🧪 SN1 vs SN2 Reactions: Key Differences for the DAT Organic Chemistry Section

Whether you’re brushing up on reaction mechanisms or identifying transition states, questions about SN1 and SN2 reactions frequently appear on the Organic Chemistry section of the DAT. These two reaction types can look similar—but they behave very differently.

⚙️ What Are SN1 and SN2 Reactions?

SN1: Unimolecular Nucleophilic Substitution

• Rate Law: Rate = ksubstratesubstratesubstrate

• Mechanism: Two steps

  • Formation of a carbocation intermediate

  • Nucleophilic attack

• Favored Conditions:

  • Tertiary substrates

  • Polar protic solvents

• Stereochemistry: Racemic mixture (due to planar intermediate)

SN2: Bimolecular Nucleophilic Substitution

• Rate Law: Rate = ksubstratesubstratesubstratenucleophilenucleophilenucleophile

• Mechanism: One concerted step

  • Backside nucleophilic attack

• Favored Conditions:

  • Primary or methyl substrates

  • Strong nucleophile

  • Polar aprotic solvents

• Stereochemistry: Inversion of configuration (Walden inversion)

🔍 How to Identify on the DAT

You may be asked to:

• Predict the major product

• Determine the reaction mechanism

• Analyze the rate law or stereochemistry

• Choose the best conditions for the reaction to proceed

📈 Quick SN1 vs SN2 Summary Table

Feature	SN1	SN2
Kinetics	Unimolecular (1st order)	Bimolecular (2nd order)
Steps	Two	One
Intermediate	Carbocation	None
Substrate Favored	Tertiary	Methyl or Primary
Nucleophile	Weak/neutral	Strong
Solvent	Polar protic	Polar aprotic
Stereochemistry	Racemic	Inversion of configuration

🧠 Memory Tip for the DAT

"SN1 needs a stage — carbocation solo!"
"SN2 needs speed — one fast punch!"

These mnemonics help you remember both the mechanism and substrate preferences.

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✅ Summary

• SN1: Two-step, tertiary, weak nucleophile, racemic

• SN2: One-step, primary/methyl, strong nucleophile, inversion

• Know when, how, and why each mechanism is favored.

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