Environ. Sci. Techno/. 1995, 29, 1933-1943
Redox Interactions of Cr(VI) and Substituted Phenols: and Mechanism M I C H A E L S. ELOVITZ' A N D WILLIAM FISH* Department of Environmental Science and Engineering, Oregon Graduate Institute of Science & Technology, 20000 N W Walker Road, P.O. Box 91000, Portland, Oregon, 97291 -1000
The mechanisms of aqueous oxidation-reduction interactions between Cr(VI) and substituted phenols (RArOH) were characterized by kinetic analysis and determinations of reaction products and intermediates. A rapid, preoxidative equilibrium between HCr04and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent ratelimiting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H'I is well accounted for by three parallel redox pathways involving zero, one, and two protons. The two-proton pathway dominates at pH 5 2, the singleproton pathway for 2 < pH < 5, and the protonindependent pathway at pH ? 5. The parallel reaction rate expression was fitted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimethoxyphenol for pH 1-6. Beside accurately predicting rates for the calibrated conditions, the model predicts a sharp decline in rates at pH ? 6. Rates subsequently measured at pH 7 agreed well with those calculated a priori. Such predictions suggest that the proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both oneand two-el ectron pathways.
Introduction Millions of tons of hexavalent Cr contaminate the environment at dozens of sites in the United States (1). Risk assessment for this contamination must account for the ability of Cr to transform between two environmentally stable oxidation states, Cr(VI) and Cr(III1, which exhibit very different toxicities and mobilities in the environment, Many inorganic and organic compounds can reduce Cr (VI) to Cr(II1). In particular, hydroxylated organic compounds such as phenols are highly reactive with CrW) in aqueous systems. Because phenols are constituents of natural * Corresponding author: telephone: 503-690- 1099; e-mail address: fishbese.ogi.edu. +Presentaddress: Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Duebendorf, Switzerland.
0013-936X/95/0929-1933$09.00/0
1995 American Chemccal Society
organic matter as well as a common component ofindustrial waste streams, their interaction with Cr (VI) is an important environmental redox reaction, the dynamics of which are not well understood. Previously, we reported on kinetic investigations of the redox interactions between Cr (VI) and substituted phenols (2).We found that within the concentration and pH range examined, Cr(VI) was reduced on time scales ranging from minutes to months. The reaction was first-order with respect to concentrations of both HCr04-, and the phenol reductant and the rate increased with decreasing solution pH. We developed an empirical rate expression, -{d[Cr(VI11 ldt = kArOfl[HCrO4-I[ArOHI,where kArOf1 is a function of the hydrogen ion concentration. This expression is useful for estimating the characteristic time scales for CrW) reduction. Its simplicity allows it to be readily incorporated into contaminant transport models, enabling redox transformations to be coupled with transport processes. Although an empirical rate law is practical for many applications, its validity is restricted to the conditions from which it was derived. Empirical laws may be invalid for other concentration ranges or more complex systems of waste mixtures. Knowledge of the redox mechanisms and reaction pathways governing the system may help us extrapolate to these situations. Investigations of Cr (VI) reduction by various organic compounds have revealed many mechanistic details of Cr redox chemistry. Unfortunately, the mechanisms of Cr (VI) reduction by phenol have not previouslybeen studied. However,it is reasonable to assume that certain features of Cr(VI) reduction by compounds such as aliphatic alcohols and thiols also apply to phenols. The accepted mechanism for the redox interaction between C r M ) and alcohols involves the rapid initial formation of a Cr(VI)ROH ester. The chromate ester decomposes with a concomitant transfer of electrons to the Cr(VI) center. Cr(VI) is an atypical oxidant because reduction to Cr(II1)involves the transfer of three electrons, which, with few exceptions, occurs in several one-electron or two-electron transfer steps. Cr, therefore, must pass through the intermediate unstable oxidation states C r O or Cr(W). The relative importance of one-electron and twoelectron transfer pathways, and accordingly the intermediate Cr(IV) and C r O species, should be governed by the properties of the reducing agent and the relative concentrations of reactants. The organic products of intermediate steps can be either unstable radicals or stable compounds. Although reactions between alcohols and Cr(VI) are analogous in some respects to reactions involving phenols, the latter reactions are apt to be strongly influenced by the high electron density of the aromatic ring. We studied Cr(VI) reduction rates for 14 mono-, di-, and trisubstituted phenols to discover the effect of phenol structure on the kinetics. Possible reaction intermediates were directly and indirectly determined. Finally, we analyzed oxidation reaction products, which proved useful for two reasons. First, products frequently suggest the mechanisms that produce them; conversely, a proposed mechanism must be consistent with observed products. Secondly, product identification reveals the new organic compounds expected to appear in waste mixtures of CrW) and phenols.
