Which of the following S_N2 reactions should proceed at a faster rate? Justify your answer on a reaction coordinate diagram.

검증된 단계별 안내

Step 1: Understand the SN2 reaction mechanism. SN2 reactions are bimolecular nucleophilic substitution reactions where the nucleophile attacks the electrophilic carbon and displaces the leaving group in a single concerted step. The rate of SN2 reactions depends on steric hindrance around the electrophilic carbon.

Step 2: Analyze the two given reactions. In the first reaction, the electrophilic carbon is part of a primary alkyl bromide (1-bromopropane). In the second reaction, the electrophilic carbon is part of a secondary alkyl bromide (2-bromopropane). Primary carbons are less sterically hindered compared to secondary carbons.

Step 3: Compare steric hindrance. The primary alkyl bromide in the first reaction has less steric hindrance, allowing the nucleophile (CN⁻) to approach the electrophilic carbon more easily. In contrast, the secondary alkyl bromide in the second reaction has more steric hindrance due to the presence of two alkyl groups attached to the electrophilic carbon, which slows down the reaction.

Step 4: Reaction coordinate diagram. On a reaction coordinate diagram, the activation energy for the first reaction (primary alkyl bromide) will be lower compared to the second reaction (secondary alkyl bromide). This is because the transition state is less sterically hindered in the first reaction, making the reaction proceed faster.

Step 5: Conclusion. The SN2 reaction involving the primary alkyl bromide (1-bromopropane) will proceed at a faster rate compared to the reaction involving the secondary alkyl bromide (2-bromopropane) due to reduced steric hindrance at the electrophilic carbon.

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주요 개념

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Sₙ2 Mechanism

The Sₙ2 (substitution nucleophilic bimolecular) mechanism involves a single concerted step where a nucleophile attacks an electrophile, leading to the simultaneous displacement of a leaving group. The reaction rate depends on the concentration of both the nucleophile and the substrate, making it second-order overall. Understanding this mechanism is crucial for predicting which reaction will proceed faster based on sterics and the nature of the nucleophile and leaving group.

Steric Hindrance

Steric hindrance refers to the crowding around a reactive center that can impede the approach of a nucleophile. In Sₙ2 reactions, substrates with less steric hindrance (such as primary alkyl halides) allow for faster reactions compared to more hindered substrates (like tertiary alkyl halides). Evaluating the steric environment of the substrates in the question is essential for determining which reaction will proceed at a faster rate.

Reaction Coordinate Diagram

A reaction coordinate diagram visually represents the energy changes during a chemical reaction. It plots the energy of the system against the progress of the reaction, showing the transition state and the activation energy required for the reaction to occur. Analyzing these diagrams helps in understanding the relative rates of reactions by comparing the activation energies and the stability of intermediates or transition states involved in the Sₙ2 processes.

Which of the following SN2 reactions should proceed at a faster rate? Justify your answer on a reaction coordinate diagram.