develoliing the color in the undiluted solution in the trap, an absorbance value of 0.080 would be obtained. LITERATURE CITED
(1) Etherington, T L., Mecarthy, L. V., A.M..4. Arch. I n d . Hyg. Occ. M e d . 5 ,
447 (1952).
(2) Hill, D. L., Gipson, E. I., Heacock, J. F . ANAL. ~ CHEM. 28, 133 (1956). (3) Hill, W. H., Johnson, 31. S., Zbid., 27, 1300 (1955). (4)Hill, K. H., Kuhns, L. J., llerrill, J. M., Palm, B. J., Seals. J.. Urouiza. U., Am. Z n d . Hyg. Assoc. J . 21; 231 (1960). ( 5 ) Hill, K. H., Merrill, J. M., Larsen, It. H., Hill, D. L., Heacock, J. F., I b z d . , 20, 5 (1959).
(6) Kuhns, L. J., Forsyth, R. H., Nasi, J. F., ANAL.CHEM.28,1750 (1956). (7) Pfitzer, E. A,, Seals, J. If., Am. I n d . Hyg. Assoc. J . 20, 392 (1949). RECEIVEDfor review Ami1 13. 1964. Accepted January 4, 1965. This effort was supported by Air Force Contract AF04(611)-5963, Edwards Rocket Base, Edwards, Calif., under the program monitorship of A . V. Jensen.
Study of the Pi Complexes of 2,4,7-Trinitrofluorenone with Phenols, Aromatic Hydrocarbons, and Aromatic Amines GEORGE H. SCHENK, PATRICIA W. VANCE, JOHANNA PIETRANDREA, and CAROLYN MOJZIS Department o f Chemistry, Wayne State University, Detroit, Mich.
b In contrast to tetracyanoethylene (TCNE), which is a T complexing agent with some selectivity for methyl substituted benzenes and phenols, 2,4,7trinitro-9-fluorenone (TNF) complexes more strongly with aromatic molecules having at least three fused ringse.g., phenanthrene, pyrene, perylene, etc. TNF and TCNE are compared as to their complexation of various ir bases, Selectivity in complexing ir bases appears to b e gained by using a ir acid of the same dimensions. To make TNF complexing more selective, a new mild nitrating reagent of nitric acid-acetic anhydride in triethyl phosphate is utilized. Also useful are 0.1M perchloric acid in triethyl phosphate, acid-catalyzed acetylation, and TCNE (Diels-Alder reaction). Plots for the spectrophotometric determination of small and large rings are obtained which do not deviate appreciably from Beer’s law. TNF obeys the same equilibrium constant expression as TCNE, and absorbances of solutions of known concentrations can b e calculated. Solubilities, precipitation effects, solvent effects, temperature effects, and acid-base-salt effects are reported. The TNF-pyrene precipitate appears to obey a solubility product expression over a limited cencentration range. The interference of molecules like phenanthrene, but not 2-naphtho1, on the complexation of pyrene-TNF appears to b e predicta ble.
I
INORGANIC analysis, some selectivity in complexometric titrations has been achieved by substitution of all-nitrogen type chelons, such as triethylenetetramine, for EDTA as titrant. The principle involved is that of selecting a chelating agent which N
372
e
ANALYTICAL CHEMISTRY
48202
coordinates more readily with a restricted set of ions because of the properties of the ligand. In organic analysis, some selectivity has been observed in the utilization of tetracyanoethylene ( T C S E ) , a T complexing agent which is especially effective for a complexing substituted benzenes (16) as compared to naphthalene and higher homologs. For instance, the stability constant, K , for the TCSE-hexamethylbenzene complex is 263 compared to a K of 11.7 for the TCSE-naphthalene complex and a K of 29.5 for the TCSE-pyrene complex (9). The principle involved seems to be that a ?r acid complexes more strongly with a ?r base of comparable size. It is desirable to investigate the analytical properties of other T acids to discover some with a selectivity for larger aromatic rings than benzene. For example, the tetracyanoquinodimethane (TCKQ)-pyrene complex has a K of 78.4 compared to a K of 14.5 for the TCXQ-hexamethylbenzene complex (8). Unfortunately, TCXQ is not nearly as soluble as TCNE in chlorinated solvents. 2,4,7-Trinitro-9-fluorenone ( T N F ) , another large ?r acid, is as soluble or more soluble than TCNE in many chlorinated solvents, and is also commercially available. Klemm and Sprague ( 5 ) have shown T N F to have some selectivity in that it complexes 2-substituted naphthalenes more strongly than 1-substituted naphthalenes because of greater overlap of a orbitals. TXF is a strong ?r acid, and its complexes have been studied from an organic ( I , 5 , 10, 11), physical ( 6 ) , and qualitative analytical (4) viewpoints. T N F complexes have also been analyzed by nonaqueous titration ( 2 ) . T S F has additional advantages over T C S E in
that it is not hydrolyzed by water and does not react with anthracene and other Diels-..llder active dienes. The work reported shows that it has an increasing selectivity for complexing aromatic systems as the number of fused benzene rings is increased from three to four to five. Sensitivity of TCNE and TNF. T C S E and T N F both form 1 : 1 complexes with A bases. The formation constant, or stability constant, expression for analytical situations where the ?r acid is a t concentrations appreciably higher than the aromatic a base is (16) :
in which (C) is the molarity of the complex a t equilibrium, ( B ) iq the analytical or initial molarity of the T base, [TI is the analytical or initial mole fraction of TCNE or TXF, and [ C ] is the mole fraction of the complex a t equilibrium. The [C] term is usually negligible compared to the [TI term. n’eglecting the [C] term, rearranging Equation 1, and (assuming 1-cm. cells) substituting A / € for (C) gives:
Given a constant lower limit of absorbance, A , Equation 2 predicts that ( B ) will be decreased by increases in E , K , and [TI. (The term K [ T ]is usually small compared to one. A relatively large value would be 0.54 for pyrene in O . l L 1 1 TSF). T N F is definitely the more sensitive reagent for pyrene if Equation 2 is used to compare the two practical reagents, 0.1 1J/ T S F in methylene chloride and 0.05.11 TCKE in methylene chloride.
