Nitrosation is the reaction introducing nitroso (-NO) to the carbon or nitrogen atoms of organic molecules. The commonly used reagents for nitrosation are nitrous acid (nitrite plus acid) and nitrite. Compared with nitro compounds, nitroso groups possess the nature of unsaturated bonds so that they can be reduced, oxidized, condensed and added to produce various types of intermediates, which have an important application value in drug synthesis. Since the nitrosation are an exothermic process, a low temperature is thought to be favorable for the reaction. Having recognized the fact that almost all of the nitro compounds are unstable and easy to be decomposed, the nitrosation should be, therefore, elaborately designed and strictly controlled or monitored.
Since phenol is nitrated with dilute nitrous acid, both ortho and para isomers are mainly produced, and there are oxidized and polymerized resinous by-products, thus reducing the yield of the main product. However, when the phenol is nitrosated, p-nitrosophenol is mainly obtained with high yields, and the p-nitrophenol can be obtained by further nitric acid oxidation of the nitrosophenol. In addition, nitrosophenol is also an important intermediate for the preparation of sulfur blue and p-aminophenol.
When nitrous acid reacts with aromatic secondary amines, it is easier to produce N-nitroso derivatives than to produce C-nitroso derivatives. The nitroso group is introduced into the ring of the secondary arylamine, and the N-nitroso derivative is always first produced, then isomerized in an acidic medium, and an intramolecular rearrangement reaction occurs to obtain a C-nitroso derivative.
When the nitroso is introduced into the ring of the tertiary aromatic amine, the corresponding para-substituted product is mainly obtained. For example, nitroso-N, N-dimethylaniline hydrochloride may be prepared by reacting N-dimethylaniline, hydrochloride with aqueous NaNO2. It is an important intermediate for the preparation of dyes, perfumes, pharmaceuticals, printing and dyeing auxiliaries.
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