steric hindrance in reactions

Question

Steric hindrance most reduces rate of which mechanism?

RankGuru · Accepted Answer

1. **$S_N2$ Mechanism**: Occurs in a single step where the nucleophile attacks from the **backside** of the leaving group.
2. **Transition State**: The carbon atom is briefly bonded to both the nucleophile and the leaving group, creating a crowded environment.
3. **Steric Impact**: If the carbon atom is 'shielded' by bulky alkyl groups (e.g., in tertiary halides), the nucleophile cannot reach the center easily. 
4. **Rate Effect**: Higher branching at the $\alpha$ and $\beta$ carbons causes a drastic drop in $S_N2$ rates ($1^\circ > 2^\circ > 3^\circ$).
5. **Check**: $S_N1$ is less affected by hindrance because the slow step is the formation of a carbocation, which actually relieves steric strain.

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steric hindrance in reactions

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