The Chemistry of Dyestuff. Intermediate Compounds. XII. Diazo Compounds

A Manual for Students of Chemistry and Dyeing
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)
The diazo compounds are obtained by the action of nitrous acid upon salts of primary aromatic amines. In most cases the amine is dissolved in excess of mineral acid, generally hydrochloric or sulphuric, and sodium nitrite, powdered or in solution, is added in equivalent amount to the amine. The temperature at which the reaction is carried out is varied according to the properties of the compound treated. The method of formation of the diazo compounds shows that the nitrogen directly attached to the ring is pentavalent and the nitrogen introduced trivalent.


Compounds of this structure are termed diazonium salts. These salts may undergo transformation to form the diazo salts, the latter existing in two stereo-isomeric modifications, viz.


The syn-diazo compounds readily couple with phenols or aromatic tertiary amines to produce azo compounds. By the addition of a large excess of alkali to the syn compound it is transformed into the more stable anticompound, which either does not condense with phenols or amines to produce azo compounds, or condenses only slowly. If hydrochloric acid is added to the antiform it is transformed into the .diazonium form, and this by the addition of a small amount of alkali gives the labile syn form (Hantzsch).


The rate of isomerisation of synto antidia/o compounds depends largely upon the groups present in the aromatic nucleus. Methyl groups hinder the rate of change, whilst halogen substituents increase it.

Diazophenols. The diazo chlorides of oand pamidophenol are obtained by diazotising the corresponding bases in alcoholic solution with amyl nitrite and hydrochloric acid at 0., and precipitating with ether. When these salts are dissolved in water and treated with caustic potash, hydrochloric acid is split off, and the free diazophenols, or quinone-diazides, are formed.


Meta-diazophenol chloride is very unstable, and loses nitrogen at °C.

By the diazotisation of substituted amidophenols quinone-diazides are obtained directly.

2.4.6-trichlorodiazobenzene acid sulphate on standing for some time loses one atom of chlorine and becomes converted into 3.5-dichloro-o-quinone-diazide.

On first consideration it appears a comparatively simple matter to produce diazo salts, nevertheless there are many amines which form diazo compounds very slowly, or resist the action of nitrous acid, or, owing to the formation of secondary products, are incapable of forming diazo salts.

For the diazotisation of amines the general method is to dissolve the amine in 2j to 3 equivalents of hydrochloric acid, cool to about 5°C. and add the calculated quantity of sodium nitrite; in most cases slowly until, after standing for about five minutes, a reaction is obtained with starch-iodide paper. In certain cases, to prevent the formation of diazoamido derivatives, the reacting quantity of nitrite is added at once, care being taken that the temperature does not rise too high.

Symmetrical trinitroaniline is very difficult to diazotise unless a large excess of sulphuric acid is used as solvent. The use of strong acid solutions will often overcome the steric hindrance of various groups in substituted benzenes.

The ortho-amidophenols may be easily diazotised, but difficulties arise with the 1.2and 2.1-amidonaphthols, owing to the oxidising action of the nitrous acid. This may often be overcome by the addition of salts of copper, zinc, nickel and mercury to the reaction mixture.

By the action of nitrous acid upon aromatic compounds containing more than one amido group, one would expect each amido group to be converted into the corresponding diazo group. In many cases secondary reactions take place with such rapidity that no diazo salt can be isolated, and special methods are employed for these compounds.

For example, orthophenylene diamine gives, on treatment with sodium nitrite and a mineral acid, aziminobenzene. Metaphenylene diamine similarly treated gives the well-known dyestuff Bismarck Brown. By the rapid addition of metaphenylene diamine hydrochloride to an excess of nitrous acid a clear yellow solution of the tetrazo compound is obtained. The substituted metaphenylene diamines, particularly 2.6-diamido-4-chlorophenol, 2.6-diamido p-cresol, 3.5-diamido-p-oxybenzoic acid, and 2.4-diamidophenol, readily tetrazotise. Only one amido group is diazotised if the theoretical amount of nitrite is mixed with the base and mineral acid then added.

Paraphenylene diamine on treatment with one equivalent of sodium nitrite and mineral acid gives a mixture of diazoand tetrazo-compounds. The diazo compound is obtained indirectly, using p-amido-acetanilide as base, or by employing p-nitraniline. The diazo compound is prepared and, after coupling to form the azo compound, the nitro group is reduced by sodium sulphide solution, or the acetyl group is removed by hydrolysis with caustic soda.

