Reactions involving α-hydrogen

The α-carbon is the carbon next to the C=O, and the hydrogens on it are α-hydrogens. They are slightly acidic: a base can pull one off, because the negative charge left behind is shared with the oxygen by resonance (the ion is called an enolate). Two important reactions depend on whether a compound has α-hydrogens.

Aldol reaction (needs α-H). With dilute NaOH, the enolate of one molecule attacks the C=O of another. The product has an aldehyde (ald) and an alcohol (ol) group: a β-hydroxy aldehyde (or ketone). Ethanal gives 3-hydroxybutanal. On heating, it loses water to give an α,β-unsaturated aldehyde, but-2-enal: this is aldol condensation.

With two different aldehydes, both having α-H, you get a cross aldol reaction. Each can act as the attacker and each as the target, so a mixture of four products forms.

Cannizzaro reaction (needs no α-H). Aldehydes with no α-hydrogen, such as methanal and benzaldehyde, cannot form an enolate. With concentrated NaOH, they disproportionate: one molecule is reduced to an alcohol and another is oxidised to the salt of an acid. Methanal gives methanol and sodium methanoate.

A compound with no α-H that is not an aldehyde, such as a ketone with a benzene ring on each side of the C=O, can do neither reaction.

So the quick check: has α-H → aldol; no α-H and an aldehyde → Cannizzaro.