Dimer (chemistry)

For other uses, see Dimer (disambiguation).

Dimers of carboxylic acids are often found in vapour phase.

A dimer is a chemical entity consisting of two structurally similar subunits called monomers joined by bonds that can be either strong or weak.

Organic chemistry

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 Diaminocarbenes, which dimerise to give tetraaminoethylenes:

2 C(NR₂)₂ → (R₂N)₂C=C(NR₂)₂

Acetic acid forms a dimer in the gas phase, the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers, e.g. the water dimer.

Dicyclopentadiene is an unsymmetrical 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

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.

See also

• Monomer
• Trimer
• Polymer

References

• "IUPAC "Gold Book" definition". http://goldbook.iupac.org/D01744.html. Retrieved 2009-04-30.