Determination of Methylbenzimidazolone Isomers in Tolylene Diisocyanate W. W. Harple, E. J. Kuchar, and R. D. Householder O h Corporation, Research and Development Department, Central Analytical Department, New Haven, Conn. COMMERCIAL TOLYLENE DIISOCYANATE (TDI) consisting of approximately 80 per cent 2,4-tolylene diisocyanate and 20 per cent 2,6-tolylene diisocyanate, is prepared from the corresponding isomeric mixture of the dinitrotoluenes by reduction to the amines and subsequent phosgenation. Commercial dinitrotoluene normally contains approximately three to four per cent of the so-called “ortho” isomers, which consist essentially of 2,3- and 3,4-dinitrotoluene. In the phosgenation step, these isomers may be partially converted to the 4-methylbenzimidazolone and 5-methylbenzimidazolone and then to polymeric reaction products of varying molecular weight by further reaction with TDI, by (I):
P O
6 I
CH3
--
NCO
Some of the ortho isomers may be converted to o-tolylene diisocyanates and polymers thereof (2) which have been postulated to contain an imidazolone structure. Regardless of the mechanism, all species containing the benzimidazolone structure should be measured by the procedures outlined. Polymeric reaction products of methylbenzimidazolone and TDI were prepared and hydrolyzed by this procedure. Tolylene diamines and methylbenzimidazolones were recovered and identified by infrared absorption. Since the object of this study was to develop a method which would detect low concentrations of benzimidazolone, it was necessary to separate the desired products from the large excess of TDI. The methylbenzimidazolone isomers were found to be stable in the presence of strong mineral acids, which led to the method described utilizing acid hydrolysis of the TDI to the corresponding amines, followed by their removal by ion-exchange and determination of the methylbenzimidazolones in the column effluent by ultraviolet spectrophotometry. The ultraviolet determination is based on the sufficiently similar molar absorptivities of the 4-methyl- and 5-methylbenzimidazolones to permit the use of a single value for measurement near 280 mp, thus giving a total concentration of the two isomers. EXPERIMENTAL
Apparatus. Hydrolysis of the samples was conducted in glass pressure tubes, 30 cm long with a wall thickness of 4 mm, (1) V. Kirss and J. C. Park, U. S . Patent 3,420,752 (1969). (2) W. J. Schnabel and E. Kober, J. Org. Chern., 34, 1162 (1969). 1658
Table I. Molar Absorptivities for Standard Solutions of the Methylbenzirnidazolone Isomers Near 280 mp Concentration Sample moles/liter ( X Log € 4-Methyl 1.41 3.75 4-Methyl 0.65 3.72 5-Methyl 2.19 3.74 5-Methyl 0.41 3.77 CMethyl 1.65 (4-methyl) 3.76 5-Methyl 0.41 (5-methyl) 4-Methyl 0.65 (4-methyl) 3.78 5-Methyl 0.73 (5-methyl) 4-Methyl 0.49 (4-methyl) 3.80 5-Methyl 1.24 (5-methyl)
+ + +
and an inside diameter of 17 mm. The tubes were sealed at one end prior to analysis. Dowex 50 W-X8 cation exchange resin (20-50 mesh) was washed with 1:l aqueous hydrochloric acid, followed by water washing until the effluent was neutral. Columns were prepared with 20 grams of the above prepared resin added to 35-cm glass columns of 2.5-cm 0.d. tubing fitted with a stopcock. Absorbance measurements were obtained using a Cary Model 14 recording spectrophotometer in 1-cm silica absorption cells. Reagents. Pure standards of the 4-methyl- and 5-methylbenzimidazolone were prepared according to the method of Clark et al. (3) from chromatographically pure 2,3- and 3,4-toluenediamine by phosgenation and recrystallization of the resulting product from benzene. Purity of both standards was greater than 99%, based on titrimetric assay, mass spectrometry, and infrared. Procedure. Molar absorptivities of each of the pure methylbenzimidazolone standards, and mixtures thereof, were obtained from standard solutions containing 6-32 ppm of the isomers dissolved in a 3:l (v/v) isopropanol-water mixture containing 1 ml of concentrated HCl per 100 ml solvent. Absorbance measurements were taken at the point of maximum absorption in the 280-285 mp region. Molar absorptivity values (reported as log e) are given in Table I. TDI samples are analyzed as follows: 0.5 ml of TDI is pipetted into a pressure tube and 15 ml of concentrated hydrochloric acid are added. A few boiling chips are added to achieve even heating. The tube is partially immersed in a silicone oil bath maintained at 180 f 10 “C, using care to avoid excessive bubbling due to carbon dioxide evolution. After the initial effervescence has subsided, the tube is completely immersed in the bath for 3 to 4 minutes or until all solids which form have dissolved. The tube is then removed from the bath, cooled under tap water, and the open end sealed carefully in an oxygen flame. The tube is again immersed in the 180 OC oil bath for 1 hour. CAUTION: The use of a safety shield is recommended in the event that (3) R. L. Clark and A. A. Pessolane, J . Amer. Client. SOC.,80,
