Here’s a clear comparison of SN1 and SN2 reactions:
1. Meaning
- SN1 (Substitution Nucleophilic Unimolecular): A two-step reaction where the rate depends only on the concentration of the substrate.
- SN2 (Substitution Nucleophilic Bimolecular): A one-step reaction where the rate depends on both the substrate and the nucleophile.
2. Mechanism
- SN1:
- Step 1: The leaving group departs, forming a carbocation.
- Step 2: The nucleophile attacks the carbocation.
- SN2:
- A single step: The nucleophile attacks the substrate at the same time as the leaving group leaves.
3. Rate Equation
- SN1: Rate = k[substrate]
- SN2: Rate = k[substrate][nucleophile]
4. Order of Reaction
- SN1: First order (unimolecular)
- SN2: Second order (bimolecular)
5. Substrate Preference
- SN1: Tertiary > Secondary > Primary (because carbocation stability matters)
- SN2: Primary > Secondary > Tertiary (because steric hindrance matters)
6. Nucleophile
- SN1: Strength of nucleophile not very important.
- SN2: Strong nucleophile required.
7. Solvent
- SN1: Favored by polar protic solvents (e.g., water, alcohol).
- SN2: Favored by polar aprotic solvents (e.g., acetone, DMSO).
8. Stereochemistry
- SN1: Racemization (mixture of enantiomers), since attack can occur from both sides of the carbocation.
- SN2: Inversion of configuration (backside attack).
9. Examples
- SN1: Hydrolysis of tert-butyl bromide in water.
- SN2: Reaction of methyl bromide with hydroxide ion.
In short:
- SN1 = Carbocation mechanism, stepwise, favored by tertiary carbons, racemization.
- SN2 = One-step, backside attack, favored by primary carbons, inversion of configuration.