Key Concept
Nucleophilic substitution of halogenoalkanes can proceed via two different mechanisms: the concerted SN2 (primary substrates) or the two-step SN1 (tertiary substrates). Secondary substrates can undergo both.
The SN2 Mechanism
SN2: Concerted Backside Attack
SN2: One step, concerted
- The nucleophile attacks from the opposite side to the leaving group (backside attack, 180°)
- The new bond forms at the same time as the C-X bond breaks. No intermediate.
- The transition state has five groups around carbon (pentacoordinate) with partial bonds shown as dashed lines
Stereochemistry: Walden inversion. The three substituents "flip" like an umbrella turning inside out. The product has inverted configuration.
Favoured by: primary halogenoalkanes (minimal steric hindrance allows backside attack)
The SN1 Mechanism
SN1: Two steps, via carbocation
Step 1 (slow, rate-determining):
The C-X bond breaks heterolytically → planar carbocation + X⁻
Step 2 (fast):
The nucleophile attacks the carbocation from either side → product
Stereochemistry: Racemic mixture. The nucleophile attacks equally from both sides of the planar carbocation, producing equal amounts of both enantiomers.
Favoured by: tertiary halogenoalkanes (steric hindrance prevents backside attack; tertiary carbocation is stabilised by inductive effect of three alkyl groups)
SN1 vs SN2 Comparison
| SN1 | SN2 | |
|---|---|---|
| Steps | 2 (carbocation intermediate) | 1 (concerted) |
| Substrate | Tertiary halogenoalkanes | Primary halogenoalkanes |
| Stereochemistry | Racemic mixture | Inversion (Walden) |
| Nucleophile | Weak (e.g. H₂O) | Strong (e.g. OH⁻) |
| Secondary substrates | Both mechanisms can occur | |
Why Does Substrate Type Matter?
Tertiary → SN1
Three bulky alkyl groups create steric hindrance. The nucleophile cannot approach from behind. The leaving group departs first to form a stabilised tertiary carbocation.
Primary → SN2
Minimal steric hindrance allows direct backside attack. A primary carbocation would be too unstable to form, so SN1 is not feasible.
Think About It
The numbers "1" and "2" in SN1 and SN2 refer to what?
They refer to the molecularity of the rate-determining step. SN2 = two species in the RDS (substrate + nucleophile). SN1 = one species in the RDS (substrate only). They do NOT refer to the number of steps.
Common Exam Mistakes
- Confusing the "1" and "2" as the number of steps. They mean unimolecular and bimolecular.
- Forgetting that SN1 gives a racemic mixture, not a single enantiomer.
- Not drawing curly arrows from the nucleophile to the carbon in SN2.
- Forgetting to show the transition state brackets [ ]‡ with partial bonds (dashed lines) and δ- charges.
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