(21) Gardner, K.,Overton, K. D., A n a l . Chim. Acta 23,271 (1960). ’ (22) Goldbaum, L. R., Williams, A., Kofpangi, T., ANAL. CHEM. 32, 81 (1960). (23)Gordon, G.E.,Zbid., 32,1325(1960). (24) Hamilton, P. B., Zbid., 32, 1779 (1960). (25) Hamilton, P. B., Bogue, D. C., Anderson, R. A., Zbid., 32, 1782 (1960). (26) Hardon, H. J., Brunink, H., VanAnalyst 85, 187 (1960). DerPol, E. W., (27) Harris, I). N., Davis, F. F., Biochem. et Bzophys. Acta 40, 373 (1960). (28) Herrett, R. J., Duyarce, J. A., ANAL. CHEM. 32, 1677 (1960). (29) Hobson, B. C., Hartley, R. S., Analyst 85, 193 (1960). (30) Homas, R. T., Zbid., 85, 551 (1960). (31) Hornstein, I., Alford, J. A., Elliot, L. E., Crove, P. F., ANAL. CHEW 32, 540 (1960). (32)Hughes, E. E., Lias, S. G., Zbid., 32, 707 (1960). (33) Kabora, J. J., McLaughlin, J. T., Riegel, C. A., Ibid., 33, 305 (1961). (34)Karr, C., Jr., Zbid.,. 32, 463 (1960). (35) Kaufman, S.,Medina, J. C., Zapata, C., Ibid., 32, 192 (1960). (36) Killheffer, J. V., Jungermann, E., Ibid 32, 1178 (1960). (37) i’iser, R. Shetlar, M. D., Johnson, G. D., B i d . , 33, 315 (1961). (38) Jakubovic, A. O.,Nature 184, 1065 (1059).
w.,
(39)Jones, M.R., Analyst 85,111 (1960). (40) Larson, L. P., Becker, H. C., ANAL. CHEM.32, 1215 (1960). (41) Leahy, J. S., Waterhouse, C. E., Analyst 85,492(1960). (42) Lijinsky, W.,ANAL.CHEW32, 684 (1960). (43) Lumokin. H.E..Xicholson. D. E.. ’ Ibid., 3 i , 74’(1960).’ (44) Mathews, J. S., Burow, F. H., Snyder, R. E., Ibid., 32, 691 (1960). 45) Metcalfe, L. D., Ibid., 32, 70 (1960). 46) Metzsch, F. A. V., Angew. Chem. 65, 586 (1953). (47) Mita, M. A., Schleuter, R. J., J . Am. Chem. SOC.81, 4024 (1959). (48) Montant, C., Touze-Soulet, J. M., BuZl. Soc. Chem. Biol. 42, 161 (1960). (49) Mueller, H. F., Larson, T. E., Ferretti, M., ANAL. CHEM. 32, 687 (1960). (50) O’Connor, J. G., Xorris, S. hl., ioid., 32, 701 (1960). (51) Oliverio, V. T.,Ibid., 33, 273 (1961). (52) Peterson, E. A,, Sober, H. A., J . Am. Chem. SOC.78,751 (1956). (53) Rajzman, 8.,Analyst 85, 116 (1960). 154) Rall. J. IT-.. ANAL. CHEM.32. 332
I
(55) Rdd, R. L., Analyst 85,265(1960). (56) Reuter, W.,Biochern. 2. 331, 337 (1959). (57) Robinson, M., ANAL. CHEM. 33, 109 (1961). (58) Rudstam, G., Zbid., 32, 1664 (1960).
(59) Sammons, H. G., Wiggs, S. M., Analyst 85,417 (1960). ( 6 0 ) Sawicki, E., Elbert, W., Stanley, T. W., Hauser, T. R., Fox, F T., ANAL.CHEM. 32, sin ( I R A O ) (61) Sherwood, R. fi Jr.. Zbid.. 32. 1131 (1960).(62) Smith, E: D .., Ibid.. 32. 1301 (1960). (63) Somers. E..Richmond. D. V.. A nalyst 85,440 ( i k m / . (64) Spell, CHEM. 32, 1811 (1’ (65) SteDhens. R.L., \ - - ,
\ - - - - ,
(67)Thompson, C. J., Coleman, H. J., Hopkins, R. L., Ward, C. C., Rall, H. T., ANAL.CHEM.32, 1762 (1960). (68) Walsh, J. T..Merritt, C.,. Jr.,. Ibid., . 32, 1378 (1960).’ (69) Williams. L. A.. Brusock. Y . M.. ’ Zak. B.. Zbid., 32, 1883 (1960).’ rom, M. L., Arsenault, G. P., < (1960). ) Wood, R., Analyst 85,21 (1960). (72) Yasuda, S. K., Rogers, R. K., ANAL.CHEM.32, 911 (1960).
