Q6(c). E2 in hexachlorocyclohexanes: anti-periplanar requirement

Introduction. KOH in methanol removes HCl from 1,2,3,4,5,6-hexachlorocyclohexane by E2. In a cyclohexane chair, E2 needs the H and the Cl on adjacent carbons to be anti-periplanar, which means both axial (trans-diaxial). Comparing A and B tests this rule.

1. A (the β isomer) has all six Cl equatorial; on every carbon the H is therefore axial. Each pair of adjacent Cl atoms is trans, so an axial H always sits next to an equatorial Cl: H–C–C–Cl dihedral about 60° (gauche), never 180°.
2. Ring flipping of A gives the all-axial chair (very high in energy), but then all H are equatorial: again no axial H next to an axial Cl. No anti-periplanar H/Cl pair exists in any conformation.

Figure: Trans-diaxial H and Cl in B (dashed): the E2 geometry.

1. B has Cl axial at two carbons (C-2 and C-4 of the drawing). Their neighbours carry equatorial Cl and therefore axial H, anti-periplanar to the axial Cl: E2 is easy, so B reacts fast.
2. A can eliminate only by slower routes (syn elimination or a carbanion (E1cb) path); it reacts thousands of times more slowly than the other isomers.

Inferences

• The reaction is a bimolecular, concerted E2 (rate depends on base and on geometry), not E1, whose rate would not depend on the H/Cl arrangement.
• E2 is stereospecific: it requires anti-periplanar (trans-diaxial) H and Cl; a geometric requirement can slow a reaction by orders of magnitude even when the bonds are the same.
• Repeated E2 steps finally give trichlorobenzenes (mainly 1,2,4-trichlorobenzene).

Conclusion. A, with all Cl equatorial, has no anti-periplanar H–C–C–Cl arrangement in any chair, so E2 is very slow; B, with axial Cl next to axial H, eliminates fast. The reaction is E2 with an anti (trans-diaxial) stereochemical requirement.

Application. One hexachlorocyclohexane isomer, lindane (γ), is the insecticide; the resistance of the all-equatorial β isomer to dehydrochlorination makes it the most persistent HCH residue in soil and fat.