Identification of Impurities in a-Trinitrotoluene by Nuclear Magnetic Resonance Spectrometry D. G . Gehring Eastern Laboratory, Explosives Department, E . I. du Pont de Nemours & Company, Gibbstown, N . J .
Characterization of the nonvolatile aromatic impurities present in crude and refined a-TNT was accomplished using nuclear magnetic resonance spectrometry (NMR). In addition to the di- and trinitrotoluene isomers identified previously, the followin oxidation by-products were identified in crude a-$NT: 2,4,6trinitrobenzoic acid (TNBA), 2,4,6-trinitrobenzaldehyde (TNBAL) 2,4- and 3,4-dinitrobenzoic acids (DNBA), a-nitrato-2,4,6-trinitrotoluene (a-nitrato-TNT), and mdinitrobenzene (MDNB). None of these oxidation by-products could be detected in some representative samples of refined a-TNT by NMR (>0.03%). It is suggested that the a-nitro-TNT previously discovered in TNT by thin-layer chromatography was probably a-nitrato-TNT.
CHARACTERIZATION OF NONVOLATILE AROMATIC IMPURITIES in crude and refined 2,4,6-trinitrotoluene (a-TNT) was required to better define the nitration and purification processes. The most prevalent impurities, the di- and unsymmetrical trinitrotoluenes, were identified previously (1-3) and quantitative procedures for determining these individual components were described (3-5). The presence of oxidation by-products and other impurities was reported previously ( I ) and some of these compounds were detected in production grade TNT by thinlayer chromatography (2). However, conclusive proof that specific oxidation by-products are in fact present in a-TNT cannot be based solely upon toluene nitration chemistry ( I ) and/or a comparison of chromatographic retention time values. A method was desired which would detect and conclusively assign the molecular structures of all significant amounts of nonvolatile aromatic impurities in a-TNT. Nuclear magnetic resonance (NMR) spectrometry was found to be a satisfactory technique for accomplishing this purpose. EXPERIMENTAL
Apparatus. All the spectra were recorded on a Varian A-60 spectrometer using thin-walled sample tubes. The chemical shifts are expressed in hertz from tetramethylsilane (TMS) internal reference, and perdeutero acetone served as the solvent. Reagents. The samples of crude and refined a-TNT were obtained from the Du Pont Barksdale TNT plant, Barksdale, Wis. 2,4,6-Trinitrobenzoic acid (TNBA) and m-dinitrobenzene (MDNB) were reagent grade compounds purchased from Eastman Organic Chemicals, Rochester, N. Y. 2,4,6Trinitrobenzaldehyde (TNBAL) was purchased from Aldrich Chemical Company, Fairfield, N. J. 2,4- and 3,4-dinitrobenzoic acids (DNBA) were purchased from K & K Laboratories, Plainview, N. Y. a-Nitrato-2,4,6-trinitrotoluene (a-nitrato-TNT) was synthesized by treating a-bromo-2,4,6TNT with silver nitrate in acetonitrile at 0 "C for several (1) C.E.Munroe, Army Ordnance, 5,507 (1924). (2) S. K.Yasuda, J. Chromatogr., 13,78 (1964). (3) F. Pristera, Appl. Specfros., 7 , 115 (1953). 39,1315 (1967). (4) D. G. Gehring and J. E. Shirk, ANAL.CHEM., (5) D. G. Gehring and G. S. Reddy, ibid., 40,792 (1968). 898
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hours (6). The a-bromo-TNT was prepared by reacting a-TNT and sodium hypobromite in tetrahydrofuranmethanol solvent at -5 "C (7). Identities and purities of all these compounds were established from their NMR and infrared spectra. Identification of Impurities in Crude TNT. Crude a-TNT is, by definition, the product recovered directly from the trinitration step. This material has not been purified except for water dilution of the excess mineral acid and should contain relatively large amounts of by-product impurities. Upon standing under water for several days, a red oily exudate appeared in each sample forming a layer on top of the a-TNT. About 5 ml of this red oil was recovered per sample and represented about 1.5% of the total TNT sample weight. Since this exudation phenomenon is analogous to a sort of in situ zone refining process, this red oil exudate appeared to be a highly concentrated source of a-TNT impurities. The NMR spectrum of this red oil is shown in Figure 1. The Ar-CHa peaks at -151, -157, -159, -162, and -168 Hz from TMS internal reference were assigned to 2,3,6-, 2,3,5-, 2,3,4-, 2,4,6-, and 2,4,5-TNT, respectively (5). Integration of these peaks indicated that a-TNT represented only 40% of the red oil exudate confirming the initial premise that this material was rich in impurities. Most of the lowfield NMR peaks were identified as the ring hydrogens corresponding to the above Ar-CH3 peaks, but the identities of signals at -370, -523, -548, -557, -560, and -638 Hz were not immediately obvious. A small portion of the red oil was washed with 10% sodium bicarbonate and the spectrum of the remaining oil (now brown colored) is shown in Figure 2. When compared with Figure 1, the high-field methyl