Diazonium compounds, coupling detection

Salts of primary aromatic amines react with nitrous acid to produce diazonium salts. The reaction is usually performed by adding a cold solution of sodium nitrite to a cold solution of the arylamine in aqueous mineral acid. The end point of the reaction is conveniently determined by the detection of excess nitrous acid with potassium iodide-starch paper. Sulfamic acid has long been used both in industry and in the laboratory to remove excess nitrous acid. It has been found to react with the more active diazo compounds. In most cases, high temperatures are avoided to prevent the formation of phenols and the decomposition of the unstable nitrous acid. An excess of mineral acid is necessary to prevent coupling between the diazonium salt and unreacted amine (cf. method 494). If the amine salt is somewhat insoluble, a fine crystalline form, which is produced by rapid crystallization from a warm aqueous solution, may be employed. ... 

Phenol readily couples with diazonium salts to yield coloured compounds. The latter can be nsed for the photometric detection of phenol as in the case of diazotized 4-nitroaniline. Sahcylic acid (2-hydroxybenzoic acid) can be prodnced by the Kolbe-Schmitt reaction (stndied by the density functional method ) from sodinm phenolate and carbon dioxide, whereas potassium phenolate gives the para compound. Alkylation and acylation of phenol can be carried out with aluminium chloride as catalyst methyl groups can also be introduced by the Mannich reaction. Diaryl ethers can only be produced under extreme conditions. 

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). 

Nonetheless, derivatization methods have been successfully employed [56-58,65], most notably via use of the diazonium salt of 4-nitroaniline, For instance, in the study of phenylamide pesticides, no resonance was observed from these colorless compounds however, after derivation to azo dyes, detection limits of (2-4) X 10 M were achieved for monuron, diuron, a-napthylacetamide, and /3-napthylacetamide. Selectivity in distinguishing derivatized products from isomers of monosubstituted phenols (i.e., ortho, meta, and para) has also been shown to be successful, as illustrated in Fig. 8. Moreover, coupling the sensitivity and selectivity of RRS to the well-known spot-test method (qualitatively identifies colorless solutions only by visual identification of the colored products), a more accurate and sensitive characterization method of trace organic compounds is achieved. Such work has been conducted by Nakamura et al. in the characterization and detection of phenols and heterocyclic aromatic compounds on fruit rinds and food preservatives [58]. 

---