Environ. Sci. Technol. 1988, 22, 83-92
Compound Properties Relevant for Assessing the Environmental Partitioning of Nitrophenols Reni P. Schwarrenbach," Ruth Stierli, Brian R. Folsom, and Josef Zeyer Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-6047 Kastanienbaum, Switzerland
Acidity constants, UV/vis absorption maxima, aqueous solubilities, octanol/water partition constants and ratios, vapor pressures, and estimated Henry's law constants are reported and discussed for 17 mono- and dinitrophenols. For substituted 2-nitrophenols in which intramolecular hydrogen bonding between hydroxyl and nitro group is not affected by proximity effects, the acidity constant K, can be estimated from the Hammett relationship pK, = 7.23 - 2.59Caf). For these nitrophenols, the aqueous activity coefficient yw of the nondissociated species is approximately independent of solute concentration, and a good correlation between octanol/water partition constant KO, and aqueous solubility of the liquid compound Cwsat(L)is - 0.04 (R = 0.98). In found: log KO,= -0.97 log CWSat(L) cases in which intramolecular hydrogen bonding is disturbed or impossible (e.g., in 4-nitrophenol), due to solute-solute interactions, Cwsat(L)does not yield correct information on ywa t low concentrations. The electronic effect of substituents on the octanol/water partitioning behavior of the dissociated 2-nitrophenols can be reasonably quantified by using Hammett substituent constants. Nitrophenols (particularly dinitrophenols) exhibit small Henry's law constants and consequently have large water/air ratios even at low pH values. In recent years, physical-chemical properties including vapor pressure, aqueous solubility, Henry's law constant, and organic solvent/water partition constants have been determined for numerous neutral, hydrophobic organic compounds. On the basis of these data, important insights into the molecular factors governing the environmental partitioning behavior of nonpolar organic compounds have been gained, and mathematical models for quantifying phase-transfer processes have been derived (e.g., 1,2). For compounds with polar groups (especially compounds with acid or base functions), very little information is available, however, although there is a great number of such chemicals that are of environmental concern (e.g., substituted phenols and anilines). In order to determine to what extent the concepts used to describe the environmental behavior of nonpolar compounds can also be applied to polar compounds, physical-chemical property data of such compounds are essential for assessing the significance of specific interactions of polar structural moieties in aqueous and nonaqueous environments. Furthermore, exact knowledge of the acidity constant of a given compound is very important, since ionized species behave very differently than their neutral counterparts. In these cases, the phase-transfer behavior and the reactivity of both the neutral and the ionized compounds have to be evaluated. Previously, we reported on the organic phaselwater partitioning behavior of hydrophobic ionizable organic compounds (Le., chlorinated phenols, ref 3 and 4). Our present work focuses on the environmental behavior and fate of a group of more hydrophilic weak acids that are of considerable environmental concern, the nitrophenols. Nitrophenols are used as intermediates in the synthesis of pesticides and dyes or are directly applied as herbicides 0013-936X/88/0922-0083$01.50/0
and insecticides (5, 6). Recently, a variety of alkyl-nitrophenols were detected in rainwater samples collected at an urban site in Switzerland. It was hypothesized that some of these compounds could have been produced by tropospheric transformation of alkylbenzenes and alkylphenols (7). Nitrophenols act as uncoupling agents in oxidative phosphorylation, and they are known to affect cell metabolism at concentrations lower than 10 pM (8,9). In this paper, we present and discuss physical-chemical properties of 17 nitrophenols. Most of the selected model compounds exhibit a nitro group in ortho position to the hydroxyl function. For all model compounds, we report acidity constants, UV/vis absorption maxima, aqueous solubilities, reversed-phase high-performance liquid chromatography (HPLC) retention times, octanol/water partition constants of the nondissociated species, and octanol/water partition ratios of the dissociated (ionic) species. Furthermore, vapor pressures derived from gas chromatographic retention data and estimates of the Henry's law constants are given for 14 of the model compounds.
