Question

Why must the sodium hypochlorite in the electrophilic aromatic substitution reaction be added to the reaction mixture in ice?

Answer 1

Sodium hypochlorite (NaOCl) is highly unstable compound and  decomposes explosively on heating. The compound decomposes rapidly at room temperature, so it must be kept under refrigeration. At lower temperatures, however, it is quite stable.

Decomposition occurs in sodium hypochlorite solutions at high or room temperature, forming sodium chlorate and sodium chloride.

3 NaOCl (aq) → 2 NaCl (aq) + NaClO_​​3 (aq)

This decomposition reaction can also competes with chlorate formation, yielding sodium chloride and oxygen gas at room or high temperature.

2 NaOCl (aq) → 2 NaCl (aq) + O2 (g)

So, in simple words we can say that to prevent decomposition of Sodium hypochlorite, it is added to the reaction mixture in ice.