Phenols nitrosophenols

The colorations produced in this reaction arise from the action of nitrous acid on the phenol, giving />-nitrosophenol (I) which then reacts with excess of phenol to form an indophenol (II) which is an acid-base indicator ... 

The theory that the catalysed nitration proceeds through nitrosation was supported by the isolation of some />-nitrosophenol from the interrupted nitration of phenol, and from the observation that the ortho.-para ratio (9 91) of strongly catalysed nitration under aqueous conditions was very similar to the corresponding ratio of formation of nitrosophenols in the absence of nitric acid. ... 

Recently it has been claimed (Ref 42) that PA may be prepd by nitrosating phenol with Na nitrate and Oxidizing the resulting nitrosophenols with nitric acid,... 

Smoking. The effects of smoking on the formation of N-nitros-amines in bacon has been investigated recently by Bharucha et al. ( ). They reported that unsmoked bacon samples generally tended to contain more N-nitrosamines, presumably because of their higher nitrite content at the time of frying. Sink and Hsu (55) showed a lowering of residual nitrite in a liquid smoke dip process for frankfurters when the pH also was lowered. The effects of smoke seem to be a combination of pH decrease and direct C-nitrosation of phenolic compounds to lower the residual nitrite in the product (56). This is an area which requires further study since certain C-nitrosophenols have been shown to catalytically transnitrosate amines in model systems (57). 

The nitrosophenol (10), which may be isolated, is oxidised very rapidly by nitric acid to yield the p-nitrophenol (11) and nitrous acid more nitrous acid is produced thereby and the process is progressively speeded up. No nitrous acid need be present initially in the nitric acid for a little of the latter attacks phenol oxidatively to yield HN02. The rate-determining step is again believed to be the formation of the intermediate (9). Some direct nitration of such reactive aromatic compounds by N02 also takes place simultaneously, the relative amount by the two routes depending on the conditions. 

Phenols are rather common antimicrobial components of metalworking fluids however, their use in recent years has been declining (36). The inhibition of nitrosation by phenols has recently been reviewed (35). In general, phenolic compounds inhibit nitrosation by reacting with nitrite (phenol reacts with nitrite 10,0 0 0 times faster than with dimethylamine), but the intermediate nitrosophenolis unstable and enhances nitrosation. "The overall effect is dependent on the steady state concentration of the nitrosophenol and the relative degrees of retardation and enhancement exerted by the phenol and the nitrosophenol, respectively ( 35)". 

The Liebermann Reaction.—Dissolve a small amount of nitrosophenol in a little molten phenol and add concentrated sulphuric acid. A magnificent cherry-red colour is obtained when the melt is diluted with water and alkali is added the colour changes to blue. 

Since phenol is converted to nitrosophenol by nitrous acid, even when the latter is combined in the form of the NO-group, labile nitroso-groups may he detected by the Liebermann reaction. 

Dimethylhydrazine yV-Nitrosoglyphosate, see Glvphosate 2-Nitroso-yV-isopropylbenzamide, see Bentazone Nitrosophenol, see Phenol Nitrosyl chloride, see Chloropicrin... 

Nitrosation of phenolic substrates usually uses nitrous acid prepared in situ from a dilute mineral acid and an alkali metal nitrite. In general, for every phenolic group present in a substrate an equal number of nitroso groups can be introduced into the aromatic ring phenol, resorcinol and phloroglucinol react with nitrous acid to form 4-nitrosophenol, 2,4-dinitrosoresorcinol and 2,4,6-trinitrosophloroglucinol respectively. 

Nitro alcohols were reduced to amino alcohols by catalytic hydrogenation over platinum [632] and with iron [JJ9], and nitrosophenols [255] and nitro-phenols [256] to aminophenols with sodium hydrosulfite, sodium sulfide [238] or tin [176]. Bromine atoms in 2,6-di-bromo-4-nitrophenol were not affected [176]. 

We note that the difference between the enthalpy of formation of the unsubstituted benzoquinone/nitrosophenol and that of its isopropyl methyl cymene-like disubstituted derivative is 146 kJmol, comparable to that of the corresponding hydrocarbons, benzene and its substituted counterpart as liquids, 127.0 1.2 kJmol, and as the corresponding phenols as solids, 144.4 9.6 kJmol . ... 

The nitrosation of phenols proceeds in a manner similar to that of tertiary amines. For example, l-nitroso-2-naphthol has been prepared from the sodium salt of /9-naphthol by treatment with sodium nitrite and sulfuric acid near 0°C [29], This general procedure, suitably modified, has been used to prepare other nitrosophenols such as p-nitrosophenol (m.p. 135°-136°C) [30]. 

At low acid concentrations, nitric oxide tends to form. This evidently may attack nitrosophenol to form diazonium compounds directly. The diazonium salts, in turn, may couple with unreacted phenol to give colored products. Nitrous acid may also produce nitrophenols from phenols. The mechanism of this reaction may involve oxidation of initially formed nitrosophenols, homolytic attack by nitrogen dioxide, or nucleophilic attack by nitrite ions [1]. 

Nitrosophenols are formed very easily by the action of nitrous acid on phenols. Some salts of heavy metals have weak initiating properties. These are lead dini-trosophenate and lead trinitrosophloroglucinate. 

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. 

If the phenol is allowed to react with nitrous acid (generated in an acidified solution of sodium nitrite), the nitrosophenol may be obtained in good yield. An example is provided by the nitrosation of 2-naphthol which yields l-nitroso-2-naphthol (Expt 6.103). 

The formation of Af-nitrosamines which usually separate as orange-yellow oils or low melting solids indicates the presence of a secondary amine. Confirm the formation of the nitrosamine by the Liebermann nitroso reaction. This consists in warming the nitrosamine with phenol and concentrated sulphuric acid. The sulphuric acid liberates nitrous acid from the nitrosamine, and the nitrous acid reacts with the phenol to form p-nitrosophenol, which then combines with another molecule of phenol to give red indophenol. In alkaline solution the red indophenol yields a blue indophenol anion. 

Along with nitration processes, isomerization processes may take place which in turn may lead to various fairly complex reactions. As a result such products as C02, CO, NH3 are formed. Such reactions are particularly notable in the nitration of phenols. Their mechanism has been explained by Seyevetz [81] in the following way. A phenol undergoes nitrosation under the influence of nitrous acid present in the nitrating acid. Nitrosophenol isomerizes to quinone oxime, which oxidizes at the double bonds to form mesoxalic acid and its oxime ... 

Nitrous acid reacts with phenol to form nitrosophenol. 

Pure p- nitrophenol may also be prepared by nitrosation of phenol, followed by oxidation of p- nitrosophenol with dilute nitric acid. The m- isomer is usually prepared by diazotization of m- nitroaniline. 

Oxidation of nitrosophenols. This method is applied when fewer nitro groups are to be introduced on the ring than those obtained by way of nitration. It is also applied when a pure product, free from its isomers, is to be prepared, e.g. p- nitro-phenol from phenol. Direct nitration of phenol yields a mixture of the o- and p- isomers, dinitrophenol being readily formed as a by-product. Similarly dinitroresorcinol may be obtained by oxidation of dinitrosoresorcinol (p. 537). 

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