Hydrogenation of Higher Alkadienes

Extension of the linear carbon chain to five or six atoms opens up additional possible reaction paths. Higher 1,3-alkadienes are expected to behave in a similar fashion to 1,3-butadiene, except the two double bonds may differ in reactivity under the influence of the alkyl substituent, and their reduction naturally affords distinguishable products. Thus for example 1,3-hexadiene can give 1- and 3-hexenes by respectively 3,4- and 1,2-addition as well as 2-hexenes by 1,4-addition or isomerisation. A new feature in the higher 1,3- alkadienes is the existence of geometrical isomerism Z- and -l,3-pentadienes have been examined separately.  

Notes Product selectivities are tiiose found at low conversions, but change with conversion is usually small. Cobalt and nickel catalysts are designated Type A or Type B, d ending on reduction temperature the latter are likely to be sulfur-contaminated. 

The four possible orientations of the two reactants are shown, and they are then represented as the possible n ia structures (714 structures are only possible in the syn conformations). We then assume that in forming the alkene the a C—M bond is first broken and the second hydrogen atom is added at one or other end of the delocalised bond. Assuming values for the relative stabilities of syn and anti conformers, and 

The seleclivilies at the bottom are obtained Iqr (i) breaking the o C-M bond by H atom addiihst, (ii) adding a second II atom with equal probability to either end of tiK bond. 

The reactions of 1,3- and 1,5-hexadienes with hydrogen have been carried out with Pd/Al203 catalysts of various dispersions the reaction was not structure-sensitive nor were product distributions affected. With the 1,3-isomer, selectiv-ities were very high until at least 95% conversion, and product selectivities were not greatly conversion-dependant. Representative values are shown in Table 8.11. The ZjE ratio of the reactant was not specified, so that model calculations cannot be made with certainty, but the predominance of Z-isomers in the product argues for the preference for anft -conformations in the adsorbed states. Pd5.2Sno.8/Al203 

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