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Dimerization

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Dimers of carboxylic acids are often found in vapour phase.

A dimer is a chemical entity consisting of two structurally similar monomers joined by bonds that can be either strong or weak, covalent or intermolecular. The term homodimer is used when the two molecules are identical (e.g. A-A) and heterodimer when they are not (e.g. A-B). The reverse of dimerisation is often called dissociation.

Noncovalent dimers

Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers, e.g. the water dimer.

Borane ("BH3") occurs as the dimer diborane (B2H6), due to the high Lewis acidity of the boron center.

Covalent dimers

The dimer of cyclopentadiene although this might not be readily apparent on initial inspection

Molecular dimers are often formed by the reaction of two identical compounds e.g.: 2A → A-A. In this example, monomer "A" is said to dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene:

2 C(NR2)2 → (R2N)2C=C(NR2)2

Carbenes are highly reactive and readily form bonds.

Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers:

C10H12 → 2 C5H6

Many nonmetallic elements occur as dimers: hydrogen, nitrogen, oxygen, the halogens, i.e. fluorine, chlorine, bromine and iodine. Mercury occurs as a mercury(I) cation (Hg2), formally heterodimer.

See also

References

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