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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.