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Advanced A Level Organic Chemistry: Reaction mechanisms - hydrogenation of alkenes

Organic chemistry Part 10: Reactivity and reaction mechanisms

10.3.8 Hydrogenation of alkenes - catalysed addition of hydrogen

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10.3 Reaction mechanisms of ALKENES

10.3.8 Hydrogenation of alkenes

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  • A HETEROGENEOUS CATALYST IS IN A DIFFERENT PHASE (often solid state') THAN THE REACTANTS (often gaseous or liquid/solution)

  • The catalyst and reactants are in different phases (usually solid catalyst and liquid/gaseous reactant)

  • The reaction occurs on the catalyst surface which may be the transition metal or one of its compounds, examples quoted above. The reactants must be adsorbed onto the catalyst surface at the 'active sites'.

  • This can be physical adsorbed or 'weakly' chemically bonded to the catalyst surface. Either way, it has the effect of concentrating the reactants close to each other and weakening the original intra-molecular bonds within the reactant molecules and so allows a greater chance of 'fruitful' collision.

    • The diagram above illustrates the catalytic conversion of an unsaturated alkene to a saturated alkane

      1. The mixture of hydrogen and alkene gas will both collide with the catalyst surface.

      2. In the manufacture of margarine, the hydrogen gas is bubbled into the vegetable oil mixed with a special nickel based catalyst.

      3. Some of the molecules of both reactants will be physical adsorbed onto the nickel catalyst surface and held there by intermolecular forces.

      4. If the adsorption, via the intermolecular forces, is strong enough, the hydrogen-hydrogen bonds and pi electron bond of the alkene are weakened sufficiently to form weak hydrogen-nickel and alkene-nickel bonds.

      5. If too strongly held, i.e. chemisorption, the reaction will halt as the catalyst surface becomes 'poisoned' with reactant molecules - the active sites on the metal surface are blocked with reactant or product molecules.

      6. The hydrogen molecule can then be split and the resulting hydrogen atoms can then bond with the carbon atoms of the 'opened' double bond of the alkene.

      7. The alkene molecule is temporarily held by the weak intermolecular forces but will then break away from the catalyst surface to be replaced by more reactant molecules.

  • Nickel, Ni(s), catalyses the addition of hydrogen to an alkene double bond, e.g. in the hydrogenation of unsaturated vegetable oils to make more saturated margarine with a slightly higher softening point making it more spreadable.

    • The addition reaction is:

    • R-CH=CH-R + H2 == Ni catalyst ==> R-CH2-CH2-R

    • e.g. ethene is hydrogenated to ethane when it is mixed with hydrogen and passed over a nickel catalyst at 150oC or over a platinum catalyst at room temperature.

    • R is the 'rest' of the molecule in this catalytic hydrogenation reaction in organic chemistry.

    • This is the reaction to 'harden' of polyunsaturated vegetable oils to margarine.

    • The hydrogenated oils become less unsaturated giving them a higher softening point and a more 'spreadable' product for the consumer.

    • See also hydrogenation of unsaturated vegetable oils.

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