Among a primary, secondary, or tertiary carbocation, which is least favored for SN2 reactions? Explain why.

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**Question:** Among a primary, secondary, or tertiary carbocation, which is least favored for SN2 reactions? Explain why.

**Answer:**

In the context of SN2 (bimolecular nucleophilic substitution) reactions, the least favored carbocation type is the tertiary carbocation. Here's why:

1. **Steric Hindrance:** SN2 reactions proceed via a backside attack mechanism, where the nucleophile must approach the carbon atom directly opposite to the leaving group. In tertiary carbocations, the carbon attached to the leaving group is surrounded by bulky alkyl groups, creating significant steric hindrance. This hinders the nucleophile's ability to access and attack the carbon, making SN2 reactions highly unfavorable for tertiary carbocations.

2. **Transition State Stability:** In an SN2 reaction, the nucleophile and the leaving group are both partially attached to the carbon in the transition state. In the case of tertiary carbocations, the presence of three bulky groups around the carbocation increases the energy and instability of the transition state due to steric interactions, further disfavoring the SN2 mechanism.

In contrast, primary carbocations have minimal steric hindrance, and secondary carbocations present moderate steric hindrance, making them more favorable for SN2 reactions compared to tertiary carbocations.
Transcribed Image Text:**Question:** Among a primary, secondary, or tertiary carbocation, which is least favored for SN2 reactions? Explain why. **Answer:** In the context of SN2 (bimolecular nucleophilic substitution) reactions, the least favored carbocation type is the tertiary carbocation. Here's why: 1. **Steric Hindrance:** SN2 reactions proceed via a backside attack mechanism, where the nucleophile must approach the carbon atom directly opposite to the leaving group. In tertiary carbocations, the carbon attached to the leaving group is surrounded by bulky alkyl groups, creating significant steric hindrance. This hinders the nucleophile's ability to access and attack the carbon, making SN2 reactions highly unfavorable for tertiary carbocations. 2. **Transition State Stability:** In an SN2 reaction, the nucleophile and the leaving group are both partially attached to the carbon in the transition state. In the case of tertiary carbocations, the presence of three bulky groups around the carbocation increases the energy and instability of the transition state due to steric interactions, further disfavoring the SN2 mechanism. In contrast, primary carbocations have minimal steric hindrance, and secondary carbocations present moderate steric hindrance, making them more favorable for SN2 reactions compared to tertiary carbocations.
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