Stereoelectronic Effects in the E2 Reaction

The effect of conformation on E2 reactions is demonstrated by the difference in the rates of the dehydrobromination of cis- and inrKf-l-bromo-d-rcrr-butylcyclohexane. The axial bromine atom of the cis isomer is eliminated about 500 times faster than the equatorial bromine atom of the trans isomer. Both isomers yield 4-fert-butyl-cyclohexene. 

Partial overlap of the developing ti orbitals in the transition state for an E2 reaction in which the leaving group and the proton are in an anti periplanar relationship. 

The stability of the cis- and trans-alkenes produced in the E2 reaction reflects the stabilities of the transition states leading to them. Steric crowing in the transition state leading to the cis isomer raises the activation energy relative to the transition state for forming the trans isomer. Therefore, the trans isomer forms faster and is the major product. 

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