Diazonium compounds Diazotisation

Some amines, such as the nitroanilines and the naphthylamines, give somewhat more stable diazonium compounds and may be diazotised at room temperature, when the reaction proceeds more rapidly. If the amine salt is only sparingly soluble in water, it should be suspended in the acid in a fine state of division (this is generally attained by cooling a hot solution and stirring vigorously), and it passes into solution as the soluble diazonium salt is formed. 

Diazotise 223 g. of 2-naphtliylamine-l-sulphonic acid as detailed under fi-Bromonaphthalene in Section IV,62. Prepare cuprous cyanide from 125 g. of cupric sulphate pentahydrate (Section IV,66) and dissolve it in a solution of 65 g. of potassium cyanide in 500 ml. of water contained in a 1-litre three-necked flask. Cool the potassium cuprocyanide solution in ice, stir mechanically, and add the damp cake of the diazonium compound in small portions whilst maintaining the temperature at 5-8°. Nitrogen is soon evolved and a red precipitate forms gradually. Continue the stirring for about 10 hours in the cold, heat slowly to the boiling point, add 250 g. of potassium chloride, stir, and allow to stand. Collect the orange crystals which separate by suction filtration recrystallise first from water and then from alcohol dry at 100°. The product is almost pure potassium 2-cyanonaphthalene-l-sulphonate. Transfer the product to a 2-litre round-bottomed flask, add a solution prepared from 400 ml. of concentrated sulphuric acid and 400 g. of crushed ice, and heat the mixture under reflux for 12 hours. Collect the -naphthoic acid formed (some of which sublimes from the reaction mixture) by suction filtration... 

Diazonium compounds are usually prepared in mineral acid solution, and the nitrous acid generated from sodium nitrite. Sufficient acid must be used to generate nitrous acid and to form the salt of the base, and still leave the solution acid. In practice 2J—2f mols. of hydrochloric acid are generally employed. In most cases it is essential that the reaction be carried out at about 0°, as many diazonium solutions decompose above this temperature. The reaction goes very readily in some cases but in others, and especially where an acid group is present, e.g., naphthylamine sul-phonic acids, the reaction is only carried out with difficulty. It is possible to diazotise a solid in suspension, but the reaction is usually very slow. If the solid is dissolved and reprecipitated in a fine state of division the action goes much more quickly. 

It is not usually necessary to isolate the diazonium salt from solution, although in some cases this separates out as the reaction proceeds. If sufficient acid is not present, an aminoazo compound may be precipitated, due to the coupling (see p. 282) of the diazonium compound with the excess of base. In fact, this is one method of forming aminoazo compounds—by diazotising in presence of about half the quantity of acid necessary for the complete diazotisation. 

Diazonium salts couple readily with aromatic primary amines, giving diazoamino compounds. If for instance an aqueous solution of aniline sulphate is diazotised with a deficiency of nitrous acid, only part of it is converted into benzenediazonium sulphate and the latter then couples with the unchanged aniline to give diazoaminobenzene. The reaction is carried out at the opti-CeHsNHj.HjSO + HONO = CbHsNjHSO, + zHaO... 

Attention has previously (see Diazonium Sails) been drawn to the fact that unless an excess of hydrochloric (or mineral) acid is used in the diazotisation process, coupling occurs between the diazonium salt and the amine to give diazoamino compounds. Thus phenyldiazonium chloride and aniline yield diazoaminobenzene. This substance may be conveniently prepared by dissolving two equivalents of aniline in three equivalents of hydrochloric acid, and adding one equivalent of sodium nitrite in aqueous solution followed by two equivalents of sodium acetate ... 

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. 

During nitration of several amino derivatives, diazotisation and oxidation occurred to produce internal diazonium phenoxide compounds. 5-Acetylamino-3-bromobenzo[h ]thiophene unexpectedly underwent hydrolysis, diazotisation and oxidation to the explosive compound below. 

For the diazotisation reaction to succeed, it is necessary that the amine should be completely converted into the hydrochloride before the addition of sodium nitrite, because any free amine can react with the diazonium salt to form a diazoamino compound (Scheme 4.7). This complication needs to be avoided in the case of amines, such as the dichloro anilines, which do not dissolve easily in dilute hydrochloric acid [17]. With such compounds it is convenient first to dissolve them in a hot acid solution, which is then cooled to 0-5 °C. This procedure ensures the absence of free amine, and even if the hydrochloride precipitates on cooling it readily redissolves as the diazotisation proceeds. Similar precautions are required with the nitroanilines and here an increased amount of mineral acid is advantageous [18]. 

For preparative diazotisations it is important to use a sufficient excess of acid and to keep the temperature down. Two moles of acid are required for each mole of amine, one for salt formation and one for liberating the nitrous acid from the nitrite. As a rule 2-5-3-0 moles are used. The excess is required to prevent condensation of the diazonium salt with unchanged base to diazoamino-compound such condensations take place in a faintly acid medium. The test for unchanged amine, accordingly, consists in buffering the free mineral acid with sodium acetate, and so providing a solution faintly acid with acetic acid, under which conditions the diazoamino-compound is formed. The latter is decomposed by mineral acids into diazonium salt and amine salt, e.g. 

Several of these internal diazonium salts, prepared by diazotisation of anthranilic acids, are explosive in the solid state, or react violently with various materials. Individually indexed compounds (including analogous sulfonates) are Benzenediazonium-2-carboxylate, 2659 Benzenediazonium-3 or 4-carboxylate, 2661... 

The diazonium perchlorates are extremely explosive, shock-sensitive materials when dry, some even when damp [1,2], The salt derived from diazotised p-phenylendiamine was considered to be more explosive than any other substance known in 1910 [3], Individually indexed compounds are 4-Aminobenzenediazonium perchlorate, 2304 Benzene-l,4-bis(diazonium perchlorate), 2160 Benzenediazonium perchlorate, 2232 4,4/-Biphenylenebis(diazonium) perchlorate, 3457... 

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