Hydrogenation

Hydrogenation is the addition of hydrogen across the carbon–carbon double bond of an alkene, using a nickel catalyst, to give the corresponding alkane. It is a particular addition reaction. Malay: penghidrogenan · Chinese: 加氢反应.

The hydrogen molecule adds across the C=C, opening the double bond into a single bond, so the unsaturated alkene becomes a saturated alkane. The reaction needs a nickel catalyst and a suitable temperature; the nickel speeds the reaction up but is not itself changed, so it is recovered at the end. In industry the same reaction is used to harden vegetable oils: an unsaturated oil, which is liquid, is hydrogenated so that its double bonds become single bonds, turning it into a saturated fat that is solid at room temperature, as in making margarine. Because a double bond is removed, a hydrogenated product no longer decolourises bromine water, which is a neat way to show that the addition has taken place.

Example. Ethene plus hydrogen over a heated nickel catalyst gives ethane: C₂H₄ + H₂ → C₂H₆. The reactive alkene becomes the unreactive alkane once the double bond has gone, so the product no longer takes part in the addition reactions typical of alkenes.

Confusion to avoid. Do not confuse hydrogenation with hydration. Hydrogenation adds hydrogen, H₂, over a nickel catalyst and makes an alkane; hydration adds water, H₂O, in the form of steam over a catalyst and makes an alcohol. The names look alike but the added molecule and the product are different.

Hydrogenation shows the alkene addition idea at work and links it to the everyday chemistry of fats and oils, a real-life application the marking scheme rewards in the Carbon Compounds questions of SPM Chemistry.