Analytical Significance of Aromatic Isocyanate-Peroxy Reaction Ronald F. Layton’ and Quentin Quick Research and Decelopment Department, Chemicals and Plastics, Union Carbide Corp., South Charleston, W. Vu.
THEWIDESPREAD USE of aromatic isocyanates as reactive intermediates, especially in urethane chemistry ( I ) , has prompted the appearance of a number of methods for their determination (2, 3). One of the most satisfactory of these appears t o be a titrimetric approach in which the isocyanates are reacted with an excess of di-n-butylamine ( 4 , 5), the excess of which is easily titrated. The dibutylamine method cannot be employed in its conventional form to determine low concentrations of isocyanate. During our studies of various modifications of the dibutylamine approach which we hoped might add greater sensitivity t o the reaction, we observed that a n intense orange color resulted when a dioxane-isopropanol solution of a n aromatic isocyanate was made basic with tetrabutylammonium hydroxide, This orange color was initially attributed to a reaction between dioxane and the isocyanate, but was later found to be associated with the peroxide content of the dioxane. It could, in fact, serve as a sensitive test for peroxides in dioxane. A study of the literature revealed that the reaction of peroxides with aromatic isocyanates t o form colored products has been studied from a synthesis standpoint (6-IO), but the analytical implications of these studies d o not appear to have been exploited. It is our intent to describe the nature of the peroxy-isocyanate reaction in the context of its analytical significance. EXPERIMENTAL
One-molar solutions of tetrabutylammonium hydroxide are available from Southwestern Analytical Chemicals, Austin, Texas. Peracetic acid, available at Union Carbide as a 30% solution in ethyl acetate was the preferred source of peroxy functionality for our studies because of its convenience and availability. Other peroxy compounds, perhaps more conveniently available, can be obtained commercially and can be employed with appropriate adjustments caused by differences in reactivities. The isocyanates employed in this study are available from commercial sources. The 1 Present address, Department of Chemistry, University of Cincinnati, Cincinnati, Ohio
Table I. Stability Data for MDI-Peroxy Complex a t 475 mp (1-cm cell) Solution, MDI, Absorbance after indicated development times 30 Sec 5 Min 10 Min 2 Days g/l00 ml 1. 0.02020 0.234 0.234 0.234 0.240 2. 0,03920 0.510 0.510 0.510 0.850 3. 0.07115 0.900 0.900 0.909 2.37 Table 11. Calibration Data for MDI-Peroxy Complex at 475 mp Absorbance, MDI, g/loO ml 1-cm cell 0.01061 0.143 0.02020 0.234 0.02997 0.384 0.03920 0.510 0.05692 0.721 0.07115 0.900
dimethylformamide (DMF) was commercial grade which had been carefully dried with molecular sieves. A typical color development scheme would consist of treating a freshly prepared solution of the aromatic isocyanate in D M F with a two- to sixfold excess of peracetic acid. This usually results in a rather weak yellow to orange colored solution. A much more intense color results when the solution is then made basic with tetrabutylammonium hydroxide which is added until no further change in color takes place. The development of the color is instantaneous upon the addition of the tetrabutylammonium hydroxide and the color can be read as soon as appropriate dilutions have been made. There is some change in color with time but the rate curve is sufficiently flat to permit a convenient absorbance measurement. The absorbance values reported here were obtained in the visible region. Higher absorptivities are available at selected wavelengths in the ultraviolet region but the increased sensitivity was not needed so these studies have been confined to the visible region. DISCUSSION
(1) J. H. Saunders and K. C. Frisch, “Polyurethanes: Chemistry and Technology,” Vol. XVI, Parts I and I1 of High Polymer Series, Herman Mark, Ed., Interscience, New York, 1964. (2) K. Marcali, ANAL.CHEM.,29, 552 (1957). (3) E. Pflueger, F.A.T.I.P.E.C. Compt. Rend. 4th Congr. Lucerne 141 (1957); Chem. Abstr., 54, 179086 (1960). (4) W. Siefken, Ann., 562, 75-136 (1949). (5) American Society of Testing Materials, Method No. D 1638-61T “Testing Urethane Foam Raw Materials,” 1967 issue, Part 26, p 175. (6) A. G. Davies and K. J. Hunter, J . Chem. SOC.,1953, 1808. (7) C. J. Pedersen, J . Org. Chem., 23,252 (1958). (8) Dr. Michael Lederer and Dr. Otto Fuchs (to Farbwerke Hoechst A.G.), German Patent 1,029,818 (Sept. 24, 1959). (9) E. L. O’Brien, F. M. Beringer, and R. B. Mesrobian, J . Am. Chem. SOC.,79, 6238 (1957). (IO) E. L. O’Brien, F. M. Beringer, and R. B. Mesrobian, ibid., 81, 1506 (1959).
