Precise Determination of the Molecular Weight of Trinitrobenzene Complexes JOHN C. GODFREYl School o f Chemistry, Rufgers, the State University of New lersey, New Brunswick, N. I .
A new method for the determination of molecular weights of trinitrobenzene complexes is described, whereby values are obtained to &OS% of the weight of the complex. The method depends upon the conversion of trinitrobenzene to a highly colored complex in the presence of strong base. Evidence is presented as to the probable nature of this complex.
C
determination of the molecular weight of compounds of unknown structure is often of considerable value in establishing their structures. An ebullioscopic method employing highly specialized apparatus has been described, b y IT-hich the maximum reported error is 0.7% (4). However, the usual ebullioscopic, tensimetric, and cryoscopic (Rast) methods yield results which are reliable only to within a few per cent of the true value. Recently a spectrophotometric method has been reported (5) which gives the molecular weight of picric acid complexes to within 1%. It employs Equation 1, which follows from Beer’s law. AREFUL
hfolecular weight
=
EA
X c X b X n/Ax
(1)
I n this equation E A is the molar absorptivity of the pure complexing agent a t wave length A, c is the concentration of the complex in grams per liter, b is the length of the light path in centimeters, n is the number of molecules of complexing agent associated rrith one molecule of unknown. and A X is the absorbance of the solution a t the same wave length. The wave length is so chosen that interference from absorption by the unknown is avoided. The value for n is usually 1, or occasionally a small integer, but it can be determined by comparing the carbon, hydrogen, and nitrogen analyses of the pure substrate (unknown) with those of the complex. d n d r e m ( 1 ) has noted that symmetrical trinitrobenzene (TXB) forms many more complexes than picric acid. Condensed aromatic and heteroaromatic systems invariably form complexes with trinitrobenzene. Additional classes of compounds which complex M ith trinitrobenzene are represented b y azo coml Present address, Bristol Laboratories, Syracuse 1, X , Y.
pounds (3, 22), carbostyril (8),coumarones (2S), indoles (24), phenylhydrazones (22), pyrogallol (25), and Schiff’s bases (22). These complexes are generally considered to be of the electron donor-electron acceptor type, which are usually described as B complexes (1). Molecules such as stilbene, which complex with more than one molecule of trinitrobenzene, nearly always have present tiTo or more structurally independent coordination sites ( 1 ) . Although picric acid and trinitrobenzene adducts are completely dissociated in very dilute solutions ( 7 ) ,trinitrobenzene itself is colorless in such solutions, and spectrophotometric measurement of its concentration in the presence of other organic substances is not practical. There are reports that in the presence of excess strong base, trinitrobenzene is converted to highly colored substances of unknown structure ( 9 , 1 4 ) . Investigation of the spectrum of highly purified trinitrobenzene in 95% ethyl alcohol in the presence of a 100fold excess of potassium hydroxide revealed absorption bands a t 428 to 429 mp ( E = 26,600 at to, the moment of addition of base) and 501 to 503 mp ( E = 17,910). The absorption band a t 429 mp decays rapidly, falling to about 26,000 after 20 minutes, and the rate of decay appears to depend upon the concentration of the base. On the other hand, the absorption band at 502 mp is constant, within experimental error, over a period of 30 minutes. This band has been chosen as a reference point for a method of determining molecular weights which involves Formation and isolation of a trinitrobenzene adduct Preparation of a dilute alcoholic solution from a weighed sample of the adduct Conversion of all the trinitrobenzene present to a highly colored complex with excess potassiuni ethoxide Spectrophotometric determination of the concentration of the trinitrobenzene-potassium ethoxide complex APPARATUS
All volumetric operations were carried out in standard glassware, which conformed in design and tolerance with specifications of the Xational Bureau of Standards (15). Spectrophotometric
measurements were obtained on a Cary recording spectrophotometer, Model 11MS, employing 1.000-em. quartz absorption cells. REAGENTS
1,3,5-Trinitrobenzene (Eastman Kodak White Label) was crystallized twice from hot 95% ethyl alcohol to a constant melting point of 123.5-124.0’ C. The compounds from which complexes were prepared mere also recrystallized from appropriate solvents to constant melting points. Ordinary 95% ethyl alcohol was used unless otherwise specified. Potassium hydroxide pellets TT ere used (11allinckrodt analytical reagent, 85%). PROCEDURE
Absorptivity of 1,3,5-Trinitrobenzene. Duplicate determinations of
