The reaction of m-chlorobenzaldehyde with a concentrated alkali (50% KOH) is an example of the Cannizzaro reaction. Aldehydes that do not have an $\alpha$-hydrogen atom (like benzaldehyde and its derivatives) undergo self-oxidation and reduction (disproportionation) on heating with concentrated alkali.

One molecule of the aldehyde is reduced to the corresponding alcohol (m-chlorobenzyl alcohol), and another molecule is oxidised to the corresponding carboxylic acid salt (potassium m-chlorobenzoate).

$2 \text{ m-Cl-C}_6H_4CHO + KOH \rightarrow \text{m-Cl-C}_6H_4CH_2OH + \text{m-Cl-C}_6H_4COO^-K^+$