Question

If we have a ketone with FOUR carbon atoms, where can the carbonyl group be placed?

Answer 1

Clearly, the carbonyl group is at `C2` in butanone.

Explanation:

For a four carbon chain to be a ketone, the only position for the carbonyl is an `C2`, i.e. to give `H_3C-C(=O)CH_2CH_3`. Substitution at `C1` gives the aldehyde, `H(O=)C-CH_2CH_2CH_3`.

Of course, for cyclohexanone, or cyclobutanone, the oxo group could be at `C1`, but these are special circumstances.