Investigate the stereochemistry of halogen addition

•	Lets look again at the addition reaction of bromine with cyclopentene. We will assume that Br+ adds to cyclopentene from the bottom face to form the carbocation intermediate shown below:

 

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Since the positively charged sp2 carbon is planar, it could be attacked by the bromide ion in the second step of the reaction from either the top or bottom face to give a mixture of products. One product has the two bromine atoms on the same side of the ring (cis), the other has the bromines on opposite sides (trans).

•	However, we actually find that only trans-1,2-dibromocyclopentene is produced. None of the cis product is formed. Therefore, the mechanism proposed above cannot be correct.

•	In order to explain the stereochemistry of halogen addition it has been suggested that the true reaction intermediate is not a carbocation, but is instead a bromonium ion. A bromonium ion is a symmetrical three-membered ring containing a bridged bromine atom carrying a positive charge.

•	If a bromonium ion is formed as an intermediate it would shield one face of the molecule. Attack by the bromide ion in the second step could then only occur from the opposite, unshielded face to give the trans product exclusively.

 

•	This mechanism would explain the observed stereochemistry of halogen addition reactions. However, can we find further evidence for the existence of the bromonium ion intermediate?