RECEIVEDfor review April 19, 1961. Accepted July 17, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
CompIexometric Determination of Aromatic Hy d roca rbo ns with Tetracya noet hylene GEORGE H. SCHENK and MARA OZOLINS Chemistry Department, Wayne Sfafe University, Detroit 2, Mich. Tetracyanoethylene (TCNE) is used to titrate aromatic hydrocarbons photometrically via A complexation. Pure aromatic hydrocarbons as basic as or more basic than naphthalene are estimated in a wholly organic complexometric titration. Mixtwes of a stronger aromatic P base in a weaker aromatic A base are assayed for the stronger base-e.g., naphthalene in phthalic anhydride, benzo(a)pyrene in benzo(e)pyrene, and 2,6-dimethylphenol in 2,6-di-terf-butyl-4-methylphenol. The calculated titration curve agrees fairly well with the experimental titration curve.
F
group approaches to the determination of aromatic hydrocarbons have been limited to such methods as base-catalyzed condensation at a methylene carbon as for cyclopentadiene or indene (16), and DielsAlder reactions as for anthracene (10, 15, 17). General colormetric determinations of aromatic? have been limited to nitration ($1 and the recent phosphorus pentachloride-catalyzed conaensation of UNCTIONAL
1562
ANALYTICAL CHEMISTRY
piperonal (14) with aromatics more basic than toluene. The above reactions depend to some degree on the Lewis or Bronsted basicity of the aromatic hydrocarbon. However, none of these methods are specifically based on taking analytical advantage of the basicity of the ?r electron system, the only “functional group” common t o all aromatic hydrocarbons. For instance, both olefins (7) and sulfides (5) have been determined by complexation with iodine, a Lewis acid. The work of Merrifield and Phillips on the colored T complexes of the strong IT acid, tetracyanoethylene (TCNE), suggested the use of T C N E as titrant in a complexometric titration of aromatic R electron systems. The procedure reported uses pure T C N E as a reference standard. A methylene chloride solution of T C N E titrant is added to a methylene chloride solution of the aromatic hydrocarbon to be estimated, and the end point 1s determined photometrically This procedure depends on the formation of a stable colored ?r complex and appears to be the first wholly organic
complexometric titration systeni. It is also necessary that the titrant be as concentrated as possible as well as a strong T acid. Other possible titrants are 7,7,8,8tetracyanoquinodimethane, whose K value for ?r complexation with pyrene is 78.4 (I), and 2,3,5,6-tetrachlorobenzoquinone, whose K value with pyrene is 23.3 (8). T C N E hm a K value of 29.5 with pyrene ( 8 ) . However, the first two a acids can be prepared only a t one tenth of the concentration of the T C N E titrant descyibed here; this is apparently not satisfactory for complexometric titration. EXPERIMENTAL
Apparatus. T o perform the photometric titrations, use a Bausch & Lomb Spectronic 20 in conjunction n i t h a 50- or 125-m1. Erlenmeyer flask (11). Use a 5- or 10-ml. buret equipped with a Teflon stopcock Reagents. Tetracyanoethylene ( T C S E ) , 0 1M. Synthesjzf, from malononitrile (4) or obtain fiom Eastman Kodak Co Sublime the TCNF and recrystallize (101. T I eizh 01’ exactly 640 nig of TCNF a n n dissolvr
1.0
0.6
0.8
4 0.4
0.6
U 0.2
.
