SN2 Reaction
SN2 stands for substitution nucleophilic bimolecular.
It is a concerted nucleophilic substitution reaction: bond formation to the nucleophile and bond breaking to the leaving group happen in the same rate-determining step.
Core idea
Section titled “Core idea”In an SN2 reaction, a nucleophile attacks the electrophilic carbon from the backside of the carbon-leaving group bond.
The leaving group leaves at the same time, so there is no carbocation intermediate.
Rate law
Section titled “Rate law”Rate = k[substrate][nucleophile]
The nucleophile appears in the rate law because the nucleophile is involved in the single rate-determining step.
Favoured conditions
Section titled “Favoured conditions”SN2 reactions are favoured by:
- methyl and primary substrates
- strong nucleophiles
- good leaving groups
- polar aprotic solvents
- low steric hindrance around the reacting carbon
Substrate effect
Section titled “Substrate effect”Steric accessibility is the main substrate issue.
General trend:
methyl > primary > secondary >> tertiary
Tertiary substrates are usually poor for SN2 because the nucleophile cannot easily reach the backside of the carbon-leaving group bond.
Nucleophile strength
Section titled “Nucleophile strength”Strong, negatively charged nucleophiles usually accelerate SN2.
Common examples include:
- hydroxide
- alkoxides
- cyanide
- azide
- thiolates
- iodide
Bulky nucleophiles can shift the reaction away from substitution and toward elimination.
Solvent effect
Section titled “Solvent effect”Polar aprotic solvents often favour SN2 because they do not strongly hydrogen-bond to anionic nucleophiles.
Common examples include:
- DMSO
- DMF
- acetone
- acetonitrile
Polar protic solvents can slow many SN2 reactions by solvating and stabilising the nucleophile.
Stereochemistry
Section titled “Stereochemistry”SN2 gives inversion of configuration at the reacting stereocentre.
This is often called Walden inversion.
If the starting material is chiral and the reacting carbon is the stereocentre, the product should be drawn with inverted stereochemistry.
Common exam traps
Section titled “Common exam traps”- Do not draw a carbocation intermediate.
- Do not use SN2 for a tertiary halide under normal conditions.
- Do not omit the nucleophile from the rate law.
- Do not predict racemisation; SN2 gives inversion.
- Check whether a strong bulky base would favour E2 instead.
- Polar aprotic solvents normally support SN2 better than polar protic solvents.
SN1 vs SN2 quick check
Section titled “SN1 vs SN2 quick check”| Question | Points toward SN1 | Points toward SN2 |
|---|---|---|
| Substrate | Tertiary or resonance-stabilised | Methyl or primary |
| Nucleophile | Weak or neutral acceptable | Strong nucleophile important |
| Solvent | Polar protic | Polar aprotic |
| Intermediate | Carbocation | None |
| Stereochemistry | Partial racemisation | Inversion |
| Rate law | k[substrate] | k[substrate][nucleophile] |
Related pages
Section titled “Related pages”Quick summary
Section titled “Quick summary”SN2 is favoured when the electrophilic carbon is accessible and the nucleophile is strong.
The key ideas are:
- one-step concerted mechanism
- second-order rate law
- backside attack
- inversion of configuration
- strongest for methyl and primary substrates