Tetrazo compounds

This is not the only method of forming tetrazo-compounds. Each amino-group of a diamine may be diazotised and coupled. Benzidine and its homologues, which ha e been utilised in this way, have a special value for the cotton dyer, as the shades produced are not only ery biilliant, but, unlike the maiorily of... 

In addition to Congo red, benzopurpurin 4 B, prepared from o-tolidine and naphthionic acid, is also of importance. In the preparation of this dye, the coupling reaction cannot be carried out at higher temperatures since the tetrazo compound from o-tolidine is too unstable. Benzopurpurin 4 B is somewhat less sensitive to acids than Congo red, and, like the latter, is widely used, particularly in the Orient. It is interesting that these dyes are found to be much more stable in nonindustrialized countries where the atmosphere contains little sulfuric and sulfurous acids. 

When the diazotization of the meta phenylene diamine is carried further so that both amino groups are diazotized a double diazo compound or tetrazo compound is obtained and this couples with two molecules of meta-phenylene diamine yielding a double azo or dis-azo compound. 

The diazotization of benzidine yielding diazo and tetrazo compounds, the coupling of these with phenols and amines yielding azo compounds, which are the benzidine dyes, should be recalled here (p. 569). 

Congo Red.—When benzidine is diazotized a double diazo or tetrazo compound results (p. 575) from the diazotization of both of the amino groups. This diazonium or tetrazonium salt reacts with two molecules of naphthylamine sulphonic acid, naphthionic acid, forming a double azo or disazo compound. The sodium salt of this compound is Congo red. The reactions are ... 

On cotton they are generally incapable of complete fixation, as most of them form no real colour-lakes. Exceptions to this rule are certain tetrazo-compounds which dye cotton directly in the form of alkali salts of their sulphonic acids. 

The tetrazo-compounds obtained from benzidine and similar bases combine -with phenols and amines to produce yelloiTj red, bluCj and violet dyestuffs. These have attained considerable importance on account of their remarkable property of dyeing (in form of alkali salts) on unmordanted vegetable fibres. 

Of these sulphonic acids the jS- and F-acids produce fine red dyestuffs with tetrazo-compounds, while the a- and 7-acids give worthless yellow ones. 

If a mixture of both acids (i. e. the so-called 8-acid) is used, a fine mixed dyestuff is produced, which contains one molecule of each acid linked to the benzene chains of the ditolyl. This body forms the principal constituent of the dyestuff known as Delta-Purpurine 5 B. Mixed dyestuffs of this class are easily obtained, on account of a peculiar property of the tetrazo-compounds. The... 

By oxidation of paraphenylene-diamine and metatoluylenediamine, a non-methylated toluylene blue is formed which yields a corresponding toluylene red on heating. This latter forms a tetrazo-compound which yields methylphenazine (benzene-tolazine) ... 

The saffranines form three series of salts. The mono-acid salts are, like the base, red and very stable j the diacid salts are blue, and the triacid green. The two latter series are decomposed by water and in fact the green salts can only exist in presence of concentrated sulphuric or hydrochloric acids. Both amido-groups may be diazotised [1]. The primary diazo-compound forms blue diacid salts, which are formed in slightly acid solutions, and correspond to the blue diacid saffranine salts. The green tetrazo-compounds have not been analysed, but probably correspond to the triacid salts. The salts of these diazo-compounds are not decomposed by water. 

A tetrazo-compound of saflranine may be obtained in solution by treating the green solution of phenosaflranine in concentrated sulphuric acid with nitrous acid [1]. The colour of the solution is not changed on dilution. On boiling with alcohol a violet base, forming yellow salts, is produced. The chloride of this compound is probably phenazinephenyl-chloride,... 

The reaction between tetrazo-compounds and y-amidonaphthol-sulphonic acid may be effected in either alkaline or slightly acid solution, and the products differ according to which condition is observed. The exact reason for this behaviour has not been explained. For example, in the preparation of diamine blades R and B, and diamine blue-black E, the combination of the tetrazo-compound with y-amidonaphtholsnlphonic acid is effected in alkaline solution, while in the cases of Diamine Violet and Fast Red a slightly acid solution is used. 

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. 

---