Sn2 Mechanism

Bimolecular nucleophilic substitution proceeds in a single concerted step where the nucleophile attacks the carbon bearing the halogen from the side directly opposite the leaving group, passing through a five-coordinate transition state. Because the nucleophile must approach the reaction centre, any bulky alkyl groups around that carbon hinder the approach and raise the transition-state energy. Consequently primary halides react fastest and tertiary halides slowest, giving the SN2 order primary > secondary > tertiary, which is governed by steric crowding. Carbocation stability is wrong because no carbocation is formed in a concerted SN2 process; that factor governs SN1 instead. Solvent polarity influences rate but is not the structural reason for the primary-to-tertiary order. Leaving-group electronegativity does not explain the order among substrates that all bear the same halogen. It is also worth noting that the SN2 order is the exact reverse of the SN1 order, precisely because the two mechanisms are controlled by opposite factors: SN2 by approach hindrance and SN1 by cation stability. Methyl and primary halides, having the most accessible carbon, are therefore the classic SN2 substrates, while tertiary halides are essentially unreactive under SN2 conditions. This reasoning follows the NCERT mechanism of SN2 reactions. Sanity check: the least hindered primary carbon reacts fastest, exactly matching a sterically controlled backside attack.