Лако-красочные материалы — производство

The optimum conditions for coupling depend largely on the nature of the coupling component, but also on the nature of the diazo com­pound.

Phenols and Enolizable Keto Compounds. These compounds are coupled in alkaline (soda) solution as a rule. If the coupler is a sul­fonic or carboxylic acid, or is soluble in water (e. g., resorcinol), it is dissolved directly in the required amount of soda solution. In other cases, the coupler is dissolved in the equivalent amount of dilute sodium hydroxide solution and the required amount of soda is added. Many couplers, such as naphthol AS, require an excess of caustic soda for complete solution; in such cases, no more is used than is absolutely required. (Solution of the 2-hydroxyl-3-naphthoic acid arylides is great­ly assisted by the addition of alcohol. In many cases, no other method of dissolving the coupler is known.) An unnecessarily large excess of alkali is to be avoided, except in special cases, since such an excess rapidly decomposes the diazo compound. It is desirable to use that amount of soda which will be converted to bicarbonate by the acid of
the diazo solution (free acid plus that combined with the diazo group). In this way, no carbon dioxide is formed and foaming is avoided. The amount of soda is calculated on the basis of the following equations (taking into account, of course, any sodium hydroxide added):

Diazotization:

X—NH, + 2.5 HC1 + NaNO, = X—NsN + NaCl + 0.5 HC1 + 2 H,0

Cl

Coupling with a phenol dissolved in an equivalent quantity of sodium hydroxide:

(И)

X—N—N + 0.5 HC1 + H—Y—ONa + 0.5 NajCOj — X—N=N—Y—OH

Cl + 1.5 NaCl + 0.5 NaHCOj

Coupling with a phenolsulfonic acid salt:

(HI)

X—N=N + 0.5 HC1 + H—Z

I

Cl

ОИ

= X—N=N—Z/ + 1.5 NaCl + 1.5 NaHCOj

SOjNa

These equations show the minimum amount of soda necessary; an excess usually does no harm.

The acid diazo solution is added from a dropping funnel, slowly and with good stirring, to the alkaline coupler solution. The coupling reaction frequently takes place instantaneously and it is possible to work at room temperature. However, if the reaction requires a longer time, it is usually desirable to cool the reaction mixture in ice and to work in the absence of strong light. On the other hand, there are cases where coupling must be brought about by gentle warming. There are no gen­eral rules, and the optimum conditions must be established by test in each individual case if they are not disclosed in the patent literature.

To determine whether the coupling reaction is completed, a drop of the reaction mixture is placed on filter paper. The colorless ring sur­rounding the speck of dye is treated with a drop of a solution of a re­active coupler such as sodium уЗ-naphtholate, R salt, or resorcinol, and then with a drop of soda solution or dilute sodium hydroxide. If unused diazo compound is present, dye is formed in the test spot. If the dye being made is easily soluble so that no colorless ring is formed, a small portion of the reaction mixture is treated in a test tube with salt to pre­cipitate the dye, and a drop of this mixture is then used for the test. The presence of unused coupler can be determined in a similar manner, using a diazo solution to test the colorless ring.

Since o — and p-nitrodiazo compounds are rearranged, even by soda, to the noncoupling isodiazotates, it is desirable to work with these in the absence of soda and to use, instead, sodium acetate, or, in special cases, an acid-binding agent such as calcium carbonate or magnesia, so that the solution is never alkaline.

Similar procedures are used with diazo compounds in which ortho substituents are liable to be replaced by hydroxyl (page 240 ff.). In these cases, it is desirable to carry out the coupling reaction at as low a temperature as possible and to avoid the use of an excess of alkali in dissolving the coupler (perhaps adding a few drops of acetic acid to neutralize any slight excess). Reactions employing particularly sensi­tive diazo compounds, such as 2,4-dinitrodiazobenzene, for example, must be carried out in mineral acid solution, and, in addition, at low temperatures and as rapidly as possible. If the coupler to be used is soluble only in caustic alkali, it is first dissolved as usual and then pre­cipitated in as finely divided a state as possible, before adding the diazo solution, by dilute hydrochloric or sulfuric acid. The addition of a dispersing agent may be beneficial.

Coupling reactions with diazo compounds which are very sensitive to alkali often are most successful when pyridine is used as an acid­fixing agent. This is especially true in the preparation of secondary polyazo dyes with l-amino-2-naphthol ethers or their sulfonic acids as intermediates.86

Amines. These compounds are coupled, as a rule, in weakly acid

66 Schweitzer (I. G.) Ger. Pat. 450,998 and 453,133 (1927) [Frdl., IS, 521-522

(1928) 1, Schweitzer (I. G.), Ger. Pat. 476,080 (1929), Schweitzer and Huis-

mann, Ger. Pat. 478,045 (1929) [Frdl, 16, 996-997 (1931); С. А., 23, 4348

(1929) 1.

or neutral solution. Different procedures must be used, depending on the solubility of the amine used.

