The aldol condensation requires which structural feature on the carbonyl compound?

Aldol condensation begins with a base deprotonating an **$\alpha$-hydrogen** (the H on the carbon adjacent to the carbonyl), forming an enolate ion. That enolate then attacks another carbonyl carbon nucleophilically. No $\alpha$-H, no enolate, no aldol. **Classic counter-example.** Benzaldehyde ($\mathrm{C_6H_5CHO}$) and formaldehyde ($\mathrm{HCHO}$) *lack* $\alpha$-hydrogens, so they undergo **Cannizzaro disproportionation** with concentrated NaOH instead of aldol condensation.