Key Definitions
- Nucleophile. An electron-pair donor that attacks a δ+ or positive carbon centre. It has a lone pair or negative charge. E.g. OH⁻, CN⁻, NH₃, H₂O
- Electrophile. An electron-pair acceptor that attacks regions of high electron density. E.g. H⁺, NO₂⁺, Br₂ (polarised)
- Leaving group. Departs with the bonding pair. Good leaving groups are stable anions (weak bases): I⁻ > Br⁻ > Cl⁻ > F⁻
- Curly arrow. Shows the movement of an electron pair from nucleophile to electrophilic carbon (full, double-barbed arrow)
- Substrate. The organic molecule being attacked (the halogenoalkane)
The SN2 Mechanism
Stereospecific SN2 Reaction
One step. Concerted
- The nucleophile attacks the δ+ carbon from the opposite side to the leaving group (backside attack)
- The C−X bond breaks at the same time as the new bond forms. No intermediate
- The leaving group departs with the bonding pair
Stereochemistry: Walden inversion. The configuration at the carbon is inverted (like an umbrella flipping inside out)
Rate law: Rate = k[RX][Nu⁻]. bimolecular, both substrate and nucleophile in the rate-determining step
The SN1 Mechanism
Two steps. Via carbocation intermediate
Step 1 (slow. Rate-determining):
The C−X bond breaks heterolytically → planar carbocation formed + leaving group (X⁻)
Step 2 (fast):
The nucleophile attacks the carbocation from either side → product formed
Stereochemistry: Racemic mixture. Equal amounts of both enantiomers (because nucleophile attacks equally from both sides of the planar carbocation)
Rate law: Rate = k[RX]. unimolecular, only substrate in the rate-determining step
SN1 vs SN2 Comparison
| SN1 | SN2 | |
|---|---|---|
| Number of steps | 2 (carbocation intermediate) | 1 (concerted) |
| Rate law | Rate = k[RX] | Rate = k[RX][Nu⁻] |
| Substrate | Tertiary halogenoalkanes | Primary halogenoalkanes |
| Stereochemistry | Racemic mixture | Inversion (Walden inversion) |
| Nucleophile strength | Weak nucleophile (e.g. H₂O) | Strong nucleophile (e.g. OH⁻) |
| Solvent | Polar protic (stabilises ions) | Polar aprotic |
| Energy profile | Two energy barriers (2 transition states) | One energy barrier (1 transition state) |
Why Does Substrate Type Matter?
Tertiary → SN1
Three bulky alkyl groups create steric hindrance. The nucleophile cannot easily attack. Instead, the leaving group departs first to form a stabilised tertiary carbocation (alkyl groups donate electron density via inductive effect).
Primary → SN2
Minimal steric hindrance. The nucleophile can easily approach the δ+ carbon from behind. A primary carbocation would be too unstable to form, so SN1 is not feasible.
Secondary substrates can undergo both SN1 and SN2, depending on other factors (nucleophile strength, solvent).
Common Nucleophiles
| Nucleophile | Formula | Product formed |
|---|---|---|
| Hydroxide ion | OH⁻ | Alcohol |
| Cyanide ion | CN⁻ | Nitrile (extends C chain by 1) |
| Ammonia | NH₃ | Amine |
| Water | H₂O | Alcohol (weak nucleophile → SN1) |
⚠️ Elimination as a Competing Reaction
Halogenoalkanes can also undergo elimination (E1/E2) instead of substitution. Elimination is favoured by:
- Strong bases (e.g. Ethanolic NaOH or KOH)
- Heat
- Tertiary substrates
Substitution favoured by: aqueous conditions, weaker/less bulky nucleophiles, primary substrates
Think About It
Why do tertiary halogenoalkanes favour SN1 rather than SN2?
Three bulky alkyl groups around the carbon create steric hindrance. The nucleophile cannot easily attack from behind. Instead, the leaving group departs first to form a stabilised tertiary carbocation, then the nucleophile attacks the planar carbocation.
⚠️ Common Exam Mistakes
- Confusing SN1 (2 steps, unimolecular) with SN2 (1 step, bimolecular). The numbers refer to the molecularity of the rate-determining step, not the number of steps!
- Forgetting that SN1 gives a racemic mixture (not a single enantiomer)
- Not drawing curly arrows from the nucleophile → not showing where the electrons come from
- Forgetting to show the leaving group departing with the bonding pair