Nitrosation of phenols and tertiary amines

By far the most stable nitroso compounds are those of the aromatic series. The smoothest C-nitrosation is achieved with phenols and tertiary amines having a free para-position. In benzene derivatives the nitroso group enters almost exclusively284 the para-position to the hydroxyl or amino group. In the naphthalene series the reaction is not always unidirectional 1-nitroso-2-naphthol is obtained almost quantitatively from 2-naphthol,285 but 1-naph-thol gives a mixture of about equal parts of 2- and 4-nitroso-l-naphthol.286 

The nitrosating agent most often used is nitrous acid itself. Phenols are usually dissolved in sodium hydroxide solution, aqueous sodium nitrite solution is added, and nitrous acid is developed by dropping in a dilute mineral acid. Tertiary amines are dissolved in an excess of mineral acid, and the calculated amount of sodium nitrite solution is dropped in. The reaction temperature should not exceed 10°. 

Bridge s classical procedure287 may be cited as an example of nitrosation of a phenol Phenol (60 g), sodium hydroxide (27 g), and sodium nitrite (54 g) are dissolved in water (1500 ml) and treated gradually with a mixture of concentrated sulfuric acid (150 g) and water (400ml) at 7-8° with cooling. After 2 h the precipitated nitrosophenol is filtered off, washed with ice-water, taken up in ether, and shaken with animal charcoal at room temperature. Evaporation of the ether affords an 80% yield of p-nitrosophenol as yellow needles that melt at 125° with decomposition. For purification it may be recrystallized from benzene or toluene. 

It is sometimes better to work with an alcoholic solution. 

According to Organic Syntheses288 nitrosothymol is prepared by adding sodium nitrite (72 g) gradually, in portions, to a solution of thymol (100 g) in a mixture of 95% ethanol 

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