SN2 Reaction Study Guide

Overview

The SN2 (substitution nucleophilic bimolecular) reaction is:

• Single-step and concerted
• Proceeds via backside attack
• A substitution reaction — nucleophile replaces the leaving group

Unlike SN1, there is no carbocation intermediate. The nucleophile attacks as the leaving group leaves — simultaneously. Because both substrate and nucleophile participate in the rate-determining step, SN2 follows second-order kinetics.

Mechanism

Backside Attack

The nucleophile approaches the electrophilic carbon from the opposite side of the leaving group. Why? The σ* antibonding orbital of the C–X bond is only accessible from the backside. As the nucleophile begins forming a bond, the C–X bond breaks simultaneously — one concerted motion.

Stereochemistry — Inversion of Configuration

SN2 reactions cause complete inversion at the stereocenter — known as Walden inversion. If the carbon is chiral: R becomes S, and S becomes R (unless the change in substituents alters the CIP priority ranking). This inversion is a defining, testable feature of SN2 and appears on virtually every organic chemistry exam.

Steric Effects — The Critical Concept

SN2 requires direct access to the electrophilic carbon from the backside. Steric bulk around that carbon directly determines reactivity:

• Methyl → fastest (no steric bulk at all)
• Primary → very good
• Secondary → slower (some hindrance)
• Tertiary → does not occur (too hindered for backside approach)

This steric sensitivity is one of the biggest distinctions between SN2 and SN1.

Kinetics

Second-Order Kinetics

Rate = k[substrate][nucleophile]

Doubling the nucleophile concentration doubles the rate. A stronger nucleophile gives a faster reaction. This is the key mechanistic difference from SN1 — the nucleophile directly participates in the rate-determining step.

Solvent Effects

Polar Aprotic Solvents Favor SN2

Examples: DMSO, DMF, acetone. These solvents do not hydrogen-bond strongly to the nucleophile, leaving it "naked" and highly reactive. This dramatically increases nucleophilicity.

Polar protic solvents (water, alcohols) hydrogen-bond to the nucleophile, reducing its reactivity and pushing the reaction toward SN1 instead.

Energy Diagram

The SN2 energy diagram shows a single energy barrier — one transition state, no intermediate. At the transition state, the carbon is partially bonded to both the nucleophile and the leaving group simultaneously, and the geometry resembles a trigonal bipyramidal structure (the three remaining substituents are roughly in a plane).

Factors Influencing SN2

Substrate Structure

Methyl > Primary > Secondary; Tertiary never undergoes SN2.

Nucleophile Strength

Strong nucleophiles favor SN2: negatively charged species (I⁻, Br⁻, CN⁻, RS⁻), less sterically hindered. Weak nucleophiles push the reaction toward SN1.

Leaving Group

Better leaving group = faster SN2. I⁻ > Br⁻ > Cl⁻ >> F⁻. Poor leaving groups (like OH⁻) must often be protonated first (converted to H₂O, a much better leaving group).

Competition: SN2 vs E2

• Strong nucleophile + primary substrate → SN2 favored
• Strong bulky base + primary substrate → E2 favored
• Secondary substrate → mixture possible
• Tertiary substrate → E2 only (SN2 impossible)
• Higher temperature → elimination favored over substitution