, Volume 16, Issue 2, p 28

The heats of hydrogenation of unsaturated hydrocarbons

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Abstract

Recent precise experimental work on the heats of hydrogenation of hydrocarbon compounds containing one or more double bonds shows a variation from compound to compound which can be correlated with the structure of the molecule. For compounds containing one double bond or several non-conjugated bonds, the heat of hydrogenation per double bond is nearly constant. However, in compounds containing conjugated double bonds the average heat of hydrogenation per bond is less than in non-conjugated systems due to the interaction energy between the double bonds tending to stabilize the system. This effect is most pronounced in aromatice compounds. Although the hydrogenation of benzene to form cyclohexane (addition of three molecules of hydrogen per molecule of benzene) is an exothermic reaction, the hydrogenation of benzene to form cyclohexadiene −1,3 (addition of one hydrogen molecule per molecule of benzen) is an endothermic reaction. This signifies that the energy associated with the aromatic character of benzene is greater than the energy of hydrogenation of a double bond, so that the resultant of these two effects is an endothermic reaction.