Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Patent
1992-06-08
1993-05-04
Raymond, Richard L.
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C07C20900
Patent
active
052083764
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a process for the continuous preparation of 3,3'-dichlorobenzidine dihydrochloride from 2,2'-dichlorohydrazobenzene by treatment with aqueous sulfuric acid in the presence of an alkali metal salt of an alkyl polyglycol ether sulfate.
3,3'-Dichlorobenzidine has the greatest economic importance of all known diphenyl bases. Both the free base and, in particular, the salts formed with mineral acids are valuable starting materials, for example for the synthesis of dyes.
3,3'-Dichlorobenzidine is usually prepared from 1-chloro-2-nitrobenzene by reduction and subsequent so-called benzidine rearrangement of the resultant 2,2'-dichlorohydrazobenzene. The rearrangement is catalyzed by mineral acid. To this end, the 2,2'-dichlorohydrazobenzene is expediently dissolved in an aromatic solvent, and aqueous mineral acid is then added. The rearrangement then occurs in the aqueous phase, the resultant 3,3'-dichlorobenzidine salt causing the formation of a suspension. With the increasing reaction duration, the viscosity of the suspension increases so much that it is virtually impossible to stir or pump. This furthermore has the consequence that material exchange can only take place by diffusion, and a long reaction time is therefore necessary for complete conversion. Undesired side reactions, such as, for example, disproportionation, thus increase in importance. The byproducts reduce the yield of 3,3'-dichlorobenzidine and, in addition, must be removed by complex purification operations.
Usually, exactly the amount of mineral acid required for the reaction is employed in order to keep the work-up costs as low as possible.
EP-A 0 045 459 describes a continuous process for the preparation of diphenyl bases and the salts formed therefrom with mineral acids from the corresponding N,N'-diarylhydrazines. The solution of the hydrazo compound in a water-immiscible aromatic solvent is admixed continuously with such an amount of mineral acid that the suspension remains conveyable. Suitable solvents in this process are all liquid aromatics which have sufficient solvency and are inert under the reaction conditions toward the substances participating in the reaction. Of industrial importance are, in particular, toluene, the isomeric xylenes and the commercially available m-xylene/ethylbenzene mixture, known as "Solvent Naphtha".
In the process of EP-A 0 045 459, the mineral acids used are from 20 to 80 % strength aqueous sulfuric acid or from 10 to 30 % strength aqueous hydrochloric acid. These acids are added in a 10- to 14-fold excess above the amount necessary for salt formation. The reaction is carried out at a temperature of from 20.degree. to 50.degree. C. The volume and design of the reaction zone is determined by the residence time necessary for the reaction mixture. The residence time itself depends on the constitution of the N,N'-diarylhydrazine and on the reaction temperature. At a residence time of from 1 to 3 hours, a cascade of at most 5 stirred reactors has proved advantageous.
The reaction mixture is then worked up in a conventional manner, for example by removing the aromatic solvent by distillation or by blowing through steam and separating the aqueous acid from the precipitated salt of the diphenyl base by filtration, and recycling the aromatic solvent and the aqueous acid, which is concentrated to the original acid content.
It is stated that a particular advantage of the process is that the salts of the diphenyl base are obtained in high yields and at high throughput with a minimum of operations, it being possible to fully recycle the assistants employed into the process.
However, to prepare the pure salts of the diphenyl bases, the crude salt initially produced must be purified either via the free base or by recrystallization from hydrochloric acid.
Although the process of EP-A 0 045 459 has provided a considerable advance, the space-time yields which can be achieved therewith no longer satisfy current requirements. In addition, in the continuous reaction of 2,2'-dichlorohydrazo
Baessler Konrad
Habig Kurt
Warning Klaus
Hoechst Aktiengesellschaft
Raymond Richard L.
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