Illustrated Glossary of Organic Chemistry

Reactant: Any entity whose chemical structure is changed in a chemical reaction or mechanism step. Even though their chemical structure may change (and then be restored) during the course of a reaction or mechanism step, catalysts are usually not considered to be reactants. There is no special notation to indicate that a reaction participant is a reactant: reactants can be written to the left of, above, or below the reaction arrow.

In this catalytic hydrogenation reaction, 1,2-dimethylcyclohexene and molecular hydrogen (H2) are reactants, ethanol (CH3CH2OH) is a solvent, and platinum (Pt) is a catalyst. The product is cis-1,2-dimethylcyclohexane.