Experimental Section Chemicals, The nitrophenols (the names and abbreviations are given in Table I) were purchased from the following companies: Fluka AG, Buchs, Switzerland (H, 4-C1, 4-NO2, 4-N02-6-Me,4NP, and 3-Me-4NP); Aldrich Chemical Co., Steinheim, Germany (&sBu, 4-Ph, 4-OMe, 4-C1-5-Me, 5-F, and 4-CF3); Ega Chemie, Steinheim, Germany (3-Me, 4-Me, 4-CHO, and &No2);and MerckSchuchardt, Darmstadt, Germany (&Me). All chemicals had the highest purity available (198%) and were used as received. [14C]-2-Nitrophenol(sp act. 21.3 mCi/mmol) and [14C]-2,4-dinitrophenol(sp act. 10.24 mCi/mmol) were obtained from Pathfinder Laboratories Inc., St. Louis, MO. The radiochemical purity of both labeled compounds was 98% according to the supplier. UV/Visible Spectra. The UV/vis spectra of the compounds were recorded in an aqueous sodium phosphate buffer solution (I = 0.05 M) at pH 1.5 (neutral species) and pH 12 (anionic species) on an Uvikon Model 810 spectrophotometer, Kontron, Zurich, Switzerland. The concentrations of the compounds were between 5 X 10" and 5 x 10-4 M. Acidity Constants. Acidity constants K , were determined at room temperature (21.5 f 1.5 "C) by measuring the concentration ratios of deprotonated to neutral nitrophenol [A-l/ [HA] by UV/vis spectrophotometry at eight different pH values in the pH range of pK, f 1. Sodium phosphate (pH 6-8) and sodium citrate (pH 3-6) buffers (ionic strength = 0.05 M) were used. Total niand 5 trophenol concentrations ranged between 5 X X M. Proton activity was determined by a standard glass electrode calibrated with buffer solutions (pH 4 and pH 8) obtained from Merck, Darmstadt, Germany. A t each pH, UV/vis absorbances were measured a t 8-10 wavelengths between 260 and 460 nm, and a nonlinear least-squares calibration and data reduction technique was used to allow determination of concentrations of both
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 1, 1988
83
Table I. Acidity Constants and UV/Visible Absorption Maxima (A,
compound
abbreviation
PK,b
2-nitrophenol
H
3-methyl-2-nitrophenol
3-Me
7.23 (7.22)f 7.00
4-methyl-2-nitrophenol
4-Me
7.63
5-methyl-2-nitrophenol
5-Me
7.34
4-sec-butyl-2-nitrophenol
4-sBu
7.59
4-phenyl-2-nitrophenol
4-Ph
6.69
4-methoxy-2-nitrophenol
4-OMe
7.40
4-chloro-2-nitrophenol
4-C1
6.44
4-chloro-5-methyl-2-nitrophenol
4-Cl-5-Me
6.84
5-fluoro-2-nitrophenol
5-F
6.30
4-trifluoromethyl-2-nitrophenol
4-CF3
5.66
4-formyl-2-nitrophenol
4-CHO
4.61
2,4-dinitrophenol
4-NO2
2,5-dinitrophenol
5-NO2
6-methyl-2,4-dinitrophenol
4-N02-6-Me
4-nitrophenol
4NP
3-methyl-4-nitrophenol
3-Me-4NP
3.94 (3.95, 4.09)fg 5.18 (5.22)b 4.31 (4.46, 4.35)fg 7.08 (7.15, 7.18)fg 7.33
emax) of the Model Compounds"
absorption maxima in water neutral sDecies (HAY anionic suecies (A? ,e, M-' cm-' emax, M-' cm-' hllaxe, Pm Amaxe, Fm 278 352 269 340(~)~ 282 367 294 348(s) 282 365 274 379 281 393 272 362 286 364 280 337 266 336 259 340 261 290(s) 273 361 270 305(s) 226 316 231 314
6250 3000 2200 1150 6400 2900 7900 4200 5050 2300 16600 3100 5650 3000 5850 2950 7400 3650 6000 3950 5100 2700 21500 2850 11650 8600 10700 3600 13800
282 416 275 408 286 433 294 416 284 432 295 438 283(s) 454 275(s) 426 289 425 283 40 1 272(s) 398 311 394 257 359 283 439 264 371 260(s) 399 256(s) 396
6600 9700 5600 7600
4000 4550 2800 1200 4250 4550 5500 5400 4350 4450 24200 6000 4300 5000 3800 4650 5000 5050 4100 5450 3100 4400 22000 5100 6700 14100 7500 4300 6400 14400 3000 18500 3800 16500
"All values were determined at room temperature (21.5 f 1.5 " C ) and at an ionic strength of 0.05 M. bValues in parenthesis are from the literature. C ~ 1.5. H d u H 12. e(sl = shoulder. fValues from Cowell and Anderson (In.gValues from Callhan et al. (18).
