A locant can be used in organic chemistry to indicate the position of a functional group within a molecule [See [http://cancerweb.ncl.ac.uk/cgi-bin/omd?query=locant&action=Search+OMD the definition at CancerWEB's online medical dictionary] ."] . For example, there are at least two isomers of the linear form of pentanone, a ketone that contains a chain of exactly five carbon atoms. There is an oxygen atom bonded to one of the middle three carbons (if it were bonded to an end carbon, the molecule would be an aldehyde, not a ketone), but where?

First number the carbons from one to five, starting at one end and proceeding sequentially along the chain. Now the position of the oxygen atom can be defined as on carbon atom number two, three or four. In fact atoms two and four are exactly equivalent - imagine turning the molecule around by 180 degrees, and you will see that this is the case. It just depends on which end you choose to start numbering the carbons.

The locant is the number of the carbon atom to which the oxygen atom is bonded. If the oxygen is bonded to the middle carbon, the locant is 3. If the oxygen is bonded to an atom on either side (adjacent to an end carbon), the locant is 2 or 4; given the choice here, where the carbons are exactly equivalent, always choose the lower number. So the locant is either 2 or 3 in this molecule.

The locant is incorporated into the name of the molecule to remove ambiguity. Thus the molecule is named either pentan-2-one or pentan-3-one, depending on the position of the oxygen atom.

See also IUPAC nomenclature.


Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • locant — noun That part of the name of a compound (often a letter or number) that describes the position of an atom, residue or functional group e.g. the 2 in hexan 2 one …   Wiktionary

  • locant — A number or letter preceding a substituent name in the name of a complex chemical that specifies the position (location) of the substituent on the parent molecule; e.g., 5 in 5 methyluridine, S in S adenosylmethionine …   Medical dictionary

  • locant — lo·cant …   English syllables

  • locant — ˈlōkənt noun ( s) Etymology: location + ant : the portion of a chemical name that designates the position of an atom or group in a molecule (as β in β naphthylamine, m in m xylene, 2 in 2 butanol, or 1 in glucose 1 C14) …   Useful english dictionary

  • hydrocarbon — hydrocarbonaceous, adj. /huy dreuh kahr beuhn, huy dreuh kahr /, n. any of a class of compounds containing only hydrogen and carbon, as an alkane, methane, CH4, an alkene, ethylene, C2H4, an alkyne, acetylene, C2H2, or an aromatic compound,… …   Universalium

  • IUPAC nomenclature of organic chemistry — The IUPAC nomenclature of organic chemistry is a systematic method of naming organic chemical compounds as recommended[1] by the International Union of Pure and Applied Chemistry (IUPAC). Ideally, every possible organic compound should have a… …   Wikipedia

  • nor- — 1. Chemical prefix denoting 1) elimination of one methylene group from a chain, the highest permissible locant being used; 2) contraction of a (steroid) ring by one CH2 unit, the locant being the capital letter identifying the …   Medical dictionary

  • Delta (letter) — Greek alphabet Αα Alpha Νν Nu Ββ Beta …   Wikipedia

  • M-Xylene — Chembox new Name = m Xylene ImageFile = m Xylene.png ImageName = m Xylene ImageSize = 100px ImageFile1 = m xylene spaceFilling.png ImageSize1 = 150px ImageName1 = m xylene 3D diagram OtherNames = m Xylol 1,3 Dimethylbenzene Section1 = Chembox… …   Wikipedia

  • Nor- — In chemical nomenclature, nor is a prefix to name a structural analog that can be derived from a parent compound by the removal of one carbon atom along with the accompanying hydrogen. The nor compound can be derived by demethylation or by… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”