Question

Will the following SN2 reaction proceed more rapidly in DMSO or H2O?

Accepted Answer

Step 1: Understand the reaction mechanism. The given reaction is an SN2 reaction, where the nucleophile (CN⁻) attacks the electrophilic carbon attached to the leaving group (Br⁻) in a single concerted step. SN2 reactions are bimolecular and proceed via a backside attack, leading to inversion of configuration at the carbon center. Step 2: Analyze the role of the solvent in SN2 reactions. Polar aprotic solvents, such as DMSO (dimethyl sulfoxide), are ideal for SN2 reactions because they do not solvate the nucleophile strongly. This allows the nucleophile to remain reactive and attack the electrophilic carbon more efficiently. In contrast, polar protic solvents, such as H₂O, can hydrogen bond with the nucleophile, reducing its reactivity and slowing down the reaction. Step 3: Evaluate the nucleophile and leaving group. In this reaction, CN⁻ is a strong nucleophile, and Br⁻ is a good leaving group. The efficiency of the nucleophile is enhanced in a polar aprotic solvent like DMSO, making the reaction proceed more rapidly. Step 4: Consider steric factors. The substrate (2-bromopropane) is a secondary alkyl halide, which is moderately hindered. SN2 reactions are sensitive to steric hindrance, but the choice of solvent can significantly impact the reaction rate. DMSO will help minimize steric effects by enhancing nucleophilic attack. Step 5: Conclude the solvent choice. Based on the principles of SN2 reaction mechanisms and solvent effects, the reaction will proceed more rapidly in DMSO compared to H₂O due to the increased nucleophilicity of CN⁻ in a polar aprotic environment.

Will the following S_N2 reaction proceed more rapidly in DMSO or H₂O?

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Key Concepts

S<sub>N</sub>2 Mechanism

Solvent Effects

Nucleophilicity