neutral and anionic species [which exhibit absorption maxima a t different wavelengths (Table I)]. The acidity constant was then determined with a linear regression by
The aqueous solubility of the subcooled liquid compound C,S"YL) was calculated by approximating the free energy of fusion 4Gm by 4G, = -(4Sm/R)(Tm/T- 11, where T , is the melting point (12) and by assuming AS,, the entropy of fusion, to be 13.5 cal mol-l K-l (13):
Note that the K, values reported here are mixed acidity constants at an ionic strength of 0.05 M. The pK, value of 2-nitrophenol was determined at 5,10, 15,20,25, and 30 "C. The proton activities of the phosphate buffer solutions were measured with a glass electrode at 20 "C. For all other temperatures, the pH was corrected according to literature values on the temperature dependence of the proton activity of the phosphate buffers (10, 11). The corrections applied were (in pH units): 5 "C (+0.07), 10 "C (+0.04), 15 "C (+0.02), 25 "C (-0.015), and 30 "C (-0.03). Aqueous Solubilities. The solubilities of the nitrophenols Cwsat in an aqueous buffer solution of pH 1.5 (HC1/NaH2P04,ionic strength = 0.05 M) were determined by equilibrating an excess of the pure compound with the aqueous phase for at least 96 h a t 20 f 0.5 "C. During equilibration the mixture was stirred vigorously. The two phases were then separated by centrifugation and subsequent filtration of the supernatant through a 0.45-pm membrane filter (Sartorius, Gottingen, Germany). The saturation concentration of the compound in the aqueous phase was determined by UV/vis spectrophotometry. All measurements were carried out in at least four replicates. The standard deviations were in most cases smaller than 5% and never greater than 10%.
log Cwsat(L)= log Cwsat 2.95(Tm/T - 1)
84
Environ. Sci. Technol., Vol. 22, No. 1, 1988
+
(2)
I t should be noted that the melting points of the nitrophenols cannot be sharply measured and that the values are, therefore, given as temperature ranges over 2-3 deg. For the calculations, the mean temperature of this range was used. The effect of temperature and of salinity on aqueous solubility was evaluated for H, 4-N02,and 4-Cl-5-Me. The effect of salt on Cwsatwas studied in 0.05, 0.2, 0.5, and 1 M NaCl solutions of pH 1.5 (HC1). The solubilities of the compounds were determined at 20 f 0.5 "C as described above. Octanol/Water Partitioning. The partition constants KO,of the neutral nitrophenols were measured between 1-octanol and an aqueous buffer solution of pH 1.5 (HC1/NaH2P04,ionic strength = 0.05 M). Before use, the octanol and the buffer solution were saturated with each other. The nitrophenol was added to the aqueous phase, and the two phases were equilibrated at room temperature (21.5 f 1.5 "C) by vigorous mechanical mixing for 24 h. Note that KO,values have only a very weak temperature dependence (14). After the two phases had separated (typically after 48 h), the concentration of the nitrophenol was determined by UV/vis spectrophotometry in both the
