SN1 and SN2 reaction mechanism

SN1 vs SN2 Reactions – Mechanism, Differences & Applications

In nucleophilic substitution reactions, a nucleophile replaces a leaving group. This happens mainly through two mechanisms: SN1 (unimolecular) and SN2 (bimolecular). The choice of mechanism depends on the substrate, nucleophile, solvent, and leaving group.

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🔹 SN1 Mechanism (Unimolecular Nucleophilic Substitution)

• Two-step process: Leaving group departs → carbocation forms → nucleophile attacks.
• Rate = k [substrate] (depends only on substrate).
• Favours tertiary substrates (stable carbocations).
• Occurs in polar protic solvents (water, alcohol).
• Can lead to racemization at chiral centers.

Example:
(CH₃)₃C–Br + H₂O → (CH₃)₃C–OH + HBr

🔹 SN2 Mechanism (Bimolecular Nucleophilic Substitution)

• One-step process: Nucleophile attacks as leaving group leaves (concerted).
• Rate = k [substrate][nucleophile].
• Favours primary substrates (low steric hindrance).
• Occurs in polar aprotic solvents (DMSO, acetone, DMF).
• Causes inversion of configuration (Walden inversion).

Example:
CH₃CH₂Br + OH^- → CH₃CH₂OH + Br^-

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📊 SN1 vs SN2 – Quick Comparison

Feature	SN1	SN2
Mechanism	Two-step (via carbocation)	One-step (concerted)
Rate Law	k [substrate]	k [substrate][nucleophile]
Preferred Substrate	3° > 2° >> 1°	1° > 2° >> 3°
Nucleophile	Weak sufficient	Strong required
Solvent	Polar protic (H2O, ROH)	Polar aprotic (DMSO, acetone)
Stereochemistry	Racemization	Inversion (Walden inversion)

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🧭 Decision Guide: When to Use SN1 vs SN2

✅ If substrate is tertiary + solvent is polar protic + nucleophile is weak → SN1

✅ If substrate is primary + solvent is polar aprotic + nucleophile is strong → SN2

✅ Secondary substrates can undergo either depending on base strength & solvent.

📌 Summary:
SN1 – Favoured by tertiary carbons, weak nucleophiles, polar protic solvents.
SN2 – Favoured by primary carbons, strong nucleophiles, polar aprotic solvents.