E2 Reaction Study Guide

Overview

The E2 (elimination bimolecular) reaction is a one-step, concerted elimination where all three events happen simultaneously:

1. A strong base removes a β-hydrogen.
2. The leaving group departs.
3. The π bond (double bond) forms.

Because both the substrate and the base are involved in the rate-determining step, E2 follows second-order kinetics.

Key Aspects

• Strong base required
• Anti-periplanar geometry between β-H and leaving group — critical for exams
• Regioselectivity: Zaitsev vs. Hofmann product
• Stereospecificity: anti elimination
• Competes with SN2

Mechanism — The Single Concerted Step

Unlike SN1 or E1, there is no carbocation intermediate. The base removes a β-hydrogen while:

• The C–H bond breaks
• The C–X bond breaks (leaving group departs)
• The π bond forms between the α- and β-carbons

All at once — this is why E2 is called a concerted mechanism.

Anti-Periplanar Requirement — Critical for Exams

For E2 to proceed, the β-hydrogen and leaving group must be anti-periplanar — exactly 180° apart in the same plane. This geometric requirement arises from orbital alignment: the σ C–H bond must overlap with the σ* C–X antibonding orbital to allow simultaneous bond breaking and π bond formation.

In Acyclic Systems

Free rotation around C–C bonds allows molecules to adopt the required anti conformation easily. Draw a Newman projection to confirm the geometry before predicting the product.

In Cyclic Systems (Cyclohexane) — Highly Tested

The leaving group and the β-hydrogen must both be axial. If the leaving group is equatorial, a chair flip must occur before elimination can take place. This is one of the most commonly tested E2 concepts — always draw the chair conformation and identify axial substituents before predicting the E2 product.

Regioselectivity: Zaitsev vs. Hofmann

When multiple β-hydrogens exist, which alkene forms as the major product?

Zaitsev's Rule — Default Outcome

The more substituted alkene is favored. More substituted alkenes have more stabilizing hyperconjugation interactions and are thermodynamically more stable. When a small, unhindered base is used, it preferentially removes the β-H that leads to the most substituted alkene.

Hofmann Product — The Exception

Forms when a bulky base is used (e.g., tert-butoxide, LDA, triethylamine). Steric hindrance prevents the bulky base from approaching the more substituted β-hydrogen, so it instead removes the less hindered, less substituted β-hydrogen — giving the less substituted alkene (Hofmann product).

Kinetics

Second-Order Kinetics

Rate = k[substrate][base]

• Stronger base → faster reaction
• Higher base concentration → faster reaction
• More substituted substrate → generally faster (3° > 2° > 1°)

Unlike SN1/E1, no carbocation stability is required. The rate depends directly on base strength — not on how well the substrate stabilizes a positive charge.

Energy Diagram for E2

E2 shows a single energy barrier with one transition state — a concerted, four-center geometry where the base, β-hydrogen, α-carbon, and leaving group are all partially bonded simultaneously. There is no intermediate.

Factors Influencing E2

Substrate Structure

• Tertiary → excellent; SN2 cannot compete
• Secondary → good
• Primary → possible with a strong bulky base

β-Branching also increases E2 relative to SN2, even with small nucleophiles, due to steric hindrance around the carbon center.

Base Strength

A strong base is required: OH⁻, OR⁻, tert-butoxide, LDA, triethylamine. Weak bases favor E1 instead (or SN1/SN2 depending on the substrate).

Solvent Effects

Polar aprotic solvents increase base strength (nucleophile/base left "naked") and favor E2. Polar protic solvents can weaken the base through hydrogen bonding, pushing the reaction toward E1 or SN1.

Competition: E2 vs SN2

• Strong base + primary substrate → SN2 favored
• Strong bulky base + primary substrate → E2 favored
• Secondary substrate → mixture possible
• Tertiary substrate → E2 only (SN2 impossible)

Practice drawing chair conformations. Practice predicting Zaitsev vs. Hofmann. Practice distinguishing E2 from SN2. The roadmap PDF (available from the overview guide) walks through the full decision tree for all four reactions.