VOL. 29, NO. 8, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1933
Experimental Section Kinetic Procedures. Materials and methods for all kinetic investigations were identical to those described previously (2)unless otherwise noted. Linear regression analysis was performed with Abacus Concepts, StatView (Abacus Concepts, Inc., Berkeley, CA),and nonlinear curve fitting was performed with DeltaGraph Professional (Deltapoint, Inc., Monterey, CAI. Selected experiments were performed with additions of 1.0 M stock solutions of I- (NaI,Mallinckrodt) or Mn(I1) (MnS04,Mallinckrodt) to act as scavengers of C r O and Cr(IV),respectively. Initial concentrations of Iand Mn(I1) were in 20-fold excess of initial [Cr(VI)I. Reactions were compared to control experiments consisting of Cr(VI) and phenol solutions without added I- or Mn(I1). Product Analysis. Reaction products of Cr(VI1oxidation of phenols were identified by GCIMS. Reactions conducted with an excess of phenol at pH 2 and 25 "C were extracted and derivatized in two ways. The reaction solution was acidified to pH 1 and extracted three times with ethyl acetate. Combined extracts were dried over anhydrous Na2S04, filtered, evaporated to dryness by gentle NZ blowdown, and dissolved in methylene chloride. Hydroxylated compounds were acyl derivatizedwith 2:l acetic anhydrideIanhydr0u.s pyridine. Alternatively, the reaction mixture was brought to pH 3 with NaOH and the quinone products were reduced to hydroquinone with sodium dithionite. Products were extracted and acetylated as described above. GC/mass spectrometry was done on a Hewlett-Packard 5790 GC operating in the splitless mode with a 30 m x 0.32 mm i.d. DB-5 fused silica column (0.25pm film thickness). The GC was interfaced with a VG 7070E HF mass spectrometer operated in the electron impact mode at 70 eV. Significant total-ion chromatogram peaks were identified by mass spectral interpretation and comparison of the mass ion distributionswith data system library matches and those reported in compendia of mass spectra. Chromate esters were investigated by UVIvisible spectrophotometry (Perkin-Elmer Lambda-6) in the 300-500 nm region in which these esters show characteristic absorption bands. An excess of phenol maximized ester formation while minimizing successive oxidation steps. Reactions were performed at pH 2 and pH 5 to ensure that ester formation was not rate limiting;at higher pH the overall reduction of CrW) is much slower than at pH 2. The instrument was zeroed to an arrangement of the reference and sample beams passing through matched cuvettes, both containing Cr(VI)inHz0 buffer. The solution in the sample cell was replacedwith a reaction solution containing CrW) and phenol in the identical buffer. Differences in the internal spectral subtraction are due to the presence of the phenol (,5I300 nm) or the formation of a Cr(VI1-phenol complex (2 = 300-500 nm). X-band EPR spectra of aqueous reaction solutions of C r w ) and 4-methylphenol were measured at room temperature on a Varian E-I09 EPR spectrometer using a flat quartz cell.
Results Chromate-Phenol Ester Formation. UV/visible spectra of mixtures of Cr(VI) and $-methylphenol (4MP) or 4-chiorophenol(4Cl) exhibited absorption bands in the 350-450 nm wavelength range (Figures 1 and 21, consistent with those ascribed to Cr(VI) oxy esters in the literature (3-9). 1934
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 6 , 1 9 9 5
0.10 M 4MP
W
-101
-20 \ , , , , 300 350
, / , , 1 , , 1 , 1 , , , ,
450
400
550
500
600
Wavelength (nm)
FIGURE 1. UVbisible difference spectra of solutions containing 0.5 mM Cr(V1) and 0.1.0.077, and 0.05 M &methylphenol at pH 5.0. Cell path length I = 1.0 cm, 25 "C,I = 0.1 M. Spectra taken after 1 min. Inset: spectrum of 0.5 mM Cr(Vl) at pH 5. 100
-
n
z
,
1,
I
60-
Y
a,
40-
a
9
20
-
0 1
300
350
~
'
"
1
"
400
'
"
'
'
"
450
1
'
"
'
500
1
~
'
'
~
550
600
Wavelength (nm)
FIGURE2. UVhrisible difference spectra of CdVI) and 4-chlorophenol at pH 2 showing the increasing absorbance bend et 397 nm for increasing Concentrations of 4-chlorophenol: 0.05,0.0375,0.025, and 0.02 M. [Cr(Vl)] = 5.0 mM, cell path length / = 0.2 cm, 25 'C, I = 0.1 M. Spectra taken after 1 min. Inset: spectrum of 0.5 mM CrfVI) at pH 2.
