Q.30

Four alkyl halides, MeBr, EtBr, i-PrBr and t-BuBr, undergo SN2 reactions in the presence

of hydroxide ion to yield the corresponding alcohols and the halide ion. The CORRECT

order of the alkyl halides based on the rates of reactions is

(A) MeBr > EtBr > i-PrBr > t-BuBr

(B) t-BuBr > i-PrBr > EtBr > MeBr

(C) i-PrBr > t-BuBr > EtBr > MeBr

(D) EtBr > i-PrBr > t-BuBr > MeBr

SN2 Reaction Rates: MeBr > EtBr > i-PrBr > t-BuBr Order Explained

SN2 reactions follow a specific reactivity order for alkyl halides due to steric hindrance effects. The correct sequence for MeBr, EtBr, i-PrBr, and t-BuBr with hydroxide ion is MeBr > EtBr > i-PrBr > t-BuBr, corresponding to option (A).

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Correct Answer: Option (A)

In SN2 reactions, the nucleophile attacks the carbon from the back, forming a pentacoordinate transition state sensitive to steric bulk around the reaction center. Methyl (MeBr) has no alkyl substituents, making it least hindered and fastest. Primary (EtBr) follows, then secondary (i-PrBr), with tertiary (t-BuBr) slowest due to three methyl groups blocking access.

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Relative rates often cited as 10,000 : 65 : 0.5 : 0.04 confirm this trend, with t-BuBr nearly unreactive under SN2 conditions.

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Why Other Options Are Incorrect

• Option (B) t-BuBr > i-PrBr > EtBr > MeBr: This reverses SN2 order, matching SN1 where carbocation stability dominates (3° > 2° > 1° > methyl). SN1 favors t-BuBr most due to stable tertiary carbocation.

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• Option (C) i-PrBr > t-BuBr > EtBr > MeBr: Secondary i-PrBr cannot outpace tertiary t-BuBr in SN2; both are slow, but tertiary is worse. This mixes SN1/SN2 traits incorrectly.

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• Option (D) EtBr > i-PrBr > t-BuBr > MeBr: Primary EtBr reacts faster than methyl MeBr in reality, violating the methyl > primary sequence. No basis in SN2 kinetics.

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SN2 Steric Hindrance Breakdown

This order holds for polar aprotic solvents with strong nucleophiles like OH⁻, emphasizing bimolecular backside attack.