Cyclohexanes halo-substituted—

If this principle is taken to its ultimate conclusion, there should be halo-cyclohexanes for which an E2 reaction is impossible. 2,6-Dimethyl-l-bromocy-clohexane (34) is such a case. To satisfy the relative stereochemistry of the two methyl groups (cis to each other) and the bromine anti to the two methyl groups), the bromine atom must be axial in one chair conformation with two axial methyl groups (34A), but equatorial in the other chair conformation that has two equatorial methyl groups (34B). Only conformation 34A has an axial bromine atom required for an E2 reaction, but both P-hydrogen atoms (Hg and Hb) are equatorial. No P-hydrogen atoms are trans, diaxial to an axial bromine, so there is no E2 reaction. When 34 is heated with ethanol KOH, there is no E2 reaction. The carbon bearing the bromide in 34 is very sterically hindered, so an Sn2 reaction is very unlikely certainly the substitution will be very slow. 

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