Substitution reactions are a type of chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms. This type of reaction is commonly seen in organic chemistry, where a functional group in a molecule can be substituted with a different functional group under specific reaction conditions. Substitution reactions can be classified into different types based on the mechanism by which the substitution occurs, such as nucleophilic substitution, electrophilic substitution, radical substitution, or free radical substitution. These reactions play a crucial role in the synthesis of various organic compounds and are studied extensively in organic chemistry research.