SN1 vs SN2 Mechanistic Overview
Nucleophilic substitution proceeds via two distinct mechanistic pathways depending on the steric and electronic structure of the halogenoalkane substrate:
- \(\text{S}_\text{N}2\): Bimolecular, concerted 1-step mechanism (favoured by 1° halogenoalkanes).
- \(\text{S}_\text{N}1\): Unimolecular, 2-step mechanism via carbocation intermediate (favoured by 3° halogenoalkanes).
The SN2 Mechanism
SN2: Concerted Backside Attack
SN2 Mechanism: Concerted Backside Attack
- Nucleophile attacks \(\text{C}^{\delta+}\) from the opposite side of the leaving group (backside attack, 180°).
- Passes through an unstable five-coordinate transition state \([\text{Nu}\cdots\text{C}\cdots\text{X}]^\ddagger\) with negative charge delocalised.
- Causes complete inversion of configuration (Walden inversion, like an umbrella turning inside out).
The SN1 Mechanism
SN1 Mechanism: Two-Step Carbocation Pathway
- Step 1 (Slow, RDS): Heterolytic fission of \(\text{C}-\text{X}\) bond → planar carbocation intermediate formed.
- Step 2 (Fast): Nucleophile attacks planar \(\text{C}^+\) with equal 50% probability from either face → racemic mixture (loss of optical activity).
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?
Substrate Preference Explanation
Tertiary (3°) → SN1
Bulky alkyl groups cause steric hindrance preventing backside attack, but stabilise the \(3^\circ\) carbocation intermediate via \(+I\) inductive effect.
Primary (1°) → SN2
Minimal steric hindrance allows easy backside attack; a \(1^\circ\) carbocation is too unstable to form via \(\text{S}_\text{N}1\).
Molecularity in Reaction Nomenclature
The numbers "1" and "2" in \(\text{S}_\text{N}1\) and \(\text{S}_\text{N}2\) refer to molecularity (the number of reactant particles in the rate-determining step), NOT the number of steps in the reaction!
Stereochemical and Mechanism Pitfalls
- Transition state brackets: For \(\text{S}_\text{N}2\), always draw square brackets with dotted partial bonds and a negative superscript \([\cdots]^-\).
- Stereochemistry: \(\text{S}_\text{N}2\) yields inversion; \(\text{S}_\text{N}1\) yields a racemic mixture.
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