Q.40 The correct statement/s for bimolecular nucleophilic substitution reactions is/are
(A) It goes through a carbocation formation
(B) There is an inversion of configuration if the reacting center is chiral
(C) Reaction is enhanced when carried out in polar solvents
(D) The reaction intermediate is trigonal bipyramidal
The correct answer is (B) and (C).
Bimolecular nucleophilic substitution (SN2) reactions proceed via a concerted mechanism without carbocation intermediates, featuring backside attack that inverts chiral centers and favoring polar aprotic solvents for optimal nucleophile reactivity.
Option Analysis
(A) It goes through a carbocation formation
Incorrect. SN2 reactions occur in one concerted step where the nucleophile attacks as the leaving group departs simultaneously, avoiding any carbocation intermediate, unlike SN1 mechanisms.
(B) There is an inversion of configuration if the reacting center is chiral
Correct. The nucleophile approaches from the opposite side of the leaving group, causing Walden inversion at the chiral carbon during the transition state.
(C) Reaction is enhanced when carried out in polar solvents
Correct. Polar aprotic solvents (e.g., DMSO, acetone) enhance SN2 rates by solvating cations while leaving nucleophiles unsolvated and highly reactive, unlike polar protic solvents that hinder nucleophiles via hydrogen bonding.
(D) The reaction intermediate is trigonal bipyramidal
Incorrect. No discrete intermediate forms in SN2; the transition state has trigonal bipyramidal geometry momentarily, but the option specifies “intermediate,” which applies to stable species, not transition states.
Bimolecular nucleophilic substitution reactions represent a cornerstone of organic chemistry, particularly for CSIR NET Life Sciences aspirants tackling reaction mechanisms. These SN2 processes involve a nucleophile displacing a leaving group in a single, concerted step, making them essential for understanding stereochemistry and solvent influences in substitution reactions.
SN2 Mechanism Essentials
The bimolecular nucleophilic substitution reactions proceed via backside attack, forming a pentacoordinate transition state where bond making and breaking occur simultaneously. Rate depends on both substrate and nucleophile concentrations (second-order kinetics), favoring primary and methyl substrates due to minimal steric hindrance.
Key Characteristics
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Stereochemistry: Complete inversion at chiral centers, converting (R) to (S) or vice versa.
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No Intermediates: Concerted process skips carbocations or trigonal bipyramidal stable species.
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Solvent Role: Polar aprotic solvents boost rates by enhancing nucleophile strength; protic solvents slow reactions via solvation.
For CSIR NET preparation, focus on distinguishing SN2 from SN1: SN2 inverts configuration and loves polar aprotic media, while SN1 racemizes via carbocations in protic solvents.
