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

Clearly, the carbonyl group is at $C 2$ in butanone.
For a four carbon chain to be a ketone, the only position for the carbonyl is an $C 2$, i.e. to give ${H}_{3} C - C \left(= O\right) C {H}_{2} C {H}_{3}$. Substitution at $C 1$ gives the aldehyde, $H \left(O =\right) C - C {H}_{2} C {H}_{2} C {H}_{3}$.
Of course, for cyclohexanone, or cyclobutanone, the oxo group could be at $C 1$, but these are special circumstances.