Nuclear Magnetic Resonance Examination and Determination of the Di- and Trinitrotoluene Isomers in 2,4,6-Trinitrotoluene D. G. Gehring and G. S . Reddyl Eastern Laboratory, Explosiues Department, E. I . du Pont de Nemours & Co., Gibbstown, N . J .
A procedure i s described for determining low concentrations of unsymmetrical trinitrotoluene (TNT) isomers and 2,4-dinitrotoluene in crude and refined 2,4,6-TNT. NMR chemical shifts and long-range H-CH, coupling constants were used to identify the individual components in mixtures of di- and trinitrotoluenes. It was found that the long-range coupling constants did not vary as the number of nitro groups in toluene increased. These results did not support earlier claims that long-range coupling constants are dransmitted and that the n-bond orders in the ring can be calculated using these coupling constants.
INAN EARLIER PAPER ( I ) a gas chromatographic method was reported for determining low concentrations of unsymmetrical TNT isomers in commercial 2,4,6-TNT. Unfortunately, the 2,3,6-TNT isomer was not separated by gas chromatography from 2,4,6-TNT, and this prompted us to consider N M R as a n alternative method. The Ar-methyl signals of the TNT isomers were found t o be sufficiently separated at 60 MHz to permit quantitative integration of the individual components of a n isomeric mixture (Figure 1). A quantitative procedure for determining low concentrations of 2,3,6-, 2,3,4-, and 2,4,5-TNT and 2,4-DNT in 2,4,6-TNT was developed combining an impurity concentration technique and N M R measurement of each component in the concentrate. Lcng-range H-CHa coupling constants in substituted toluenes were reported by other workers (2). Several investigators (3-5)have suggested that hyperconjugation is primarily responsible for these long-range couplings, while recently others have demonstrated a relationship between mobile T bond orders and long-range H-CHP coupling constants in aromatic systems (6, 7), showing that these couplings are essentially a-electron transmitted. McConnell carried out theoretical calculations for the long-range H-H coupling constants in aromatic systems using both molecular orbital (8) and valence bond (9) treatments. Recently, Dewar and Fahey (10) extended McConnell’s treatment t o calculate the long-
’ Present address, Central Research Dept., Experimental Station, E. I. du Pont de Nemours & Co., Wilmington, Del. (1) D. G. Gehringand J. E. Shirk, ANAL.CHEM., 39, 1315 (1967). (2) Donald T. Witiak, Dhun B. Patel and Youlin Lin, J. Am. Clzern. SOC., 89, 1908 (1967). ( 3 ) P. L. Corio and I. J. Weinberg, J . Chem. Phys., 31, 569 (1959). (4) E. B. Whipple, J. H. Goldstein, and L. Mandell, [bid., 30, 1109 (1959).
(5) E.B. Whipple, J. H. Goldstein, and W. E. Stewart, J. Am. Chem. SOC.,81, 4761 (1959). (6) P. M. Nair and G. 709. (7) H. Rottendorf and S .
Gopakumar, Tetrahedron Letters, 1964,
Sternhell, Aust. J. Cl~em.,17, 1315 (1964). (8) H. M. McConnell, J. Mol. Spec., 1, 11 (1957). (9) H. M. McConnell, J . Chem. Phys., 30, 126 (1959). (10) M. J. S. Dewar and R. C . Fahey, J. Am. Chem. SOC.,85, 2704 (1963).
