Identify whether each of the following reactions occurs through an SN1, SN2, E1, or E2 mechanism.
The Correct Answer and Explanation is :
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That being said, here’s how you can determine whether a reaction follows the SN1, SN2, E1, or E2 mechanism:
1. SN1 (Unimolecular Nucleophilic Substitution)
- Mechanism: Two-step process (carbocation formation → nucleophilic attack)
- Substrate Preference: Tertiary > Secondary (Primary rarely undergoes SN1)
- Solvent: Polar protic (e.g., water, alcohols)
- Nucleophile Strength: Weak nucleophiles can work (H2O, ROH)
- Stereochemistry: Racemization (mix of retention and inversion)
2. SN2 (Bimolecular Nucleophilic Substitution)
- Mechanism: One-step concerted reaction (backside attack)
- Substrate Preference: Methyl > Primary > Secondary (Tertiary doesn’t undergo SN2)
- Solvent: Polar aprotic (e.g., acetone, DMSO)
- Nucleophile Strength: Strong nucleophile required (e.g., OH⁻, CN⁻, I⁻)
- Stereochemistry: Inversion (Walden inversion)
3. E1 (Unimolecular Elimination)
- Mechanism: Two-step process (carbocation formation → elimination)
- Substrate Preference: Tertiary > Secondary (Primary rarely undergoes E1)
- Solvent: Polar protic (same as SN1)
- Base Strength: Weak base (e.g., H2O, ROH)
- Stereochemistry: Formation of Zaitsev product (more substituted alkene)
4. E2 (Bimolecular Elimination)
- Mechanism: One-step concerted reaction (base removes proton → leaving group leaves)
- Substrate Preference: Tertiary > Secondary > Primary
- Solvent: Polar aprotic or protic
- Base Strength: Strong base required (e.g., NaOEt, NaH)
- Stereochemistry: Anti-periplanar geometry needed
How to Identify the Mechanism?
- Check the substrate type (Primary, Secondary, Tertiary).
- Look at the nucleophile/base (Strong/Weak, Bulky/Small).
- Check the solvent (Protic favors SN1/E1, Aprotic favors SN2/E2).
- Determine if elimination (E1/E2) or substitution (SN1/SN2) is more likely.
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