A reaction in which the functional group of one chemical compound is substituted by another group is called _______.

Substitution reaction is the substitution of the functional group of one chemical compound by another group. It involves the replacement of an atom or group of atoms which are present in a compound by some other atom or group of atoms. In a substitution reaction, replacement occurs without bringing any change in the structure of the compound. “ During this reaction, the functional group present in the molecule of one compound gets detached from it and combines with the molecule of another element or compound”. This transfer of functional groups is determined by the reactivity of the two substances with respect to the functional group. 

For example, when CH₃Cl reacts with the hydroxyl ion, it leads to the formation of an original molecule called methanol with that hydroxyl ion. 

${\mathrm{CH}}_3\mathrm{Cl} +{\left(\mathrm{OH}\right)}^{-} \to {\mathrm{CH}}_3 \mathrm{OH} + {\mathrm{Cl}}^{-}$