Amines which “are soluble in water as well as in dilute mineral acids (e. g., m-phenylenediamines, many naphthylaminepolysulfonic acids, etc.) are dissolved in water. If the coupler is capable of multiple cou­pling, the diazo solution must be added slowly and with stirring to the coupler solution. It is simpler, in other cases, to add the coupler to the diazo solution. With actively coupling diazo compounds, the coupling reaction begins immediately and is completed in a short time. With more sluggish diazo compounds, the mineral acid is neutralized with sodium acetate. If the coupling is still too slow, the mixture is neutralized with soda and kept as nearly neutral as possible throughout the reaction. Ice cooling is recommended when the coupling reaction requires a long time.

Amines which are difficultly soluble or insoluble in water, but solu­ble in dilute mineral acid (most of the unsulfonated monoamines), are dissolved in dilute hydrochloric or sulfuric acid and mixed with the diazo solution as described above. If coupling proceeds slowly under these conditions, the mixture can be neutralized with sodium acetate or soda, but must be kept fairly acid to Congo red in order to keep the coupler in solution. Since acid is set free in the coupling reaction:

X—Nsstt + H—Y—NH3—Cl = X—N= N—Y—NH’jCl + HC1

I

Cl

successive additions of acetate or soda must be made from time to time as the reaction proceeds. It is usually necessary to neutralize the mix­ture completely toward the end of the reaction in order to use the last portion of the diazo compound; this is done, if possible, only when the point is reached where so little coupler remains that it stays in solu­tion despite its low solubility.

The addition of alcohol or some other water miscible solvent for the coupler is recommended when very sluggish coupling reactions are in­volved. It is then possible to have the solution neutral or acid (acetic) from the start. If the coupler tends to form a diazoamino compound (e. g., m-toluidine), the coupling reaction is always carried out in as strongly acid solution as possible.

Amines which do not dissolve in water or dilute acids (diphenyl — amine, phenylnaphthylamine, etc.) are most satisfactorily coupled, in the laboratory, in an organic solvent (alcohol, methyl alcohol, acetone, pyridine, etc.). In industrial preparations, such couplers are emulsified with soap solution or other dispersing agents and very good results are obtained when the diazo compound is sulfonated and the resulting dye is therefore water soluble.

Sulfonated amines are dissolved as their alkali salts in water, and acetate solution, equivalent to the free mineral acid in the diazo solu­tion plus that formed in the coupling reaction, is added. The diazo solution is added to the coupler solution with stirring. If necessary, the mixture is partially or completely neutralized with soda to acceler­ate the coupling.

With aminophenols, and especially aminonaphtholsulfonic acids, the position where coupling occurs depends on the reaction conditions. In acid solution, the amino group, and in alkaline solution, the hydroxyl group, exerts the determining orienting influence. Hence, in order to obtain a single dye, coupling is carried out either in distinctly alkaline solution or in strongly acid solution, depending on the desired result. If a primary disazo dye is desired, the first coupling reaction is always carried out in acid solution and the second in an alkaline medium.

For coupling in acid solution, the coupler is dissolved in water as its alkali salt, and hydrochloric acid is added until the mixture is just acid to Congo red. The resulting suspension is added to the diazo solution and the mixture is stirred in the cold until dye formation is completed. If the reaction is very slow, it can be accelerated by neutralizing the ex­cess mineral acid with sodium acetate. The reaction mixture must always be kept weakly acid to Congo red, however.

Alkaline coupling is carried out, as in the case of phenols, with the addition of soda. If the diazo compound is sensitive to alkali, it is desirable to add only a small portion of the soda at the start to dissolve the coupler and the remainder simultaneously with the diazo solution. In this way, the solution is kept only weakly alkaline throughout the course of the reaction. The addition of the diazo solution should be made slowly and with vigorous stirring.

Benzidine Dyes. In the preparation of mixed disazo dyes from ben­zidine and similar bases, the first coupling reaction must be carried out under conditions which will prevent the formation of the symmetrical disazo dye. Hence, the first coupling reaction is done with the coupler which reacts less readily, and the more active coupler is used in the second coupling step. If there is not a large difference between the two couplers with respect to coupling reactivity, the first coupling reaction is carried out under the mildest conditions possible (low alkalinity or

strong acidity). Furthermore, contrary to the usual practice, the solu­tion of the first coupler is added with good stirring to the tetrazo solu­tion since the reverse addition could lead to a temporary excess of the coupler in the reaction mixture. This procedure involves no difficulties because solutions of the tetrazo compounds of benzidine and its deriva­tives can be made alkaline with soda at low temperatures without caus­ing decomposition. The second coupling reaction is started only after the first one is fully completed and all of the tetrazo compound has reacted. The second coupling is then carried out by the usual methods for monoazo dyes.