Nitro para phenyleue diamine hydrochloride by treatment with excess of acetic acid and excess of sodium nitrite gives only the diazo compound, which is probably 3-nitro-4-amido-diazobenzene chloride. If, however, this diazo compound is coupled with a component, then the free amido group may be diazotised.

The benzidine derivatives may be either diazotised or tetrazotised.

The orthoand the peri-diamido derivatives of naphthalene react with nitrous acid to give azimino compounds. With 1.4-naphthalene diamine it is necessary to shield one amido group, by acylation, diazotise, couple, hydrolyse, and again diazotise. This is necessary to prevent oxidation with formation of naphthoquinone. With some of the sulphonic acid derivatives of 1.4-naphthalene diamine one amido group only is attacked when treated with nitrite and acetic or oxalic acid; after coupling the other amido group may be diazotised.

Sulphur dioxide or sulphites react with diazo compounds to produce series of compounds depending upon the conditions. Sulphazides, diazobenzene sulphones, sulphuric acids, or diazo-sulphites may be produced. The diazo-sulphites are important as intermediate products for the preparation of hydrazine and its derivatives.

The sulphides and disulphides are produced from diazo salts by interaction with sulphuretted hydrogen in nearly neutral or in acid solution. The disulphides are important intermediate products for the preparation of mercaptans, the latter being employed in the production of many vat dyestuffs.

Thiosalicylie acid [-] is obtained thus:

Anthranilic acid (1 mol.) is diazotised with the theoretical quantity of sodium nitrite (1 mol.) in hydrochloric acid solution, the temperature being kept below 5°C.

This solution is now run into an ice-cold solution of sulphur (1 atom) dissolved in sodium sulphide (3 molsj and caustic soda (1 mol.) of 76°Tw. During the addition of the diazo solution ice is added to keep the temperature below 5°C. Nitrogen is now rapidly given off, the temperature rising quickly; after allowing to stand some hours the solution is acidified with hydrochloric acid, using Congo Red paper as indicator. Dithiosalicylic acid [-] separates, it is filtered off and washed with water. The precipitate is dissolved in a solution of sodium carbonate, filtered from sulphur, and, after the addition of iron or zinc dust (1J mols.), heated to boiling until sulphuretted hydrogen is no longer recognisable on acidifying and boiling. The iron or zinc is then precipitated with caustic soda, boiled and filtered. The thiosalicylic acid is then precipitated from the filtrate by addition of sulphuric acid.

Some curious reactions have been observed when certain dinitro derivatives of aniline are diazotised. When 3.4-dinitro-o-anisidine is diazotised in acetic acid solution, one of the nitro groups is eliminated during the operation, and is replaced by the hydroxyl group, the compound formed having the composition [-]

The diazotisation of 2.3-dinitro-p-anisidine in acetic acid solution also causes one nitro group to be replaced
by an oxy group. In nitric or sulphuric acid solution the nitro group remains unaffected, but in presence of hydrochloric acid the nitro group adjacent to the diazo group is replaced by chlorine.

It has also been observed that when a methoxy group is in the para position to the amido group, and at the same time has a nitro group in an adjacent position, demethylation takes place on diazotisation. Thus 3.5and 2.5-dinitro-p-anisidines yield the corresponding quinonediazides of dinitrobenzene.

With derivatives ot napththylamines the number of these peculiar reactions is higher. Chlorine, bromine, and sulphonic acid groups are also readily replaced by hydroxyl.

Diazo derivatives of amido-anthraquinones have not yet been employed for the manufacture of dyestuffs, but their diazo compounds may be obtained by diazotisation in concentrated sulphuric acid solution or suspension in the same.


For a detailed treatment of this subject see Cain's Chemistry of Diazo Compounds (1907).

To the class of intermediate products belong also many of the explosives, disinfectants, synthetic drugs, perfumes etc. manufactured by the dyestuff firms. Separate sections cannot be devoted to these classes in a text-book such as this. Reference may however be made to Thorpe's Dictionary of Applied Chemistry, to literature specially devoted to these subjects and articles in technical journals.

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