 Butan-2-one

Butan-2-one is the four-carbon ketone $\mathrm{C}{\mathrm{H}}_3\mathrm{C}(=\mathrm{O})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. Its two sides are constitutionally different, so its two alpha positions need not react equivalently.

Formation and trapping of different enolates can give distinct substitution products. The example connects a simple molecular structure with enolate regioselectivity.