EXPERIMENTAL
Apparatus. The transistor regulated model Bausch and Lomb Spectronic 20 Colorimeter with 11.7-mm. test tubes and 11.6-mm. square cells, as well as the Beckman Model D B spectrophotometer were used for spectrophotometric measurements. The Temp-Blok Module Heater (module No. 2076) was used for the temperature stability studies. The Farrand spectrophotofluorometer was used for studying the fluorescence of the p y r e n e T N F complex. Reagents, 2,4,7-Trinitro-9-fluorenone ( T N F ) , obtained from the Eastman Kodak Co., was recrystallized from 3 : 1 nitric acid-water ( 6 ) , and dried over phosphorus pentoxide. Recrystallization of T N F from organic solvents is not always satisfactory since frequently a colored T N F A base complex precipitated with the first crystals of T N F . A better method is that of purification from 3: 1 nitric acid-water. This apparently nitrates any mono- or dinitrofluorenone, but does not form 2,4,5,7-tetranitrofluorenone. The latter results only when T N F is treated with fuming nitric acidsulfuric acid (IS). 2,3,5,6-Tetrachlorobenzoquinonewas obtained from Matheson, Coleman & Bell Co. 2,3-Dichloro-5,6-dicyanobenzoquinone was obtained from Arapahoe Chemicals, Inc. Tetracyanothiophene was kindly supplied by the Central Research Dept. of E. I. du Pont de Nemours & Co. All were used as received. Triethyl phosphate was Matheson, Coleman & Bell practical grade and was not purified further. Aromatic hydrocarbons, phenols, and amines were Eastman White Label grade or the equivalent in most cases. The 0.1M perchloric acid was prepared by mixing 4.3 ml. of 72Yq perchloric acid in 200 ml. of triethyl phosphate with 10 ml. of acetic anhydride and diluting to 500 ml. with triethyl phosphate. The 2M acetylating reagent was prepared from 0.47 ml. of 72% perchloric acid, 30 ml. of triethyl phosphate, and 10 ml. of acetic anhydride. The 1.6M and 0.2M nitric acid reagents were prepared by cooling mixtures of 0.5 ml. of concentrated nitric acid in 1.5 ml. and 36.5 ml. of triethyl phosphate, respectively, and then adding 3 ml. of acetic anhydride to each. The nitric acid-acetic anhydride reagent of Malins, Wekell, and Houle (7) was also prepared. Procedure. The tests of Equation 4 as well as the determination of the stability constants were performed on the Spectronic 20 using the square cells. I n the determination of the K ’ s (9) of TKF-pyrene, TKF-2-naphtho1, and tetracyanothiophene-pyrene, final A acid concentrations were 1.5 X M , and 5 X 10-4M, 10-4M, 1.0 X respectively. Mole fractions of pyrene ranged from 0.02 to 0.05; mole fractions of 2-naphthol ranged from 0.035 to 0.05. Blanks were used to correct
Table 1.
7
Comparison of Complexes of Pyrene and Various a Acids in CHpClp Solvent
Acid and K of its pyrene complex
Dichlorodicyanobenzoquinone >80 73 Trinitrofluorenone Tetracyanoethylene 29. 5a Tetracyanothiophene 29 Tetrachlorobenzoquinone 23. 3a 0
M of 7r acid in CH2Cl2
5 x n - .~ 3 ~ .. i . pyrene 7 acid, A (wavelength, mp)
0.08 0.15 0.10 0.10 0.01
~2 (540) 1 . 4 (520) 0 . 8 (720) 0 . 4 8 (480) 0 . 1 2 (610)
+
Determined by Merrifield and Phillips (9).
for the side band absorption of pyrene at 520 and 480 mp. Solvent effects were determined by measuring the absorbance of solutions containing an excess (0.02M) of pyrene over T N F (0.002M) to avoid precipitation in some solvents. Temperature effects were investigated by reading the initial absorbance of about 3 ml. of solution in an 11.7-mm. Spectronic 20 test tube, inserting the tube into the Module heater for 30 or 60 minutes, cooling and reading the absorbance again. Solutions of T N F used to set 1 0 0 ~ oT were not so treated. Adherence to Beer’s law was investigated by always setting 1 0 0 ~ oT with a solution of T N F in the appropriate solvent. Because of the broad absorption spectrum of T N F , it is necessary to purify it carefully and choose somewhat arbitrary wavelengths for measurement of its A complexes. The absorption spectrum a t 350 to 410 and 450 to 520 mp of a O.llM solution of unpurified T N F varied as much as 0.15 absorbance unit from that of purified TNF. The use of 0.1M perchloric acid in triethyl phosphate to avoid amine interference was investigated by making up a standard solution of pyrene in the 0.1M perchloric acid and always diluting to volume with this solvent. I n contrast, only 0.3 ml. of the acetylating reagent was added to 5 ml. of methylene chloride solutions. The use of the nitric acid-acetic anhydride reagents to reduce interferences was investigated by adding 0.2 ml. of the reagent to about 4 ml. of a methylene chloride solution of pyrene and the interference. After the appropriate time, 0.1 ml. of methanol was added followed by solid T N F . The solution was diluted to 5 ml. with methylene chloride, and the absorbance was read. RESULTS AND DISCUSSION
Acid Strengths. Since a n object of this study is to find A complexing agents with a selectivity for larger aromatic rings, the data in Table I have been collected to give a comparison of the strengths of various A acids toward pyrene in methylene chloride. The TNF-pyrene complex is obviously a relat,ively strong complex.
