Лако-красочные материалы — производство

The diazotization reaction proceeds according to the equation:

R—NHj + NaNO, + 2 HC1 = R—N==N + NaCl + 2 H,0

Cl

Hydrochloric acid is usually employed in diazotizations in aqueous solutions because it gives, in general, the most soluble salts with aromatic amines. If sulfuric acid is used, only 0.5 mole is used in place of 1 mole of hydrochloric acid.

If the molecule contains a free sulfonic acid group, only 1 mole of HC1 is necessary:

NHj. N-N

r/ + NaNOj + HC1 = r/ + NaCl + 2 H20

SO, H so3

While the nitrite must be used in exactly the amount theoretically required, an excess of acid is always used. For laboratory preparations, as a rule, the excess acid used is about 0.5 equivalent over the theoretical­ly required 2 equivalents. The excess of acid must be increased to 1 to 3 equivalents in diazotizations of weakly basic amines whose salts are readily hydrolyzed (e. g., chloro — and nitroanilines). In some cases where even this larger excess of acid is insufficient to dissolve the base (e. g., nitrochloroaniline), the diazotization may be carried out in suspension provided that the base is present in a finely divided state. Thus, a solu­tion of the base in concentrated sulfuric acid may be poured into ice water, the base filtered off and washed and then made up into a paste. Even this method fails to work with some bases such as dinitroaniline and nitrodichloroaniline; these compounds can be diazotized smoothly only in concentrated sulfuric acid solution.

Aminosulfonic and carboxylic acids can be diazotized in the normal way if they are readily soluble in acid solution. Usually, however, this is not the case, and it is recommended that diazotization be carried out by dissolving the alkali metal salt of the amino acid and the required amount of nitrite in water and adding the resulting neutral solution, with good stirring, to dilute hydrochloric acid (reversed diazotization). In this way, the free amino acid and the nitrous acid, formed simultane­ously in the reaction mixture, react before the amino acid is precipitated. When, however, a free aminosulfonic acid is very insoluble, as is often the case with aminoazo dyes which are to be diazotized further in the preparation of polyazo dyes, the sulfonic acid may be precipitated before diazotization can occur. In these cases it is very difficult to obtain complete diazotization, especially if the diazo compound is also very insoluble. Under such circumstances, it is often advantageous to use a considerable excess of nitrite and add a small portion of it to the hydrochloric acid first. In addition, the solution of the amino acid is made very dilute and is added very slowly to the well stirred acid con­taining some nitrite. When the diazotization is completed, the diazo com­pound is filtered off to separate it from the excess nitrous acid.

None of the methods mentioned gives satisfactory diazotization of

1,2- and 2,1-aminonaphthols or their sulfonic acid and other derivatives, all of which are readily oxidized to quinones by nitrous acid in acid solution. Diazotization of these compounds can be carried out success­fully by using just enough acid to make the aminonaphthol salt, or, in the case of the sulfo derivatives, to make the compound containing one free sulfo group, and treating with nitrite in the presence of one equival­ent of a zinc salt or a small amount of a copper salt. If a copper salt is used, the copper must be removed when diazotization is complete, but the presence of zinc in a diazo solution usually does no harm.

Special techniques must be used in cfiazotizing bases which contain a secondary amino group in addition to the primary group, such as monoethyl-p-phenylenediamine and especially p-aminodiphenylamine. These compounds can easily be diazotized in such a way that the sec­ondary amino group is unattacked; the resulting diazo compounds, however, couple very slowly and are unstable. A procedure is used, therefore, which involves the action of 2 moles of nitrite for each mole of base, diazotizing the primary amino group and simultaneously nitro — sating the —NH[49] group. The resulting nitrosated diazo compound has much better coupling properties and much higher stability. After coupling, the nitroso group must be split off from the dye molecule, usually by warming with acids or bases or by treatment with weak reducing agents such as bisulfite (variamine dyes).

It should be noted that concentrated hydrochloric acid reacts with nitrite to give chlorine, which, of course, leads to side reactions. For this reason, the diazotization mixture should never contain more than 20 per cent of free HC1.

With diazotizations carried out in dilute solution, it is not necessary to add the nitrite solution slowly. On the contrary, it may be desirable to add the nitrite all at once, as in the case of bases which tend to form diazoamino compounds or to couple with themselves. It is only neces­sary to provide adequate cooling, and this is accomplished most easily by adding ice directly to the diazotization mixture.