Benzidine tetrazotization

A mixture of 280 g. (1.52 moles) of commercial benzidine and 880 cc. (10.23 moles) of concentrated hydrochloric acid (sp. gr. 1.182) is placed in a 5-I. round-bottomed flask and wanned on a steam bath for one to two hours, with occasional shaking, to form the dihydrochloride. The flask is then equipped with a mechanical stirrer and a dropping funnel, and cooled, with stirring, to — ro° in an ice-salt bath. When this temperature has been reached, the benzidine dihydrochloride is tetrazotized over a period of two hours with a solution of 232 g. (3.19 moles) of 95 per cent sodium nitrite in 800 cc. of water, until a faint test for nitrous acid with starch-iodide paper is obtained after twenty minutes. During this reaction, the temperature is kept below —5 °. 

Difluorobiphenyl has been prepared from 4,4 -biphenyl-bis-diazonium piperidide (by diazotizing benzidine and coupling with piperidine) and concentrated hydrofluoric acid 1 by the action of sodium on -fluorobromobenzene in ether 2 from benzidine by tetrazotization and decomposing the biphenyl-bis-diazonium salt with concentrated hydrofluoric acid 3 by the above method in the presence of ferric chloride 4 and by the prolonged contact of the vapors of fluorobenzene with a red-hot wire.5 The method described here is the most satisfactory for... 

Pigment Orange 13 [3520-72-7] 21110 disazo pyrazolone (Pyrazolone Orange) coupling of tetrazotized 3,3 -dichloro-benzidine with l-phenyl-3-methyl-pyrazolone... 

Type E1 D-+E1 Synthesis. Dyes of this type require the conversion of an aryldiamine to a tetrazonium compound (one that has two diazonium groups), in a process know as tetra-zotization. See Fig. 13.93, which involves environmentally friendly alternatives to benzidine. Following tetrazotization, one tetrazonium molecule reacts with two coupler molecules to produce the target dye, examples of which are provided in Fig. 13.94 (11 C.I. Direct Red 28 (Congo Red), 12 C.I. Direct Yellow 12, and 13 C.I. Acid Yellow 42). Disazo dyes prepared this way include dye 14 (C.I. Direct Blue 15), which is converted to the important bis-copper complex, C.I. Direct Blue 218 (see Fig. 13.95). 

Pyrazolone Red (Cl Pigment Red 38, Cl No. 21120). The structure of this pigment typifies the nonmetallized azo reds as used in the plastics industry. The pigment is prepared in an aqueous environment by the tetrazotization of 3,3 -dichloro-benzidine followed by coupling the resulting tetrazonium salt onto phenyl-carbethoxy-pyrazolone under controlled pH. 

It would lead much too far afield to discuss even the most important variations of these intermediates, so only a few examples will be mentioned. Of special importance is the intermediate formed by monocoupling tetrazotized benzidine with salicyclic acid. This product is formed in high purity, since a second coupling reaction with another molecule of salicyclic acid can be made to take place only with difficulty, by using an excess of sodium hydroxide, whereas the first coupling takes place in soda solution. Furthermore, no difficulties are encountered in coupling tetrazotized benzidine with one molecule of the monoazo dye from p-nitroaniline and H acid, or with one molecule of H acid itself in mineral acid solution. Both of these latter reactions are discussed in more detail later. 

Direct dyes. The first direct dye was Congo Red (Figure 2.26) obtained by tetrazotizing benzidine and coupling the tetrazo compound with 2 molecules of naphthionic acid (as sodium salt) in the presence of sodium acetate to act as buffer, i.e. 

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