Since the molar absorptivities of complexes in Table I are all of the order of los, the sensitivity of these reagents for pyrene will depend on K and on the [TI term in Equation 2. For actual comparison, the absorbances of solutions of pyrene saturated with the respective A acid are given in Table I. This comparison overrates T C N E because 0.144 T C N E must be prepared by heating, making it impractical to use with dilute solutions of a bases. Although dichlorodicyanobenzoquinone is more sensitive for pyrene than T N F , the former also has a more appreciable absorption spectrum. This makes the setting of 100% T with the A acid solution alone more subject to error, etc. Choice of Wavelengths. The use of T N F as a complexing agent greatly depends on a n appreciation of the spectra of T N F alone and of its complexes with molecules such as pyrene and 2,6-dimethylphenol, as shown in Figure 1. T C N E differs from T N F in t h a t it is colorless and absorbs in the ultraviolet region with a peak a t 265 mp. An inspection of Figure 1 leaves no doubt that measurements are only practical a t wavelengths a t or longer than 450 mp. Of interest is the fact that molecules containing one or two rings show absorption spectra similar to that of 2,6-dimethylphenol: a sharp peak near 450 mp and a moderate tailing off. The shape of absorption spectra of molecules containing three or more fused aromatic rings is somewhat similar to that of pyrene: one or more (410 to 520 mp) broad maxima and very gradual tailing. In these cases, the greater overlap of A orbitals possible is apparently responsible. Spectrophotometric determination of pyrene and similar molecules is thus more selective a t 520 mp. The spectra of the complexes of carbazole, fluorene, and dibenzothiophene (Figure 2) are presented because of the structural similarity of these 7 bases to T N F , all molecules having a central five-membered ring fused to two benzene rings. There are some differences in the spectra, particularly in VOL. 37, NO. 3, MARCH 1965
e
373
0.4
0.2
C
4lOrn1.1 1
I 450
.
am
3701.1
410
Figure 1 . Absorption spectra of trinitrofluorenone (TNF) and its a complexes in C Curve A.
0.1 5 M recrystallized TNF
Curve 6. 0.025M 2,6-dimethylphenol-O.I 5M TNF Curve C. 0.001 M pyrene-0.1 1 M TNF Both curves 6 and C were recorded by setting 100% T with a solution of the corresponding concentration of TNF. The dotted portion of curve C i s an extrapolation from more dilute solutions
the case of carbazole. This difference suggests that T K F may be somewhat more sensitive a reagent for aromatic amines such as carbazole than for the corresponding aromatic hydrocarbon. As shown in parentheses in Table 11, thelongest wavelengthabsorptionpeaks of T N F complexes of A bases having a n increasing number of fused aromatic rings, are not as well separated as those of TCNE. The TCNE-pyrene and
Table 11.
A
=
Table 111.
Solvent CCla CHC1, CHBr3 CHzClz CHClzCHClp CHZClCH2Cl HOAc
EtOAc (EtO)3PO
CH3N02 CH3CN
(CH3)zCO (CH3)W CHaOH
374
A
Bases by TCNE and TNF
0.10, b
= 1.17 cm., CHZClp solvent Detected by Detected by 0 11M TNF, M 0 05M TCNE, M (wavelength) (wavelength) 3 0X (450 mp) 8X (520 mp) 2 5X (440 mp) 3X (480 mp) 2X (450 mp) 14 X (580 mp) 4 X (450 mp) 1X (520 mp) (520 mp) 1X (724 mp) 2X 5 X 10-6 (620 mp) 1X (890 mp)
Compound 2,6-Dimethylphenol Durene 2-Naphthol Phenanthrene Pyrene Perylene
Solubilities of TNF and Effect of Solvent on TNF Complexation 0.002.M TNF, 0.02M pyrene, ( b = 11.7 mm.)
Solubility O . 04M 0.05M >O 15.M
o
02M
13M 03M 07.M 12M
41M 12M 14M 5M
02M
ANALYTICAL CHEMISTRY
A620 m p
. (pptn.) 0.43 0.35 0.27 0.32 0.20 . . . (pptn.) . . . (pptn.) . .
0.14
0.30 . , .
0.20 0.18 . ,.
I 530
I
570
Curve A. 0 . 0 0 3 M carbazole-0.03M TNF Curve 6. 0 . 0 0 2 M dibenzothio~hene-0.02M TNF Curve C. 0.005M fluorene-0.03M TNF All curves were recorded by setting 100% T with a solution of the corresponding concentration of TNF
TCNE-perylene complexes have peaks located in the near infrared region in contrast to the T X F complexes. Lower Limits of Detection. The data in Table I1 compare the experimentally determined lower limits of detection ( A = 0.10) by T C N E and T N F of aromatic species consecutively increasing in size by one ring. T C N E obviously has the greater sensitivity for substituted benzene rings
Comparison of Lower Limits of Detection of
I 480
Figure 2. Absorption spectra of TNF a complexes of three similar molecules in CHzClz
520
480
I
450
(pptn.) (pptn.)
