E. PRICE,R. D. BAREFOOT, A. S . TOMPA, AND J. U. LOWE,JR.
1608
The Position of Protonation of 1,1,3,3-Tetrarnethyl-2-nitroguanidine in Strong Acids
by E. Price, R. D. Barefoot, A. S. Tompa, and J. U. Lowe, Jr. Research and Development Department, Naval Ordnance Station, Indian Head, Maryland (Receised July 18, 1966)
Evidence is presented to establish that protonation of 1,1,3,3-tetramethyl-2-nitroguanidine in strong acids occurs primarily at the dimethylamino site. This conclusion is based on the following nmr data: (1) in 37.5% HC1 and at temperatures above -15", one singlet is observed for the methyl groups; (2) for the same solutions and at temperatures below -15", a doublet is observed for the methyl protons; (3) in 38% DC1 in DzO and at -20 and -35", no doublet is observed for the methyl protons; (4)the addition of HzO to tetramethylnitroguanidine in 38% DC1 in D 2 0 at these temperatures causes the methyl protons to split into a doublet. The splitting of the N-CH3 resonance into a doublet is attributed to spin-spin coupling of the adjacent N-H proton with the methyl protons due to protonation a t the dimethylamino site.
The structure of certain nitroguanidines (I) and their conjugate acids has been discussed and ultraviolet absorption spectra data have been presented t o support protonation a t the nitrimino site, a' (R = methyl or
NO,
/ a+rj
I/
b R-N
.N-R
I
I
R
R
I hydrogen). This conclusion was drawn from the following facts: (1) in all the nitroguanidines studied the presence of a strong absorption band a t 26002700 A has been observed, the exact position and intensity of which depends on the substituent groups; (2) the absorption spectrum of nitram'ide (HZNN02) is quite different with a broad band at 2250 A ( E 5900 in water) ;2 (3) the absorption band characteristic of nitramide appears in the spectra of nitroguanidines in strong acidic solutions and reaches maximum intensity in 4040% sulfuric acid corresponding to The Journal of Physical Chemistry
complete conversion of the nitroguanidine into its conjugate acid. It has been reported that the change is accompanied by the disappearance of the absorption band characteristic of the free base, nitroguanidine.' I n our laboratories, preliminary studies of the absorption spectra of l,lJ3,3-tetramethyl-2-nitroguanidine (TMNG) in various nitric acid solutions are in agreement with the latter fact. However, we offer evidence to show that protonation of nitroguanidines does not occur primarily a t the nitrimino site, a, but rather a t the amino site, b. From nuclear magnetic resonance (nmr) measurements, the following observations have been obtained : (1) for l,lJ3,3-tetramethyl-2-nitroguanidine in 30.5% HNOa or 37.5% HCl and a t temperatures above - 15", a singlet is observed for the methyl groups; (2) for the same solutions and at temperatures below -15", a doublet is observed for the methyl protons. It is recognized that these facts may be interpreted in terms of one of the following explanations. The existence of the doublet below -15" may indicate (1) that hindered rotation produces nonequivalent methyl
(1) T.G. Bonner and J. C. Lockhart, J. Chem. Soc., 3858 (1958). (2) R. N. Jones and G . D. Thorn, c a n . J . Res., B Z ~828 , (1949).
