There must be few organic syntheses of any complexity that do not involve a reduction at some stage. Reduction is used in the sense of addition of hydrogen to an unsaturated group (such as a carbon–carbon double bond, a carbonyl group or an aromatic ring) or addition of hydrogen with concomitant fission of a bond between two atoms (such as the reduction of a disulfide to a thiol or of an alkyl halide to a hydrocarbon).

Reductions are generally effected either by catalytic hydrogenation or by a reducing agent (such as lithium aluminium hydride). Complete reduction of an unsaturated compound can generally be achieved without undue difficulty, but the aim is often selective reduction of one group in a molecule in the presence of other unsaturated groups. The method of choice in a particular case will often depend on the selectivity required and on the stereochemistry of the desired product.

Catalytic hydrogenation

Of the many methods available for reduction of organic compounds, catalytic hydrogenation is one of the most convenient. Reduction is carried out easily by simply stirring or shaking the substrate with the catalyst in a suitable solvent (or even without a solvent if the substance being reduced is a liquid) in an atmosphere of hydrogen gas. An apparatus can be used that measures the uptake of hydrogen. At the end of the reaction, the catalyst is filtered off and the product is recovered from the filtrate, often in a high state of purity.