A 4 6 0 mp
...
0.65 0.57 0.41 0.53 0.26 ...
... 0.21
0.43 ... 0.28 0.33
such as 2,6-dimethylphenol and durene. I t s sensitivity is somewhat lower and is relatively constant for molecules with two (2-naphthol) to five (perylene) fused aromatic rings. The sensitivity of T N F for benzene and naphthalene rings is not high, but its sensitivity continues to increase as the number of fused aromatic rings incremes. Thus T N F shows & certain selectivity for three or more aromatic rings as compared to benzene or naphthalene rings. This is probably because of the increase in A orbital overlap as the size of the A base increases. An examination of models reveals that maximum overlap of A orbitals is probably not yet attained with TNF and perylene, but might be attained with T X F and an aromatic containing 6 or 7 (coronene) rings. It seems probable that the detection limit for such aromatics would be lower than that for perylene. I t is also apparent that T N F is a more selective A acid when the A base is large enough to overlap the nitro groups as well as the ring system of T N F . Some of the TCNE detection limits in Table I1 have already been calculated from Equation 2 (16). Using Equation 2, a K of 73 and an E of 103 for GrreneT N F , a detection limit ( A = 0.10) of 2.3 X 10-4M was calculated for pyrene. Somewhat lower limits than those in Table I1 would be found on prism spectrophotometers where the molar absorptivities are some 10% higher than on the Spectronic 20. Solvent Effects. Table I11 gives the solubilities of T N F in various solvents and the effect of solvent on absorbance, and presumably K , of the pyrene-TXF A complex. The effect of solvent on complexation of pyrene by T N F is somewhat different
from that on complexation of durene by TCNE. I n the latter case, it appears that solvent hydrogen bonding to the aromatic x electron system competes favorably with complexation by T C N E
Table IV.
Thus the absorbance of the TCKEdurene complex is predictably higher in bromoform than in chloroform, but the reverse is true for the TNF-pyrene complex. I t may be that in the complex there is a solvent molecule(s) hydrogen bonded to the ring and that the chlorine or bromine atoms of the solvent provide some steric hindrance to T N F complexing. The larger size of bromine would explain why bromoform is a poorer solvent for complexation than chloroform. Dissolution of T N F does not appear to depend on hydrogen bonding alone since it has limited solubility in acetic acid, ethyl acetate, and methanol, all solvents which readily dissolve T C S E largely because of hydrogen bonding. Nitromethane, methylene chloride, and triethyl phosphate appear to be the most suitable solvents for analytical work, The latter is useful for utilizing acid-catalyzed acetylation and protonation by perchloric acid. Precipitation of the pyrene-TNF complex resulted in many solvents; this effect was investigated further. Precipitation. Many phenols and aromatic hydrocarbons form insoluble complexes (10, 11) ; this phenomenon has been utilized for analytical purposes (2, 4 ) . Precipitation occasionally limited the upper range of concentration in the investigation of adherence to Beer's law discussed below. The precipitation of the pyrene-TSF complex was briefly investigated to see whether a solubility product expression would be valid for a 1 : 1 precipitate. The data in Table IV demonstrate this may be true over a limited concentration range. As long as the T N F concentration was higher than, or about the same as that of pyrene, a somewhat constant "Kgp" value was obtained in methylene chloride. When the concentration of pyrene was appreciably greater than that of T N F , no precipitate was formed a t the predicted concentrations. A similar but more limited investigation in chloroform revealed that the K,, became progressively larger as the concentration of pyrene increased and that of T N F decreased. Acid, Base, and Salt Effects. Bases, but not acids, tend to decompose both T C N E (16) and T K F . Only one experiment was made on the effect of acid on a T X F complex because of the insolubility of many complexes in acetic acid. The 0.1M perchloric acid reagent in triethyl phosphate did lower the absorbance of the TNF-pyrene complex by some 25y0 a t
Pyrene, M Precipitation CHZC12 solvent, 22" C. 2 x lo-' Slight 2 x 10-3 None
TNF, M
(16).
Precipitation of Pyrene-TNF Complex
0.10 0.09
... 0,035 0.031
"K,," 2
x 10-4 x \10-4
x'io-a x 10-3
Slight None
2
1 . 5 X'10-2 1.2 x 10-2
2 X'io-2 2 x 10-2
Slight None
3 x 10-4
1 . 1 X'10-a
2
t0'8 x
None
...
5 5
10-1
1.7 X 1.5 X
CHCL solvent, 22" C. 4 x 10-3 Slight 4 x 10-3 None
1.5 X' io1.0 x 10-2
8 X'10-3 8X
x'io-2
1.0 x
8.0
io-2
x 10-3 Table V.
2 2
x 10-2
7
x 10-6
Slight None
I
x
10-4
Slight None
2
x
10-4
Effect of Temperature on TNF Complexes
O.llM TNF, 520 mp, b
=
11.7 mm.
Absorbance CH&lCHzCl 60 min., Initial 80" C.
T Base, M None Nonea Aniline, 1 X 10-2M Pyridine, 2 X 10-2Ma 2,6Dimethylphenol, 5 X 10-2M Pyrene, 1 X lO-3M Absorbance measured at 440 mp.
