Hydrogenation of Multiply Unsaturated Hydrocarbons

The reaction between deuterium and 1-butyne, 2-butyne, 1,2-butadiene, and 1,3-butadiene, respectively, was conducted in a flow system at near ambient temperatures. The catalyst (0.03 wt % palladium on alumina) was prepared by impregnating hard alumina pellets with palladium chloride so that the metal was probably confined to an outer shell of each particle. 

The reactions are highly selective in the sense that little if any of the product butene reacts further as long as any of the more saturated substrate remains. 2-Butyne yields ci -2-butene-2,3- ij almost exclusively  

The first sequence includes one reaction (the last) which requires a greater amount of molecular reorganization than any of the other steps which are needed to explain the major course of the reaction. A perhaps more satisfactory sequence is the following  

The last step is analogous to the reaction which accounts, in part, for multiple exchange in methane (59), e.g.. 

The proportion of products obtained from 1,2-butadiene are 0.53 cis-2-butene, 0.07 Jrarts-2-butene, and 0.40 1-butene. The predominant deuterated species are again those which result from simple 1,2 addition to one or the other double bond. However 1-butene is more extensively exchanged than cis-2-butene. 

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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. 

TABLE 8.1. Heats of Hydrogenation of Multiply-Unsaturated Hydrocarbons (kJ mol ) ... 

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K jK or KiK IK K depending upon whether the principal surface species are the TT-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78), but not if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. 

The choice of catalyst applied in the hydrogenation of a certain unsaturated hydrocarbon depends on several factors, such as the reactivity of the substrate and the experimental conditions (pressure, temperature, solvent, liquid- or gas-phase reaction). Multiply unsaturated compounds may require the use of a selective catalyst attaining the reduction of only one multiple bond. The use of suitable selective catalysts and reaction conditions is also necessary to achieve stereospecific hydrogenations. 

When applying this principle to replacement names generated from fusion nomenclature, it is essential to keep in mind that fusion names for hydrocarbons ending in -cycloalkene are for fully unsaturated skeletons the -ene ending implies whatever number of double bonds may be necessary, without a multiplier. Thus (117) has six double bonds in the twelve-membered ring, and one must add ten hydrogens to saturate it to the stage of a simple benzene derivative, compound (118). 

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