Dimer
Definition and meaning of Dimer in chemistry.
A dimer is a chemical species formed when two molecules of the same substance (or two closely related monomers) join together, held by either covalent bonds or noncovalent interactions such as hydrogen bonds or van der Waals forces.
In more detail
Dimerization can be covalent, as when two alkene molecules react to form a ring or chain twice the size, or noncovalent, as when two molecules associate through hydrogen bonding or dipole interactions without forming new sigma bonds. The process is often reversible and depends on concentration, temperature, and phase, since noncovalent dimers can dissociate back into monomers under different conditions. Dimerization matters throughout chemistry: it affects vapor density and boiling point measurements, drives certain polymerization reactions, and produces functional protein dimers in biochemistry.
Key facts
| Field | General Chemistry |
|---|---|
| General formula | (monomer)2 |
| Bonding types | Covalent or noncovalent (H-bonds, van der Waals) |
| Common example | 2 NO2 ⇌ N2O4 (dinitrogen tetroxide) |
In the gas phase and in nonpolar solvents, two acetic acid molecules form a cyclic hydrogen-bonded dimer, (CH3COOH)2, held together by two O–H···O=C hydrogen bonds; this is why acetic acid vapor has an apparent molar mass roughly twice that of a single molecule.
Frequently asked questions
How is a dimer different from a polymer?
A dimer is made of exactly two monomer units, while a polymer consists of many repeating monomer units, often thousands, linked in a chain or network.
Must the two units in a dimer be identical?
No. A homodimer forms from two identical monomers, while a heterodimer forms from two different but chemically related molecules, as seen with many dimeric proteins.