of the diadsorbed buta-1 3-diene. Consequently, the trans cis ratio will depend upon the relative rates of these reversible steps and the ratio may be much lower than would be expected if the relative surface concentrations of anti- and syn-diadsorbed buta-1 3-diene, species I and III, respectively, in Fig. 37, were similar to the relative amounts of anti- and syn-buta-1 3-diene in the gas phase. It was also suggested that the relative importance of the various steps in mechanism A may be different for different metals. Thus, for example, the type A behaviour of nickel and cobalt catalysts, as deduced from the butene distributions and a detailed examination of the butene AAprofiles [166], was... 

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