Continuous diazotization of amines

The invention provides a process for the continuous diazotization of primary aromatic amines by reacting an aqueous solution or suspension of the amine in a mineral acid with an aqueous sodium nitrite solution. To this end, the lower portion of a cylindrical diazotization vessel placed in upright position is fed continuously with an aqueous mineral acid solution or suspension of a diazotizable primary aromatic amine. At the same time, the vessel is fed, via one or more inlets arranged one above the other so as to open laterally thereinto, with an aqueous sodium nitrite solution, the amine and nitrite being used in stoichiometric proportions, or the nitrite being used in a stoichiometric deficiency and the acid being used in an excess of about 1 to 3 equivalents per amine equivalent in the mineral acid solution. The resulting mixture is reacted while producing a laminar flow of liquid matter at temperatures of about 5.degree. to 30.degree. C. Next, reaction mixture is removed, in accordance with the diazotization velocity of the particular amine used, from the upper portion of the diazotization vessel at a place where the reaction mixture is substantially free from nitrous acid, said place being situated in the vessel at a level which is the higher the lower the diazotization velocity of the amine used. The reaction mixture removed is finally filtered and diazonium salt-containing solution is delivered to a sojourn zone.

This invention relates to the continuous diazotization of amines. 
The diazotization of primary aromatic amines so as to obtain diazonium 
salts is of considerable commercial interest as regards the production of 
azo dyestuffs. 
Primary aromatic amines are normally diazotized discontinuously. To this 
end, the amine is placed in an agitator-provided vessel and dissolved or 
suspended therein in an aqueous mineral acid, e.g. hydrochloric or 
sulfuric acid, the mineral acid being commonly employed in an excess of 
2.5 to 3 equivalents per equivalent of amine. Next, the solution or 
suspension is admixed with an aqueous concentrated sodium nitrite solution 
for as long as necessary to reach the point of equivalence. By the 
addition of ice, the reaction mixture is maintained at a temperature of 
about 0.degree. to 5.degree. C. The resulting diazonium salt or its 
aqueous solution is generally filtered and used in coupling reactions for 
making azo dyestuffs. 
This is a commercial process which suffers from serious deficiencies in 
respect of the following points: The space/time-yield is unsatisfactory. 
This is a result of the fact that use is made of the agitator-provided 
vessel (a) for dissolving or suspending the amine in the aqueous mineral 
acid, (b) for effecting the diazotization reaction, and (e) for storing 
the resulting diazonium solution therein until work-up. In other words, 
the vessel is at least temporarily not available for effecting further 
diazotization reactions therein, which is disadvantageous. A further 
technically adverse effect of discontinuous diazotization resides in the 
relatively long periods over which the diazonium salt, once it has been 
formed, is allowed to remain in the agitator-provided vessel until the 
reaction is terminated. It is well known in the art that diazonium salts 
are more or less readily decomposable and that the quality of secondary 
products made therefrom is seriously impaired if the diazonium salts 
actually decompose. It is therefore highly desirable for the residence or 
storage period of diazonium salt solutions in the diazotization vessel to 
be kept constant and as short as possible, or for that period to be at 
least variable without blockading the use of the vessel for diazotizing a 
fresh batch. 
A process for the continuous diazotization of amines has already been 
described in German Pat. No. 960 205, wherein a diazotizable amine, which 
may be used in salt form and in admixture with a suitable solvent, is 
introduced continuously and jointly with a diazotizing agent and with or 
without one or more acids as diluents into a precooled solvent or diluent, 
in which reaction mixture gradually concentrates as the reaction proceeds, 
the introduction of the various reactants being controlled depending on 
the composition selected for the final product which is continuously 
removed. 
