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A Level Organic Chemistry: Reaction mechanisms - hydrogenation of alkenes

Doc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

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

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

      3. If the adsorption, via the intermolecular forces, is strong enough, the hydrogen-hydrogen bonds and pi electron bond of the alkene are weakened to form weak hydrogen-nickel and alkene-nickel bonds. If too strongly held, i.e. chemisorption, the reaction will halt as the catalyst surface becomes 'poisoned' with reactant molecules.

      4. 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.

      5. 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.

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

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

keywords phrases: catalytic hydrogenation reaction nickel catalyst ethene hydrogen conditions formula intermediates organic chemistry reaction mechanisms steps



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