| Revision as of 08:42, 22 January 2012 edit86.1.171.218 (talk) →Organic chemistry← Previous edit | Revision as of 08:58, 3 March 2012 edit undo82.132.236.168 (talk) Dimer explanationNext edit → | ||
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| ]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 '''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-). | ||
| == Organic chemistry == | == Organic chemistry == | ||
Revision as of 08:58, 3 March 2012
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A dimer is a chemical entity consisting of two structurally similar monomers 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-).
Organic chemistry
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
- "IUPAC "Gold Book" definition". Retrieved 2009-04-30.