Write a note on the addition of sodium hydrogen sulphite $(NaHSO_3)$ to the carbonyl group.

(N/A) Sodium hydrogen sulphite $(NaHSO_3)$ adds to aldehydes and ketones to form the corresponding addition products.
The reaction is as follows:
$R-C(=O)-R' + NaHSO_3 \rightleftharpoons R-C(OH)(SO_3Na)-R'$
$1$. The nucleophilic part of $NaHSO_3$ is the sulphite ion $(SO_3^{2-})$,which attacks the electrophilic carbonyl carbon.
$2$. This is followed by a proton transfer to form the stable crystalline bisulphite addition compound.
$3$. This reaction is reversible and is used for the separation and purification of aldehydes and ketones,as the addition product can be decomposed back to the original carbonyl compound by treatment with dilute mineral acid or base.