Diazonium salts formation

Azo pigments, subdivided into the monoazo and disazo pigments, have the azo group (—N=N—) in common. The synthesis of azo pigments is economically attractive, because the standard sequence of diazonium salt formation and subsequent reaction with a wide choice of coupling components allows access to a wide range of products. 

The reaction proceeds via two main steps a diazotization reaction, which is the basis of diazonium salt formation, followed by azo coupling, which is responsible for the formation of the azo compound. 

On an industrial scale, diazotization reactions are carried out by dissolving the aromatic amine in hydrochloric or sulfuric acid. Despite the fact that 2 equivalents of acid per equivalent of amino group should theoretically suffice, as much as 2.5 to 3 equivalents per amino function are actually required to ensure complete diazonium salt formation. One equivalent of an aqueous sodium nitrite solution is added to the resulting mixture at 0 to 5°C. The exothermic nature of the reaction, combined with the heat sensitivity of most diazonium salts, makes it necessary to provide cooling, usually by direct addition of ice. 

Diazonium salt formation Primary arylamines react with nitrous acid (HNO2) to yield stable arenediazonium salts, Ar—N+=NX . Alkylamines also react with nitrous acid, but the alkanediazonium salts are so reactive that they cannot be isolated. 

The aminophenols are chemically reactive, undergoing reactions involving both the aromatic amino group and the phenolic hydroxyl moiety, as well as substitution on the benzene ring. Oxidation leads to the formation of highly colored polymeric quinoid structures. 2-Aminophenol undergoes a variety of cyclization reactions. Important reactions include alkylation, acylation, diazonium salt formation, cyclization reactions, condensation reactions, and reactions of the benzene ring. 

Microreactors provide a safe means by which reactions, including multistage schemes, can be undertaken where, otherwise, products involving unstable intermediates may be formed. This is exemplified by Fortt who showed that for a serial diazonium salt formation and chlorination reaction performed in a microreactor under hydrodynamic pumping, significant yield enhancements (15-20%) could be observed and attributed them to enhanced heat and mass transfer [77]. This demonstrates the advantage of microreactor-based synthesis where diazonium salts are sensitive to electromagnetic radiation and static electricity, which in turn can lead to rapid decomposition. Microreactors facilitate the ability to achieve continuous-flow synthesis, which is often not possible with conventional macroscale reactors and batch production. 

R—N=N. This procedure is called diazotization of an amine. Diazonium salts are the most useful products obtained from the reactions of amines with nitrous acid. The mechanism for diazonium salt formation begins with a nucleophilic attack on the nitrosonium ion to form an A-nitrosoamine. 

Study the mechanism of diazonium salt formation in Chapter 42 and then propose a mechanism for diazotization using isoamyl nitrite. 

The mechanism of diazonium salt formation involves the N-nitrosation of aniline,... 

Amines are characterized by their basicity and, thus, by their ability to dissolve in dilute aqueous acid (Sections 20.3A, 20.3E). Moist pH paper can be used to test for the presence of an amine functional group in an unknown compound. If the compound is an amine, the pH paper shows the presence of a base. The unknown amine can then readily be classified as 1°, 2°, or 3° by IR spectroscopy (see below). Primary, secondary, and tertiary amines can also be distinguished from each other on the basis of the Hinsberg test (Section 20.9A). Primary aromatic amines are often detected through diazonium salt formation and subsequent coupling with 2-naphthol to form a brightly colored azo dye (Section 20.8). 

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