What will be the substitution product formed when KNO₂ reacts with an alkyl group? 

Alkyl nitrite 

The substitution product formed when KNO₂ reacts with an alkyl group is alkyl nitrite. The reaction is as follows:

Where R is an alkyl group, X is a halogen atom, and KX is the potassium salt of the halogen atom.

The reaction proceeds through a nucleophilic substitution mechanism. The negatively charged oxygen atom of KNO₂ attacks the alkyl halide, displacing the halogen atom. The resulting alkyl nitrite is a relatively unstable compound that can decompose to form nitrous oxide and the alkyl halide.