...
, . .
..,
, . .
0.45 0.09
1.2 0.11
0.38
0.140 0.37
0.140
Absorbance ( E t 0 ) 3 P 0 60 min., Initial 115' C. 0.05 0.08 0.37 0.47 ... 0.05
...
0.33
1.05 0.41
0.28
0.38
5
both 450 and 520 mb. Because this reagent contained acetic acid from the reaction of acetic anhydride and water, it is not possible to state that this effect arises entirely from perchloric acid. Salts also effect the absorbance of T N F complexes. An experiment involving 0.3M sodium perchlorate revealed that this salt increased the absorbance of the TPiF-pyrene complex by some 10% a t both 450 and 520 mb. Temperature Effects. Gordon and Huraux ( 4 ) reported t h a t the colors of T I W complexes of substituted anilines and substituted phenols disappear when the complexes are heated on filter paper 5 minutes a t 110' C. However, in solution a t 80' or 115' C. no such decoloration is found, as shown in Table V. Pyridine apparently causes some decomposition of T N F a t 115' C., and some decomposition of T N F appears to occur a t 115' C. when it is heated alone in solution. Similar differences in reactivity have been noted with T C N E (16) where the colors of T C N E complexes disappear with heating on paper but not in solution. Evidently the chemistry of the complexes on paper is different than that in solution. At any rate, no preferential destruction of the color of selected
complexes to permit measurement of other complexes, seems possible. Agreement with Beer's law. Table VI lists various 7 complex systems which do not deviate significantly from Beer's law from concentrations of x base a t which the absorbance is 0.10 to the stated upper limits. All experiments involved using a high concentration of T N F relative to that of the x base. Deviations from Beer's law a t high concentrations of x base may be the result of a significant [C] term in the following equation:
(This equation is derived from Equation 1 by substituting A / € for (C), assuming 1-cm. cells, and rearranging.) I t has been pointed out that neglecting the [C] term in Equation 1 will cause an error in the determination of K (3, 18). The same should hold true for agreement with Beer's law. The instrumental value of E should be constant over a concentration range only if the right side of Equation 3 is sensibly constant over this range. One way of measuring this is to calculate VOL. 37, NO. 3, MARCH 1965
375,
2 for BZand Equation 2 for B1 permits one to predict the relative concentration of Bz that will cause a given error in absorbance (&/AI) :
Table VI.
Agreement with Beer’s Law 520 mp, b = 11.7 mm.
Compound Concentration range= 4 to 32 x 1 0 - 4 ~ Carbazole 1 6 to 8 X 10-4Mb Dibenzothiophene 3 to 20 x 1 0 - 3 ~ b 2,6-l)imethylphenol 1 to 8 X 10-3M Diphenylamine 5 to 37 5 x 1 0 - 3 ~ Durene 2-Kaphthol 2 to 12 x 10-3’14 Pyrene 2 to 15 x 1 0 - 4 ~ Pyrene 0 , 3 to io x 1 0 - 4 ~ b a The lower figure quoted corresponds to A = 0.10. 450 mu. Table VII.
...
0 6 1 4 ...
0.6
1.4 ...
0.6 1.4 1.0 1.0 1.0
=
11.6 mm. Rel. error,
Pyrene, M 3 x 10-4 3 x 10-4 3 x 10-4 6 X 6 X loT4 6 X
9 9 9 9 9 9
x x x x x x
10-4 10-4 10-4 10-4 10-4 10-4
Weaker base None 2-Naphthol Phenanthrene None 2-Naphthol Phenanthrene None 2-Naphthol Phenanthrene Chrysene Anthracene Carbazole
the right hand side of Equation 3 for the highest and lowest concentrations of the range and then compare their relative difference with instrumental error. This relative difference for pyrene between the two points A = 0.10 and 0.70 given in Table VI is about 0.5 p.p.h., compared to a relative error in concentration (Ac,/c.) of 1.6 p.p.h. resulting from instrumental reading error at A = 0.70. (This relative error was calculated by assuming an instrumental reading error of O.5y0T.) The plot of the pyrene-TNF complex at 450 mp did not deviate significantly from Beer’s law but did not pass through the intercept. Since in this case, as for all systems listed in Table 111, 100% T was set with a solution of T N F alone at the final concentration listed, the higher absorbance of T N F at 450 mp (than at 520) is probably responsible. The plot of the 2-naphthol-TNF complex also did not pass through the intercept. Since Gordon and Huraux (4) noted fluorescence of many T N F complexes, the activation wavelength for the TNFpyrene complex was investigated. This complex was found to be activated for fluorescence in the 310- to 360-mp region. Since absorbance measurements are made a t 450 or 520 mp, no activation to fluoresce is to be expected because the optical system of the 376
Solvent
Complexation of a Stronger x Base in Weaker a Bases
O.llM TNF, CHzClz,b (Weaker ______ base) (Pyrene)
TNF, final M 0.11 0.30 0.10 0.11 0.11 0.11 0.11 0.11
ANALYTICAL CHEMISTRY
ALZO mp
%
0.106 0,119 0.117
...
0,243 0.265 0.267 0,389 0.403 0.425
0.68 0.76 0.59
12
10.5 ...
