Which type of reaction involves the removal of functional groups from a reactant?

Elimination reactions are characterized by the removal of functional groups from a reactant, leading to the formation of a double bond or a ring structure in the product. In these reactions, two atoms or groups are eliminated from the molecule, typically resulting in the production of a small molecule such as water or hydrogen halide. This contrasts with substitution reactions, which involve the replacement of one functional group with another, rather than removal. Rearrangement reactions involve the reorganization of the molecular structure without the addition or removal of atoms, and hydrogenation specifically refers to the addition of hydrogen to unsaturated compounds. Thus, elimination reactions are distinct in their mechanism of removing functional groups to create unsaturation or to form new structures.