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#1 2010-08-18 13:50:55


Cyclohexene with bromine and water

Re: unsymmetrical alkenes
The opening of epoxides and bromonium ions can be thought of as paradoxical to the rules for SN1/SN2 reactions. Methylcyclohexene is often used as an example as the stereochemical effects can also be learned. The reaction of methylcyclohexene with Br2/H2O will give the intermediate bromonium ion. This will be opened with water. The opening of the bromonium ring occurs on the tertiary carbon, but this is not an SN1 opening. If it were a classic SN1 opening, then water could attack from either side of the carbocation. If it were a classic SN2 opening, then water should attack at the least hindered side of the bromonium ion. The actual result is an anti opening of the bromonium ion at the tertiary carbon. The result is an SN2-like opening of the bromonium ion at the tertiary carbon with inversion. The net addition is anti to give the trans product.

This example is excerpted from A Handbook of Organic Chemistry Mechanisms, available at or



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