octanol and the aqueous phase. Depending on the compound, the concentrations of the nitrophenols were between and lo-* M in the aqueous phase and between M in the octanol phase. Mass balance and 5 X between compound added and compound found agreed to within 2% in most cases and was no worse than 5% in any case. The octanol/water volume ratios were chosen such that the amount of the nitrophenol in either phase was generally at least 20% and, only in a few cases, 10% of the total amount added. Measurements were carried out in a t least two replicates. The standard deviations were in most cases smaller than 5% and never greater than 10%. The effect of concentration on KO,was evaluated for H, 3-Me, 4-NO2, 4NP, and 3-Me-4NP. 14C-LabeledH and 4-N02were used to determine the KO,of these two comM). pounds at very low aqueous concentrations (lo4 For these experiments, the aqueous phase (500 mL) containing the "cold" compound at the desired concentration was spiked with 10 pL of a -1.6 mM [14C]-2-nitrophenol or 2.5 mM [14C]-2,4-dinitrophenol solution in ethanol corresponding to a total activity of 750000 dpm for H and 560000 dpm for 4-NO2. The aqueous phase was then equilibrated with 5 mL of 1-octanol. The amount of radioactivity in each phase was determined by mixing either 100 pL of the octanol phase or 1mL of the aqueous phase, respectively, with 10 mL of Kontrogel (Kontron, Zurich, Switzerland) and counting the samples in a liquid scintillation counter. The distribution ratios of the deprotonated nitrophenols, Do,"-, between octanol and an aqueous buffer solution of pH 12 (NaOH/Na2HP04,ionic strength = 0.05 M) were determined with the same procedure as used for the neutral phenols. For five compounds (H, 4-sBu, 4-C1, 4-CF3, and 4-NO2),the effect of ionic strength on the Do,*- value was investigated in a 0.01 M aqueous NaOH solution containing 0.01, 0.05, 0.10, and 0.20 M NaC1. Solubilities in Water-Saturated Octanol. The solubilities in octanol (preequilibrated with the aqueous buffer solution of pH 1.5) were determined for H, 4NP, and 3-Me-4NP with the same procedure as for the aqueous solubilities, except that the solutions were not filtered. HPLC and GC Retention Times. HPLC retention times of all compounds were measured on a C12 reversed-phase column (stainless steel, 120 X 4.6 mm, RP12, 5 pM spheres; Permacoat, Bern, Switzerland) connected to a Perkin-Elmer pumping system (Series 4 liquid chromatograph, Perkin-Elmer AG, Ueberlingen, Germany) supplemented with a Rheodyne 7125 injector and a Kratos Spectroflow 773 variable-wavelength UV/vis detector (both from Kratos AG, Riehen, Switzerland). The mobile phase was methanollwater, 312 v/v, acidified with concentrated phosphoric acid to an apparent pH of -2. The flow rate was 1.3 mL min-l. The GC retention times were determined a t 55,60,65, 70,75, and 80 "C for the mononitrophenols and at 65,70, 75, and 80 "C for the dinitrophenols. The measurements were carried out on a 21 m X 0.32 mm i.d. 2%0PS-089 (similar to SE54) glass capillary column by using direct on-column injection and flame ionization detection. The carrier gas was H2 (0.32 atm). The gas chromatograph was a Carlo Erba Model 5160 Mega Series (Brechbuhler AG, Schlieren, Swhzerland). No decent chromatographic peaks could be obtained for 4-Ph, 4-CHO, and 3-Me-4NP. Vapor Pressure Data from Retention Times. As demonstrated by Hamilton (15) and Bidleman (16),one can obtain a fairly good estimate of the vapor pressure of a compound from its gas chromatographic retention behavior, provided that vapor pressure data of a suitable
2.0
1
= I I
I
I
I
- 6.0
-5.5
In
PP
-5.0 (atm)
Flgure 1. Plot of the natural logarithms of the relative GC retention times of some substituted mononitrophenols versus the natural logarithm of the vapor pressure of the reference compound 2-nitrophenol at 55, 60,65, 70, 75,and 80 OC.
