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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 elimination reaction removes groups from a substrate while forming a new multiple bond. Removing a hydrogen and a leaving group from neighboring carbons can produce an alkene.

An E2 elimination combines bond cleavage and bond formation in one elementary step; an E1 elimination passes through an intermediate, usually a carbocation. These alternatives predict different rate laws and stereochemical requirements. In a concerted elimination, orbital alignment can make an antiperiplanar conformation decisive, linking product formation to molecular geometry.

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.

A substitution reaction replaces an atom or group in a molecular entity with another. The replacement can occur through one elementary step or several; the definition specifies the net chemical change rather than a unique mechanism.^[ref]

For example, in $R-X+N{u}^{-}\to R-Nu+X^{-}$, a nucleophile replaces a leaving group. Concerted displacement and a pathway through a carbocation can have different concentration dependences and stereochemical outcomes even when the written products match. The reacting functional group, solvent and competing elimination reactions help determine which pathway is observed.