of solutions of trinitrobenzene in 95% ethyl alcohol containing a 100-fold excess of potassium hydroxide (one pellet in 50 ml. of solution) gave identical values of 17,910. As confirmation, a n average “best value” for E502 was back-calculated from the nine most reliable determinations in Table I, and found to be 17,910 + 40. Preparation of Adducts. (Preparation of t h e carbazole complex illustrates t h e general method.) T o a boiling solution of 0.3 gram of carbazole in 30 ml. of absolute ethyl alcohol were added 10 ml. of a saturated solution of trinitrobenzene in absolute ethyl alcohol (ea. 2% solution at 25’ C.). The solution was boiled for 1 minute and set aside to crystallize. The complex was recrystallized to a constant melting point from absolute ethyl alcohol containing a small excess of trinitrobenzene. It was usually necessary to add to the recrystallization liquid about 5% by volume of the saturated trinitrobenzene in absolute ethyl alcohol solution in order to prevent dissociation of the complex. The product was then filtered, m-ashed with cold absolute ethyl alcohol, and vacuum-dried over phosphorus pentoxide a t room temperature. Molecular Weight. A solution was prepared from a carefully neighed (microbalance) amount of complex so t h a t t h e concentration was 20 t o 30 mg. per liter (ca. lo-4-V). Fifty milliliters of the solution were measured in a graduated cylinder and added t o one crushed potassium hydroxide pellet (ca. 0.15 gram) in a glass-stoppered 125-ml. Erlenmeyer flask, which had previously been flushed with nitrogen. (Sodium hydroxide gives identical reVOL. 31, NO. 6, JUNE 1959
1087
sults, but is very slow to dissolve.) The spectrum of the deep orange solution was observed over the range 475 to 526 mp about 12 times in the course of 20 minutes, and the average value of the maximum absorbance (at 601 to 503 mp) was employed in Equation 1 to calculate the molecular weight of the complex. K h e n limited amounts of material are available, excellent results may be obtained with 2 t o 3 nig. of the complex. Solutions should be used promptly, as they show some tendency to decompose after several hours. The slow decomposition may result from the slight alkalinity of the soft glass of which volumetric flasks are generally made. RESULTS
The results of determinations carried out on 15 complexes are recorded in Table I. Ten of the 15 results are within 0.5% of the expected value, and obvious sources of uncertainty may account for most of the errors greater than 0.5%. The complex containing two molecules of 1-naphthoic acid and one of trinitrobenzene is unusual in several respects. ilromatic molecules having only electronegative substituents seldom form adducts with trinitrobenzene. Furthermore, formation of complexes containing more than one molecule of trinitrobenzene per aromatic molecule is not uncommon, but the reverse is very rare. From the value consistently obtained on three independent samples, which is well below the probable experimental error, it appears that there may be a small tendency to form a 1 to 1 complex. DISCUSSION
This determination is subject t o several types of uncertainty, some of which may be minimized. The most common source of error is the occasional instability of the organic substrate-trinitrobenzene complex. TFO types of inTable 1.
b c e
-._-1.0
2.0 TOTAL
5
Of
TMl,
1.0
W L X IO*
stability mere observed. Most of the complexes tended to dissociate during purification by recrystallization, but this difficulty was easily avoided by adding a small excess of trinitrobenzene to the recrystallization liquor. Such dissociation, if not overcome, would give high results. Certain volatile organic compounds quickly evaporate from the solid complex, leaving behind excess trinitrobenzene which leads t o low results. Such evaporation is difficult t o suppress, and the consequences are exemplified by the very low value obtained for N,W-dimethylaniline. The greatest uncertainty is in the value a t 502 mp for ETNB = 17,910 =t 40. The weight of sample and the absorbance are good t o a t least 0.001. I n practice, the absorbance could always be read to =tO.OOl. The greatest probable error is therefore =t0.5%, neglecting possible error in dilution. Most of the values reported in Table I fall within these limits. The dissociation constant of the complex trinitrobenzene-potassium ethoxide was determined a t various total concentrations of trinitrobenzene in 95% ethyl alcohol in order to demonstrate that the excess of base used in the molecular
weight measurements was sufficient to repress the dissociation of the colored complex, and to determine the stability of the solution of trinitrobenzene-potassium alkoxide toward decomposition, as opposed to dissociationof the complex. If the dissociation constant were "constant," it could be concluded that the dissociation of trinitrobenzene-potassium alkoxide was the only reaction occurring. Measurement of reported in Table I1 and Figure 1, showed that i t is not in fact constant, nor does it depend in any simple way upon the concentration of the base. The lack of constancy may be considered to be a result of base-catalyzed reactions which occur a t a slow rate in the essentially neutral solutions employed in determining KdlSs. Fortunately, the rate at which side reactions occur in ethyl alcohol a t room temperature is small enough that it does not affect the determination of molecular m-eight when carried out as described. Decomposition is very rapid in methanol, and solvents less polar than ethyl alcohol are not useful, because of the low solubility of potassium hydroxide. The nature of the side reactions is of interest. Their study permits a conclu-
