Substitution reactions are a type of chemical reaction where one atom or group in a molecule is replaced by another atom or group. This process often occurs in organic chemistry, particularly with compounds like alkanes and aromatic compounds. The new atom or group can change the properties of the original molecule, leading to different chemical behaviors. In a typical substitution reaction, a reactant undergoes a transformation, resulting in a product that contains the substituent. For example, in the reaction of chloromethane with sodium hydroxide, the chlorine atom is replaced by a hydroxyl group, forming methanol. This illustrates how substitution can create new compounds with distinct characteristics.