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ANALYTICAL CHEMISTRY
Dimethylformamide was chosen as the basic solvent in these studies because of its excellent solvating behavior toward the reactants and products. In addition, the reaction seemed to be faster and less sensitive to experimental parameters when conducted in D M F . The stability data for the peroxy complex of a typical aromatic isocyanate, methylene-p,p’-diphenyl diisocyanate (MDI) are presented in Table I. The test solutions of M D I in DMF were treated with 10 drops of 26 per cent peracetic acid and, after being made basic with tetrabutylammonium hydroxide, were diluted to 100 ml with D M F . The more highly-colored solutions are seen to increase in absorbance with time but, within normal analytical requirements, the colors are completely stable. The absorbance values also
Aromatic isocyanate Phenyl n~T01yl p-Tolyl p-Nitrophenyl I-Naphthyl Bitolylene diisocyanate (TODI) Tolyl diisocyanate (TDI) Methylene p,p’-diphenyl diisocyanate (MDI) a V = Violet. Ov = Olive. c Y = Yellow. It 0 = Orange. e R = Red. J B = Blue. G = Green.
Table 111. Color Reactions of Aromatic Peracetic acid Base Acid Va Ovb V Y Re Y V Y B’ Y
Isocyanates with Peroxy Compounds Cumene r-Butyl hydroperoxide - hydroperoxide Base Acid Base Acid ov YC Y ov ov Od-Y ov Y ov Y 0 Y Y R R Y ov Y 0 Y
R-V 0-Y
Y
R R-0
0 0
GQ
Y
R-0
0 0
R-V
Y
R-0
Y
R
Y
Q
seemed insensitive t o the usual fluctuations in room temperature. The calibration data for the MDI-peroxy color after 5 minutes development time are presented in Table 11. A straight-line calibration curve results. An estimate of the precision of the method is available from the results of 7 samples each containing 0.03920 gram of MDI. The mean absorbance value at 475 mp was 0.510. The standard deviation was 0.0067 absorbance unit for a relative standard deviation of 1.31 %. The spectrophotometric curve for the MDI-peracetic complex is presented in Figure 1. The slope of this curve in the 400-500 m p region permits a choice of wavelength, depending upon degree of sensitivity needed, with no apparent loss in accuracy. Other aromatic isocyanates give different colors but the general spectral characteristics of the curves appear t o be retained. The base-form color of the MDIperacetic complex is seen to present a well defined maximum absorbance a t 321 mp with a strong molar absorptivity of approximately 14,000. Several bases were tested t o form the complex, but tetrabutylammonium hydroxide seemed t o possess the desired basicity to produce a rapid color change and give a resultant clear solution. All of the aromatic isocyanates studied have an acid-form color which is weaker in the visible than the baseform color. Both the acid-form and base-form colors of a variety of isocyanate-peroxy combinations are listed in Table 111. In all cases a weak solution of the isocyanate-peroxy compound in D M F was treated with tetrabutylammonium hydroxide to give the base-form color, then acetic acid was added to give the acid-form color. A color transition invariably occurs when the systems are made acid so they serve as their own indicators. The acid-form color of the combinations listed tended to be yellow while the base-form color varied from blue to orange-yellow. An additional dimension of qualitative significance is the additional colors made possible by changing the peroxy compound used in the colordeveloping reaction. Several other peroxides were found t o be effective in this reaction. It appears that the only requirement which must be satisfied is that the peroxide must possess a labile hydrogen. The distinctive character of the 1naphthyl isocyanate-peroxy complex is particularly striking inasmuch as several different colors were obtained, depending upon the peroxy compound used and the acidity of the system. Some difficulty was obtained in reproducing the colors
2.2 I
2 5 0 300
400
500
700
600
WAVELENGTH IN MILLIMICRONS
Figure 1. Spectrophotometric peracetic acid complex
characteristics
I
of
MDI-
Base-form color (conc. of 0.0477 gram MDI/liter) (b) Acid-form color (conc. of 0.0350 gram MDI/liter) (a)
obtained with peracetic acid and the limitations of this system have not been fully investigated. Aliphatic isocyanates, such as methyl and ethyl, d o not undergo the characteristic color reaction and no doubt the presence of an aromatic ring is necessary to give sufficient conjugation for a colored species to be obtained. We feel that this reaction will be positive for all aromatic isocyanates and have not, t o date, encountered any other aromatic species which will present this behavior. Any substance which destroys the isocyanate functionality prior t o its reaction with peroxide will, of course, interfere in this determination. The aromatic isocyanate-peroxy reaction has also been considered in terms of its significance in the determination of low VOL. 40, NO. 7, JUNE 1968
1159
Table IV. Calibration Data for Low Concentrations of Peracetic Acid Peroxide, ppm Absorbance, 500 mp 40 0.094 50 0.077 89 0.129 99 0.166 148 0.252 198 0.315 297 0.495 346 0.550
concentrations of peroxy compounds. The quantitative aspects of this reaction are exemplified in Table IV in which the concentration of peracetic acid cs. absorbance at 500 mp is recorded. The concentrations of peracetic were determined by the conventional sodium iodide in glacial acetic acid procedure. A Beer’s law plot of this data presents a straightline calibration curve. Again, much greater levels of sen-
sitivity are available by the simple expedient of lowering the wavelength for the absorbance data. From this data, one can easily project a sensitivity for peracetic acid in the subppm level. CONCLUSION The base-catalyzed aromatic isocyanate-peroxy reaction has been demonstrated to have quantitative significance in the determination of aromatic isocyanates and peroxy compounds. It should have widespread analytical utility. The qualitative aspects of the reaction provide an interesting tool for the classification of aromatic isocyanates and, t o some degree perhaps, for the classification of peroxy compounds. Neither the quantitative nor the qualitative aspects have been explored in sufficient depth to fully appraise the potential as well as the limitations of this reaction. Further investigations to provide a better understanding of the theoretical aspects of this reaction are being conducted, RECEIVED for review January 26, 1968. Accepted April 5, 1968.