Molecular Weights of Trinitrobenzene Complexes
Molecular Weight Observed Calcd. Error, % +0.46 369.0 367.31 25.96 1.260 -0.03 392.32 1.468 392.2 32.15 391.33 -0.38 29.90 1.374 389.8 24,68 1.261 350.5 350.24 +0.09 22.73 1.072 379.8 380.31 -0.13 19.60 1.100 319.1 334.29 -4.5b -0.38 26.11 1.231 379.9 381.29 25.40 1.335 340.8 341.27 -0.15 46.82 1.542 271. gC 278.7 -2.5' 20.95 1.041 360.4 357.27 + O . 86 23.36 1.171 357.3 356.29 +0.28 23.43 1.075 390.4 391.33 -0.23 -0.07 415.35 0,908 415,O 21.04 Pyrene +0.61 606.46 1.417 610,2d trans-Stilbene 24.14 + O , 8% 294.8 292.25 1.282 TSB.KOCH3. '/zH20 21.10 All melting points were in sealed capillaries, and all readings below 220" C. are corrected. At room temperature dimethylaniline evaporated from complex. 1-Naphthoic acid forms a 2 to 1complex with TNB. Stilbene forms a 1 to 2 complex with TNB. Crude complex was used, as attempts at purification resulted in decomposition. Compound Acenaphthene Acridine Anthracene Anthranilic acid Carbazole N,N-Dimethylaniline Diphenylene oxide Naphthalene 1-Kaphthoic acid 1-Yaphthol 1-Naphthylamine Phenanthrene
a
Figure 1. Dissociation constant vs. trinitrobenzene concentration
c, Mg./Liter
~~
1088
0
ANALYTICAL CHEMISTRY
A502
Meltinog Point,a C. 167.0-169.0 115.0-115.5 163.3-164.0 191.5-192.7 200.3-200.8 107.5-109.5 98,0-99.0 153.5-154.5 190.0-192.5 175.0-176.0 218.2-218.6 162.7-163.7 250.0-250.5 122.0-123.0 Dec.
Reference (21, $5) ($2) (7) ( 1 7 , 82) (2@ (6) ( 25) (2, 7 ) (21) (d, 23) (20) ( 7 , 16, 21) ( 7 , 19) (18, 21) (dl1
which the molecular weight determination depends might be an equilibrium mixture of T- and u-complexes (f), structures I and 11,respectively. Support for this view was found upon investigating the visible spectra of a large number of compounds which complex with trinitrobenzene. (These data will be reported in detail elsewhere.) I n 95y0 ethyl alcohol containing trinitrobenzene, aromatic nuclei which carry electron-donating substituents and which are not capable of significant hydrogen-bonding with the solvent, induced the trinitrobenzene to show only a single low intensity broad maximum, corresponding to the 502 mfi absorption band of trinitrobenzene in the presence of strong base. On the other hand. aliphatic amines n-hich are capable of hydrogen-bonding with solvent induced trinitrobenzene to show both the sharp, intense, short ITave length band near 430 mp, and the broad, less intense band near 505 mp. It seems clear therefore that the 502 mu band of trinitrobenzene in basic ethyl alcohol is due to the 7r-comlslex I. while the 429 mu band is characteristic of the a-complex, 11. I n the presence of base, complex I
sion to be drawn on the nature of the colored substance upon which this method depends. It has long been knoivn that trinitrobenzene is converted to 3,5-dinitroanisole on short refluxing with a sodium methoxide solution (12. 13) and that 3,3',5,5'-tetranitroazoxybenzene (TAB) is obtained on refluxing an aqueous solution of trinitrobenzene with a dilute sodium carbonate solution or upon standing in cold, dilute potassium hydroxide ( I O , If). Furthermore, nitrobenzene is knonn to be disproportionated to a mixture of azoxybenzene and 2- and 4-nitrophenols when heated with pondered potassium hydroside in the absence of oxygen ( 5 ) . The latter obsermtion suggested that picric acid should be produced along n i t h tetraiiitroazoxybenzeiie in the decomposition of trinitrobenzrne catalyzed by aqueous base. Picric acid as indeed found, in a n amount corresponding to 20% of that calculated from the apparent stoichionietryr. 5 TNB + 1 TAB 3 picric acid
+
while the amouut of tetranitroazoxybenzene represented an SO% yield, on the same basis. r
r
m
I n 95% ethyl alcohol, a solvent much less polar than XTater, the base catalyzed reaction of trinitrobenzene took an entirely different course. When a solution of 520 mg. of T S B . KOCH,. l2 H20 in 500 nil. of ethyl alcohol mas allowed to stand for 21 hours a t 27" C., a 54% yield of 3,s-dinitrophenetole was obtained. These observations suggested that the colored form of trinitrobenzene upon
L
d - ~
J
may dissociate by path 1; it may undergo nucleophilic substitution if R = H or alkyl, path 2; if R = H and the aqueous solution is n-eakly basic, disproportionation of the u-complex, 11, may occur along path 3. Where R = alkyl, and the solution is strongly basic, the second step along path 3 is blocked, leaving as the only possibility the slow substitution reaction, 2.
