Reduction (chemistry)

A reduction decreases an oxidation state, recording gain in the formal electron assignment. For ${\mathrm{Cu}}^{2+}+2e^{-}\to \mathrm{Cu}$, the decrease is $+2\to 0$. The reduced chemical species serves as the oxidizing agent in the coupled redox.^[ref]

In the electrochemistry examples here, reduction is gain of electrons by a chemical species; it occurs at the cathode. That electrode definition holds in both a galvanic cell and an electrolytic cell; it does not fix terminal polarity.

Removal of a sulfone substituent by a reducing process, replacing its carbon–sulfur connection with a carbon–hydrogen connection under an appropriate work-up. Compatibility with other reducible groups must be considered.

Reduction at a carbonyl group, for example conversion of a ketone to an alcohol. In an enone, 1,2- and 1,4-reduction produce different functional-group patterns.

Conversion of an aldehyde or ketone carbonyl into a methylene through hydrazone formation and base-promoted nitrogen extrusion. The strongly basic, often heated conditions must be compatible with the other functional groups.

A cerium(III)-assisted borohydride reduction often used to favour 1,2-reduction of an enone to an allylic alcohol. Actual selectivity remains substrate- and condition-dependent.

Addition of hydrogen to an unsaturated substrate. Catalyst and conditions determine which unsaturated groups are reduced; the mere presence of H2 does not guarantee chemoselectivity.

A porous nickel catalyst prepared by selective removal of aluminium from a nickel–aluminium alloy. It can catalyse hydrogenations; its use does not guarantee that every other reducible functional group will remain unchanged.

Reduction using hydrogen and a catalyst. Selectivity depends on the catalyst, pressure, solvent and functional groups: reducing an alkene while retaining a ketone is a distinct requirement from reducing a nitrile while retaining an ester.