Aromatic and heterocyclic double bonds

Zelinsky118 found that the three isomeric methylcyclohexenes are dehydrogenated to toluene when passed over a palladium catalyst at the relatively low temperature of 116-118°, with simultaneous formation of methylcyclo-hexane. This phenomenon, that unsaturated compounds give aromatic and perhydrogenated systems at relatively low temperatures, i.e., disproportionation, is often observed during catalytic dehydrogenation, so that the theoretical 

Considerably more energetic conditions are required for dehydrogenatioa of perhydrogenated compounds. Thus, whereas tetralin is dehydrogenated in the liquid phase in the presence of platinum or palladium catalysts at 185°, a temperature of at least 300° is necessary for dehydrogenation of decalin.119 

Dehydrogenation, like hydrogenation, is temperature-dependent at any given temperature there exists an equilibrium and this is shifted by a decrease in temperature in favor of hydrogenation and by an increase in favor of dehydrogenation. Thus hydrogenation catalysts will often also effect dehydrogenation. 

The catalysts most often used for dehydrogenation are those from platinum or palladium.120 Nickel catalysts are less useful for dehydrogenation because carbonization may readily occur at the necessary high temperatures. Osmium can also be used as dehydrogenation catalyst in some cases.121 

High-boiling substances can be dehydrogenated by simple heating with the catalyst in the liquid phase. More volatile compounds are dehydrogenated in a sealed tube or are passed as vapor over the catalyst.119 

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