A carbonyl compound or group is a double-bonded oxygen atom with a carbon atom. A carbonyl group can classified as an aldehyde, which has one hydrogen atom and one carbon atom bonded to it; or it can be classified as a ketone, which has two carbon atoms bonded to the carbonyl group. The first step in naming a ketone is to identify the carbonyl group and identify the compound as a ketone instead of an aldehyde.
To get the base name of the compound, count the longest hydrocarbon chain in the compound. For instance, a chain with five hydrocarbons (carbon/hydrogen bonds) would be identified as a pentane molecule. The ketone will take the base name of the hydrocarbons that are attached to it.
To determine the functional name of the molecule, you must begin counting from the end of the hydrocarbon closest to the carbonyl compound. While it won't change the base name of the molecule, the carbonyl must be located at the lowest numbered carbon. Therefore, in a five-carbon molecule, pentanone would be numbered according to the carbon molecule with the oxygen attached, whether it is the carbon to the left or to the right of the middle carbon. This molecule would be called 2-pentanone.
If other compounds are connected to the hydrocarbon chain that take stronger naming precedence, the ketone is indicated by the phrase "oxo" in the name. For instance if a cyclic hydrocarbon chain is included, the ketone would by identified by "oxocyclohexane" signification.