Analytical Determination of p-Toluidine in the Presence of Its Isomers C. H; BENBROOK AND R. H. KIENLE Calco Chemical Division, American Cyanamid Company, Bound Brook, N. J.
T
HERE has long been a need for a n analytical method for
ilpparatus
the determination of p-toluidine, particularly when present in small quantities in mixtures of the isomeric toluidines. Commercial o-toluidine contains small quantities of both the meta and para isomers, b u t accurate methods are available for the determination of only the meta isomer ( 3 ) . The present investigation supplies a method for the determination of the para isomer.
The earlier work carried out in these laboratories on the stability of diazo compounds involved the use of an automatic recording nitrometer ( I ) , which was invaluable in working out the basis for the present analytical method. In the present work, horever, a sim ler apparatus has been found equally satisfactory. The setup uselis shown in Figure 1 and consists of a cylindrical flask fitted with a ground-glass joint, a bubble counter or vapor trap, and a conventional nitrometer. A constant-temperature bath set a t 45" * 0.1' C. is also required but is not shown in Figure 1.
Principles of the Method
Details of Procedure
I n earlier work on the stability of diazo compounds (2) i t was shown that at a temperature of 45' C. the stabilities of 0- and m-toluenediazonium chlorides are identical and very much lower than the stability of the para isomer. For example, 0- or m-toluenediazonium chloride is 99.9 per cent decomposed at the end of 36 minutes, whereas the para isomer requires 21 hours to reach 99.90 per cent decomposition. Thus, if a mixture of the isomeric toluidines is diazotized and allowed to decompose, virtually all of the 0- and m-toluenediazonium chlorides will be decomposed in less than an hour, and all nitrogen which is evolved from the reaction mixture subsequently m a y be attributed to the decomposition of p-toluenediazonium chloride. Since careful determinations have been made of the rates at which the various diazonium compounds decompose ( 2 ) , i t is possible to calculate what percentage of the p-toluenediazonium chloride originally present will remain a t the end of a n y chosen period of time. It was found, for example, that p-toluenediazonium chloride is 62 per cent decomposed after 3 hours at 45" C. Thus, if the nitrogen subsequently evolved from a n unknown mixture of isomeric toluenediazonium chlorides after 3 hours of decomposition at 45' C. is gathered and measured, i t will represent 38 per cent of t h e total para isomer which was present in the unknown mixture. B y choosing 3 hours for the decomposition time, inaccuracies due t o the presence of aniline are avoided, since 2.5 hours are required for benzenediazonium chloride to reach 99.90 per cent decomposition a t 45" C. The analytical method described in detail below is a n application of these principles.
A 0.05-mole sample of the mixed toluidines (5.354 grams) is dissolved in 90 ml. of 5 N hydrochloric acid. The solution is cooled to 5" C. and N sodium nitrite added from a buret, the tip of which extends below the surface of the solution. The solution is stirred mechanically and the sodium nitrite solution added a t a rate not greater than 3.5 ml. per minute at the beginning of the reaction and becoming even slower after the addition of 40 to 45 ml. of nitrite (theoretical requirement 50 ml.). It is very important to avoid an appreciable excess of nitrite at any time, as this seriously affects the final results. The temperature must not exceed 8" C. The pro ress of the diazotization is observed by dipping a glass stirring rof into the solution and touching the rod to white starchiodide paper. An immediate development of blue color indicates the presence of excess nitrite and the end point is reached when the immediate blue color which appears can be obtained repeatedly during a period of 5 minutes without further addition of nitrite. At the end of the 5 minutes the solution is transferred to flask C and diluted to 250 ml. with distilled water a t 5" to 8" C. and the flask is immersed up to the neck and clamped in a constant-temperature bath held at 45" * 0.1' C. (not shown in Figure 1). The flask is left open to the air and provided with a high-speed electric stirrer. The solution is protected