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Step-by-Step Explanation
Step 1: Understanding the SN2 Mechanism
The SN2 (bimolecular nucleophilic substitution) mechanism proceeds in a single step where the nucleophile attacks the electrophilic carbon at the same time as the leaving group departs. The rate of this reaction depends on both the concentrations of the nucleophile and the substrate, and it is represented by:
Rate ∝ [Substrate] × [Nucleophile]
Step 2: Role of Steric Hindrance
Because the nucleophile has to attack the carbon bearing the leaving group, any bulky substituents around this carbon will hinder the approach of the nucleophile. Therefore, the order of reactivity for SN2 decreases as steric hindrance around the reactive carbon increases:
CH3–X > 1° alkyl halide > 2° alkyl halide > 3° alkyl halide
This means a primary alkyl halide (1°) reacts faster in SN2 than a secondary (2°), which in turn reacts faster than a tertiary (3°) alkyl halide.
Step 3: Comparing the Given Options
Among the provided options, CH3CH2Br (ethyl bromide) is a primary alkyl halide with minimal steric hindrance. The other structures either have more substituents or ring structures that reduce the efficiency of the nucleophile’s backside attack. Hence, CH3CH2Br will undergo SN2 substitution at the highest relative rate.
Step 4: Conclusion
Based on SN2 steric constraints, the correct answer is CH3CH2Br, as it experiences the least steric hindrance among the choices, thus showing the highest SN2 reactivity.
