Hydrogenation of Alkenes

Hydrogenation of Alkenes

Alkenes do not react with hydrogen unless the reaction is catalyzed. However, in the presence of a suitable catalyst occurs.

Fig.1
Hydrogenation catalysts

Heterogeneous catalysts such as Pd or Pt on activated carbon normally are used for hydrogenation, but homogeneous catalysts are also known, e.g. the Wilkinson´s catalyst chlorotris(triphenylphosphine)rhodium(I), RhCl(Ph3P)3. For more detailed information about the heterogeneous hydrogenation of alkenes with metal catalysts click here. Hydrogenation of cycloalkenes in the presence of heterogeneous catalysts selectively yields the syn product, i.e. the hydrogen atoms are added to the same side of the ring (Fig. 2 shows deuteration).

Fig.2
Syn selectivity of deuteration

Details of the mechanism of catalytic hydrogenation are not known. Probably, both hydrogen and alkene are bound to the surface of the catalyst activating the π bond of the alkene as well as the σ bond of the hydrogen. The alkane product only leaves the surface of the catalyst after the transfer of hydrogen (deuterium) atoms to the alkene is completed.

Fig.3
Mechanism of catalytic hydrogenation

The reaction rate of hydrogenation depends on the degree of substitution at the double bond. Higher-substituted alkenes show a lower reaction rate than less substituted. This fact can be explained by larger steric hindrance as well as better stabilization of the π bond in higher-substituted alkenes. Therefore, selective hydrogenation of differently substituted double bonds is often possible. For an overview of alkene hydrogenation click here.

Hydrogenation of cinnamic acid

The movie shows hydrogenation of cinnamic acid on a laboratory scale.

Fig.4
Fig.5

Estimation of reaction energy

The reaction energy of hydrogenation can be estimated from binding and dissociation energies.

Tab.1
Estimation of hydrogenation reaction energy
Dissociation energiesBinding energiesTotal energy difference
Alkene (π bond) and $H2$ (σ bond)Two new carbon-hydrogen bondsBinding energies - dissociation energies
276 $kJmol-1$ and 435 $kJmol-1$ -837 $kJmol-1$ -837+(276+435) = -126 $kJmol-1$

According to the estimation, the following values are obtained for some alkenes.

Tab.2
Calculated heats of hydrogenation
Ethylene Propylene $CH2=CH2$ 136.9 $kJmol-1$ $CH3CH2=CH2$ 125.6 $kJmol-1$ $CH3CH=CHCH3$ 115.6 $kJmol-1$ $CH3CH=CHCH3$ 119.7 $kJmol-1$

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