region was unchanged, but the lowfield multiplet at -523 Hz and the singlet at -557 Hz were gone. The aqueous bicarbonate extract was acidified to pH 7 with acetic acid and extracted several times with chloroform. The spectrum of the concentrated chloroform extract appears in Figure 3. The peaks designated X were assigned to 2,4-DNBA when compared with an NMR reference spectrum of this compound. This red-colored acid accounted for most of the chloroform-recovered material and was primarily responsible for the color of the oil. The bicarbonate-removed singlet at - 557 Hz did not reappear in the chloroform extract, indicating that the acid molecule responsible for this peak was either decarboxylated during the work-up or more soluble in water than in chloroform, or both. TNBA was known to be water-soluble and easily decarboxylated. A small amount of this acid was added to some of the original red oil and the resulting spectrum (Figure 4) showed a relative increase in the -557 Hz peak; hence, this peak was assigned to the ring protons of TNBA. A second portion of the chloroform extract was esterified using diazomethane, and gas chromatographed over a 24-ft X 0.25-inch stacked column consisting of 12 ft of 5 Apiezon L (injection port end) and 12 ft of 10% DC-LSX-3-0295 on Anakrom-ABS support. In addition to the methyl ester of 2,4-DNBA, two smaller peaks were isolated and identified (6) L. F. Fieser and W. V. E. Doering, J. Amer. Chem. SOC.,68, 2253 (1946). (7) K. G . Shipp and L. A. Kaplan, J. Org. Chem., 31,857(1966).
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Figure 1. NMR spectrum (60 MHz) of crude TNT red oil exudate
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Figure 2. NMR spectrum (60 M H z ) of NaHCOrextracted red oil exudate showing disappearance of low-field multiplet at -523 Hz and singlet at -557 Hz ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Figure 3. Spectrum (60 MHz) of NaHC03 extract from crude oil exudate. Peaks designated X were assigned to 2,4-DNBA
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Figure 4. NMR spectrum (60 MHz) of : crude red oil illustrating relativeincreasein -557 ; Hz peak after dosing 5 a, with TNBA -2
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Figure 5. MNR spectrum (60 MHz) of crude red oil showing relative increase in -560 and -638 Hz peaks after dosing with TNBAL from their infrared reference spectra as the methyl ester of 3,4-DNBA and MDNB. These impurities would account for most of the small aromatic peaks in Figure 3 which are not assigned to 2,4-DNBA. Because of the very small amounts present, it is uncertain if the MDNB was actually present in the original red oil or if some of the 2,4-DNBA was decarboxylated during the sample preparation and gas chromatography. The chemical shift of the low-field resonance at -638 Hz (Figure 1) was indicative of an aldehydic proton. By washing a second portion of the red oil with a saturated solution of sodium bisulfite, the two signals at -638 and -560 Hz disappeared. The compound responsible for these signals was thought to be TNBAL because the -560 and -638 Hz signals were in 2 : l intensity ratio, the ring protons were chemically equivalent, and no spin-spin coupling was observed in either signal. The red oil was dosed with some purchased TNBAL and the two NMR signals increased relative to the other resonances as illustrated in Figure 5 . Subsequently, a portion of the red oil was gas chromatographed using a 8-ft X 0.25-inch column consisting of 10% DC-LSX-3-0295 on Anakrom-ABS support. A peak eluting after the TNT isomers was collected and identified as TNBAL from infrared and NMR reference spectra. The remaining unidentified singlets at 370 and - 548 Hz (Figure 1) had a 1:1 intensity ratio and appeared to originate from the same molecule. The singlet at -548 Hz indicated that the molecule was either symmetrically 2,4,6trinitrosubstituted or, less likely, tetranitrosubstituted. The singlet at -370 Hz was indicative of either vinyl hydrogens or CH, protons adjacent to two, or perhaps three, deshielding functional groups. The presence of unsaturation was tested first by dissolving the red oil in CClr and treating with excess bromine. A second portion of red oil was treated with aqueous permanganate. The NMR spectra of the recovered oil in both cases indicated no change in the shifts or relative intensities of either peak. It was thought that the
