Regiochemistry and Stereochemistry of the El Reaction

When regioisomers are possible in an El reaction, the product distribution is found to follow Zaitsev s rule. The reaction of 2-bromo-2-methylbutane under Sn1/E1 conditions (in a polar solvent mixture of ethanol and water with no good base or nucleophile present) gives 64% of the substitution products (water acts as the nucleophile to give an alcohol, or ethanol acts as a nucleophile to give an ether), 30% of the more highly substituted alkene, and 6% of the less highly substituted alkene. 

As in the E2 reaction, the relative yields of the alkenes are under product development control—the more stable alkene predominates. 

Unlike the E2 reaction, the relative stereochemistry of the leaving group and the hydrogen is not important in the El elimination reaction. In the first step of the reaction, the leaving group departs, producing a planar carbocation. Only at this point must the C—H bond be aligned parallel to the empty p orbital of the carbocation so that pi over- 

The bromine is bonded to a tertiary carbon and there is not a strong base present, so the reaction will proceed by an SN1 /E1 mechanism. The substitution product should predominate. The El reaction follows Zaitsev s rule, so more 1-methylcyclohexene should be formed than methylenecyclohexane. 

When this mechanism does occur, the second step often determines the rate. Because this step involves a unimolecular reaction of the conjugate base of the initial reactant, the mechanism is designated as elimination, unimolecular, conjugate base—or Elcb. 

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