Requirements: Aldehyde with NO alpha-hydrogen + concentrated NaOH.

Examples of qualifying aldehydes: HCHO, PhCHO, (CH3)3CCHO, furfural Non-qualifying: CH3CHO, C2H5CHO, PhCH2CHO (all have alpha-H → undergo aldol instead)

Mechanism: Hydride (H:-) transfer from one aldehyde molecule to another. One gets oxidized (→ carboxylate), one gets reduced (→ alkoxide/alcohol).

Crossed Cannizzaro: HCHO + RCHO → RCH2OH + HCOONa. HCHO is always preferentially oxidized (to HCOONa) because it's the better hydride donor. The other aldehyde is always reduced.

Intramolecular Cannizzaro: Possible with dialdehydes like glyoxal.