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An addition reaction joins reacting molecular entities while reducing bond multiplicity and retaining their atoms in the combined product. For example, adding hydrogen across an alkene's carbon–carbon double chemical bond produces a saturated product. The reverse transformation is an elimination reaction.^[ref]

The net transformation does not uniquely specify the mechanism. Electrophilic, radical and concerted routes can give addition products with different rate laws and stereochemistry. Tracking both the atoms and the changing bond order separates an addition reaction from a replacement or rearrangement.

An organic reaction transforms organic compounds by changing their bonds, charge, or stereochemistry. Useful families include substitution reactions, addition reactions, elimination reactions, and rearrangements.

A reaction mechanism follows electron movement and intermediates rather than only comparing starting and final formulas. For instance, replacing a leaving group by a nucleophile can occur by a concerted pathway or through a carbocation intermediate; the two pathways predict different rate laws and stereochemical outcomes. Reaction classes organize transformations, while mechanisms explain how particular conditions produce them.