Para -Bromophenol

Fig. 1. Plots of dielectric constant vs selectivity of bromination of phenol with Br2 (left) and NBS (solid powder) (right). ort/io-Bromophenol (A), para-Bromophenol ( ).

Phenols undergo electrophilic substitutions. In phenol, the substitutions take place in ortho and para positions. As the —OH group is an activating group, the reaction rate is much faster than usually observed with benzene. For example, the bromination of phenol produces ort/io-bromophenol (12%) and para-bromophenol (88%). 

More recently, (26) salts of para-bromophenols have been reacted with a variety of oxidizing agents as initiators under anhydrous conditions in the presence of aprotic, dipolar solvents. Under these conditions it is remarkable that the product from 2,6-dichloro-4-bromophenol appears to be the high molecular weight, linear 1.4-phenylene ether (VI). 

The shifts are smallest in the ortho and para positions so these are where there is greatest electron density and hence these are the sites for electrophilic attack. The shifts in the meta positions are not significantly different from those in benzene. If you want to put just one bromine atom into a phenol, you must work at low temperature (< 5°C) and use just one equivalent of bromine. The best solvent is the rather dangerously inflammable carbon disulfide (CS2), the sulfur analogue of C02. Under these conditions, para bromophenol is formed in good yield as the main product, which is why we started the bromination of phenol in the para position. The minor product is ortho bromophenol. 

TABLE 20. Some harmonic vibrational frequencies, IR intensities and assignments for para-bromophenols"... 

Following are a few typical examples of medicinal compounds that are prepared by the help of bromination method, namely para-Bromoacetanilide para-Bromophenol Tetrabromofluorescein (or Eosin). 

Phenol is taken up in dry carbon disulphide (CS2) which is subsequently reacted with a solution of Br2 in CS2 at a controlled temperature ranging between 0-5°C with vigorous constant stirring in an efficient fume cupboard. para-Bromophenol is obtained as the major product, whereas ori/io-bromophenol is also produced inevitably to a much lesser extent. 

At bp 145-150°C/25-30 mm Hg. Which being a reasonably pure para-bromophenol. 

The high reactivity of aniline can actually be a problem. Suppose we wanted to put just one bromine atom onto the ring. With phenol, this is possible (p. 480)—if bromine is added slowly to a solution of phenol in carbon disulfide solution and the temperature is kept below 5 °C, the main product is para-bromophenol. Not so if aniline is used—the main product is the triply substituted product. 

---