9 10 ... 3.5 9.5 74 95 51
Spectronic 20 focuses light on the sample only after the light has been dispersed by the grating. INTERFERENCES AND SELECTIVITY
Of the many approaches to removing interferences that are possible with TCNE (16), some could not be evaluated readily because of solubility problems. Acetic acid and acetic anhydride were not investigated for this reason. The effect of water was also not investigated as WELS done for TCNE because there is no hydrolysis of TNF. The approaches investigated are treated individually below. Strength of A Bases. Some aromatic hydrocarbons, phenols, and aromatic amines may be determined in the presence of others by virtue of being much stronger x bases. Obviously, large amounts of benzene, halogenated aromatics, and nitroaromatics can be tolerated in the determination of stronger T bases such as durene (16) or pyrene. If it is assumed that there is no interaction between two different x complexes, it is possible to use Equation 2 to predict interferences. Suppose that a strong a base, B1 is to be determined in the presence of a weak x base, Bz. Writing a ratio of Equation
Equation 4 predicts that in the determination of B1, the error caused by the weaker x base, Bz,is constant if the ratio (Bz)/(B~)is constant. This means the error is independent of the actual concentrations. This prediction was tested by measuring pyrene in 2naphthol and in phenanthrene as shown in Table VII. To use Equation 4, K for the 2naphthol-TKF complex was measured and found to be about 9; e at 520 mp was found to be about the same as E at 520 mp for the pyrene-TNF complex. A value of 0.0073 was used for [TI. The solution to Equation 4 predicts that a relative error of 1Oyo (Az/A1 = 0.10) will occur at a 2naphthol/pyrene ratio of 0.6/1. As seen in Table VII, the relative error is somethat higher than 10% at low concentrations but is much lower than 10% a t higher concentrations. Two explanations seem plausible: one is that there is an interaction between the two pi complexes, and the other is that the hydroxyl group of 2-naphthol is exerting some unpredictable influence. It seems likely that a t higher concentrations 2-naphthol can hydrogen bond to itself to a greater extent. Certainly if this hydrogen bond involves the x electron system of 2-naphthol, this would reduce R complexation of 2-naphthol by TSF and would reduce the relative error. As a further test, K for the phenanthrene-TNF complex was measured and found to be about 13; E a t 520 mp was found to be about one fourth of E a t 520 mp for the pyrene-TNF complex. The value of 0.0073 was again used for [TI in solving Equatim 4. The solution predicts that a relative error of 10% ( A ~ / A I= 0.10) will occur in the measurement of TXF-pyrene at 520 mp at a phenanthrene/pyrene ratio of 1.4 to 1. As seen in Table VII, the relative error in absorbance is sensibly constant for the measurement of various concentration levels of 1.4 to 1 ratios of phenanthrene to pyrene. The relative error in absorbance decreases only very slightly with an increase in concentration. Equation 4 appears to hold for a mixture of phenanthrene and pyrene with T N F , and it may possibly hold for other mixtures of aromatic hydrocarbons. Phenanthrene and 2-naphthol are both fairly weak T bases compared to pyrene and might not be expected to cause appreciable error. As seen from
Table VIII. 0.1 M Perchloric Acid/(EtO)3P0 as Solvent for Selective Complexation
0.08M TXF, 0.0006M pyrene, b = 11.7 mm. A
Base, M
None @-Naphthylamine, 0.003M
6-Xaphthylamine, 0 006M
Pyridine, 0 006M Triethvlamine, 0 05M Carbaiole, 0 . OOlM Diphenylamine, 0.001.Rri
A520 rnp
Alw
mp
0.22
0.. 2 1
0.22
0 22
0.33
0.25 0 24 0.22 0.75
0.26
0.27
0.235 0 21
Acid-Catalyzed Acetylation a t Room Temperature for Selective Complexation 0.0006M pyrene, 0.11M TNF, CH2Cl2,0.3 ml of 2M A C ~ O / ( E ~ O ) ~ P O
Am
Ah20 mu
0 21 ~~
Table IX.
~~
Table VII, anthracene, chrysene, and carbazole, a bases of strengths comparable to pyrene, interfere seriously in the complexation of pyrene. Perchloric Acid. Perchloric acid was the first of a number of chemical reagents investigated for t h e reduction of interferences. Table VI11 indicates t h a t 0.1M perchloric acid in triethyl phosphate as solvent materially reduces the interference of amines in the complexation of pyrene. Perchloric acid in acetic acid (16) could not be used because the pyrene-TNF complex precipitates in that solvent. Apparently protonation reduces the a basicity of amines with a p% (H20) of a t least 10 but not the a basicity of more weakly basic amines such as carbazole or diphenylamine. Acid-Catalyzed Acetylation. The substitution of a n electron-withdrawing group like t h e acetyl group on a phenol or aromatic amine markedly reduces t h e a basicity of these compounds (16). This is accomplished by uncatalyzed acetylation of amines or acid-catalyzed acetylation of amines, phenols, and mercaptans. Instead of employing a dilute 0.06M acetylating reagent in ethyl acetate as solvent (16), a small volume of a 2M acetylating reagent (16) was added to a methylene chloride solution of T N F complexes. Triethyl phosphate was used as the solvent since the pyreneT N F complex precipitates in ethyl acetate. Table IX illustrates the usefulness of this acetylating reagent in reducing the interference of phenols and amines. Once again the interference of carbazole cannot be reduced. The acetylation of aryl mercaptans was not investigated, but it seems likely that this would reduce their interference. Dodecyl mercaptan did not appear to interfere with the complexation of pyrene at concentrations up to 0.01M. It seems probable that acetic anhydride could be used in small amounts in triethyl phosphate as sol-
Interference, M None 2,6--Dimethylphenol, 0.004M 2-Naphthol] 0.003M Carbazole, 0 . OOlM 2-Naphthalamine, 0 . OOlM
Initial 0.23 0.25 0 47 0.58 0.72
vent to reduce the interference of amines in the complexation of phenols or aromatic hydrocarbons. Diels-Alder Reaction. Neither of the above reagents reacts a t more t h a n one atom of t h e large molecules investigated and therefore does not appreciably reduce the a basicity of carbazole, 2-naphthylamine] etc. [Acetylation of resorcinol, for instance, apparently reduces its T basicity more than that of p-methoxyphenol ( I C ) . ] A better reagent would be one which reacts at more than one atom of a molecule to reduce its T basicity either by attaching some electron-withdrawing substituent or by attaching a substituent which will offer steric hindrance to complexation. A DielsAlder dienophile would be ideal in that it would do both. Such a dienophile is T C N E which, in contrast to T N F , reacts rapidly in Diels-Alder fashion with such dienes as cyclopentadiene, anthracene, naphthacene (tetracene), etc. (12). This permits the addition of micro amounts of T C X E to mixtures of aromatic hydrocarbons to react selectively with dienes such as anthracene. The resulting adduct is much too weak a a base to complex with a macro amount of T N F . Aromatics which are not dienes, such as pyrene, can then be determined spectrophotometrically, as shown in Table X. Although the interference of anthracene is readily removed, benz(a)anthracene appears to react slowly and incompletely at this concentration with TCNE. It reacts quantitatively
Table XI.
