Bi-phenyl \[{{C}_{6}}{{H}_{5}}\text{ }{{C}_{6}}{{H}_{5}}\]
Category : JEE Main & Advanced
It occurs in coal-tar. It is the simplest example of an aromatic hydrocarbon in which two benzene rings are directly linked to each other.
(1) Methods of formation
(i) Fittig reaction : It consists heating of an ethereal solution of bromobenzene with metallic sodium.
(ii) Ullmann biaryl synthesis : Iodobenzene, on heating with copper in a sealed tube, forms biphenyl. The reaction is facilitated if a strong electron withdrawing group is present in ortho or para position.
(iii) Grignard reaction : Phenyl magnesium bromide reacts with bromo benzene in presence of \[CoC{{l}_{2}}\].
(2) Properties : It is a colourless solid, melting point 71°C. It undergoes usual electrophilic substitution reactions. Since aryl groups are electron withdrawing , they should have deactivating and m-orientating effect. But, it has been experimentally shown that presence of one benzene ring activates the other for electrophilic substitution and directs the incoming group to o- and p- positions. It has been shown that monosubstitution in the bi-phenyl results in the formation of para isomer as the major product.
Another special feature of the biphenyl is the behaviour towards second substitution in a monosubstituted biphenyl. The second substituent invariably enters the unsubstituted ring in the ortho and para position no matter what is the nature of substituent already present.
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