792
ANALYTICAL CHEMISTRY
range coupling constants between the methylene and ring protons in acenaphthenes and concluded that these couplings are a-transmitted. The results presented in this study revealed that H-CHI coupling constants d o not vary with the number of nitro substituents in the ring and, hence, d o not support the earlier claims. Based upon the long-range coupling constants obtained from some of the pure mono-, di-, and trinitrotoluenes, identification of unknown isomeric components in a mixture was facilitated. EXPERIMENTAL
Apparatus. All the spectra were recorded on a Varian A-60 spectrometer equipped with a Varian V-6040 variable temperature regulator. The H-CHa coupling constants were measured on a Varian HA-100 spectrometer. The chemical shifts are expressed in cycles-per-second from tetramethylsilane internal reference and perdeutero acetone served as the solvent. Reagents. The 2,4,6-TNT was laboratory refined DuPont material. The 2,3,4- and 2,4,5-TNT isomers were laboratory synthesized and purified by selective solvent extraction (11). A purchased sample of 2,4,5-TNT (K & K Laboratories, Inc., Plainview, N. Y . ) contained 11% 2,3,6-TNT by N M R analysis. This sample was recrystallized from carbon tetrachloride. The solids recovered from the mother liquor were 2,3,6-, 2,4,5-, assayed by G C and NMR as 66, 29, and and 2,3,5-TNT, respectively, and served as a source of 2,3,6and 2,3,5-TNT for calibration curve preparation and for NMR study. The DNT isomers were purchased from K & K Laboratories, Inc. Identities and purities of all the isomers were established from GC ( I ) , NMR, and infrared (11) techniques. Calibration and Procedure. Calibration standards were prepared by weighing into three 250-ml Erlenmeyer flasks the amounts of each standard given in Table I ; a fourth flask served as the blank. Then to each flask was added 15 f 0.1 grams of laboratory-refined 2,4,6-TNT, 80 ml of reagent grade carbon tetrachloride, and a magnetic stirring bar. The flasks were warmed on a steam bath until dissolution was complete, then transferred to a magnetic stirrer and vigorously stirred until the solution temperatures reached 30” to 35” C. The flasks were placed in a n ice bath which was positioned atop four magnetic stirring motors and, with continuous stirring, cooled to 4 ” f 1 ” C. The mixtures were rapidly vacuum-filtered through Whatman #41 filter paper in a 6-cm i.d. Buchner funnel into 250-ml filter flasks. A glass rod was used to compress the TNT crystals in order to recover rapidly the maximum amount of filtrate (about 80 ml). The filtrates were quantitatively transferred into 150-ml graduated beakers and evaporated to 30 =t1 ml. The solutions were removed
5z
C. Conklin and F. Pristera, “Preparation and Physical Properties of Di- and Trinitrotoluene Isomers,” Tech. Rep?. No. 2525 (1958), Picatinny Arsenal, Dover, N. J.
(11)
Grams of Components Added to Erlenmeyer Flasks for Calibration Curve Preparation Standards Component 1 2 3 0.05 0.10 0.20 2,4-DNT 0.02 0.04 0.06 2,3,6-TNT 2,4,5-TNT 0.02 0.05 0.10 0.02 0.05 0.10 2,3,4-TNT
Table I.
Table 11. Procedure for Integration of Sample Components Component Peak integrated Sweep direction 2,4-DNT Highest Field Ar-H Doublet Reverse (high-to-low field) 2,3,4-TNT Highest Field Ar-H Doublet Reverse 2,3,6-TNT Ar-CH3 Reverse 2,4,5-TNTn Ar-CH3 Forward
f
W v)
c 0
n
C
v)
Integrated at 60" C if water (present in de-acetone) signal should interfere at normal probe temperature.
W
a L
W -0 L
0
TNT samples were analyzed by weighing 15 =t0.1 grams into 250-1111 Erlenmeyer flasks and proceeding as described above. The per cent concentration was calculated as:
V 0)
U
Isomer = grams of component in 1-ml X 100/grams of sample
I
-168 -162 -150.7 Frequency ( HZ 1 from TMS Internal R e f e r en c e Figure 1. Methyl proton region of synthetic mixture of TNT isomers at 60 MHz Peak A B C
D E
Isomer 2,4,5-TNT 2,4,6-TNT 2,3,4-TNT 2,3,5-TNT 2,3,6-TNT
from the steam bath, air-cooled to about 25" C, then placed in an ice bath and, with occasional agitation, cooled to 4" f 1 O C. While cold, the solutions were rapidly filtered through Whatman #41 filter paper in a 4-cm i.d. Buchner funnel into 125-ml filter flasks. The filtrates were quantitatively transferred into 30-1111 beakers and evaporated to dryness on a steam bath with assistance from an air or nitrogen jet. The residues were dissolved in 0.5 ml of de-acetone and, via transfer pipets, these solutions were quantitatively transferred into glass-stoppered, 1-ml volumetric flasks. Two or three additional 0.2- and 0.3-ml portions of ds-acetone were added to the beakers and these washings were transferred to the 1-ml flasks. Solution volumes were adjusted to exactly 1 ml by carefully evaporating some of the acetone with a jet of air or nitrogen. The flasks were stoppered, mixed, and portions of each solution transferred into standard thin-walled NMR sample tubes, and the 60-MHz spectra were recorded. The component peaks were integrated as in Table I1 under identical instrumental conditions. Calibration curves were prepared by plotting integration intensities us. concentrations and were found to be linear for all components. To check daily instrumental response, the methyl doublet of the acetaldehyde standard (supplied with the A-60) was integrated using standard conditions prior to the analysis of each sample or group of samples. Hence, calibration curve slopes were related to, and sometimes altered by, changes in the acetaldehyde integration value.
