Electrophilic addition to alkenes can produce stereoisomers

When cyclohexene is treated with bromine in carbon tetrachloride, the racemic rmfz- 1,2-dibroinocy-clohexane is obtained exclusively. 

The result is no surprise if we think first of the formation of the bromonium ion that is with inversion in an Sn2 reaction. Here is the mechanism drawn flat , which is all we need to the stereochemistry of the product. The fact that this reaction (like other similar ones) gives diastereoisomer is one of the best pieces of evidence that electrophilic additions of Br2 to proceed through a bromonium ion. 

We don t need to write ( ) next to the isomer that isn t formed, because it is an achiral structure—it has a plane of symmetry and is a meso compound. See p. 396. 

This part of the discussion Is a revision of the material in Chapter 18, When dealing with six-membered rings, you should always aim to draw their conformation, though in this base you can explain the result adequately without conformational diagrams. 

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When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

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