POSITION O F PROTONATION O F 1,1,3,3-TETRAMETHYL-2-NITROGUANIDINE
1609
protons in these media are largely attributed to deshielding caused by the positive charge developed at the amino nitrogen. A more substantial demonstration of protonation at the amino site is indicated as follows. For TMNG one singlet resonance a t 183.9 cps is observed for the methyl protons in water, while in 37.5% by weight hydrochloric acid one singlet is observed at 196.6 cps (reference, internal sodium 2,2-dimethyl-2-silapentane5-~ulfonate).~In the latter solution, it has been observed that the N-CH3 resonance splits into a doublet in the temperature range 8 to -30". This doublet is partially resolved with a separation ranging from 1.7 to 3.0 cps a t 8 to -30", respectively. A partially re+ solved doublet is also observed in 23% hydrochloric this group [(CHS)~NH-]acts inductively as an elecacid and in 30-70y0 nitric acid solutions at temperatron-withdrawing group and would also make the tures below -15". The splitting of the N-CHI nitrimino site, a, even less b a s k 4 resonance into a doublet is attributed to spin-spin It has been observed t.hat the character of the N-CHS coupling of the adjacent N-H proton with methyl resonances of aqueous solutions of methyl derivatives protons owing to protonation at the dimethylamino of nitroguanidine is dependent on the acidity of the site. This is confirmed (Figure 1) by the absence of medium (Table I). For 1,1,3,3-tetramethyl-2-nitrothe doublet in 38y0 DC1 in DzO solutions of TASNG guanidine the methyl resonance shifts approximately at -20 and -35°.6 The addition of HzO to TMNG 16 cps downfield in going from 0 to 70 wt % nitric 38% DC1 in D20 at these temperatures causes the in acid. The chemical shifts observed for the methyl methyl protons to split into a doublet. The formation of a singlet for the N-CH, resonance a t temperatures above - 15" is attributed to rapid proton exchange Table I: N m r Chemical Shifts at 60 Mc/sec for between the amino sites and the solvent. Methylnitroguanidines in Aqueous Acids We believe that protonation of 1,l-dimethyl-2at Room Temperature nitroguanidine (DMNG) occurs mainly a t the priWt % ------ CHa re80nancea------mary amino site rather than the tertiary amino site of HNOs TMNG~ DMNGC MNG~ because the former is a more basic site.' No doublet 70.0 199.8 204.1 194.8 for the methyl resonance for the dimethylamino group 56.9 197.0 201.1 192.4 in DMNG in 70'% H N 0 3 was observed at tempera46.7 ... ... 189.2 tures as low as -35". Under no conditions in aqueous 45.2 196.5 198.8 ... media could the N-H protons of methylnitroguanidines 39.4 195.9 ... ... 37.9 195.4 ... 188.0 36.6 ... 195.8 ... groups, (2) that the methyl groups are spin-spin coupled with the proton that adds to the amino site, b, and that proton exchange is rapid at temperatures above -15", or (3) that protonation is occurring a t both the nitrimino and amino sites. We believe that the second explanation is more in agreement with all the facts. From our knowledge of the base-strengthening effects of methyl groups in guanidines3 and the base-weakening effect of a nitro group, one would expect protonation of 1,1,3,3-tetramethyl-2-nitroguanidineto occur at the dimethylamino site, b, rather than the nitrimino site, a. Once protonation occurs a t the dimethylamino site,
30.5 23.1 12.6 10.5 6.4 5.2 4.9 3.2 1.4 37.570 HCl
CH&OOH HzO
194.5 193.0
... 191.3 188.3
183.4 182.1 179.0
188.5 186.1
...
...
... ...
177.7
.
.
I
184.8 183.9 196.6 183.9 183.9
...
... ... ... 200
... ...
... . I .
177.7
... ... 188.2 177.0
...
' Cyclies per second downfield from sodium 2,2-dimethyl-2silapentane-5-sulfonate used as internal standard. T h e conm. 1,1,3,3centration of the nitroguanidines was -0.04 Tetramethyl-2-nitroguanidine. 1-Methyl-2-nitroguanidine.
l,l-Dimethyl-2-nitroguanidine.