0.4
.~ Titration of durene, 101% recovery
Figure 1.
as much as possible in 40 ml. of reagent grade methylene chloride, heating if necessary. Add no more than 0.5 ml. of dimethoxyethane to complete dissolution. Dilute to exactly 50 ml. with methylene chloride. The titrant is slightly yellow; this results from a weak TCNE-dimethoxyethane complex. Make the titrant fresh daily, since there will be some overnight evaporation of the volatile methylene chloride from the nearly saturated solution. If desired, check the purity of the T C N E against pure anthracene via the Diels-Alder reaction (10). Aromatic Hydrocarbons. Use E a s e man White Label grade or purify by sublimation or recrystallization. Procedure. Weigh o u t from 0.3 mmole of aromatic hydrocarbon having a K value close t o 120 to 0.6 mmole of aromatic hydrocarbon having a K value close t o 12 (Table I). Transfer t o a 50- or 125-ml. Erlenmeyer flask (11) connected by Tygon tubing to the test tube cell of the Spectronic 20. Add 60 ml. or more of reagent grade chloroform and circulate the solution through the flask and cell by magnetic stirring. Adjust the per cent transmittance to read 100% a t the appropriate wave length (Table 111). Add 0.1M T C N E in 0.3-ml. portions. Use 0.5ml. portions for naphthalene. Stir thoroughly and then record the absorbance. Try to record four absorb-
Figure 2. A. 5.
0.8
/ 0.4
/
0.2
4.0
6.0
ml. 0.10M TCNE
Figure 3. A. 5.
Titration of naphthalene
Usual method, no definable end point Trace analysis method, 90% recovery
8.0
Titration
Alone, 99% recovery In equimolar amounts wi!h
ance values before the halfway point in the titration. Continue the titration until two or three times the stoichiometric amount of T C N E is added, recording a t least four points after one and a half times the stoichiometric amount of T C N E is added. Plot the absorbance readings ws. milliliters of 0.lM T C N E as shown in Figure 1, and determine the end point graphically and the volume of 0.1M T C N E corresponding to it. It is not necessary to correct for dilution (6) for a n estimation. Do not use the 0 intercept as a point for naphthalene. T o determine naphthalene in phthalic anhydride, extract the anhydride once with 60 ml. of methylene chloride or chloroform. Filter and titrate the soluble naphthalene as above. Titrate mixtures of strong aromatic n bases in weak aromatic n bases where there is a large difference in their basicities, as shown in Figures 2 and 5. About 0.15 to 0.3 mmole of aromatic may be titrated by adapting trace analysis methods (12). Set the 0% T reading with a solution of the unknown containing a two- to threefold excess of TCNE, titrate as above, and
I
2.0
3.0 ml. 0.1OM TCNE
1.0
0.4 {
t
V
0.2
7.0
5.0
3.0 ml. 0.10M TCNE
1.0
“
7.0
5.0
of benzo(a)pyrene benzo(e)pyrene, 99%
recovery
estimate the end point graphically, as illustrated in Figure 3. T o determine anthracene in another aromatic, add the T C N E as above, allowing enough stirring time for the Diels-Alder reaction to proceed to completion. Near the anthracene end point, allow 3 minutes for complete reaction. Then continue the titration as above to estimate the other aromatic hydrocarbon complexometrically, as shown in Figure 4. TITRATION THEORY
To understand the complexometric titration of aromatic hydrocarbons, it is necessary to consider the nature and strength of the T C N E n complexes as well m the titration curve. Nature of Complexes. Merrifield and Phillips (8) studied the complexes a t high ratios of aromatic hydrocarbon to T C N E and concluded that their linear plots indicated that complexes other than the simple 1 to 1 need not be considered. However, Andrews (9)has reviewed
4
1
1
1
w/ -, 2.0
4.0 6.0 ml. 0.10M TCNE
8.0
Figure 4. Titration of anthracene (99.5% recovery) and fluoroanthene (99% recovery) VOL 33, NO. 11, OCTOBER 1 9 6 1
1563
Table I.
1.0
TCNE Complexes in Order
4
I
OH
of Their Strength ?r
Base
K
0.8 ..