This prior process does, however, not favorably compare with the 
discontinuous diazotization inasmuch as it is necessary over the entire 
reaction period to use a diazonium salt solution with an excess of nitrous 
acid therein, which promotes the decomposition of the diazonium salt and 
impairs the quality of secondary products made therefrom. Last but not 
least, the process just described is rendered disadvantageous by the fact 
that it is necessary for the bulk of the diazonium salt solution to be 
recycled so that the tendency of the diazonium salt to undergo 
decomposition is even increased. It is therefore an object of the present 
invention to provide a process for the continuous diazotization of 
diazotizable amines in good yields, which avoids the technically adverse 
effects described hereinabove. 
The present invention relates more particularly to a process for the 
continuous diazotization of primary aromatic amines by reacting an aqueous 
solution or suspension of the amine in a mineral acid with an aqueous 
sodium nitrite solution, which comprises: supplying continuously the lower 
portion of a cylindrical diazotization vessel placed in upright position 
with an aqueous mineral acid solution or suspension of a diazotizable 
primary aromatic amine and supplying the vessel simultaneously, via one or 
more inlets arranged one above the other so as to open laterally 
thereinto, with an aqueous sodium nitrite solution, the amine and nitrite 
being used in stoichiometric proportions, or the nitrite being used in a 
stoichiometric deficiency and the acid being used in an excess of about 1 
to 3 equivalents per amine equivalent in the mineral acid solution; 
reacting the resulting mixture with agitation and producing a laminar flow 
of liquid matter at temperatures of about 5.degree. to 30.degree. C.; 
removing reaction mixture, in accordance with the diazotization velocity 
of the particular amine used, from the upper portion of the diazotization 
vessel at a place where the reaction mixture is substantially free from 
nitrous acid, said place being situated in the vessel at a level which is 
the higher the lower the diazotization velocity of the amine used; 
filtering the reaction mixture removed and delivering diazonium 
salt-containing solution to a sojourn vessel. 
The process of the present invention can be modified in various ways 
depending on the diazotization velocity of the particular aromatic amine 
used. In all those cases in which use is made of an amine which rapidly 
undergoes diazotization, it has been found good practice, for example, to 
introduce the aqueous sodium nitrite solution exclusively into the 
diazotization vessel through the lowermost inlet possible. 
On the other hand, in all those cases in which the amine to be diazotized 
is one which undergoes diazotization at a reduced velocity, it is good 
practice to introduce the aqueous sodium nitrite solution into the 
diazotization vessel, through a plurality of inlets disposed at different 
levels, the sodium nitrite solution being admitted through the individual 
inlets in quantities decreasing from above to below so as to have a minor 
excess proportion of nitrous acid just in the reaction mixture formed 
within the region of the uppermost inlet. In the case just described, it 
is also possible to supply the diazotization vessel with a stoichiometric 
deficiency (with respect to the amine) of aqueous sodium nitrite solution, 
to filter off unreacted amine from the reaction mixture removed, and to 
recycle the filter residue to the diazotization vessel. 
A further preferred feature of the present invention provides for the amine 
to be diazotized at a temperature of 10.degree. to 20.degree. C., 
irrespective of the nature of the particular amine concerned. To ensure 
reliable operation, it is an important requirement for the reaction 
mixture to contain, at the discharge place from the diazotization vessel, 
some minor excess proportion of nitrous acid, which is recognizable by 
spot reaction on potassium iodide starch paper, and which is an index of 
the completeness of the reaction. The present process permits unreacted 
amine to be post-diazotized outside the diazotization vessel, namely in 
the sojourn vessel placed downstream thereof, by admixing it with aqueous 
sodium nitrite solution, and thus provides for the diazotization to be 
controlled. 
Finally, the present process permits the quality of the diazonium salt 
solution and azo dyestuffs which are made therefrom to be influenced in 
reproducible manner by varying the residence time of the reaction mixture 
in the sojourn vessel. Generally, the residence time for diazonium salt 
solutions made from readily diazotizable amines is about 1 to 10 minutes, 
and for those which are made from difficultly diazotizable amines is about 
10 to 30 minutes.