At pH 5, the redox reaction with 4MP proceeds very slowly with negligible reduction of Cr(VI) within the first hour. Thus, at this pH, the ester formation step can be clearly distinguished from the electron transfer reaction. Spectra obtained within 1 min after mixing revealed a distinct absorption band in the region of 380-550 nm and a diminished absorption in the 350-380 nm region (Figure 1). Continued scanning for 30 min showed no further change in the spectra. Cr(VI) measurements confirmed that essentially no Cr(VI)was reduced within this time. The 380-550 nm band is asymmetrical and suggests a combination of increased absorbance at 400 nm by an ester and diminished absorbance around 350 nm due to the loss
*
Are* &OR + 0
OH
OH
+ POLYMERS
0 ’
A
A I: RICH:, II: R=OCH3 111: R = CI
x1
P2
P3
++-$
I R
R
R
I CH20H
x2
P4 OH
P5
OH
+
I
R
P6
OH
P7
FIGURE 3. Reaction products and partial mechanisms for the oxidation of 4-methyb. 4-methoxy-, and 4-chlorophenol by CdVI); adapted from refs 29-32. Compounds designated P’l-P9 were identified by GC/MS.
of free HCr04-. Without Cr(VI) reduction, Cr(VI) and its complexes are the only species that absorb in this spectral region (phenols do not absorb significantlyin this region). It follows that the spectral shifts observed are due to a rapid formation of a Cr(VI1-phenol complex. At pH 2, successive spectrophotometric scans showed development of a strong absorbance band in the region 410-450 nm with a concurrent absorbance increase in the 280-340 nm region. These absorbance bands continued to grow as Cr(VI) was consumed, although more slowly as the reaction approached completion. The noticeable formation of colloidal precipitates, presumably phenolic polymerization products, over the course of the reaction was most likely responsible for the gradually developing absorbance bands. Overall, it was impossible at pH 2 to distinguish between the spectral absorbance of ester intermediates and that of oxidation products. Therefore, under conditions where the redox reaction is relatively fast, rapid formation of oxidation products interferes with observations of the initial ester formation. Because oxidation products interfere with ester identification, we examined the slowly oxidized 4-chlorophenol spectrophotometrically at pH 2 and pH 5. Reduction of Cr(VI) by 4C1 is very slow at pH 2 and negligible at pH 5. Scans performed within minutes after mixing the reactants showed discernible absorbance changes for both pH 2 (Figure2) and pH 5 reactions (not shown). The absorbance bands in the 397 nm region agree with the slightly longer wavelength bands in the 4MP systems.
Varying the excess concentration of 4MP at pH 5 and 4C1 at pH 2 showed that the absorbance increased in proportion to the phenol concentration [ArOHl (Figures 1 and 2). Because 4C1 and 4MP do not absorb in the 320400 nm region, we can interpret the increased absorbance as a result of a shift toward the ester in the esterification equilibrium. Intermediate C r O and Cr(W Species. EPR and competitive reactions with I- were used to explore the role of C r O . The scavenging of C r O by I- has been demonstrated for reductions of CrW) by metal centers (3, 4, 10-12). In moderately acidic solution, CrW) very slowly oxidizes I- to 12 (I3-). In contrast, C r O oxidizes I- orders of magnitude faster (13, 14). Addition of I- to reactions of Cr(VI)and excess 4MP showed indirect evidence of induced oxidation of I- by a C r O intermediate. Direct spectral detection of triiodide (13-1 formation in the 350 nm region was masked by the strong absorbance of HCr04- at the same wavelength. However, reactions with and without Ishowed a marked difference in organic reaction products. In particular, reactions with I- yielded 4-hydroxybenzaldehyde as a major product but only traces of Pummerer’s ketone (P1 in Figure 3) and trimeric coupled product(s) (P3,Figure 3; see the following products section). In contrast, for reactions with no I-, Pummerer’s ketone was the major reaction product, and only trace amounts of the 4-hydroxybenzaldehyde product were detected. Because I- is virtually unreactive with either CrW) or 4MP, its substantial effect on products is most likely due to its VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
193s
IV: R V: R
0
= OCH3
P10
P9
CH3
H
3
c
~
T
c
H
Cr(V1) 0
VI
P12
Cr(V1)
-
hCq I 1
3
on
3
H~O
CH2
OH
+
' CH3
CH3
CH3
CHI
CHI
'0
."'UzdP11
P i3
H3c7@ OH
CHzOH
POLYMERS
CH3
P i5
P14
-
-
H 3 C , ( 7 OH
H&y$CH3
CHO
CWH
VI1 x3 P16 P17 P18 FIGURE 4. Reaction products and partiel mechanisms for the oxidation of various multisubstituted methyl and methoxy phenols by Cr(VI); adapted from refs 29-32. Compounds designated PIO-P19 were identified by GCIMS.