RESULTS AND DISCUSSION
The impurity enhancement technique previously described is actually a carefully-controlled crystallization procedure designed to reduce the 2,4,6-TNT/isomeric impurity concentration ratio. This was necessary because of the inherent insensitivity of the NMR method and because the large 2,4,6-TNT Ar-CH3 matrix obscured the methyl resonances of the isomeric impurities when samples were examined as received. By reproducing the preparation steps from sample to sample, essentially constant recoveries of each component were obtained, even when concentrations of the individual components and total impurity levels were varied (Table 111). The chemical shift of the methyl peak of 2,4-DNT (- 163 Hz) is nearly identical to that of 2,4,6-TNT (- 162 Hz), and integration of this peak at 60 MHz was not possible. Also, the ratio of 2,4,6-TNT to 2,3,4-TNT is normally large in the sample preparations and the 2,3,4-TNT methyl peak cannot be accurately integrated because of its proximity to the 2,4,6-TNT peak. For these reasons, the 2,4-DNT and 2,3,4TNT concentrations were determined by integrating their respective high-field ring proton doublets centered at -470 and -489 Hz. The methyl peaks of 2,4,5- and 2,3,6-TNT were easily integrated except for occasional interference of a water signal which is present in d6-acetone. This problem was easily surmounted by shifting the water signal to higher field at higher temperature, and integrating the peaks at 60" C. Acetone was used as the solvent because of the high solubility of the TNT isomers and because concentration effects are small. Calibration of 2,3,5-TNT and the other DNT isomers was not attempted since significant amounts (>0.05%) of these isomers were not found in any production samples which were examined ( I ) . NMR data for 3,4,5-TNT was not obtained as sufficient amounts of this isomer were not available. This presents no serious analytical problem as this isomer has never been detected in production samples ( I ) and, based upon VOL. 40, NO. 4, APRIL 1968
793
Table 111. Results of Some Synthetics and TNT Production Samples Synthetic mixture I4 Component 2,4-DNT 2,3,4-TNT 2,4,5-TNT 2,3,6TNT
Per cent added
Synthetic mixture II4 2,4-DNT 2,3,4-TNT 2,4,5-TNT 2,3,6TNT
1.25 0.30 0.66 0.33
Per cent found 1.24 0.28 0.64 0.36
A Z -0.01 -0.02 -0.02 $0.03
0.33 0.30 0.33 0.13
0.35 0.30 0.34 0.14
-0.02
...
$0.01 +0.01
Crude product TNT
Sample #
2,4-DNT 0.17 0.42
1 2
2,3,4-TNT
Ar-CHa, Hz
PNT 2,4DNT 3,4-DNT 3,5-DNT 2,6DNT 2,3,4-TNT 2,3,5-TNT 2,4,5-TNT 2,3,6TNT 2,4,6TNT 0
1.10
2,4,5-TNT 0.34 0.28 0.35
2,3,6ThT 0.03 0.04 0.04
1 2 0.40 3 0.34 Prepared by adding known weights of each component to refined 2,4,6TNT.
Isomer
b
0.84
2,3,6TNT 0.13 0.15
Refined product TNT 2,3,4-TNT 0.12 0.20 0.20
2,4-DNT 0.26
4
0.40
2,4,5-TNT 0.71
- 146 -162.0 -155.0 - 160.0 -152.0 -159.0 -157.0 -168.0 -150.7 -162.0
Table IV. Observed Chemical Shifts. and Coupling Constantsb Ring protons, Hz (+OS0 Hz) 2 3 4 5 6 -442 - 484 ... -484 -442 -525 ... - 506 - 470 ... -474 ... ... - 483 - 466 - 510 -510 *.. - 524 *.. ... ... -491 -465 -491 - 489 ... ... -511 .*. *.. -530 ... -498 ... - 501 ... -529 ... ...
... ...
...
-541
-509
...
-509 - 541
... ...
Measured from tetramethylsilane internal reference at 60 MHz using dgacetone solvent. Measured on the HA-100spectrometer.
toluene nitration chemistry, only trace amounts would be expected (12). The integrated spectrum of a typical TNT sample preparation is illustrated in Figure 2 and some results of synthetic and production samples are listed in Table 111. The lowest detectable concentration of any component was about O.O2Z, although greater sensitivity was possible (if desired) by increasing instrumental rf power. The precision of the method was found to be *O.O4Z at the two standard deviation level within the 0.02 to 1.50% concentration range. The ring protons of the unsymmetrical TNT isomers are of first order AB type split by the methyl hydrogens and are well separated from the methyl resonances which occur at much higher field. The mono- and dinitrotoluenes gave somewhat complicated patterns in the aromatic regions depending upon the positions of the nitro groups in the ring. At 100 MHz, long-range couplings were resolved between the methyl and ring protons. All of these long-range couplings are readily obtainable from the methyl resonances except in the case of oand m-nitrotoluene (Figure 3). In these two cases the methyl
(12) P. de Beule, Bull. SOC.Chim., Belges, 42, 21 (1933).