An aqueous solution or suspension of a primary aromatic amine and mineral 
acid in an agitator-provided vessel 1 is delivered by means of a dosing 
pump 4 and via a conduit 3 to an agitator-provided diazotization vessel 2. 
At the same time, a predetermined quantity of aquous sodium nitrite 
solution coming from a reservoir 6 is introduced by means of a pump 5 via 
a conduit 7 and supply inlets 8, 9 and 10 into the diazotization vessel 2, 
the quantity supplied being controlled by a rotameter 11. The 
diazotization vessel 2 is provided with a cooling jacket (not shown in the 
drawing) permitting the reaction mixture to be cooled. The blades of the 
agitator in the diazotization vessel 2 are arranged so as to ensure a good 
mixing effect in the plane which runs transversely to the agitator axis, 
the agitating velocity being sufficient to produce a laminar liquid flow 
in the direction of the agitator axis. As a result of the laminar flow 
conditions which are established inside the diazotization vessel 2, it is 
possible for the diazonium salt to remain over a defined period of time in 
the vessel, the residence time depending exlusively on the dimensions of 
the diazotization vessel and the quantity of liquid which is admitted 
thereto per unit time. 
Reaction mixture issuing through a conduit 12 is filtered in a filtering 
device 13 and collected in a sojourn vessel 14. 
In the event of undissolved unreacted amine being contained in the aqueous 
diazonium salt solution collected in the sojourn vessel 14, it is possible 
to subject the solution to post-diazotization in the sojourn vessel 14 by 
introducing aqueous sodium nitrite solution thereinto, the latter solution 
coming from a tank 15 and travelling through a conduit 16. A magnetic 
agitator 17 is used for thoroughly mixing the material in the sojourn 
vessel 14. The final diazonium salt solution is removed through a conduit 
18 by means of a pump 19. Conduit 20 can be used for operation of the 
filter 13 under vacuum, if desired or necessary. 
The process of the present invention compares favorably with the prior art 
in respect of the following points: As a result of the short residence 
time of the reaction mixture in the diazotization vessel, it is possible 
in the present process to effect the diazotization of the amines while 
cooling with water, without the need to use ice as in the prior art 
methods. The use of the nitrite in a stoichiometric deficiency and the 
step of recycling unreacted amine, which is not separated in the filter, 
to the diazotization vessel has turned out beneficial in the diazotization 
of difficultly diazotizable amines. More specifically, the diazonium salt 
remains in the reactor over a short while only, and the process is easy to 
control just by regulating the quantity of nitrite admitted to the 
reactor. 
Last but not least, it is possible by the arrangement of a sojourn vessel 
downstream of the diazotization vessel to regulate the residence time of 
the diazonium salt solution, and in this manner reproducibly to influence 
under control the quality of the diazonium salt solution and/or azo 
dyestuffs which are made therefrom. 
EXAMPLE 1 
253 g (1 mol) of 3,3'-dichlorobenzidine was suspended in vessel (1) in 2500 
ml of water and the resulting suspension was stirred for 15 minutes. Next, 
it was admixed with 510 ml of hydrochloric acid of 31 weight % strength, 
and the whole was stirred for a further 15 minutes. By means of dosing 
pump (4), the suspension was pumped continuously within 1 hour into the 
base portion of the cylindrical diazotization vessel (2) which was 4 cm 
wide and 25 cm high. At the same time, a 20 weight % aqueous sodium 
nitrite solution coming from reservoir (6) was continuously introduced 
through inlet (10) into the diazotization vessel (2); the nitrite solution 
was used in a quantity sufficient for the diazonium salt solution issuing 
from the diazotization vessel (2) to contain a minor excess proportion of 
nitrite. 600 ml or 2 mols of sodium nitrite solution was consumed. The 
temperature inside the diazotization vessel was maintained at about 
15.degree. C. by means of water. A clear bright yellow diazonium salt 
solution was obtained after filtration. The conversion rate was 100 %. 