Nitric Acid-Acetic
30 min.
Initial 0.29 0.40 0.82 0.96 1.00
...
0.23 0.26 0.58 0.35
mp
30 min. ...
0.31 0.46 0.96 0.49
Table X. Diels-Alder Reactions Using TCNE for Selective Complexation Room temperature, O.llM TNF, 5 X
lO-'M pyrene, CH2C12 (Purity of TNF varies) 5 ml. total volume 0.0005M diene A520 m p None (blank) 0.31 Anthracene 0.91 Anthracene 5 mg. TCNE: 5 min. 0.335 Anthracene 5 mg. TCNE: 10 min. 0.325 Benz( a)anthracene 0.51 Benz(a)anthracene 5 mg. TCNE:5 min. 0.53 Benz(a)anthracene 5 mg. TCNE: 5 hr. 0.43 Naphthacene 0.48 Xaphthacene 5 mg. TCNE: 5 min. 0.26 Naphthacene 5 mg. TCNE: 10 min. 0.265
+ +
+ +
+ +
at semimicro levels (1%'). Naphthacene appears to react quantatively with T C N E : the absorbance of the final solution is somewhat lower than that of TNF-pyrene alone. Some variation in the purity of the T N F may be partly responsible. Nitric Acid-Acetic Anhydride. T h e 2.3M nitric acid-acetic anhydride (no solvent) reagent of Malins, Wekell, and Houle (7) was investigated because nitration will introduce more than one electron-withdrawing substituent on a ring and tend to reduce T basicity more than perchloric acid or acetic anhydride. Nitric acid can also reduce T basicity by oxidation, but this frequently produces colored products. This reagent
Anhydride Reagent for Selective Complexation
Room temperature, 0.2 ml. reagentl5 ml CH2C1,, 0.0006M pyrene, O.llM TNF Aslo
rnp
After Interference, M reagent Initial reaction (time) None 0.24 &Naphthol, 0.003M 1:6M 0.47 0 . 4 8 (lb'min.) Carbazole, 0.001M 1.6M 0.58 0.45 (40 rnin.) Dibenzothio hene, 0.001M 1.6M 0.31 0.28 (40 rnin.) Phenyl sulfiie, 0.02M 2.3M 0.43 0.20 ( 0 . 5 min.)a 2-Naphthylamine, 0.001M 0.2M 0.72 0.38 ( 5 rnin.) To reduce oxidation of pyrene, 0.05 ml. of triethyl phosphate was added immediately after the nitric acid reagent. Not less than 30 seconds later, 0.1 ml. of methanol was added. "0s
VOL. 37, NO. 3, MARCH 1965
b
377
presumably nitrates by acid-catalyzed formation of the nitronium ion: H + KO-NO2 A c ~ O+ Noif 2HOAc (5) To control the nitration to avoid production of yellow nitrated pyrene compounds (IC), triethyl phosphate solvent was used to prepare 1.6M and 0.2M nitric acid reagents. Turbak (19) has found that 1 : l complexes of triethyl phosphate and sulfur trioxide will not attack aromatic rings but will react with other functional groups. The 1.6M reagent (1.5:1 triethyl phosphatenitric acid) did attack carbazole but not pyrene as shown in Table XI. It would appear as though the phosphoryl oxygen of triethyl phosphate controls the reactivity of nitric acid because reagents containing 0.5:1 or 1: 1 triethyl phosphate-nitric acid did react rapidly with pyrene. The 0.2144 reagent was mild enough to reduce the interference of 2naphthylamine, as shown in Table XI, without producing the excessive colored oxidation products encountered with the more concentrated reagents. To eliminate slow side reactions of nitric acid, excess methanol was added to react with the nitric acid and thus quench the reaction ( 7 ) :
+
+
+
NO2+
+ MeOH
+
MeONOz
+ Hf
(6)
This was critical in the oxidation of phenyl sulfide with the 2.3-44 reagent to avoid appreciable nitration of pyrene. ACKNOWLEDGMENT
The authors thank Norman Radke for investigating the fluorescence of the TNF-pyrene .complex, Jean Jessup for some experimental measurements, hlilagros Santiago for preliminary work on the nitric acid-acetic anhydride reagent, and Howard Simmons of E. I. du Pont de Nemours & Co. for furnishing a sample of tetracyanothiophene. LITERATURE CITED
(1) Briegleb, G., “Elektronen-Donator-
Acceptor Kornplexe,” p. 176, SpringerVerlag, Berlin, 1961. (2) Cundiff, R. H., Markunas, P. C., ANAL.CHEM.35, 1323 (1963). (3) Drago, R. S., Rose, N. J., J . Am. Chem. Soc. 81, 6141 (1959). (4) Gordon, H. T., Huraux, M. J., ANAL. CHEM.31, 302 (1959). (5) Klemrn, L. H., Sprague, J. W., J . Org. Chem. 19, 1464 (1954). (6) Lepley, A. R., J . Am. Chem. SOC.84, 3577 (1962).
