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{{refimprove|date=April 2009}} {{refimprove|date=April 2009}}
]s are often found in vapour phase.]] ]s are often found in vapour phase.]]
A '''dimer''' is a ] entity consisting of two structurally similar ]s joined by bonds that can be either strong or weak. A weak bond is shown in the diagram above, as hydrogen bonds form between the C=O bond and the O-H bond. This is possible because both bonds are polar, whereby the H in the O-H bond is slightly positive, and the O in the C=O bond is slightly negative. In the case of two a-amino acids, for example, a dimer can only be synthesised using high temperature and pressure in a closed system. This causes two strong covalent bonds to form between the molecules, forming two amide links (-NH-CO-). A '''dimer''' is a ] entity consisting of two structurally similar ]s joined by bonds that can be either strong or weak.


What you have in the diagram below is two cyclopentene molecules which are joined two each other by two single covalent bonds. Structurally this has resulted in the removal of two hydrogens per covalent bond formed (four in total).


== Organic chemistry == == Organic chemistry ==

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For other uses, see Dimer (disambiguation).
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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.

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 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

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

References

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