794
ANALYTICAL CHEMISTRY
resonances show the couplings with the aromatic hydrogens, but the proximity of the aromatic proton chemical shifts will undoubtedly introduce appreciable perturbation in the methyl resonances and hence the splittings observed in the spectra are not necessarily the coupling constants. However, the ortho- and meta-coupling constants are readily obtained from p-nitrotoluene where this perturbation is negligible. These long-range H-CH, coupling constants are essentially identical in mono-, di-, and trinitrotoluenes, and were found to be 0.75,0.35, and 0.60 Hz, respectively. These valuts are believed to be accurate to +0.03 Hz. Observed chemical shifts and coupling constants for the nitrotoluenes are given in Table
IV. It has been established that in aromatic systems short-range coupling constants are mostly a-transmitted whereas longrange couplings are essentially n-transmitted (8-10). Also, long-range couplings have been calculated for numerous aromatic systems (6, 7, 10) based upon McConnell’s theoretical relationship between n-bond orders and long-range a-transmitted couplings (8). Using the Dewar and Fahey (10) modification of McConnell’s equation (8) and the observed longrange coupling constants, a-bond orders (electron correlations) were estimated to be 0.770,0.412, and 0.687 between the
D
E
a v) c
1
0
n
Y aJ)
a L
aJ L
0
u aJ
a
-489 Frequency (
-470
Hz 1
-168 -162
-150.7
from T M S I n t e r n a l Reference
Figure 2. NMR spectrum of “prepared” sample of crude production 2,4,6-TNT Peak A B
C
D E
F
Isomer 2,3,4-TNT 2,4-DNT 2,4,5-TNT 2,4-DNT & 2,4,6-TNT 2,3,4-TNT 2,3,6-TNT
Isomer present 0.33 0.66 0.26
-
0.12
’A H
Figure 3. Methyl resonances of representative nitrotoluenes at 100 MHz A o-Nitrotoluene B rn-Nitrotoluene C p-Nitrotoluene
D 33-DNT E 2,4-DNT
Ar-CH3 and the ortho, meta, and para ring protons. The methyl group was assumed to be freely rotating and assigned a hyperfine splitting of 25 gauss (IO)and A E was taken as 4 eV (8). These bond order values, therefore, are identical for all the mono-, di-, and trinitrotoluenes reflecting the constancy in the long-range coupling constants. If the presumption that long-range coupling primarily through the a-bonds is valid, then the constancy in the couplings and bond orders observed here is evidence that T electron delocalization and ring-current density in mononitrotoluenes is unchanged with multiple nitro group substitution. However, it is common knowledge that additional nitration in fact considerably lowers ring-current density (difficulty of trinitration compared to di- and mononitration, K , of trinitrophenol compared to di- and mononitrophenol, etc.). Hence, if these long-range couplings are primarily a-bond transmitted, then it would be expected that J and bond order values would significantly decrease as follows: TNT < DNT < MNT. Apparently, a large u-transmission exists in the nitrotoluenes and additional work beyond the scope of this report would be necessary to resolve the extent of U-T contribution to these couplings. The value of utilizing long-range coupling constants in isomer identification was demonstrated when an unknown resonance (presumably Ar-CH3) was observed at - 151 Hz
F 3,4-DNT G 2,4,5-TNT H 2,3,4-TNT I 2,4,6-TNT
during analyses of refined TNT samples. Ring protons associated with this peak were obscured because relatively large amounts of other TNT isomers and 2,4-DNT were present in these samples. The 100-MHz spectrum resolved the unknown peak into a quartet with inner and outer spacings of 0.35 and 0.60 Hz, respectively. Therefore, this peak was assigned to 2,3,6-TNT based upon the long-range coupling constants derived from other isomers which were commercially or otherwise available in pure form. Subsequently, a gas chromatographic cut was collected from a sample of commercially available 2,4,5-TNT (K & K Laboratories, Inc.) and identified as pure 2,3,6-TNT from the infrared (11) and NMR spectra. By comparing the observed methyl signal with that of the known 2,3,6-TNT, the initial identification was confirmed. ACKNOWLEDGMENT
Frank Pristera of Picatinny Arsenal, Dover, N. J., generously supplied pure samples of 2,3,4- and 2,4,5-TNT. Martin J. Dipper of Eastern Laboratory, Explosives Department, Du Pont Co., prepared and analyzed many of the TNT standards and samples. RECEIVED for review December 18, 1967. Accepted February 6, 1968. VO?. 40 NO. 4 APRIL 1968
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