When cyclohexene is treated with bromine in carbon tetrachloride, the racemic anti-1,2-di bromocyclohexane is obtained exclusively.

The result is no surprise if we think first of the formation of the bromonium ion that is opened with inversion in an SN2 reaction.

Bromination of alkenes is stereospecific because the geometry of the starting alkene deter mines which product diastereoisomer is obtained. We couldn’t demonstrate this with cyclohexene because only a Z double bond is possible in a six-membered ring. But bromination or chlorination of Z and E-2-butene in acetic acid produces a single diastereoisomer in each case, and they are different from each other. Anti-addition occurs in both cases—more evidence that a bromonium ion is the intermediate.

The stereochemistry of the products is a bit clearer if we redraw them, and in the scheme below the product of each reaction is shown in two different ways. Firstly, the products have been rotated to place the carbon chain in the plane of the paper: in this conformation you can clearly see that there has been an anti-addition across the E double bond. Secondly, the middle bond has been twisted 180° to give an (unrealistically) eclipsed conformation. We show this conformation for two reasons: now you can clearly see that there has been an anti-addition across the Z double bond too. It also makes it quite clear that the product of the E-butene bromination is achiral: you can see the plane of symmetry in this conformation, and this is why we haven’t placed (±) signs next to the products from the E alkene.

Note that in all three different views of each product the same stereoisomer is represented. There is no change of configuration, only changes of conformation to help you understand what is going on. If you cannot follow any of the ‘redrawing’ steps, make a model. With practice, you will soon learn to manipulate mental models in your head, and to see what happens to substituents when bonds are rotated. Most importantly, don’t let all of this more subtle stereochemical discussion cloud the simple message:

●Bromine undergoes anti-addition to alkenes.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic additions to alkenes can be stereospecific

The regioselectivity of epoxide opening can depend on the conditions