CH3CHO + 4 NaOH + 3 I2 → CHI3 + HCOONa + 3 NaIH2O
This is an oxidation-reduction (redox) reaction:
CI - 1 e- → CII (oxidation)
C-III - 5 e- → CII (oxidation)
6 I0 + 6 e- → 6 I-I (reduction)
CH3CHO is a reducing agent, CH3CHO is a reducing agent, I2 is an oxidizing agent.