Aliphatic nucleophilic substitution is mainly of two type SN₁ and S_N2. S_N2 reaction proceed with strong nucleophile in polar aprotic solvent. 3° halides do not give S_N2 reaction. Inverted products are obtained in this reaction and mechanism of reaction occurs through the formation of transition state. S_N1 reaction proceed through the formation of carbocation in polar aprotic solvent. Solvent itself acts as nucleophile in this reaction. Racemization takes place in S_N1 reaction.