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S_Ni reactions are a type of chemical reaction in organic chemistry where one atom or group within a molecule is substituted with another atom or group that is already part of the same molecule. This occurs through the action of a nucleophile, which is a molecule or atom with an extra pair of electrons. S_Ni reactions are less common than other substitution reactions and often require specific conditions, such as steric hindrance, leaving group ability, and intermediate species stability.