Alkynes and alkadienes

A remarkable and potentially very useful property of gold catalysts is that they are essentially fully selective for alkene formation.20 This is because alkynes and alkadienes are much more strongly adsorbed than the derived alkenes, which are therefore displaced from the surface and unable to return, at least until the first stage of the reaction is complete. The 2-butenes were however found to isomerise after butadiene had completely disappeared.38 

S % selectivity to total butenes Si %1-butene S2Z %Z-2-butene S e - %E-2-butene. a0.02% AU/AI2O3 in multichannel reactor. 

Norbornadiene has been hydrogenated on Au/MgO above 353 K, but no details have been given.33 

There have been two recent studies of the hydrogenation of ethyne on gold catalysts. With 1% Au/TiC 2 made by thermally decomposing the supported complex [Au2(PPh3)6](BF4)2 and having particles of average size 4.6 nm, reaction occurred at 453 K with about 90% selectivity to ethene, 

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Values for hydrocarbons other than alkynes and alkadienes can be predicted by the method of Suzuki et al. The best model includes the descriptors T, P, the parachor, the molecular surface area (which can be approximated by the van der Waals area), and the zero-order connectivity index. Excluding alkynes and alkadienes, a studv for 58 alkanes, aromatics, and cycloalkanes showed an average deviation from experimental values of about 30 K. 

The main products of the irradiation of beef fat at high doses are such hydrocarbons as alkanes, alkenes, alkynes, and alkadienes. Of these, the alkanes and alkenes consitute the most significant part of the volatile products. 

The metal-catalysed hydrogenation of multiply unsaturated hydrocarbons is, of necessity, more complex than that of monoolefins. The problems encountered in alkyne and alkadiene hydrogenation are essentially similar and it is appropriate, therefore, that the two systems are considered together. 

A further problem arises when one considers the hydrogenation of alkynes and alkadienes containing four or more carbon atoms. In such cases, it is possible that the intermediate olefin may be formed in more than one isomeric form. Hence, the stereospecificity of the hydrogenation must be considered and attempts made to explain the observed stereospecificity in terms of the mechanism (see Sect. 4.3). 

As with monoolefins, it is generally considered that, before undergoing reaction, the hydrocarbon is adsorbed at the metal surface. However, compared with monoolefins, the adsorption of alkynes and alkadienes has been little studied. 

The interpretation of the experimentally. observed selectivity and stereoselectivity depends to some extent on the assumed nature of the adsorbed alkyne and alkadiene. 

Figures 3 to 5 show some effects of hydrocarbon structure on maximum burning velocity (30, 32, Jfi, 72). Figure 3 deals with straight-chain alkanes, alkenes, alkynes, alkadienes, and allenes Figure 4 shows the effects of methyl substitution in four-carbon-atom alkane, alkene, alkyne, and alkadiene molecules. In general, the burning velocity decreases with increased chain length or methyl substitution, except for the alkanes,...

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

Alkynes and alkadienes (alkenes with two double bonds) have the general formula C H2n-2- Alkenynes (hydrocarbons with one double bond and one triple bond) and alkatrienes (alkenes with three double bonds) have the general formula Cn 2n-4 and so forth. 

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