By Team Aakash Byju's
The locants are distributed in such a way that the main functional group receives the lowest locant number.
For example, consider 3-pentanone (C5H10O) which contains a chain of exactly five carbon atoms.
Number the carbons from one to five, beginning from one end and making your way down the chain.
If the oxygen is bonded to an end carbon, the molecule would be an aldehyde, not a ketone.
Hence the locant is 2 or 4 if the oxygen is linked to an atom on either side and the locant is 3 if the oxygen is linked to the middle carbon.
Always go for the lower number. In this molecule, the locant is either 2 or 3.
Depending on the location of the oxygen atom, the molecule is called Pentan-2-one or Pentan-3-one.