E2 in chairs: menthyl vs neomenthyl chloride template

Neomenthyl chloride has Cl axial in its stable chair, with an axial H anti on both neighbours, so with NaOEt it eliminates fast to menth-3-ene (Zaitsev, about 75 %) and menth-2-ene. Menthyl chloride must flip to the chair with all three groups axial; only C2–H is then anti, so it gives only menth-2-ene, about 200 times more slowly.

What the template shows

• E2 needs the H and the leaving group anti-periplanar; in a cyclohexane chair that means both axial (trans-diaxial)
• Which H is anti-periplanar to Cl?
• Neomenthyl chloride: Cl axial in the stable chair (fast)
• two axial H (red) anti to the axial Cl
• NaOEt, EtOH
• menth-3-ene
• trisubstituted (Zaitsev)
• menth-2-ene
• neomenthyl chloride
• Menthyl chloride: all equatorial, must flip first (about 200 × slower)
• ring flip
• Cl equatorial: no anti H, no E2
• minor chair, all axial: only the C2 H (red) is anti; the C4 H (blue) is equatorial
• menthyl chloride
• green = Cl · red H = axial, anti-periplanar to Cl · blue H = equatorial · red dashes join each anti pair
• In E2 the C–H and C–LG bonds must be anti-periplanar (180°) so σ(C–H) overlaps σ*(C–Cl). On a chair only trans-diaxial pairs qualify; equatorial H or Cl never do.
• Neomenthyl chloride: Cl is axial in the stable chair (isopropyl and CH₃ equatorial) and both neighbouring carbons have an axial H, so it reacts fast and mostly gives the more substituted menth-3-ene (about 75 : 25).
• Menthyl chloride: every group is equatorial, so it must ring-flip to the all-axial chair (a tiny fraction) to react; the C4 H is then equatorial and only the C2 H is anti: menth-2-ene only, about 200 times slower.
• Conformation beats Zaitsev’s rule: the alkene formed is set by which β-H can lie anti to the leaving group (Hughes and Ingold, NaOEt in EtOH).
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 52 atoms, 99 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 17 Textbook
• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 11 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 17 Textbook