(3) S. J. Angyal and W. K. Warburton, J. Chem. Soc., 2492 (1951). (4) The presence of any diprotonated form of TMNG in strong acids is unlikely in our experiments. It follows from the order of magnitude of the pK. for the guanidinium cation (pK, N - 11) that guanidine does not form a doubly charged cation t o any considerable extent in acids weaker than 99% HzSO4. By analogy the weakly basic TMNG is not expected t o become doubly charged; cf., G. Williams and M. L. Hardy, ibid., 2560 (1953). (5) Two doublets ( J = 5.00 cps) a t 179 and 188.5 cps have been observed for N,N'-dimethylacetamidinium chloride solutions in 14% HzS04 and in 60% DzSOc a t 184.0and 194.0cps, respectively: R. C. Neuman, Jr., and A. S. Hammond, J. Phys. Chem., 67, 1659 (1963). (6) The effect of the deuterium atom on the methyl proton spectra is to cause the methyl proton line t o split into three lines by spinspin coupling t o the deuterium nuclei, Since the coupling is small, the observed effect is t o broaden the methyl proton resonance line into an unresolved multiplet. (7) H.C. Brown. J. Am. Chem. SOC.,67. 378 (1945). For the behavior of DMNG in 70-90% HzSO4, see J. C. Lockhart, J. Chem. Soc.,
1174 (1966).
Volume 7 1 , Number 6 May 1967
E. PRICE,R. D. BAREFOOT, A. S. TOMPA, AND J. U. LOWE,JR.
1610
I
Ma Temperatwe: -20' C 60
A
Figure 1. Nuclear magnetic resonance spectra of N-methyl protons in TMNQ; A, 38% DC1 in D 2 0 ; B, 26.7y0DCl-HCl in DzO-HZO.
be observed because the solvent resonance obscures the N-H signal.
Experimental Section DC1 (38%) (99% isotopic purity) in DzO (99.5% purity) was obtained from Volk Radiochemical Co. 1,l,S1S-Tetramethyl-2-nitroguanidine( T M N Q ). TRlNQ was prepared by the mixed acid nitration of 1,1,3,3-tetramethylganidine(Eastman White Label) by the procedure of Kirkwood and Wrighta8 Several recrystallizations from absolute ethanol-ethyl acetate gave a sample whose melting point was 84.5°.9 Anal. Calcd for C5H12N402:C, 37.49; HI 7.55; N, 34.99.
The Journal of Physical Chemistry
Found: C, 38.02; H I 7.73; N, 34.05. TMNQ was dried for 20 hr at 56" in an Abderhalden just before the solutions were prepared for deuterium experiments. Methylnitroguanidine (mp 161.8-162"; lit.'o 160161") and l,l-dimethyl-2-nitroguanidine(mp 197198" ; lit.lo 193.5-195") were conveniently prepared from ethanolic solutions of 2-methyl-l-nitro-2-thiopseudourea with methyl- and dimethylamines, respectively. Analytically pure samples were obtained by repeated crystallization of newly synthesized methylnitroguanidines from absolute ethanol-ethyl acetate. 1,1,3,3-Tetramethyl-2-nitroguanidinewas dissolved in 38% DC1 in D20 to give a 1.92 M solution. The nmr spectrum of this solution was then taken immediately a t -35 and -20". Afterward, the 1.92 M solution was diluted with water to give a 1.28M solution of 1,1,3,3-tetramethyl-2-nitroguanidine in 26.7% DC1 HC1 in D20-H20 mixture. The nmr spectrum of this solution was also recorded immediately. The nmr measurements were performed with a Varian DA-60-Elspectrometer equipped with a superstabilizer. The chemical shifts were measured with a precision of 0.05 cps by placing side bands on both sides of the signal. The side-band frequency was measured with a Hewlett-Packard Model 522-B electronic frequency counter. The temperature was kept constant to within *0.2" by the use of a Leeds and Northrup Azar H recorder-controller. The temperature was varied with dry nitrogen gas and the use of a Varian 4340 variabletemperature probe assembly and a hlodel V-4331T H R spinning sample dewar probe insert.
Acknowledgment. We gratefully acknowledge support of this work by the Foundational Research Program of the Xaval Ordnance Systems Command. ~
~
~~
~~
(8) AI. W. Kirkwood and G. F. Wright, Can. J . Chem., 35, 527 (1957). (9) All melting points were taken on a micro Kofler hot stage. (10) T. G. Bonner and J. C. Lockhart, J . Chem. Soc., 3852 (1958).
