open-chain compound
{{Short description|Type of organic molecule with a linear structure}}
In chemistry, an open-chain compound (or open chain compound) or acyclic compound (Greek prefix α 'without' and κύκλος 'cycle') is a compound with a linear structure, rather than a cyclic one.{{GoldBookRef|file=P04724|title=polycyclic system}}
An open-chain compound having no side groups is called a straight-chain compound (also spelled as straight chain compound).{{cite journal|last1=Coles|first1=Lydia|title=A chromatographic method for the separation of branched-chain and straight-chain compounds of columns containing urea|journal=Journal of Chromatography A|volume=32|pages=657–661|year=1968|issue=4|doi=10.1016/S0021-9673(01)80544-6|pmid=5645558}}{{GoldBookRef|title=silazanes|file=S05669}} Many of the simple molecules of organic chemistry, such as the alkanes and alkenes, have both linear and ring isomers, that is, both acyclic and cyclic. For those with 4 or more carbons, the linear forms can have straight-chain or branched-chain isomers. The lowercase prefix n- denotes the straight-chain isomer; for example, n-butane is straight-chain butane, whereas i-butane is isobutane. Cycloalkanes are isomers of alkenes, not of alkanes, because the ring's closure involves a C-C bond. Having no rings (aromatic or otherwise), all open-chain compounds are aliphatic.
Typically in biochemistry, some isomers are more prevalent than others. For example, in living organisms, the open-chain isomer of glucose usually exists only transiently, in small amounts; D-glucose is the usual isomer; and L-glucose is rare.
Straight-chain molecules are often not literally straight, in the sense that their bond angles are often not 180°, but the name reflects that they are schematically straight. For example, the straight-chain alkanes are wavy or "puckered", as the models below show.
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| branched-chain | straight-chain | cyclic |
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| colspan=2 | open-chain |
File:HexosesHemiacetal.png and mannose. The lower row shows the cyclic forms.]]
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