Chlorinated hydroxybiphenyls

Chlorobiphenyts. These compounds can be synthesized by direct chlorination of biphenyl in [he presence of iron or other calalysts. Other means of preparation include reaction of diazotized aminobiphenyl with copper chloride. Treatment of chlorobiphenyls at elevated temperatures (300—400°C] with strong caustic yields hydroxybiphenyls. Various reactions, normal to aromatic systems, will occur—usually on the unsubstlluted ring. 

Figure 6.17 Stop-flow NMR spectra of the chlorination products of 2-hydroxybiphenyl. Conditions column, LiChrospher RP18e, 125 x 4 mm, 5 pm eluent, acetonitrile (A), D20 (B) gradient, t = Omin A/B (55/45), t = 35min A/B (75/25) flow, l.Oml/min spectrometer, Bruker DRX 600 probe head, 4 mm z-gradient LC probe between 24 and 256 scans were acquired...

Figure 6.18 Stop-flow 2D WET-TOCSY spectrum of 3,5-dichloro-2-hydroxybiphenyl. Protons (H4.H6) of the chlorinated ring can be identified due to the magnetization transfer between the signals at 7.23 (H4) and 7.41 (H6) ppm. Conditions spectrometer, Bruker DRX 600 probe head, 4mm z-gradient LC probe. 13C decoupling was applied during the WET-pulse train and acquisition. The spectral size was 8k x 512 data points, with 16 scans per increment, and a 64 ms TOCSY mixing time...

Alkali hypochlorite solutions can be employed in chlorinating hydroxy- or methoxybiphenyl. One mole of 4-hydroxybiphenyl is dissolved in 1 mole of caustic soda and 6 liters of water. The solution is cooled, and 1 mole of sodium hypochlorite in solution is added slowly, with constant stirring. The reaction mixture is allowed to stand for about 1 hr and then warmed to 40 C. After filtration and cooling, 3-chloro-4-hydroxybiphenyl is precipitated by the addition of hydrochloric acid. 

Chloro-, bromo- and fluorobenzenes were hydrolyzed to phenol upon photolysis in water at different pHs chlorobenzene reacted markedly faster than the other halo derivatives [228]. Photosubstitution likewise occurred to some extent with various mcta-substitutedhalobenzenes [229]. Interesting was the case of monochlorobiphenyls in water in all cases variable amounts of hydroxybiphenyls were formed, but through a different mechanism according to the position of the chlorine on the aromatic ring [230,231]. 

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