Hexamethylbenzene 263 Pentamethylbenzene 123 -90 Fluoranthene Durene 54.2 Benzo( a)pyrene N40 Pyrene 29.5 2,6-Dimethylphenol -20 Mesitylene 17.3 Naphthalene 11.7 Hexaethylbenzene 5.11 2.00 Benzene 2,6-Di-tert-butyl-4-methylphenol -0.5
0.6
1
a 0.4
0.2
3.0
1.0
of other T acids complex 1 molecule of aromatic x base; Merrifield and Phillips did not report any study involving high ratios of T C N E to aromatic. Using their method, a check of the complex of durene with excess T C N E under conditions approaching titration conditions indicated only 1 to 1 complexes for substituted benzene rings. -4 simple experiment illustrates the dual response of these 1 to 1 complexes: 2.5 x l O + M solutions of durene and T C N E in separate flasks, when complexed with fivefold excesses of T C N E and durene, respectively, have the same absorbance. As in the conventional photometric titration, the concentration of the absorbing species is limited chiefly but not exclusively by the reagent present at the lower concentration. We11 before the end point the absorbance is limited by the T C N E titrant, and well past the end point i t is limited by the aromatic being titrated. Strength of Complexes. Table I lists most of t h e aromatic hydrocarbons titrated, with their association constants as determined by hlerrifield and Phillips (8) or as determined approximately by us wing their method. These constants are not conventional equilibrium constants, since they contain a mole fraction term for the association:
Table 11.
Calculated Titration Curves of Aromatics (6)
0.3 Mmole 0.6 Mmole Durene Naphthalene 80 M1.CHPClz 50 M1. CH&lz
TCNE 'ritrant
Aoalod
0 . 1 (B) 0 . 3 (B) 0 . 5 (B) 1 . 5 (B)
0
Aoalod
Aabsd
0.023 0.030 0.031 0.035
0.070 0.085 0.085 0.10
0.11 0.32 3 . 0 (B) 0.40 2 . 3 3 ( B ) 0.47
1564
Aabad
0.13 0.34 0.43 0.49
0.14 0.33 0.40 0.44
ANALYTICAL CHEMISTRY
0.15 0.33 0.41 0.45
5.0
1.0
ml. 0.10M TCNE
a number of cases in which 2 niolecules Figure 5.
Titration of 2,6-dimethylphenol
Alone, 99% recovery In 25-fold excess of 2,6-di-terf-butyl-4-methylphenol, 99% recovery
A. 6.
TCNE
+ base e complex
(1)
Merrifield and Phillips (8) defined K , the association constant for complex formation, as : K =
{(TCNE)-
( C)
( e ) ([Bl ) - [el)
(2 )
in which (TCNE) is the initial molar concentration of TCNE, (C) is the molar concentration of the complex a t equilibrium, and [B] and [C] are the mole fractions of the initial aromatic x base and the complex a t equilibrium, respectively. Merrifield and Phillips determined the constants under conditions where [ B ]>> [C], but did not report any determination of K where T C N E mas present in large excess over the aromatic. An approximate check of K for durene was obtained b y us using the analogoiis equation where [TCNE]>> [C] :
-
For the relatively dilute titration conditions, [ B ] (B)/15in Equation 2 , and substitution of molarity for the mole fraction term in that equation results in A value of about K/l5 for the conventional stability constant. An approximate determination of the conventional stability constant by the graphical method of Andrcws and Kecfcr (3) substantiates the substitution. This indicates that even well beyond the end point not all of the aromatic base is complexed and that the absorbance should keep increasing as T C N E is added. Figure 1 substantiates this. This is also substantiated by substitution into Equation 3. JSTell past the end point [TCWE]approaches only 0.001; the ratio of complex to uncomplexed durene a t equilibrium will be no more than 0.0542
Titration Curve. T h e titration curve, such as the portions used for extrapolation, can be calculated using the constants in Table I if methylene chloride is the exclusive solvent. Since the illustrated titrations were perfoinied in chloroform, different K values ( 8 ) mu+t be determined to calculate theni. Table I1 contains absorbance data calculated by using Equations 2 and 3 for the titration curve before the titration mid-point and after the end point, rmpectively, and by using Beer's I%\\.. The follon ing approximations were used: I n both Equations 2 and 3, [C] WLS neglected, the effect of the dimethoxyethnne in the titrant was ignored, and the molar absorptivities used on the Spectronic 20 for durene and naphthalene were 1870 and 1110, respectively. The 2.5-em. test tube cell of the Spectronic 20 was used in obtaining the experimental data. The agreement between the calculated and observed absorbances may be homewhat fortuitous, but it is obvious that the calculated absorbance parallels the observed absorbance and thus predicts the shape of the titration curve. End Point. The extrapolated end point obviously does not represent 1007, roniplexation b u t is simply the point a t M hich the slope of the titration curve changes. Well before the end point the slope of the curve depends chiefly on the concentration of the tetracyanoethglene; well after the end 1)oint the slope of the curve has clianged, since it now depends chiefly on the concentration of the nromatic hydrocarbon. Mixtures. In Figures 2 and 5 it is obvious t h a t the titration curves before the end points of the mixtures have cityiter slopes than for thesc
