How does Benzene React with Cl2 in presence of halogen Carrier
Answers
Explanation:
The halogenation of benzene
Substitution reactions
Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom.
The reactions happen at room temperature. The catalyst is either aluminium chloride (or aluminium bromide if you are reacting benzene with bromine) or iron.
Strictly speaking iron isn't a catalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorine or bromine to form iron(III) chloride, FeCl3, or iron(III) bromide, FeBr3.
2 Fe + 3 Cl -----> 2 FeCl3
2 Fe + 3 Cl -----> 2 FeBr3
These compounds act as the catalyst and behave exactly like aluminium chloride, AlCl3, or aluminium bromide, AlBr3, in these reactions.
The reaction with chlorine
The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene.
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or, written more compactly:
C6H6 + Cl2 -----> C6H5Cl + HCl
The reaction with bromine
The reaction between benzene and bromine in the presence of either aluminium bromide or iron gives bromobenzene. Iron is usually used because it is cheaper and more readily available.
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or:
C6H6 + Br2 -----> C6H5Br + HBr
Addition reactions
In the presence of ultraviolet light (but without a catalyst present), hot benzene will also undergo an addition reaction with chlorine or bromine. The ring delocalisation is permanently broken and a chlorine or bromine atom adds on to each carbon atom.
For example, if you bubble chlorine gas through hot benzene exposed to UV light for an hour, you get 1,2,3,4,5,6-hexachlorocyclohexane.
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Bromine would behave similarly.
The chlorines and hydrogens can stick up and down at random above and below the ring and this leads to a number of geometric isomers. Although there aren't any carbon-carbon double bonds, the bonds are still "locked" and unable to rotate.
One of these isomers was once commonly used as an insecticide known variously as BHC, HCH and Gammexane. This is one of the "chlorinated hydrocarbons" which caused so much environmental harm.