lodometric Method for Determining Alkyl Anthraquinones and Alkyl Anthrahydroquinones Scott Lynn1 and H. H. Paaiman Western Research Laboratories, The Dow Chemical Co., Pittsburg, Calif. ALKYLANTHRAQUINONES,and especially 2-ethyl anthraquinone (EAQ), are widely used in the production of hydrogen peroxide (1). In a typical process an organic solution of EAQ is hydrogenated to 2-ethyl anthrahydroquinone (H,EAQ) catalytically. In a separate step H2EAQ is autoxidized back to EAQ and H202is formed. The EAQ is then recycled. The reactions are carried out in a solvent system capable of dissolving both EAQ and H2EAQ. Two solvent components are usually required. Alternative Analytical Methods. EAQ and, presumably, other alkyl anthraquinones may be determined polarographically (2) using a dropping mercury electrode. Dimethyl formamide is used as solvent with LiCl and LiOH as supporting electrolytes. This method suffers as an analytical procedure by being relatively tedious, not highly precise (i2 7 3 , and unsuitable for the determination of H2EAQ. The authors have found vapor phase chromatography useful in many respects for studying the reaction system. However, EAQ cannot be determined with a precision of greater than + 2 z by the method used and the peaks for EAQ and HzEAQ were found to be superimposed. Reduction of EAQ and Oxidation of Na2EAQ. The reaction of EAQ with sodium dithionite can be written 1 Present address, Department of Chemical Engineering, University of California, Berkeley, Calif 94720
(1) W. C. Schumb, C. N. Satterfield, and R. L. Wentworth, “Hydrogen Peroxide,” A.C.S. Monograph No. 128, Reinhold Publishing Co., New York, 1955. (2) R. L. Edsberg, D. Eichlin, and J. J. Garis, ANAL.CHEM., 25, 798 (1953).
1 160
e
ANALYTICAL CHEMISTRY
EAQ
+ Na2S2O4+ 4 NaOH
+
Na2EAQ
+ 2 Na2SOz + 2H20 (1)
where Na,EAQ represents the disodium salt of H2EAQ. The bright red sodium salt js very soluble in aqueous solution above pH 12 while below pH 10 hydrolysis to the virtually insoluble H2EAQ is essentially complete. As might be expected from the solubility behavior and the stoichiometry involved, Reaction 1 proceeds a t a negligible rate when the p H is less than 10. An aqueous solution of Na2EAQ reacts very readily with oxygen. Together with a basic solution of sodium dithionite it is called Fieser’s solution (3) and is used as an oxygen scavenger in applications where water vapor is of no concern. NazEAQ also reacts rapidly and completely in aqueous solution with hydrogen peroxide and sodium hypochlorite. The latter reaction, which may be written Na2EAQ
+ NaOCl + H 2 0
+
EAQ
+ NaCl + 2 NaOH (2)
forms the basis of the analytical procedure described below. EXPERIMENTAL For the determination of an alkyl anthraquinone, it is necessary first to reduce it quantitatively to the corresponding hydroquinone by contacting a solution of it with an excess of basic sodium dithionite. Upon acidification the bright fluorescent yellow hydroquinone is precipitated from the aqueous phase and must be dissolved in the organic phase. (3) F. L. Fieser, and M. Fieser, “Topics in Organic Chemistry,” 2nd Ed., D. van Nostrand Co., Inc., Princeton, N. J., 1951.