EXAMPLE 2 
304 g (2 mols) of 3-nitro-4-aminotoluene, 850 ml of water and 500 ml of 
hydrochloric acid of 31 weight % strength were stirred together for 15 
minutes in the vessel (1). By means of dosing pump (4), the suspension so 
obtained was pumped continuously within 1 hour into the base portion of 
the diazotization vessel (2). At the same time, the diazotization vessel 
(2) was continuously supplied, through inlets (8, 9 and 10) with a 
quantity of a 20 weight % sodium nitrite solution necessary for the 
reaction mixture to remain free from nitrite in excess within the region 
of inlets (9) and (10) and to contain some minor excess proportion of 
nitrite within the region of inlet (8). The bulk of nitrite solution was 
introduced into the base portion, a small quantity was introduced into the 
center protion, and a minor quantity was introduced into the head portion 
of the diazotization vessel (2). A temperature of about 20.degree. C. was 
maintained inside the vessel (2) by means of cooling water. A clear 
reddish to bright violet diazonium salt solution was obtained downstream 
of the filter. The conversion rate was 100%. 
EXAMPLE 3 
The procedure was as described in Example 2, but a temperature of only 
15.degree. C. was maintained inside the diazotization vessel (2). In 
addition to this, only 90 weight % (based on the quantity used in Example 
(2) of sodium nitrite solution was used, and the solution was introduced 
into the diazotization vessel exclusively through the lowermost inlet 
(10). 3-nitro-4-amino-toluene in excess was separated from the reaction 
mixture in filtering device (13), recycled to the vessel (1) and used for 
preparation of the next batch, which was composed of 90% of fresh and 10% 
of recovered and recycled 3-nitro-4-aminotoluene. 
Recovered amine was recycled nine times, without any adverse effect on the 
quality of the diazonium salt. Amine obtained as filter residue after the 
10th cycle was discarded to remove impurities. The filtrate was a 
diazonium salt solution brown colored by 3-nitro-4-aminotoluene which was 
dissolved therein. The solution was postdiazotized in the sojourn vessel 
(14) by admixing it with a minor quantity of sodium nitrite solution. The 
diazonium salt solution turned reddish-violet. After having been allowed 
to remain in the sojourn vessel (14) over 15 minutes, it was taken 
therefrom for work up. 
EXAMPLE 4 
The procedure was as described in Example 3, but the residence time of the 
diazonium salt solution in sojourn vessel (14) was once reduced to 5 
minutes and once increased to 30 minutes. The different residence times 
resulted in the formation of qualitatively different diazonium salts as 
was evidenced by the tint of the azo dyestuffs made from the respective 
diazonium salt. 
EXAMPLE 5 
345 g (2 mols) of 4-chloro-2nitraniline was made into a paste with 136 ml 
of water and the paste was admixed with 520 ml of hydrochloric acid of 31 
weight % strength. The whole was stirred for 15 minutes and then admixed 
with a further 662 ml of water and 8 mols of a 10 weight % solution of 
dibutylated naphthalene sulfonate (LEONIL DB, this is a registered Trade 
Mark of Hoechst Aktiengesellschaft, Frankfurt/M., Federal Republic of 
Germany). The suspension so obtained was continuously introduced within 1 
hour into the base portion of the diazotization vessel (2) which was 4 cm 
wide and 70 cm high. At the same time, the diazotization vessel (2) was 
fed with sodium nitrite solution, through inlets (8, 9 and 10). A 
temperature of 10.degree. C. was maintained inside the diazotization 
vessel (2) by cooling with brine. The conversion rate was 93% under these 
conditions. Unreacted 4-chloro-2-nitraniline was cycled as described in 
Example 3. The diazonium salt solution obtained downstream of the filter 
was post-diazotized in sojourn vessel (14) with a small quantity of sodium 
nitrite solution to effect reaction of dissolved amine.