What causes catalytic hydrogenation

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If alkenes are fumigated with hydrogen, no reaction occurs. In the presence of a suitable catalyst, however, the alkene is hydrogenated.

Influence of the catalyst
An alkene is only reduced to an alkane with the help of a catalyst using hydrogen.

Usually heterogeneous catalysts such as or on activated carbon are used, but there are also homogeneous (soluble) catalysts, e.g. the so-called Wilkinson catalyst, a tris-triphenylphosphine-rhodium chloride. A longer learning unit dealing with hydrogenation with soluble metal catalysts can be found here. If this reaction is carried out with a cyclic alkene, it can be seen that the hydrogen transfer occurs with a syn arrangement, i.e. both atoms are transferred from the same side:

The mechanism of the catalytic hydrogenation is not exactly known, but obviously both the hydrogen (or here deuterium) and the alkene are bound to the surface of the catalyst, which on the one hand activates the π bond of the alkene and on the other hand the σ bond of the hydrogen becomes. Only after the -atoms (D-atoms) have been transferred to the alkene does the product (the alkane) diffuse from the catalyst again.

Not all double bonds are equally well hydrated. More highly substituted alkenes show a slower reaction rate. This is due on the one hand to the steric hindrance and on the other hand to the better stabilization of the π bond. One double bond can therefore often be selectively hydrogenated in the presence of another. There are a large number of methods for hydrogenating multiple bonds, an overview can be found here.

Hydrogenation of cinnamic acid

The hydrogenation of cinnamic acid with measurement of the hydrogen consumption on a laboratory scale is demonstrated.

Energy estimation

By considering the binding and dissociation energies, energetic estimates of the hydrogenation can be made.

Tab. 1
Energy estimate for the addition reaction
Energy for breaking bondsEnergy for new bondsTotal energy difference
Alkene (π bond) and (σ bond)Two new C-H bondsBond formation - bond breaks
276 and 435 -837 -837+(276+435) = -126

This results in the following values, for example:

Tab. 2
Calculated heats of hydrogenation