SN2 Reaction
SN2 Reaction: The nucleophile attacks the electrophilic carbon from the side directly opposite the leaving group in a single concerted step, displacing the…
What happens
The nucleophile attacks the electrophilic carbon from the side directly opposite the leaving group in a single concerted step, displacing the leaving group as the new bond forms.
Reaction summary
CH₃Br + OH⁻ → CH₃OH + Br⁻
Reagents and conditions
- Starting material: Bromomethane
- Reagents: Hydroxide
- Products: Methanol
Mechanism
- Backside attack: Hydroxide approaches from 180° opposite the C–Br bond; as the new C–O bond forms, the C–Br bond breaks simultaneously.
- Inversion of configuration: The carbon's other substituents flip through the transition state like an umbrella turning inside out (Walden inversion), then the product forms with inverted stereochemistry at that carbon.
Selectivity
- Stereoselectivity: Complete inversion of configuration at the reacting carbon (Walden inversion), always, in a single concerted step.
Common mistakes
- Trying to run SN2 on a tertiary substrate, steric hindrance from three alkyl groups blocks backside attack almost completely.
- Forgetting SN2's rate depends on BOTH the substrate and nucleophile concentration (second-order overall).
Memory trick
"SN2 = 2 things collide in 1 step": nucleophile and substrate react together in one concerted, second-order step with backside attack.