SULFONATION OF ANILINE b b b The production of sulfanilic acid by a partial-pressure evaporation system is shown to be feasible on a laboratory scale. The difficulties of local overheating and complete conversion of the aniline sulfate to sulfanilic acid are overcome so that a yield of over 95 per cent sulfanilic acid is produced when stoichoimetric amounts of aniline and sulfuric acid are used. The small loss of diluent (7 per cent) can easily be lowered with a more efficient condenser and better sealing of the joints than is possible in the experimental work. Larger scale operation will tend to minimize the loss of diluent. These results suggest the possibility of producing sulfanilic acid b y the partial-pressure evaporation system continuously on a larger scale. However, the problem of the sulfanilic acid caking in larger scale operation must be met. By the use of proper agitating equipment, it may be possible to prevent this caking and thereby make possible a continuous process for the sulfanilic acid production.
Partial- Pressure Distillation Joseph
Jr., Donald F. Othmer, and Allan Hokanson
POLYTECHNIC INSTITUTE, BROOKLYN, N. Y.
In the dehydration stage the sulfonic acid group shifts to the para position, and water is removed to form sulfanilic acid:
o s4 ’‘ It was felt that the partial-pressure method might facilitate removal of the water of reaction as it has in previously reported cases (8, 8). Accordingly, the feasibility of carrying out the reaction was investigated, using an acid-treated hydrocarbon fraction in the kerosene range as an entrainer.
NE of the important intermediates in the production of
azo dyes is the organic acid p-aminosulfonic, generally termed “sulfanilic”, acid. These dyes are prepared by coupling diazotized amines with other amines and with sulfonic acids. The United States produces’about 1.5 million pounds of sulfanilic acid annually. I n making sulfanilic acid, water formed by the reaction between aniline and sulfuric acid must be removed. One commercial process does this by baking the aniline sulfate (the initial reaction product) in vacuum oven; another process uses 50 per cent excess concentrated sulfuric acid to withdraw the water. The over-all yield of pure sulfanilic acid on the basis of aniline charged from the “bake” process is over 80 per cent. The yields by the sulfonation process (1) are equivalent to 92 and 93 per cent on the basis of aniline charged. Although the commercial methods do result in high yields of sulfanilic acid, both of them have drawbacks. Both processes are batch operations; the bake process requires considerable handling before the final product is obtained; and the sulfonation process uses a large excess of sulfuric acid which cannot be recovered. The formation of sulfanilic acid by the reaction of sulfuric acid and aniline may be considered to t,ake place in two steps ( I ) :
Sulfanilic A c i d Production
Kerosene fractions of different boiling ranges were used. They were always pretreated by boiling under reflux while agitating with concentrated sulfuric acid for several hours. A heavy scum of carbonaceous matter was removed, the two layers were separated, and the kerosene layer was washed with sodium carbonate solution and water, and finally distilled. The apparatus consisted of a one-liter three-neck flask fitted with a moisture determination tube, a condenser, and a thermometer. (See Figure 1of the paper by Othmer, Jacobs, and Buschmann, page 326). In the flask were placed the sulfuric acid (commercial 93 per cent) and the diluent. The aniline was poured in slowly over a period of 2 minutes. I n all runs 40 grams (39 cc.) were used. Immediately the aniline and the sulfuric acid reacted to form a pinkish-white precipitate, aniline sulfate, with the evolution of heat. The mass was agitated violently and heated gently. Vapors of diluent and water bubbled through the molten mass of aniline sulfate and passed up to the condenser where both the diluent and the water condensed. The water collected a t the bottom of the moisture determination tube, and the diluent refluxed back into the flask.
The sulfuric acid and aniline first react to produce the salt of the dibasic acid:
Effect of Variables on A c i d Yield
Several variables affect the production of sulfanilic acid by the partial pressure method; they were investigated individually in so far as it was possible to maintain the other variables constant. 321
INDUSTRIAL AND ENGINEERING CHEMISTRY
yo Excess Has04 0 10
20 30 40
C O L L E C T E D - C C.
