Organic Reactions & Mechanisms (CHEM 220) · Module 3 of 9
The SN2 Mechanism in Depth
The concerted, backside-attack substitution pathway: kinetics, sterics, stereochemistry, and solvent.
Kinetics & the Concerted Mechanism
- SN2 (substitution nucleophilic bimolecular) occurs in a single concerted step: the nucleophile attacks as the leaving group departs, with a rate depending on the concentration of both the substrate and the nucleophile (rate = k[substrate][nucleophile]).
What does the second-order rate law of SN2 (rate = k[substrate][nucleophile]) tell you about its mechanism?
Solution. It indicates the rate-determining step involves both the substrate and the nucleophile simultaneously, consistent with a single concerted step where bond-breaking and bond-forming happen at the same time.
Steric Effects & Substrate Reactivity
- The nucleophile attacks from the backside, opposite the leaving group, so steric hindrance around the carbon dramatically slows SN2; reactivity order is methyl > primary > secondary ≫ tertiary, which essentially doesn't undergo SN2.
Why does a tertiary substrate essentially not undergo SN2 reactions?
Solution. The backside attack required for SN2 is blocked by the three bulky alkyl groups surrounding the carbon, making it sterically inaccessible to the incoming nucleophile.
Walden Inversion & Stereochemistry
- Because the nucleophile attacks from the side opposite the leaving group, the stereochemistry at that carbon inverts — like an umbrella flipping inside out — known as Walden inversion.
What happens stereochemically at the reacting carbon during an SN2 reaction?
Solution. The configuration inverts (Walden inversion) because the nucleophile attacks from the side directly opposite the leaving group, pushing the other three substituents to flip to the other side, like an umbrella turning inside out.
Solvent Effects
- Polar aprotic solvents, like DMSO or acetone, favor SN2 by dissolving the nucleophile's counter-cation without strongly solvating (and thus weakening) the nucleophile's anion.
Why do polar aprotic solvents speed up SN2 reactions compared to polar protic solvents?
Solution. Polar aprotic solvents lack O–H or N–H bonds, so they can't hydrogen-bond with and "cage" the nucleophile the way protic solvents do; this leaves the nucleophile more reactive ("naked") and better able to attack the substrate.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the rate law for an SN2 reaction?
2. What is the reactivity order of substrates for SN2?