This invention relates to a novel process for the preparation of aqueous diazonium salt solutions. Aqueous diazonium salt solutions are used, for example, for the preparation of azo compounds, in particular of dyes and pigments. In the literature only a few methods for the preparation of aqueous diazonium salt solutions, starting from aromatic nitro compounds, are described, and even these have only been developed for the synthesis of small amounts for scientific purposes.
Usually, the aromatic nitro compound is firstly reduced to give the corresponding aromatic amine which, following work-up and isolation, is reacted in a second, separate step with an alkaline earth metal nitrite to give the desired diazonium salt. Here, the reduction is usually carried out as a catalytic hydrogenation using a suitable catalyst. The solvents normally used are organic solvents, in particular low molecular weight alcohols, such as methanol, ethanol or propanol, and also corresponding esters, such as, for example, ethyl acetate. After the aromatic amine formed in the process has been isolated, it is reacted in aqueous solution with nitrite to give the corresponding diazonium salt.
A procedure of this type is described in U.S. Pat. No. 5,051,131. Here, aromatic nitro compounds prepared by reacting nitrobenzoyl chloride derivatives and nitrobenzenesulfonyl chloride derivatives with aliphatic bisamines are reduced in ethanolic solution using a palladium catalyst to give the corresponding aromatic amine, and the latter is isolated from the solution. In the next step, the isolated amine is reacted in aqueous acetic acid solution with sodium nitrite to give the desired diazonium salt solution. A disadvantage of this procedure is the use of different solvents in the two reaction stages since, following the first stage, the organic solvent used has to be removed in a separate step and is formed as a waste product.
Because of the combustibility and possible formation of ignitable solvent vapor/air mixtures, this step is complex on an industrial scale and is also unacceptable from a safety viewpoint.