Effect of Amount of Diluent on Yield of Sulfanilic Acid
Effect of Amount o f Water Removed on Yield o f Sulfanilic Acid
Effect o f Temperature o f Reaction on Yield o f Sulfanilic Acid
400 450 500
61.3 54.3 64.3
Cc. Water Removed
Table Co. Water Removed
TEMPERATURE OF DILUENT
Effect o f Amount o f Sulfuric Acid over That Required Stoichiometrically on Yield o f Sulfanilic Acid
Effect of Excess Sulfuric A c i d Temppature, C.
Effect of Volume of Diluent
34.4 40.3 50.5 51.9
(Temperature, 169' C.: excess HaSOa, 1%: water removed, 9.0 cc.) Volume Yield of. Volume Yield of o f Diluent, Cc. Sulfanilic Acid, yo of Diluent, Cc. Sulfanilic Acid, a00 250 300 350
DILuENT \ r ~ ~ ~In ~some f ~ of . the initial experiments the aniline sulfate first formed tended to cake and char. This difficulty was overcome by adding larger volumes of hydrocarbon diluent (boiling range 155-190" C.). In order to test quantitatively the effect of larger amounts of diluent, a series of runs was made in which the volume of diluent was varied. It might ordinarily be expected that the amount of hydrocarbon preqent would have little effect upon yield. But in this case the caking of the aniline sulfate and the consequent reduction in yield seems to be influenced by the quantity of diluent. The effect of increasing the volume of diluent is shown in Table I and Figure 1. EXCESS SULFURIC ACID. The use of the partial pressure method is predicated upon the ability of the diluent to remove the water of reaction as it is formed and hence maintain a high concentration of sulfuric acid. If this concentration is maintained, the necessity for using excess sulfuric acid is eliminated since its only function in ordinary sulfonations is to keep the desired high acid concentration. A series of runs was made in vihich the quantity of excess sulfuric acid was varied while the temperature and amount of water removed were kept substantially constant. The results are shown in Table I1 and plotted in Figure 2; they indicate that the sole function of excess acid in a sulfonation is t o remove chemically the water formed by the reaction and
IO 20 EXCESS SULFURIC
Vol. 35, No. 3
Effect of Amount of Water Removed Temperature, O c.
7.3a 9.oa 10.0 10.4 10.7 11.1 a These r u m were made lower recovery.
IV. Effect of Reaction Tem erature (Boiling Point of Diluent) on g e l d
Cc. Water Removed
168 169 169 170 170 171 172 179 183
10.4 10.7 9.8 10.4 11.1 11.3 10.0 11.0 11.0
57.0 67.2 64.3 58.4 71.7 75.3 65.8 81.0
% Diluent Recovered 85.0 84.0 63.2 86.0 86 0 77.3 86.0 85.0 93.0
prevent the sulfonation acid from falling below a limiting concentration. WATERREMOVAL. Different amounts of water were withdrawn from the reactants in order to observe the effect on sulfanilic acid yield. The conditions mere kept constant for these runs, including the optimum amount of kerosene (500 cc.) and the theoretical amounts of sulfuric acid for the fixed amount of aniline. The results are shown in Table I11 and Figure 3; they indicate that the reaction yields are practically proportional to the amount of water removed within the range studied in this series. REACTION TEMPERATURE.Higher boiling fractions of kerosene were tried in order to find the effect of reaction temperature on the yield of sulfanilic acid. I n previous runs a simple single-blade agitator operating a t low speeds was used. However, high-speed agitation served to produce a more uniform reaction mass and, consequently, allowed the sulfonation to proceed more smoothly. The runs shown in Table IV were made with this improved procedure. Again 500 cc. of diluent were used with the stoichiometric amount of sulfuric acid for the aniline. However, instead of feeding aniline into the kerosene and sulfuric acid mixture as in pre-