Ortho-nitrobiphenyl

Ortho-nitrobiphenyl (C12H9NO2) is an amber crystalline plasticizer compatible with practically all of the thermoplastics, with the possible exception of polyamides. 

Electrophilic aromatic substitution in biphenyl is best understood by considering one ring as the functional group and the other as a substituent. An aryl substituent is ortho, para-directing. Nitration of biphenyl gives a mixture of o-nitrobiphenyl and p-nitrobiphenyl. 

In support of the free radical mechanism is the observation that the usual directive influences are not operative in these reactions. Substitution usually takes place para and ortho to the substituent in the benzene ring, irrespective of the nature of the groups. Even with nitrobenzene, para and ortho derivatives are formed. Thus, N-nitrosoacetanilide and nitrobenzene give 2- and 4-nitrobiphenyl. 

M. Klein et al., From mutagenic to non-mutagenic nitroarenes Effect of bulky alkyl substituents on the mutagenic activity of 4-nitrobiphenyl in Salmonella typhimurium. Part I. Substituents ortho to the nitro group and in 2 -position. Mutat. Res. 467, 55-68 (2000)... 

Photochemical reactions involving ortho-ortho interactions have been reported.80 The anil (39) gives phenanthridone (25%) when irradiated in ethanol, and 2-(cyanoanilinomethyl)-2/-nitrobiphenyl (40) forms 6-cyanophenanthridine under similar conditions. The same product is also formed when the biphenyl (40) is treated with sodium ethoxide.80... 

Biphenyls containing four large groups in the ortho positions cannot freely rotate about the central bond because of steric hindrance. For example, the activation energy (rotational barrier) for the enantiomerization process was determined, AG = 21.8 0.1 kcal mol , for the chiral 2-carboxy-2 -methoxy-6-nitrobiphenyl. In such compounds, the two rings are in perpendicular planes. If either ring is symmetrically substituted, the molecule has a plane of symmetry. For example, consider the biaryls ... 

We will first consider the synthesis of carbazole (2) from 2-nitrobiphenyl (1), in view of the information that can be gained from this reaction on the mechanism of the carhonylation of organic nitro compounds having an unsaturated substituent in the ortho position. 

Good yields o-f azobenzenes were also obtained -from several ni trobenzenes substituted with meta or para electron-releasing substituents. On the other hand, ortho substituents -favoured the -formation o-f anilines, perhaps becouse o-f their steric e-f-fect in retarding the competing reaction leading to the azo compound. 2-Nitrobiphenyl gave 2-aminobiphenyl as the major product, together with a small amount o-f carbazole ... 

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