locant

The locant is incorporated into the name of the molecule to remove ambiguity.

In organic chemistry, a locant is a figure to indicate the position of a functional group within a molecule.

If the oxygen is bonded to the middle carbon, the locant is 3.

The prefix is preceded by the position number (locant) of the carbon atoms that disappear.

Has the lowest-numbered locant (or locants) for the suffix functional group.

The locant is the number of the carbon atom to which the oxygen atom is bonded.

Has the lowest-numbered locants for multiple bonds (The locant of a multiple bond is the number of the adjacent carbon with a lower number).

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.