O- and p-Nitrophenols

N.C6H3(0H).S03Na, mw 241.16, N 5.8%, orange-red crysts. Can be prepd by treating the product obtained on sulfonation of phenol with Na nitrate and an excess of sulfuric acid. This salt, which is a mixt of Na o- and p-nitrophenol-sulfonates, was patented in 1911 by A. Voight for use in expl mixts such as Na nitrophenol-sulfonate 22.5 to 25, K or Na nitrate 45 to 65,... 

Carbo- and heterocyclic nitroarenes react with anions of tert-butyl and cumyl hydroperoxides in the presence of strong bases to form substituted o- and p-nitrophenols. Regiochemistry of the hydroxylation can be controlled to a substantial extent by selection of proper conditions (Eq. 9.52).85... 

Not much is known about the gas-phase oxidation of the phenyl radical. In the Cl-atom initiated oxidation of 0-CHO in the presence of NO, Niki et al. [148] identified o- and p-nitrophenol (HO-0-NO2) as the major products. The most likely precursor for HO-0-NO2 appears to be the phenoxy radical 0-0 which is formed by ... 

Phenol may be converted into a mixture of o- and p-nitrophenols (Expt 6.102) by reaction with dilute nitric acid the yield of p-nitrophenol is increased if a mixture of sodium nitrate and dilute sulphuric acid is employed. Upon steam distillation of the mixture of nitrophenols, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent hydrochloric acid. The mechanism of the substitution probably involves an electrophilic attack (cf. Section 6.2.1, p. 851) by a nitrosonium ion at a position either ortho or para to the activating hydroxyl group, to yield a mixture of o- and p-nitrosophenols, which are then oxidised by the nitric acid to the corresponding nitrophenols. The reaction depends upon the presence in the nitric acid of traces of nitrous acid which serve as the source of the nitrosonium ion. 

As a result of nitrating phenol by means of N204 at a low temperature, a mixture of o- and p- nitrophenol was obtained. 

Thus, p- xylene yielded 2,5-dichloro-p-xylene. The principal reaction with anisole was also chlorination and with phenol nitration when nitration temperature was low (-80°C) both o- and p- nitrophenols were formed. At room temperature chlorination also occurred yielding 2,4-dichloro-6-nitrophenol and 3,6-dichloro-2-nitro-phenol. 

Simpson and Evans [I 31 reported that nitrophenols such as o- and p-nitrophenols, 2,4-dinitrophenol or such a nitro compound as chloramphenicol could be destroyed by some Pseudomonas species to form nitrous acid salts. 

When treated with alkalis, o- and p- dinitrobenzenes are slowly converted to o- and p- nitrophenols, while with ammonia they form o- and p- nitroaniline. These reactions proceed slowly at room temperature, and more rapidly on heating. 

Since apart from the nitrophenols some dark coloured, often resinous by-products are formed, o- and p- nitrophenols are Frequently prepared on an industrial scale From the corresponding chloronitrobenzene derivatives. 

Negative substituents enhance the acidic properties of phenols, an effect opposite to that produced with aromatic amines. o and p-Chloro-phenols are considerably stronger acids than phenol itself, and o- and p-nitrophenols are still stronger. Trinitrophenol, picric acid, is a strong acid whose salts are neutral and not decomposed by carbonic acid or by ammonium salts. These salts of picric acid can be salted out of neutral solutions by sodium or potassium chloride. With negatively substituted phenols, it may be possible to separate the phenolate from solutions which are neutral or weakly alkaline to litmus. In doubtful cases, just as with the amines, the precipitated material must be studied to determine whether it is the free phenol or one of its salts. The color of the precipitate gives an indication in the case of the nitrophenols, since the free phenols have only a weak yellow color, whereas the alkali salts are deep yellow. Solubility tests with indififerent solvents may be used in the case of uncolored compounds. Only the free phenol can be separated from acidic solutions. 

The alkyl ethers of o- and p-nitrophenols can also be prepared from o- and p-nitrochlorobenzenes bv the action of sodium alcoholates (see page 97). 

Distillation with steam in plant operations is a relatively expensive process because it requires large volumes of steam and cooling water. For laboratory work, however, it is a very suitable method for the smooth separation of organic products, which are volatile with steam, from non-volatile inorganic impurities and high molecular tarry byproducts. The separation of isomers is also possible in those instances where only one of the isomers is volatile with steam (e.g., o- and p-nitrophenols, page 147). Volatile acids and basic compounds can be separated from each other, and from neutral products, by successive steam distillations from acid and alkaline solution. 

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