SN2 Reaction Recap
- SN2 = Substitution Nucleophilic Bimolecular
- It happens in one step: the nucleophile attacks the substrate from the opposite side of the leaving group.
- The rate depends on both the substrate and the nucleophile.
Role of Steric Hindrance
Steric hindrance means the blocking of the nucleophile’s approach because bulky groups are crowding the reactive carbon.
- In SN2, the nucleophile must directly attack the carbon bonded to the leaving group.
- If there are bulky groups (like alkyl substituents) around that carbon, the nucleophile finds it hard to approach from the backside.
- This slows down or prevents the reaction.
Substrate Reactivity Trend (due to steric hindrance)
- Methyl halide (CH₃–X): No hindrance → SN2 is very fast.
- Primary alkyl halide (R–CH₂–X): Slight hindrance → SN2 works well.
- Secondary alkyl halide (R₂–CH–X): More hindrance → SN2 is slower.
- Tertiary alkyl halide (R₃–C–X): Very crowded → SN2 almost impossible.
In short:
Steric hindrance makes it difficult for the nucleophile to attack the carbon in SN2 reactions. The more substituted the carbon, the harder the attack, so the reaction rate decreases.