reference compound is available for the temperature range of interest. Of critical importance is that the reference compound and the compound of interest exhibit very similar interactions with the stationary phase. The vapor pressures Po of two compounds at the same temperature are related through In Pi"= [(Mv)i/(AHv)r] In P," C (3)
+
where i and r refer to test and reference compound, respectively, and AHv is the heat of vaporization. The parameters (AHv)i/(AHv)rand C can be determined from GC retention data obtained at various temperatures by a linear regression with the relationship In [(tR)i/(t~)rl= [(I - ( a v ) i / ( A f f v ) r l
In
Pro
-C
(4)
where t R is the retention time. Figure 1shows the natural logarithms of the relative retention times at six different temperatures of some substituted mononitrophenols plotted against In P," of the reference compound (2nitrophenol) a t the corresponding temperatures. Expressed in decadic logarithms, the vapor pressure-temperature relationship of 2-nitrophenol in the temperature range of interest is (11) log Pro = -2776/T 5.735 (5)
+
where Prois expressed in atm. From the slope and the intercept of the regression analysis of eq 4, the vapor pressure of the test compound at a given temperature can be calculated by log Pi" = -A/T B (6)
+
where A = 2776 (1 - slope) and B = I5.735 (1 - slope) intercept/2.303]. It should be noted that, at temperatures below the melting point, eq 6 yields the vapor pressure PO (L) of the subcooled liquid compound and that, over the temperature range considered, it is assumed that AHv (of the liquid) is constant. the vapor pressure of the (solid) compound can be approximated in analogy to eq 2 by log Po = log Po(L) - 2.95(Tm/T- 1) (7)
Results and Discussion UV/Visible Absorption Spectra. Both neutral and anionic forms of all nitrophenols investigated show a strong Environ. Sci. Technoi., Vol. 22, No. 1, 1966
85
Table 11. Estimated and Experimental pK, Values of Some Substituted 2-Nitrophenols
PK,
I
compd
experimental (ref)
5.47 4.59
5.40 (17) 4.62 (22)
2.23 0.78
2.10 (17) 0.80 (11)
6-chloro-2-nitrophenol 6-sec-butyl-2,4-dinitrophenol (Dinoseb) 6-chloro-2,4-dinitrophenol 2,4,6-trinitrophenol
5-NO2
4-CF3
1
/
estimated from eq 8 O ( p = 2.59)
a For ortho substitution, apparent Barlin and Perrin (23).
values were taken from
4-Ph@
4-Cl- 5 - M e
3-Me@
0
-1 E , f
(inductive
bilize the nondissociated form by intramolecular hydrogen bonding:
i 1 +
2
resonance 1
Figure 2. Hammett plot for a series of substituted 2-nkrophenols. Note were not included in the linear that 3-Me and 4-Ph (both indicated by 0) regression.