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- 370 Hz singlet might represent methylene protons adjacent to a 2,4,6-trinitro-substituted aromatic ring and either a nitro or nitrato substituent group. Neither compound was commercially available for purposes of comparison. Attempts were made to synthesize first the a-nitro-2,4,6-TNT because of a report ( 2 ) indicating that this compound was present in TNT. Attempts to synthesize a-nitro-2,4,6-TNT included nitration of 2,4,6-TNT, trinitration of a-nitrotoluene, oximation of TBAL followed by treatment of the oxime with H 2 0 2in acetic acid, and reaction of a-bromo2,4,6-TNT with silver nitrate. cu-Nitro-2,4,6-TNT could not be synthesized by any of these reactions and, because of this apparent difficulty, it was speculated that the unknown structure might be a-nitrato-2,4,6-TNT which might be formed by nitration of the initial short-lived oxidation intermediate a-hydroxy-2,4,6-TNT. A portion of the crude red oil was dosed with a small amount of this compound and the NMR spectrum obtained. The two singlets at -370 and - 548 Hz increased proportionately (Figure 6), hence, these peaks were assigned to a-nitrato-TNT. This completed the identification of all detectable impurities in the crude red oil exudate and assignment of all the peaks in Figure 1 . In order to determine the concentration of each component in a sample of crude TNT, a 15-gram portion of the melt-solid TNT (below the red oil) was analyzed for impurities employing the method described in Reference 5. These results and the red oil assay were combined to give an approximate weight percentage of each impurity in the entire sample. These results appear in Table I. It is concluded that the percentages in Table I probably indicate typical concentrations of these impurities in crude TNT as similar results were obtained for each of the examined samples. Small amounts (-0.0273 of 2,5- and 3,5-DNT were detected in crude TNT by gas chromatography ( 4 ) , but these compounds were not observed here, presumably because of the relative insensitivity of the NMR method. Impurities in Refined TNT. Several samples of production ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Figure 6. NMR spectrum (60 MHz) of crude red oil showing relative increase in -370 and -548 Hz peaks after dosingwith a-nitrato-2,4,6-TNT
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Table I. Impurities Detected in Crude TNT by NMR Approximate weight percentage Impurity Sample 1 Sample 2 Sample 3 1.80 2.00 2.20 2,4,5-TNT 1.50 1.50 1.70 2,3,4-TNT 1.30 1.40 1.25 2,3,6-TNT 0.05 0.05 0.06 2,3,5-TNT 0.22 0.25 0.20 2,4-DNT 0.30 0.35 0.30 a-Nitrato-TNT 0.20 0.20 0.22 TNBA 0.25 0.24 0.20 TNBAL 0.05 0.05 0.06 2,4-DNBA 0.01 0.01 0.01 3,4-DNBA 0.005 0.003 0.005 MDNB
grade TNT were examined by NMR for oxidation by-products using the impurities enhancement technique and analysis previously described in Reference 5. Except for the presence of 2,4-dinitrotoluene (2,4-DNT) and the unsymmetrical TNT isomers (4, 5), no other impurity signals could be detected. By adding known amounts of TNBA, TNBAL, and a-nitrato-TNT to samples of a-TNT, the lower limit for detecting these impurities was found to be about 0.03z. DISCUSSION The presence of TNBAL, TNBA, 2,4- and 3,4-DNBA, and a-nitrato-TNT in the crude product shows that some oxidation of the Ar-CH3 group takes place during nitration. The fact that none of these oxidation by-products were observed (>0.03%) in refined TNT can be attributed to the efficiency of the sellite (sodium sulfite) purification process. Several compounds suspected as possible impurities in a-TNT included trinitrobenzene, a-hydroxy-2,4,6-TNT, picric acid, and other 902
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nitrophenols, but none of these were detected within the NMR sensitivity range. Using a sensitive thin-layer chromatographic detection technique, Yasuda reported the presence of 2,5-DNT, trinitrobenzene, and a-nitro-2,4,6-TNT, in addition to the usual unsymmetrical TNT isomers (2). These components were found by zone refining a sample of production grade (refined) TNT and examining theimpurity concentrate. In view of our findings here, it is suggested that the a-nitro-TNT, identified by Yasuda may, in reality, have been a-nitrato-TNT. This seems plausible considering the fact that the a-nitro-TNT comparison standard described by Yasuda was of uncertain purity, origin, and structure (2). Unlike the unsymmetrical TNT isomers, a-nitrato-TNT does not react during sellite purification to form a water-soluble sodium sulfonate. However, as with most trinitroaromatics, it combines slowly with sellite to form the base stabilized “nitronic” acid salt (8). Hence, complete removal of this impurity during sellite purification may be difficult, and trace amounts of a-nitrato-TNT are, no doubt, present in refined TNT. ACKNOWLEDGMENT The author thanks Martin J. Dipper and John L. Hadfield of Eastern Laboratory, Explosives Department, E. I. du Pont de Nemours & Company, for the NMR and gas chromatographic analysis, respectively. William T. Boyce of the Du Pont Seneca Works, supplied the a-TNT samples and assisted in the synthesis of a-nitrato-TNT. RECEIVED for review March 16,1970. Accepted May 8,1970. (8) N. V. Sidgwick, “The Organic Chemistry of Nitrogen,” 34rd ed, Clarendon Press, Oxford, 1966, p 395.
