ELECTROPHILLIC ADDITION ------HALOGEN ADDITION

     

v Addition of Bromine and Chlorine  Cl₂ or Br₂ is used to convert an

alkene into a vicinal dihalide.

v The mechanism involves attack by the alkene pi  bond on one atom of X₂ to give a bridged halonium ion intermediate (a cation) that is, in turn, attacked by X₂ from the backside to give the vicinal dihalide. Rearrangements do not occur.

Not all electrophilic additions necessarily involve carbocations, although they  involve the development of positive charge on the alkene, because it is serving as a nucleophile.

The addition of bromine, chlorine , or iodine to an alkene pi bond proceeds via an intermediate which has the positive charge mainly on the halogen.

This is because the positive charge is more stable on the halogen than on carbon.

The primary reason is that , an extra bond (a carbon-halogen bond) is formed in this intermediate, which is called an EPIHALONIUM ION .

v The reaction displays anti addition stereoselectivity because of the halonium ion intermediate .

v The reaction is stereospecifi c because Z alkenes give different products  than E alkenes.

The addition of bromine,  to trans-2-butene yields only meso-2,3-dibromobutane and no traces of the enantiomeric pair.

In contrast, cis-2-butene yields only the enantiomeric pair as a racemate, and none of the meso isomer.

By comparing the structure of the alkene (which we have drawn as a Newman projection) with that of the product corresponding to it, we can see that the bromines had to add to opposite faces of the double bond. This is termed anti stereospecific addition.

The addition of bromine,  in the presence of solvents such as WATER

Or METHANOL will also occur via a cyclic bromonium intermediate.

But, instead of bromine nucleophile attacking ,due to high concentration of solvent ,the solvent used acts as anucleophile ,and in this case WATER or METHANOL .

Now that we have established that bromine addition to alkenes is anti ,let us explore bromine addition to cyclic compounds .

v Usually the halogen is dissolved in some inert solvent such as tri- or tetrachloromethane,and then this solution is added dropwise to the alkene.

v Reaction is nearly instantaneous, even at room temperature or below. No light or heat is required, as in the case of substitution reactions.

v The addition of bromine can be used as a chemical test for the presence of unsaturation in an organic compound. Bromine solutions in tetrachloromethane are dark reddish-brown, and both the unsaturated compound and its bromine adduct are usually colorless.

v As the bromine solution is added to the unsaturated compound,

the bromine color disappears.

v    If the compound being tested is saturated, it will not

react with bromine under these conditions, and the color will persist.