compounds alone. This is explicable, considering t h a t before t h e end point, complex formation depends chiefly but not exclusively on the ( T C N E ) term. The weaker T base in the mixture simply forces more of the T C N E into the complex; the [ B ] - [C] term in Equation 2 is larger and increases (C), If the second K base is appreciably weaker and present in a correspondingly low concentration, the end point for the stronger A base \Till not be high, because the change in the slope of the titration curve should come at the point where (TC’VE) equals the concentration of the strongcr i~base. RESULTS
Results for the estimation of a number of pure aromatic hydrocarbons and some mixtures are given in Table 11. -1pparently aromatics which have a K of 11.7 or more can be titrated. The marginal case is that of naphthalene. Figure 1 illustrates the photometric end point obtained for the titration of durene. The estimation of stronger A bases in the presence of weaker A bases is also possible. Figure 2 illustrates the estimation of benzo(a)pyrene alone and in the presence of the weaker A base, benzo(e) pyrene, Ailthough the titration determines only as low as 0.3 mmole of strong T base or 0.6 mmole of weak A base, a trace analysis (12) technique of setting the O%T reading with a solution of the aromatic being titrated permits the estimation of half as much aromatic. Figure 3 illustrates the photometric titration curve for the estimation of 0.3 rnmole of naphthalene by the usual procedure and b y the trace analysis technique. Aromatics which react rapidly via a reaction such as the Diels-Alder reaction need not interfere. As shown in Figure 4, anthracene is actually estimated in the presence of fluoranthene, which is also estimated in the same titration by the usual procedure. The extrapolation of the initial part of the titration curve to the approximate zero absorbance line permits the estimation of anthracene. The accuracy and precision of the estimation performed b y an experienced operator are probably no better than 1 to 3% for aromatic hydrocarbons n i t h reasonable K values and higher molar absorptivities. An inexperienced operator may incur an average deviation of as high as 7%, however. Naphthalene is the marginal case, and the average deviation is much higher for its estimation. Admittedly, many of the end points are not as sharp as desired, but the estimation iq certainly esthetically at-
Table
111.
Estimation of Hydrocarbons
Aromatic Hydrocarbon Benzo( a)pyrene Chrysene 2,6-Dimethylphenol Durene Fluoranthene Hexamethylbenzene Mesitylene Naphthalene Pent amethylbenzene Perylene Phenanthrene
Amax,
hIp
520 560 520 480 540 545 460 550 525 460 530
Aromatic
50 Recovery
( Av. 99 100 99 99 100 99 99 90 97 101
Dev.) i2 i1 i3 =t2 i1 +1 i8 i2 i1
100
Mixtures Mole 7; Anthracene and 50 99 5 i 0 . 5 fluoranthene 50 99 =k 1 Anthracene and 50 99 5 phenanthrene 50 101 Benzo(a)pyrene in 50 99 i. 1 benzo(e)pyrene 50 2,GDimethylphenol 5 99 in 2,6-di-tert-butyl9,5 4-methylphenol Fluoranthene in 10 95 pyrene 25 Naphthalene in 5 $10 phthalic anhydride 95
tractive in that it presents a functional group method for the T electron qystem of the aromatic hydrocarbon. Mixtures. Although phenolb are alkylated a t t h e macro level in pyridine catalyst b y TCNE ( I S ) , 2,6dimethylphenol can be estimated alone or in mixtures without apparent alkylation at this semimicro level. Figure 5 illustrates its titration alone and in the presence of a 25-fold excess of 2,6-ditert-butyl4-methylphenol. Table I reveals that Zi for 2,6dimethylphenol is about 40 times that this of 2,6-di-tert-butyl-4-methylphenol; is probably the most favorable case for a mixture. The latter phenol should be a stronger A base than the former on electronic grounds, but the tert-butyl groups prevent T C K E from approaching close enough to complex strongly. The photometric end points in this case and for the titration shown in Figure 2 are surprisingly improved by the addition of a much weaker A base than the A base being titrated. It may be desirable in other cases to add judicious amounts of a very pure weak A base to improve the extrapolation to the end point. Another favorable case is the estimation of naphthalene in phthalic anhydride, as shown in Table 11. The latter is a n extremely weak A base ( K
