Question

How do you name ketones?

Answer 1

As alkanones, with a number specifying the position of the carbonyl group on the chain.

Explanation:

Now clearly I need a three-carbon backbone to construct a ketone. Why?

For `H_3C-C(=O)CH_3`, `"2-propanone"` is clearly a misnomer because I don't have to specify the position of the carbonyl; it must be at `C2` to be a ketone. Of course we could (and would) call it acetone. Butanone is likewise unambiguous without a number to specify the position of the carbonyl. Why not?

But when we get to pentanone, we could have 3-pentanone, the symmetric ketone, `H_3C-CH_2C(=O)CH_2CH_3`, or 2-pentanone, `H_3C-C(=O)CH_2CH_2CH_3`. Why can I not have 1-butanone?

Are these principles clear?