Butan-2-one
= Butan-2-one
<Butan-2-one> is the four-carbon <ketone> $\mathrm{CH_3C(=O)CH_2CH_3}$. 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>.
<Organic Chemistry by John McMurry>[Chapter22, Carbonyl Alpha-Substitution Reactions: enolate formation]{ref}