Diazotisation aromatic amines

It is well known that nitrite diazotises aromatic amines in acidic media and that the resulting diazonium ions are reduced by hypophosphorous acid to form benzene derivatives. The reactions are formulated as follows. 

Many terphenylquinones have been synthesised by arylation of 2,5-dichlorobenzoquinone (17) with iV-nitrosoacetanilides (5,9,10,80,149, 150) or diazotised aromatic amines (9, 80, 148, 149, 150, 205, 206) followed by alkaline hydrolysis. The procedure is illustrated in Scheme 4 for the preparation of leucomelone (19) (205). 

The determination of primary amines on the macro scale is most conveniently carried out by titration in non-aqueous solution (Section 10.41), but for small quantities of amines spectroscopic methods of determination are very valuable. In some cases the procedure is applicable to aromatic amines only, and the diazotisation method described for determination of nitrite (Section 17.38) can be adapted as a method for the determination of aromatic primary amines. On the other hand, the naphthaquinone method can be applied to both aliphatic and aromatic primary amines. 

Discussion. The formation of coloured compounds by coupling phenols with diazotised primary aromatic amines has long been recognised as a method of determining phenols, and procedures have been evolved whereby the phenol solution is titrated with a diazonium solution which has been calibrated against known concentrations of the phenol. The resultant reaction products are coloured, but many are only sparingly soluble in water and organic solvents and do not therefore lend themselves to colorimetric determination. 

Studies of Trifluoroacetic Acid. Part VIII. Diazotisations of Aromatic Amines in Aqueous Trifluo-roacetic Acid and Other Perhalogeno-carboxylic Acids, M. R. Pettit, M. Stacey, and J. C. Tatlow, / Chem. Soc., (1953) 3081-3084. 

Shortly after Perkin had produced the first commercially successful dyestuff, a discovery was made which led to what is now the dominant chemical class of dyestuffs, the azo dyes. This development stemmed from the work of Peter Griess, who in 1858 passed nitrous fumes (which correspond to the formula N203) into a cold alcoholic solution of 2-aminO 4,6 dinitrophenol (picramic acid) and isolated a cationic product, the properties of which showed it to be a member of a new class of compounds [1]. Griess extended his investigations to other primary aromatic amines and showed his reaction to be generally applicable. He named the products diazo compounds and the reaction came to be known as the diazotisation reaction. This reaction can be represented most simply by Scheme 4.1, in which HX stands for a strong monobasic acid and Ar is any aromatic or heteroaromatic nucleus. 

Of some importance as textile dyes are aza analogues of polymethine (cyanine) dyes. Azacarbocyanines result when Fischer s aldehyde is heated with primary aromatic amines. Thus Cl Basic Yellow 11 (6.220) is obtained when Fischer s aldehyde is condensed with 2,4-dimethoxyaniline. The equivalent reaction with 2-methylindoline gives Cl Basic Yellow 21 (6.221), which has superior light fastness but has been classified by ETAD as toxic [73]. The tinctorially strong golden yellow diazacarbocyanine dye Cl Basic Yellow 28 (6.222) is prepared by coupling diazotised p-anisidine with Fischer s base (6.223), followed by quaternisation with dimethyl sulphate. Some triazacarbocyanine dyes are also used commercially. 

Sparingly soluble salts of primary aromatic amines are diazotised in suspension, with vigorous stirring. Very weak bases such as halo-genated anilines and nitroanilines require a large excess of acid for salt formation they are first dissolved in just sufficient hot concentrated hydrochloric acid, which is then simultaneously cooled in ice and diluted. In this way the salts, which are mostly sparingly soluble, are precipitated in a finely divided condition. Dissolution in concentrated sulphuric acid and direct diazotisation of the sulphate, precipitated as a fine powder by means of ice, is often to be recommended. The free amines, however, should never be diazotised in acid suspension because they react far too slowly. Salt formation should first be ensured. 

Benzenedlazonlum chloride is prepared by the reaction of anUme with nitrous acid at 273-278K. Nitrous acid is produced In the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The conversion of prlmaiy aromatic amines into diazonium salts is known as diazotisation. Due to its Instability, the diazonium salt Is not generally stored and is used immediately after its preparation. 

Reactions on or close to solid surfaces maybe inhibited by deposition ofinsoluble or poorly soluble products on the reactant surface, a phenomenon referred to as overgrowth. Examples include the reaction of amines with chloranil [7], the diazotisation of poorly soluble aromatic amines in which the product diazonium salt is also insoluble and halogen exchange reactions of chloroaromatic compounds using potassium fluoride in dipolar aprotic solvents where the potassium chloride product may coat the potassium fluoride [8]. 

Lawson (1994) examined levels of aromatic amine migration from a number of laminate samples. Specially prepared laminate pouches containing distilled water were boiled for one hour and tested for primary aromatic amine (PAA) migration using a diazotisation procedure. Olive oil migration data at the same temperature was also obtained. Results are shown in Table 16.5 (detection limit 0.3 t,g dm ). These results, if expressed using the conventional surface area to volume ratio of 6 dm per kg of food, would range between 3 and... 

Up to now, photochemical studies are still confined to the reactions of simple azobenzene compounds, such as those derived by coupling a phenol or aromatic amine with a diazotised aniline. Azobenzene is isoelectronic with stilbene, its derivatives undergo facile trans cis photo isomerisation reactions in a manner analogous to that of stilbene. Azo dyes which possess a hydroxyl or amino substitutent ortho or para to the azo group... 

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