Table II. Dissociation Constant for Reaction TNB.KOC2H5 + TNB KOCzH5 c Complex, x 104~ A ~ P K ~ ~ ~1 0~ 3 ~ ~ . 1.20 0.150 1.48 1.42 0.254 1.15 1.57 0.328 1.05 1.95 0.600 0.84 2.14 0.674 0.82 3.65 1.930 0.61
+
a To assure uniform comparisons, values of Asas were read 15 minutes after solution of sample.
LITERATURE CITED
(1) &drew, L. J., Chem. Revs. 54,713-76 (1904). (2) Asahina, Teichi, Shinomiya, Chiro, J . Chem. SOC.J a p a n 59, 341-51 (1938). (3) Cunningham, K. G., Dawson, W., Spring, F. S., J . Chem. SOC. 1951, 2305-6. (4) Dimbat, Martin, Stross, F. H., AXAL. CHEM.29, 1517-20 (1957). (5) Fieser, L. F., Fieser, M.,"Organic Chemistry," 1st ed., pp. 574-5, Heath and Co., X e r York, 1944. (6) Hibbert, H., Sudborough, J. J., J . Chem. SOC.1903,1334-42. (7) Jones, R. C., Seuworth, h i . B., J . Am. Chem. SOC.66, 1497-9 (1944). (8) Kent, Andrew, McKeil, Donald, Cowper, R. hi., J . Chenz. SOC.1939, 1858-62. (9) Kortum, Gustav, Ber. 74B, 409-16 (1941). (10) Lobry de Bruyn, C. A., Rec. trav. chim. 14, 89-94 (1895). (11) Lobry de Bruyn, C. A., van Leent, F. H., Zbid., 13, 148-54 (1894). (12) Zbzd., 14, 150-5 (1895). (13) Marvel, C. S., Bateman, D. E., "Organic Syntheses," pp. 219-20, Col. Vol. I, \\?ley, New York, 1941. (14) Mudge, C. S., Food Znd. 1 , 613-15 (1929). (15) Natl. Bur. Standards, Circ. C-434. (16) Noller, C.,,R., "Chemistry of Organic Compounds, 2nd ed., p. 460, Saunders Co., Philadelphia, 1957. (17) Ostromisslenskii, I., J . prakt. Chem. 84, 495-506 (1911). (18) Pfeiffer, Paul, et al., Ann. 412, 25363 (1916). (19) Sen Gupta, S. C., Chatterjee, D. N., J . I n d i a n Chem. SOC.31, 285-90 (1954). (20) Sudborough, J. J., J . Chem. 8oc. 1901,522-33, (21) Zbid., 1916, 1339-43. (22) Sudborough, J. J., Beard, S. H., Zbid., 1910,773-98. (23) Ibid., 1911, 209-17. (24) \Teller, L. E., Rebstock, T. L., Sell, H. M., J. Am. Chem. SOC.74, 2690 (1952). (25) Whitmore, F. C., "Organic Chemistry," 2nd ed., p. 636, Van Nostrand, New Pork, 1951.
RECEIVED for review May 6, 1958. Accepted December 24, 1958. Division of Analytical Chemistr , 133rd Meeting, ACS, San Francisco, Zalif., Bpril 1958.
VOL. 31, NO. 6, JUNE 1959
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