(7) Malins, D. C., Wekell, J. C., Houle, C. R.,ANAL.CHEM.36, 658 (1964). (8) Melby, L. R., Harder, R. J., Hertler, W. R., Mahler, W., Benson, R. E., Mochel, W. E., J . Am. Chem. SOC.84, 3374 (1962). (9) Merrifield, R. E., Phillips, W. D., Ibid., 80, 2778 (1958). (10) Orchin, At., Reggel, L., Woolfolk, E. 0.. Ibid.. 69. 1225 (1947). (11) Orchin, 61., Woolfolk, E.‘ O., Ibzd., 68, 1727 (1946). (12) Ozolins, M., Schenk, G. H., ANAL. CHEM.33, 1035 (1961). (13) Ray, K. E., Francis, W. C., J . Org. Chem. 8. 52 (19431. (14) Sawicki, E., Stanley, T. W., ChemistAnalyst 49, 77 (1960). (15) Schenk, G. H., Fritz, J. S., ANAL. CHEM.32, 987 (1960). (16) Schenk, G. H., Santiago, M., Wines, P., Zbid., 35, 167 (1963). (17) Siggia, S., Stahl, C. R., Ibid., 35, 1740 i19631. (18) Tamres,’M., J . Phus. Chem. 65,654 ( 1961).
(19) Turbak, A. F., Division of Polymer Chemistry, ACS, Preprints 2, S o . 1, 140 (1961); through C.A. 57: 153319. RECEIVEDfor review August 10, 1964. Accepted December 17, 1964. Work supported by Public Health Research Grant GM-07760 from the National Institutes of Health, Public Health Service. Presented in part at the 15th Pittsbureh Conference on Analvtical Chemistry and Applied Spectroscopy, March 1964.
Isolation and Identification of Alcohols in Cold-Pressed Valencia Orange Oil by Liquid-Liquid Extraction and Gas Chromatography G.
L.
K. HUNTER and M. G. MOSHONAS
Fruit and Vegefable Products laboratory, Winter Haven, Flu.
b A procedure for the qualitative analysis of alcohols in cold-pressed Valencia orange oil is presented. The method utilizes the partitioning of the oil between carbon tetrachloride and propylene glycol. Water is added to the propylene glycol layer and the essential oil alcohols are extracted with ether. The alcohols are further purified by column chromatography, separated by gas chromatography, and identified by infrared and mass spectrometry. Nineteen alcohols were isolated from Valencia cold-pressed orange oil and identified as n-octanol; n-decanol; linalool; citronellol; a-terpineol; nnonanol; trans-carveol; geraniol; nerol; heptanol; undecanol; dodecanol; elemol; cis- and frans-2,8-pmenthadiene-1-01; cis-carveol; 1 -pmethene-9-01; 1,8-p-menthadiene-9-01; and 8-p-methene-1,2-diol. The first nine alcohols have been reported to be in orange oil or essence by others. 378
ANALYTICAL CHEMISTRY
The remaining 10 have not been reported to be constituents of orange oil, and the last four alcohols of these 10 have not been reported in natural products.
V
INVESTIQATORS attempted to determine the alcohols present in citrus essential oils or essence directly from mixtures either by column chromatography (7, 8), by column chromatography followed by gas chromatography on total terpenoids (2), or by subtractive analyses of the whole oil in conjunction with gas chromatography (15). Recently, Attaway et al. (1) described a procedure in which alcohols were isolated from orange essence oil through removal of the carbonyls by Girard-T reagent ( I $ ) followed by solvent gradient elution through columns of either activated alumina or silicic acid. Frilette, Mower, and Ruben (4) have subsequently shown that the conARIOUS
ditions under which the Girard-T procedure is performed are sufficient to promote dehydration of some tertiary alcohols. The present paper describes a procedure for isolation of the alcohols from cold-pressed orange essential oil using the nonaqueous extraction procedure of Suffis and Dean (12) with some modifications. Yonalcoholic materials which resulted from incomplete partitioning were removed from the alcohol fraction by column chromatography. The alcohols were subsequently separated by gas chromatography and identified spectrometrically. The partitioning of citrus oils between carbon tetrachloride and propylene glycol is particularly effective because the major portion of the oil is nonalcoholic and expeditiously removed by the carbon tetrachloride. The alcohols are isolated from the oil by preferential solubility in propylene glycol, eliminating the danger of artifact formation.
