Ketone Naming

[stephanieyang_3F]

Why is it for ketones only you don't have to start numbering until there are five or more carbons? How would we differentiate a "butane" which has the ketone on the second carbon compared to the first carbon?

[Sophia Rawson 1C]

The best way to understand this would probably be to draw out the structures. You will notice that up until there is 5 carbons, the location is obvious simply by the name butane or propanone. With propanone, only one carbon is available for the oxygen to be on because if it was on either of the end carbons then it would be considered an aldehyde. With butanone, either middle carbon is considered the same location because when you name you will give the ketone priority. So both of them can be thought of as position 2... and you can't put an O atom on the end carbons for the same reason as before- it would make it an aldehyde.