Halogen molecules such as Cl₂ and Br₂ also add to alkene double bonds. Here we need not be concerned with orientation since the two ends of the adding molecule are identical, but the electrophilic addition mechanism helps us understand another characteristic of this reaction, its stereochemistry.

If bromine is added to cyclopentene, we might anticipate two products which differ in how the bromine atoms are geometrically related to each other. If the two bromines are on the same face of the ring, the compound is called cis. If they are on opposite faces, the compound is called trans. The experimental result is that only the trans product is formed.

One of the bromine atoms is acting as an electrophile. If we apply the usual mechanism, its first step it would go like this:

As we learned in our study of S_N1 reactions, carbocations are attacked by nucleophiles on both faces. If a carbocation is present in this system, we’d expect to find both the cis and trans products.

This is not what happens when the experiment is done, so we conclude that the carbocation is not present, or it reacts immediately to form a new cation. Something is happening which prevents the nucleophilic bromide ion from attacking the face of the carbocation which already is attached to the first bromine. We understand that by envisioning that the electrophilic bromine atom attaches itself to both alkene carbon atoms. One bond is made using the electrons from the pi bond, and the other is made using an unshared electron pair from the bromine. This results in a new ring formed from the bromine and the two alkene carbon atoms. It is called a “bromonium” ion.  Note that the mechanism for forming the three-membered ring has some parallels with cyclopropanation, described above.

Attack by the nucleophilic bromide ion on the bromine in this ring would only result in cyclopentene and bromine, so no reaction would occur. Attack by the bromide ion on the either of the alkene carbons would be like an S_N2 reaction. The attacked carbon would invert, and the product would have the trans configuration. (Notice that there are two such products, which are enantiomers, so we get a racemic mixture.)