from strong direct daylight and held thus at 45" C. for 180 ('1) minutes. U on removal it is placed immediately in the ice and water bath Figure 1) and cooled as quickly as possible to 0" C. buring the cooling period the assembly shown in Figure 1 is completed. Pinchclamp B is closed, stopcock A is opened to the air, and carbon dioxide from a pressure cylinder is run throu h the system in a very rapid stream and allowed to escape into t i e air a t A . When the system has been thus flushed for 15 minutes, the rate of flow of the carbon dioxide is reduced to about one bubble er second, the pinchclamp a t B is opened, and simultaneousg the stopcock a t A is closed. The carbon dioxide is dissolved in 41.6 per cent potassium hydroxide solution which a i s the nitrometer. The air and nitrogen trapped in the system are collected over this potassium hydroxide solution. The value of the zero reading is not influenced measurably by the extremely slow evolution of nitrogen at 0' C. When a constant zero reading has been obtained (requiring about 3 minutes), the bulb is leveled, the reading is recorded, the flow of carbon dioxide is almost stopped (one bubble every 5 or 6 seconds), and the ice bath is replaced by a warm water bath. A flame is immediately applied to the water bath and the m-ater is heated to the boil. After 10 or 15 minutes of boiling the flow of carbon dioxide is increased to 2 or 3 bubbles per second and the system flushed until there is no further increase in the volume of gas in the nitrometer. The bulb is then leveled and the volume of gas is read and recorded. The room temperature and barometric pressure are also recorded. The barometric pressure is corrected for the vapor pressure of potassium hydroxide solution and, knowing the temperature and corrected pressure, the number of milligrams of nitrogen collected is calculated. This value is divided by the weight of the sample in grams and the value obtained is then divided by an empirical factor-namely, 96.06 (loglo = 1.98254). This factor represents the milligrams of nitrogen per gram of sample if the sample had been pure p-toluidine, corrected by the extent to which p-toluidine is incompletely diazotized under the prescribed
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0ECOMPOS;TION
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 14, No. 5
conditions. The figure thus obtained is multiplied by 100 to give the per cent of p-toluidine in the sample.
the nonvolatile impurity in the commercial sample had no effect on the results of the determination.
The following typical example will serve to illustrate the method of calculation:
Precautions The most critical aspect of t,he analytical method is the diazotization. Apparently 0- and m-toluidine diazotize much more readily than p-toluidine; however, the resulting diazonium chlorides vary in stability as stated above. Thus, if complete diazotization of the para isomer is to be attempted, considerahle decomposition of the other two isomeric diazonium chlorides will result. during diazotization, even at very low temperatures, and the lower the temperature the slower the diazotization of the para isomer. Theoretically, the decomposition of 0- and,/or rn-toluenediazonium chloride during diazotization of the mixture should be of no consequence, since complete decomposition of the ortho and meta isomers is to be carried out before any measurements are made. Practically, however, if the ortho and meta isomers decompose excessively and an excess of nitrous acid is present, side reactions occur which appear to involve p-toluenediazonium chloride, decomposed o-toluenediazonium chloride, and probably nitrous acid, since very low values are obtained if appreciable excesses of nitrous acid are allowed t,o accumulate during diazotization.
Weight of mixed toluidines diazotized = 5.354 graniz Zero nitrometer reading = 1.30 ml. Nitrometer reading after complete decomposition = 26.25 ml. Volume of evolved nitrogen = 26.25 - 1.30 = 24.95 ml. Room temperature = 25.0" C. Barometric pressure = 758.0 mni. of mercury Vapor pressure of 41.6 per cent potassium hydroxide at 25" C. = 7 mm. of mercury ( 5 ) Corrected vapor pressure = 751 mm. of mercury Since the density of nitrogen at standard conditions is 1.25055 ( 4 ) , therefore at 25' C. and 751 mm. pressure 1 ml. of nitrogen = 1.132 mg.
