Cyclopentanone

Cyclopentanone is a five-membered cyclic ketone with formula ${\mathrm{C}}_5{\mathrm{H}}_8\mathrm{O}$. Its carbonyl carbon is part of the ring, rather than attached to a separate cyclic substituent.

The neighboring carbons can form enolates, so the molecule is a useful substrate for carbonyl addition, reduction, and alpha-carbon functionalization. Ring geometry constrains the conformations available to those reacting groups.