Лако-красочные материалы — производство

The following types of compounds undergo coupling reactions with diazo compounds: (a) Phenols, (fe) Enolizable keto compounds of an aliphatic character, having a “reactive methylene group” which may be part of an open chain or a ring system, either homocyclic or hetero­cyclic. This group of compounds may be represented by the formula:

X—СН4—CO—V ±=Ї X—CH — C(OH)—Y

in which X is a negative group, such as —COR, —C02R, —CN, —S02R, etc. The pyrazolones belong to this group, (c) Primary, secondary, or tertiary amines in which the amino group is attached directly to an aromatic ring. (d) Pyrrole, indole, and similar ring systems.

Phenol derivatives, in which the hydroxyl group has been converted to an ether or an ester, are incapable of coupling under normal condi­tions. Similarly, acyl derivatives of amines (benzoyl, acetyl, etc.) are unreactive. The sulfonyl derivatives of primary amines, such as p-tolu — enesulfonanilide, are exceptions; these compounds are soluble in caustic alkali and behave as phenols toward diazo compounds.

Aromatic compounds which contain two amino groups, two hydroxyl groups, or one amino group and one hydroxyl group, located para or ortho to each other, are not suitable as coupling components. These compounds reduce diazo compounds, being oxidi2ed to quinones in the process. (Smooth coupling may be achieved in many cases by the addi­tion of thiosulfates or thiocyanates.[52])

Primary monoamines of the benzene series react with diazo com­pounds to give diazoamino compounds, along with, or instead of, the isomeric aminoazo compounds. The formation of diazoamino com­pounds is favored by low acidity of the coupling solution. Methyl or alkoxy groups in the ortho, or, more effectively, the meta position rela­tive to the amino group favor the formation of aminoazo compounds; particularly good coupling is obtained with bases having such substi­tuents in both an ortho and a meta position, for example:

If it is desired to obtain aminoazo dyes from bases which tend to form diazoamino compounds, the amines can first be converted to their w-sulfomethyl derivatives by reaction with formaldehyde and bisulfite:

R—NH2 + CH20 + NaHS03 -> R—NH—CH2—SOaNa + H20

The sulfomethyl derivatives couple with diazo compounds to form azo compounds which yield aminoazo dyes on hydrolysis.[53]

Coupling with phenols is usually effected more easily than coupling with amines, and naphthols couple considerably more readily than hydroxybenzene derivatives. Reactive methylene compounds behave similarly to phenols of the benzene series. Negative substituents (halo­gen, nitro, sulfo, carboxyl, carbonyl, etc.) accelerate coupling when present in the diazo compound, and retard the reaction when present in the coupling component. Positive substituents have the opposite ef­fect. A second hydroxyl or amino group, in the meta position in the coupler, or in the other ring in the case of naphthalene compounds, has a strong accelerating effect on the coupling reaction.

Coupling can be accelerated by warming the solution, increasing the alkalinity (lowering the acidity), increasing the concentration, or add­ing water-binding agents. The first two changes accelerate not only the coupling reaction but also the decomposition of the diazo compounds. It is necessary, therefore, to establish by trial whether any given slow coupling reaction is benefited by increasing the temperature or by rais­ing the alkalinity. Generally, but not always, the best results are ob­tained in such cases in the cold at a point as nearly neutral as possible, but the coupling reaction requires a long time under these conditions. No disadvantages attend an increase in concentration of the coupling solution, and it is always desirable to conduct sluggish coupling reac­tions in as concentrated a solution as possible. If the diazo compound is difficultly soluble, as is the case with many sulfonic acids, it is fre­quently advantageous to filter it off and thus remove most of the diluting liquid. Much the same effect is produced by adding indiffer­ent salts, especially common salt, or water-miscible organic solvents such as alcohol or pyridine. Obviously, these addition agents bind some of the water and thus effectively raise the concentration of the reactants. Pyridine also acts as an acid-fixing agent.

Diazo compounds always couple with amines and phenols of the benzene series to introduce the azo group para to the amino or hydroxyl group if this position is unoccupied. If the para position is occupied, the azo group enters the ortho position, but never the meta position. Phenols are capable of reacting with two or three molecules of the diazo compound, forming dis — and trisazo dyes, if both ortho and para posi­tions are free. In some cases, substituents (e. g., —S03H or —C02H) in the ortho or para position may be replaced by the azo group.

/З-Naphthol and /J-naphthylamine and their derivatives always cou­ple in the adjacent a position. If this position is occupied, either the coupling reaction does not occur or the substituent is replaced by the azo group (e. g., with 2-naphthol-l-sulfonic acid). A sulfo group in the 8 position retards or prevents the introduction of an azo group in the 1 position.

a-Naphthol and a-naphthylamine and their derivatives usually couple in the 4 position, but the azo group may easily be introduced partially, or exclusively in exceptional cases with a-naphthol, into the 2 position.

a-Naphthol also tends to form disazo dyes by coupling in both the 2 and 4 positions. a-Naphthol and a-naphthylamine derivatives which are substituted in the 4 position couple exclusively in the 2 position; if both the 2 and 4 positions are occupied, coupling does not take place at all. A sulfo group in the 3 or 5 position interferes strongly with the introduction of an azo group into the 4 position, and therefore 3- or 5- sulfonic acid derivatives of a-naphthol or a-naphthylamine couple only in the 2 position with most diazo compounds. A mixture of the 2 and 4 azo compounds is formed, however, with diazo compounds which cou­ple energetically (such as o~ or p-nitro — or polyhalogen diazo compounds, or, to a smaller degree, diazotized derivatives of p-aminoazobenzenes, etc.).

With couplers containing both an aromatic amino group and a phenolic hydroxyl group, the amino group determines the position taken by the azo residue when coupling is carried out in acid solution, and the hydroxyl group is determinative in alkaline media. If coupling is carried out first in acid solution (directed by the amino group), a subsequent coupling reaction in alkaline solution can be used to intro­duce a second azo group (directed by the hydroxyl). The reverse order of coupling is not used successfully in any technical preparations.