Elimination E2 Zaitsev

Alkyl Halide Reactions: Alkene formation using Strong Bases (E2 Zaitsev Product)

Alkyl Halide Reactions: E2 Zaitsev (Strong, Small Bases)

Strong, compact bases such as sodium ethoxide or sodium methoxide remove a β-hydrogen while the leaving group departs in the same step, delivering the Zaitsev alkene. The outcome hinges on anti-periplanar geometry: the base must approach a β-hydrogen that is anti to the C–X bond, and cyclohexane systems require the classic trans-diaxial arrangement. Use this guide to keep the geometry, regioselectivity, and common pitfalls front-of-mind when teaching or drawing E2 pathways with small bases.


Quick Summary

  • Class: E2 (bimolecular, concerted); rate = k[substrate][base].
  • Geometry: Requires anti-periplanar alignment of the β-hydrogen and the leaving group; in cyclohexanes this translates to a trans-diaxial pairing.
  • Regiochemistry: Small bases favor the more substituted (Zaitsev) alkene unless anti access is blocked; conjugation can override to give the most stabilized π-system.
  • Substrate order: Tertiary ≳ secondary > primary; methyl lacks a β-hydrogen.
  • Solvent/temperature: Polar aprotic or alcohol solvents support basicity; heat shifts substitution–elimination equilibria toward E2.
  • No intermediates: The reaction is concerted—no carbocation, no rearrangements.


Mechanism (3 Steps)

  1. Step 1 – Anti alignment – Rotate the substrate so the target β-hydrogen sits anti to the C–X bond. Cyclohexanes must adopt the conformer with an axial leaving group and an axial β-hydrogen on the adjacent carbon.
  2. Step 2 – Concerted elimination – The base removes the β-hydrogen while electrons flow into the C=C bond and the leaving group departs simultaneously.
  3. Step 3 – Alkene outcome – Label the product as Zaitsev when the anti pathway reaches the more substituted alkene; annotate the E or Z geometry dictated by the anti trajectory.
E2 Step 1: anti alignment of beta hydrogen and leaving group
Step 1 — Confirm the β-hydrogen is anti to the leaving group (trans-diaxial in rings).
E2 Step 2: concerted elimination to form the alkene
Step 2 — Push all three arrows in one motion: base to β-H, C–H to C=C, C–X to X⁻.
E2 Step 3: Zaitsev alkene outcome after using sodium ethoxide heat
Step 3 — Zaitsev alkene highlighted; note the geometry badge that reports the anti trajectory.


Worked Examples

Reactant panel showing 2-bromohexane poised for elimination Reagent panel showing sodium ethoxide with heat Product panel showing the Zaitsev alkene
2-bromohexane + sodium ethoxide (ethanol, reflux) — hex-2-ene (Zaitsev) dominates; cooling or switching to polar aprotic solvent re-opens SN2.
Reactant panel showing cyclohexyl bromide in the axial conformation Reagent panel showing sodium ethoxide Product panel showing the cyclohexene formed via trans-diaxial elimination
Exocyclic bromide on cyclohexane + sodium ethoxide — methylidenecyclohexane appears after the chair flip delivers the trans-diaxial β-hydrogen.
Reactant panel showing tert-pentyl chloride Reagent panel showing sodium ethoxide with heat Product panel showing 2-methyl-2-butene
Tert-pentyl chloride + sodium ethoxide (reflux) — 2-methyl-2-butene emerges as the Zaitsev alkene; highlight how bulky β-sites still favor substitution unless the base is heated.


Practical Tips

  • Choose strong, compact bases (ethoxide, methoxide, hydroxide) and add heat to favor elimination over substitution.
  • Draw the anti arrangement before arrow pushing; this saves time and prevents stereochemical mistakes.
  • Mention conjugation: forming a conjugated alkene may override Zaitsev/Hofmann expectations.
  • Remind students that there are no rearrangements in E2—if a rearranged product appears, they likely drew E1 by mistake.


Exam-Style Summary

Anti-periplanar geometry plus a strong, small base gives a concerted E2 that favors the more substituted alkene. Show the correct conformation, push the three arrows in one step, and label the Zaitsev product along with any E/Z outcome dictated by the anti path.

Watch for these pitfalls:

  • Lack of an anti β-hydrogen (e.g., fully substituted adjacent carbon) blocks E2; substitution or no reaction may result.
  • Constrained systems may push Hofmann if the Zaitsev β-hydrogen cannot align anti.
  • Poor leaving groups such as chlorides demand higher temperatures or a better leaving group (sulfonate, bromide, iodide).
  • Benzylic and allylic halides can still perform SN2 with small bases at low temperature—state conditions clearly when predicting E2 dominance.


Interactive Toolbox

  • Mechanism Solver — step through anti-alignment, concerted elimination, and product steps for Zaitsev-focused presets.
  • Reaction Solver — compare substitution and elimination outcomes as you change base strength, sterics, solvent, and temperature.
  • IUPAC Namer — practice naming the alkene products (include E/Z descriptors when needed).


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