In organic chemistry nomenclature, the locant is used to show where a functional group is attached, on the parent chain.
Let’s say we have a 5-carbon molecule with a ketone functional group. According to ketone nomenclature, the name will be pentanone. Pentan- for the 5 carbon chain, and -one for the ketone primary functional group.
But, the name pentanone doesn’t give us enough information to draw the molecule, because we don’t know where the ketone is on the carbon chain.
In this pentanone, the ketone is attached to carbon #2In this pentanone, the ketone is attached to the middle carbon
In this example, a locant can be used to specify where the ketone is attached. The above examples can be renamed 2-pentanone (top) and 3-pentanone (bottom).
If a compound has multiple functional groups, then a locant can be used for each one:
6-bromo-2-methyl-1-hexene
In this example, locants are used to show that a Bromine is attached to carbon #6, a methyl is attached to carbon #2, and the double bond starts at carbon #1.
Not all locants are numbers! When naming amides, the locant “N” is often used.