1657 (1958).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
the tube is not properly sealed. The tube is then removed from the bath, cooled, opened, and the contents transferred t o a 250-mi beaker using water to effect complete transfer. The solution is taken to dryness at which time 30 ml of 3 :1 isopropanol-water (v/v) is added with warming until all salts have dissolved. The solution is added to the cation resin in the column and the effluent collected in a 100-ml volumetric flask. This solution is then scanned from 300 mfi to 250 mp, using 3 :1 isopropanol-water as a reference blank. The total amount of methylbenzimidazolone is determined from the predetermined absorptivity, reading the absorbance at the point of maximum absorption. RESULTS AND DISCUSSION
4-Methylbenzimidazolone exhibits two absorption peaks of nearly identical absorptivity at 278 mp and 282 mp; 5-methylbenzimidazoline exhibits one peak at 285 mp with a weak shoulder at 290 mp. The peak maximum of mixtures of the two isomers varies from 282 mp to 285 mp, depending on isomer ratio. In practice, the absorption peak is read at peak maximum and the average molar absorptivity of the two isomers is used to calculate total methylbenzimidazolone content (Table I). The peak shape and position has been used in these laboratories to estimate qualitatively the approximate ratio of the two isomers. No attempt was made, however, to determine the precise ratio. During the development of the method, a study was conducted to ascertain the optimum conditions for complete hydrolysis. No improvement in recovery of methylbenzimidazolone isomers was found by increasing hydrolysis time beyond 1 hour or by raising the bath temperature beyond 180 O C . No interferences have been encountered provided removal of the amines is complete. The use of insufficient resin results in slightly colored solutions and strong interfering absorption in the ultraviolet region. Under these conditions, the solution is passed through a fresh resin bed which, in most cases, has been found to satisfactorily remove the interference. Recovery data, as shown in Table 11, were obtained by complete hydrolysis and separation of synthetic standards prepared by adding known amounts of each isomer to ortho-free 80/20 TDI.
Table 11. Recovery of Methylbenzimidazolone Isomers from 80/20 TDI Total recovered, Recovery, Isomer Added, mg mg 4-Methyl 1.20 1.105 91.6 1.13 94.2 1.16 96.7 5-Methyl 1.30 1.32O 101.5 1.30 100.0 1.36 104.6 Mixed isomers 0.80 (4-methyl) 2. lob 100.0 1.30 (5-methyl) 2.20 104.7 Mixed isomers 1.80 (5-methyl) 2.52b 96.9 0.80 (4-methyl) 2.56 98.5 Mixed isomers 2.70 (5-methyl) 5 . 05b 99.0 2.40 (4-methyl) 5.01 98.2 a Triplicate determinations. * Duplicate determinations.
z
Table 111. Repeatability of Methylbenzimidazolone Content in TDI MBA found, mg Std dev Sample 1 0.56 0.61 0.53 h0.06 0.47
Sample 2
1.90 2.00 1.67 1.92
10.14
The method has been applied routinely to determination of methylbenzimidazolones in commercial 80/20 TDI at an average time of 21/2hours per sample. Table I11 shows the repeatability which can be expected. These data were obtained on two samples of commercial TDI containing two levels of methylbenzimidazolone. The data indicate that the method has applicability for routine determinations. RECEIVED for review June 16, 1970. Accepted August 31, 1970. The authors thank Olin Corporation for allowing us to publish this work.
Fast-Response Differential Amplifier for Use with Ion-Selective Electrodes M. J. D. Brand and G . A. Rechnitz Department of Chemistry, State University of New York, Buffaa[o,N . Y . 14214
DESPITETHE RAPID ADVANCES which have occurred in ionselective electrode technology in recent years ( I ) , few attempts have been made to measure electrode response times or to use membrane electrodes to follow fast reactions in solution. (1) R. A. Durst, “Ion Selective Electrodes,” National Bureau of
Standards Special Publication 314, Washington, D. C., 1969. c. Kugler, ANAL. CHEM.. 39. 1682 ~, (1967). (3) G. Johansson and K. Norberg, J. Elecfroanal. Chem., 18, 239 (1968). (4) F. J. W. Roughton and B. Chance, in “Technique of Organic Chemistry,” S. L. Fries, E. S. Lewis, and A. Weissberger, Ed., Interscience, New York, N. Y.,1963, Vol. VIII, Part 11, p 784. (2) G. A. Rahnitz and G.
Measurements of glass electrode response times have been made and values in the range 10-2 to 10-3 second reported (2, 3). Reliable measurements have not usually been made on other electrodes, but in most cases the response times are long (seconds to minutes) at low concentrations and become fast at the highest usable concentrations (0.1 to 1.OM). Kinetic studies of fast reactions in solution using glass ( 4 ) and liquid (5) membrane electrodes have avoided problems associated with the electrode response times by rapid mixing continuous flow techniques. ( 5 ) B. Fleet and G. A. Rechnitz, ANAL. CHEM., 42, 690 (1970).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
1659