light absorbance above 290 nm (see Table I). These compounds should, therefore, be quite sensitive to direct photolysis. However as shown very recently by Faust et al. (24),anions of some of the 2-nitrophenols have rather low quantum yields. Thus, despite the strong sunlight absorbance of the 2-nitrophenols, their rates of direct sunlight photolysis may be slower than rates of other degradative processes such as biological transformation (25). Acidity Constants. Table I shows that the pKa values of the nitrophenols investigated in this study span a range of about four pKa units. The few reported pKa values for nitrophenols are in good agreement with those determined in this study, particularly, when considering that most of these values represent mixed acidity constant that were generally measured at a somewhat different ionic strength and/or temperature. The pKa values of 2-nitrophenol at various temperatures are 7.40 (5 "C), 7.35 (10 "C), 7.30 (15 "C), 7.24 (20 "C), 7.20 (25 "C), and 7.15 (30 "C). Thus, over the temperature range considered, the pKa of 2-nitrophenol decreased by about 0.01 units for each degree increase in temperature, which is slightly less than the effect observed for phenol (0.012 pKa units per degree, ref 19). For the more acidic nitrophenols, an even weaker temperature dependence can be assumed (19). The electronic effects of various substituents on the pKa of the 2-nitrophenol acid function can be quantitatively evaluated by using the Hammett relationship (e.g., 20): pKa = PKa, - P E ~ { - ) i
(8)
where pK& is the pKa of 2-nitrophenol, c$) are electronic substituent constants, which are readily available in the literature (20,21), and p is the susceptibility factor. The p value is a measure of how sensitive the dissociation reaction is to substitution as compared with benzoic acid. Figure 2 shows that, except for 4-Ph and 3-Me, a very good correlation ( R = 0.99) between ApK, and c a f ) was found. The linear regression analysis yielded a p value of 2.59 (zkO.Ol), which is significantly higher than the 2.25 commonly derived for meta- and para-substituted phenols (20). This difference might be due to the fact that a nitro substituent in ortho position to the phenolic group may sta86
Environ. Sci. Technol., Vol. 22, No. 1, 1988
An electron-withdrawing substituent on the ring will now weaken this hydrogen bond, which means that, in addition to its stabilizing effect on the anionic form, the substituent will destabilize the nondissociated form (the opposite effect will be observed for an electron-donating substituent). Thus, the effect of a substituent on the pKa of the phenolic group will be enhanced by the presence of a nitro group in ortho position. While we have no obvious explanation for the deviation of 4-Ph from eq 8 (Figure 2), the lower than expected pKa of 3-Me can be rationalized by proximity effects that are not included in the Hammett relationship. In this case, coplanarity (Le., lying in the same plane) of the atoms of the nitro group with the atoms of the aromatic ring could be somewhat hindered due to steric interaction of the nitro group with the methyl group. Some indications of such an interaction are given by the UV/vis spectra (Table I). For both the neutral and the anionic species, the absorption maxima of 3-Me are at shorter wavelengths than those of all other alkyl-substituted 2-nitrophenols, and the molar extinction coefficients are significantly smaller. The consequences of some loss in coplanarity would be a weakening of the above-mentioned hydrogen bond, and probably, a more optimal solvation of the hydroxyl functionality. Since the latter factor should be more important for the dissociated species, both effects should decrease the pKa. However, this decrease would be partially compensated by the smaller resonance effect of the nitro group. A quantification of these and other possible proximity effects is, of course, very difficult. Nevertheless, as shown by the examples given in Table I1 for compounds not investigated in this study, if proximity effects are not too important, eq 8 with p = 2.59 should yield a good estimate of the pKa value of a given substituted 2-nitrophenol. Aqueous Solubilities, Aqueous Activity Coefficients, and Octanol/Water Partition Constants of Nondissociated Species. Experimentally determined aqueous solubilities, calculated (eq 2) aqueous solubilities of the (subcooled) liquid compounds, HPLC retention times, and octanol/water partition constants of the nondissociated nitrophenols are summarized in Table 111. When choosing the pure organic liquid as the reference state, the activity coefficient of a compound in water at saturation ywsatis related to its (subcooled) liquid aqueous solubility Cwsat(L)by (26) ywsat
= 1/C,S"t(L)
v,
(9)
Table 111. Water Solubilities, HPLC Retention Times, a n d Octanol/Water Partition Constants of t h e Neutral Species (HA); Octanol/Water Distribution Ratios of t h e Ionic Species (A-) neutral species (HA)
compd
T,, "C
H 3-Me 4-Me 5-Me 4-sBu 4-Ph 4-OMe 4-C1 4-Cl-5-Me 5-F 4-CF3 4-CHO 4-NO2 5-NO2 4-N02-6-Me 4NP 3-Me-4NP
45-46 35-39 32-35 53-56
