Substitution Nucleophilic Internal (S_Ni) reactions are a type of organic chemical reaction where an atom or group within a molecule is replaced by another atom or group already present within the same molecule. These reactions occur under specific conditions and with particular types of molecules, and involve a nucleophile attacking a polarized bond within the molecule. Examples of S_Ni reactions include the conversion of alkyl halides into alkenes through β-elimination and the transformation of epoxides into alcohols. The occurrence of these reactions depends on factors such as steric hindrance, leaving group ability, and the stability of the intermediate species formed during the reaction. S_Ni reactions are important in the synthesis of complex organic compounds.