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.
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