The Addition of a Halogen to an Alkene

The halogens Br2 and CI2 add to alkenes. This may surprise you because it is not immediately apparent that an electrophile—which is necessary to start an electrophilic addition reaction—is present. 

The reaction is possible because the bond joining the two halogen atoms is relatively weak and therefore is easily broken (Table 5.1) and because Br2 and CI2 are polarizable. As the nucleophilic alkene approaches Br2 (or CI2), it induces a dipole, which causes one of the halogen atoms to have a partial positive charge. This is the electrophile that adds to the sp carbon of the alkene. The mechanism of the reaction is shown on page 257. 

When CI2 (or Br2) adds to an alkene, the final product of the reaction is a vicinal dichloride (or dibromide). Vicinal means that the two bromines are on adjacent carbons (vicinus is the Latin word for near ). Reactions of alkenes with Br2 or CI2 are generally carried out by mixing the alkene and the halogen in an inert solvent, such as dichloromethane (CH2CI2), which readily dissolves both reactants, but does not participate in the reaction. 

As the TT electrons of the alkene approach a molecule of Br2, one of the bromines accepts those electrons and releases the shared electrons to the other bromine, which leaves as a bromide ion. Because bromine s electron cloud is close enough to the other sp carbon to form a bond, a cyclic bromonium ion intermediate is formed rather than a carbocation intermediate. 

The cyclic bromonium ion intermediate formed from the reaction of Bt2 with c/s-2-butene shows that the electron-deficient region (the blue 

---

Normally, addition reactions are more readily accomplished than substimtion reactions. This is hinted at in the reaction conditions specified above. The addition of a halogen to an alkene will occur easily at room temperature, whereas the substimtion of a halogen for a hydrogen in an alkane requires either high temperature or ultraviolet light. This shows that unsaturated hydrocarbons are more reactive than saturated hydrocarbons. 

The absence of rearrangement and the anti stereochemistry of the addition product must be accommodated in a proposed mechanism for the addition of a halogen to an alkene involving a carbocation intermediate. The first step of the reaction mechanism is the electrophilic addition of bromine to the Jt bond to give a three-membered ring called a tydic bromonium ion. 

---