The diketone shown is heated with dilute NaOH. The intramolecular aldol condensation closes a ring of which size?

A3-membered
B4-membered
C5-membered
D6-membered

Answer: C

The drawn compound is hexane-2,5-dione. The enolate from C1 attacks C5: the ring contains C1, C2, C3, C4, C5, i.e. five atoms. Dehydration gives 3-methylcyclopent-2-enone. Closure to 3- or 4-membered rings is strained.

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