Bi-phenyl (C₆H₅  - C₆H₅)

         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 giving temperature with copper in a freeze tube, forms biphenyl. The reaction is completed 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 CoCl₂.

         (2) Properties : It is a colourless solid, melting point 71°C. It undergoes usual electrophilic replacement reactions. Since aryl set are electron withdrawing , they should have deactivating and m-orientating effect. But, it has been usually given 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 given 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 replacement invariably involves the unsubstituted ring in the ortho and para position no matter what is the nature of substituent already present. 

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