Locant
Encyclopedia
In organic chemistry
, a locant is a figure to indicate the position of a functional group
within a molecule
.
s 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.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, a locant is a figure to indicate the position of a functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
within a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
.
Example
There are at least two isomerIsomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
s of the linear form of pentanone
Pentanone
Pentanone may refer to any of three organic compounds containing five carbons and a ketone functional group.There are two isomeric linear pentanones:*2-Pentanone*3-PentanoneThere is one cyclic pentanone:*Cyclopentanone...
, a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
that contains a chain of exactly five carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms. There is an oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atom bonded to one of the middle three carbons (if it were bonded to an end carbon, the molecule would be an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
, 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
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
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
2-Pentanone
Pentan-2-one or 2-pentanone or methyl propyl ketone is a colorless liquid ketone with an odor resembling that of acetone. Its formula is C5H10O. It is sometimes used in very small amounts as a flavoring food additive. Two other ketones, 3-pentanone and methyl isopropyl ketone are isomers of...
or pentan-3-one
3-Pentanone
3-Pentanone is a simple, symmetrical dialkyl ketone. It is a colorless liquid ketone with an odor like that of acetone. It is soluble in about 25 parts water, but miscible with organic solvents. It is mainly used as a solvent in paint and a precursor to vitamin E...
, depending on the position of the oxygen atom.