what is substitution reaction give example

A substitution reaction is a type of chemical reaction where an atom or functional group of a molecule is replaced by another atom or functional group. There are two types of substitution reactions: nucleophilic and electrophilic reactions. In nucleophilic reactions, an electron-rich species replaces a leaving group, while in electrophilic reactions, an electron-deficient species replaces a leaving group. An example of a substitution reaction is the conversion of methane to carbon tetrachloride, where hydrogen atoms are replaced by chlorine atoms. Another example is the chlorination of methane, where chlorine breaks the carbon-hydrogen bond to bond with the carbon to form chloromethane. Substitution reactions are divided into three general classes, depending on the type of atom or group that acts as the substituent. These classes are nucleophilic substitution, electrophilic substitution, and free-radical substitution.