% p-toluidine
=
corrected ml.of S, evolved x 100 wt. of sample X factor
=
Experimental Results I n commercial o-toluidine the para content is seldom in excess of 10 per cent. Hence for purposes of testing this method i t was considered sufficient to analyze synthetic mixtures of toluidines made up to contain from 0 to 15 per cent of p-toluidine. I n Table I are given results of analyses of such mixtures of pure 0- and p-toluidines. The major part of the work in testing the method involved the use of mixtures of the ortho and para isomers. Two experiments were carried out, however, in which the para isomer was mixed with m-toluidine; as is indicated in Table 11, the method operates as well in the presence of the meta isomer as in the presence of only the ortho isomer. TARMI .
ASAI.I.SES
OF SYNTHE'I.I(~
i r I x ~ r ~ ~ tO.Y: s0- .\SI) p-
Tor.viars ~ ; s Para Found
Para Present
%
%
1.31 1.68 4.92 5.13 13.81 14.19
1.34 1.44 4.86 5,17 13.80 14.59
As other impurities may be present in commercial o-toluidine, steps were taken to determine whether these impurities would interfere with the successful operation of the analytical method. A common impurity is moisture. Obviously, since the method is carried out in aqueous solution, moisture does not interfere with the reactions involved.. However, for accurate work the total diazotizable material present should first be determined. This may be done by means of wellestablished procedures for determination of aromatic amines. TABLE 11. ANALYSESOF SYKTHETIC MIXTURESOF ~TOLUIDISES Para Found
-PresentPara
0-, m-, .ASI)
Meta
%
%
%
4 86 5 36
4 74 6 18
4 07
4.37
Certain high-boiling impurities are sometimes present in commercial material. I n order to test the effect of these materials on the analysis, 75 ml. of a commercial o-toluidine were distilled until only 2 or 3 ml. of a highly-colored, nonvolatile liquid remained. This remaining impurity was mixed ~ i t half h of the distillate and the resulting mixture analyzed by the present method. The remaining half of the distillate was likewise analyzed and since, as shown in Table 111, there was no difference in the values obtained, it waq inferred that
T.IR1.E
111. ANALYSISOF F:NPb: .4ND
~ O H M E K ( ' l . % IO-'rOLUIDIXE . IN PKc:SHICrH-HoIr.rr,-.(: IMPL-RITIES
ABSENCEO F
Pure distillate A>,. Pure distillate Av.
+ residue
Para, % 0 . 9 2 , 0.90 0.91 0.90, 0 . 9 2 0.91
Consequently, the authors' inet'liotl prescribes the addition of nitrite so slowly that the troublesome side reactions are virtually eliminated. The prescribed temperature is high enough so that the greater part of the para isomer diazotizes, and yet not enough of the ortho and meta isomers decompose to cause significant errors. The side reactions mentioned can be recognized b y the development) of a deep coloration during diazotization and/or decomposition. Thus, by use of the diazotization procedure outlined the p-toluidine is incompletely diazotized, but if the directions are carefully followed, the reproducibility of the degree of diazotization appears to be well within the limits of accuracy stated. The accuracy of the analytical method depends not upon complete diazotization but upon di:izotization to the same extent every time. It thus becomes necessary for the calculat,ions to include a term which will embrace the extent to which diazotization of the p-toluidine is complete. The term 96.06 in the above calculations is the milligrams of nitrogen actually obtained per gram of pure p-toluidine when diazotization has been carried out as described herein and has been found to hold over t'he range of mixtures reported.
Acknowledgments The authors wish to acknowledge the assistance of Charles D. Compton in the development of the details of this method and to express thanks to A. R . Sorton and Wlliam Seaman, who have kindly permitted the authors to cite the results of critical tests carried out on the method by members of their staff. (1)
Literature Cited Crossley, Kienle, and Benbrook, I s u . ENU.CHEM.,ANAL.ED..
(2)
Crossley, Kienle, and Benbrook, J . A m . Chem. SOC.,62,
12, 216 (1940).
1400
(1940).
Evers and Strafford, J . SOC.Chem. I d . . 46, 114T (1927). Handbook of Chemistry and Physics. 21st ed., p. 1187, Chemical Rubber Publishing Co., 1936. ( 5 ) Milner and Sherman, IND. EKG.CHEM., A N A L . ED.,8, 331 (19.36).
(3) (4)
