Hydrogen is added to alkenes and alkynes by hydrogenation, an addition reaction in which hydrogen adds across a carbon-carbon multiple bond, usually using a nickel, platinum, or palladium catalyst. This is classified as reduction because the carbon atoms gain hydrogen and the degree of unsaturation decreases.
How Hydrogenation Works
An alkene contains one carbon-carbon sigma bond and one pi bond. During hydrogenation, the weaker pi bond is broken, the hydrogen-hydrogen bond is broken, and two new carbon-hydrogen bonds form. The carbon-carbon double bond therefore becomes a single bond.
For example, ethene is reduced to ethane:
The metal catalyst provides a surface on which the reactants are adsorbed. It weakens the hydrogen-hydrogen bond and provides an alternative pathway with lower activation energy; it is not consumed.
Alkynes contain two pi bonds, so complete hydrogenation requires two moles of hydrogen per mole of alkyne:
Partial hydrogenation stops after one mole of hydrogen has added, producing an alkene. Complete hydrogenation adds a second mole of hydrogen and produces an alkane. Therefore, one mole of ethyne consumes two moles of hydrogen during complete reduction.
| Starting compound | Hydrogen required | Product |
|---|---|---|
| Alkene | 1 mol per mol of alkene | Alkane |
| Alkyne, partial reduction | 1 mol per mol of alkyne | Alkene |
| Alkyne, complete reduction | 2 mol per mol of alkyne |
A common misconception is that hydrogen simply attaches without changing the carbon-carbon bond. In fact, each addition of removes one pi bond and lowers the degree of unsaturation.
IB Exam Technique
This is assessed at both SL and HL under Reactivity 3.2.11. When asked to deduce an equation, preserve the carbon skeleton, reduce the multiple bond by the correct amount, and balance the number of hydrogen atoms. If asked why the reaction is reduction, state explicitly that hydrogen is gained and the degree of unsaturation decreases.