In what type of reaction does a nucleophile replace a leaving group?

In a nucleophilic substitution reaction, a nucleophile directly attacks an electrophilic center, typically a carbon atom that is bonded to a leaving group. This process involves the nucleophile replacing the leaving group in a single concerted step (in the case of an SN2 mechanism) or through a two-step process where a carbocation intermediate forms first (in the case of an SN1 mechanism).

The term "nucleophile" describes a species that has a high electron density and is attracted to positive or partially positive centers, enabling it to form bonds by donating an electron pair. The "leaving group" is a portion of the molecule that can depart with its electrons, often being a weaker base than the nucleophile or stabilized by resonance or inductive effects.

In contrast, nucleophilic addition typically refers to reactions where a nucleophile adds to a carbonyl group or multiple bonds, nucleophilic oxidation involves the addition of oxygen or the removal of electrons, and nucleophilic elimination describes a process where bonds are broken to form double or triple bonds, often eliminating small molecules like water or halides. Thus, nucleophilic substitution specifically captures the essence of the question, where a nucleophile replaces a leaving group directly.