reduction of C r O . Addition of I- could alter the products either by diminishing a CrO-mediated pathway or by creating a new I--mediated oxidation. The apparent scavenging of C r O by I-, however, failed to alter measurably the rate of Cr(VI) reduction by excess 4MP, 4-methoxyphenol,or 2,6-dimethoxyphenol. We conclude that any step involving C r O as a reactant must occur after the ratelimiting step. C r O produces a strongX-band EPR signal that has been used to detect C r O formation during chromate reduction by aliphatic alcohols (13,hydroxy acids (13, 14, 1@,and thiols (17-25). Reactions with 4MP were conducted at pH 2, and the Cr(VI):4MPmolar ratio varied from 2:l to 1:20. All reactions of CrW) and 4MP were EPR silent. For comparison, a reaction of CrW) and glutathione with a molar ratio of 2.5:1 yielded a strong C r O EPR signal. A supplementary experiment utilizing dissolved humic acid as the reducing agent displayed a short-lived but distinct EPR signal with similar spectral bands as the glutathione system. Evidently, glutathione and humic acid stabilize C r O through chelation or ligand exchange (18-24). The failure to detect a C r O EPR signal for the 4MP reactions supports the theory that if C r O is produced, it does not accumulate, which is consistent with its involvement after a rate-limiting reaction step. Cr(W ScavengingwithMn(I1).Selected reactions were conducted with additions of Mn(I1) to scavenge possible Cr(W intermediates. The ability of Mn(l1) to scavenge Cr(W has been shown previously (4, 26-28) for Cr(VI) reductions by alcoholswhere the elimination of Cr(W from the reaction sequence halves the CrW) reduction rate. For reaction conditions ranging from a 5-fold excess of CrW) to a 5-fold excess of 4MP, rates of 4MP and CrW) consumption were unaffected by Mn(II),and the apparent reaction stoichiometry (A[Cr(VI)]/A[4MP])was consistently 213. Reactions of CrW) and 2,6-dimethoxyphenol(26DMX) at a CrW):26DMXmolar ratio of 1:4 mM also showed no change in the apparent reaction stoichiometry or rates of CrW) or 26DMX depletion. The absence of a Mn(I1)effect on rates does not necessarily exclude Cr(W involvement 1938 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8 , 1 9 9 5
in the reaction sequence, but it does suggest that, if Cr(IV) is produced, it is consumed in a step succeeding the initial CrW) reduction step. Product Analysis. Phenolic compounds typically are oxidized to a phenoxy radical or cation, which can undergo coupling reactions to numerous end products including polymers (29-32). The proposed reaction schemes and principal aqueous products of CrW) oxidations of several phenols are shown in Figures 3 and 4 . Complete oxidation of 4-methylphenol (I, Figure 3) with CrW) in aqueous solution yielded a final reaction mixture of soluble products and a reddish brown resinous precipitate. The amorphous, probably polymeric precipitate was insoluble in ethyl acetate, methylene chloride, and hexane. Shorter reaction times yielded considerably less polymer. We therefore restricted product analyses to moderate reaction times and focused on the initial, low molecular weight products. The dimeric ketone (Pl, Pummerer's ketone) was the most abundant product, followed by dimeric P2 and trimeric P3 coupled phenols. Trace amounts of coupled phenol tetramer also were observed. Coupled products are presumably ortho-linked phenols (30) and are believed to form via a one-electron production of a phenoxy radical El) and subsequent radical-radical coupling (29,30,33). A strong oxidant like CrWj and a low pH could also induce coupling via a phenoxy cation (X2)/phenol interaction (29, 31). Trace amounts of the monomeric aldehyde (P4)and the carboxylic acid (P5)were observed and are evidence of a two-electron transfer. The formation of dimeric ketone and coupled phenols for 4MP oxidationis well-known (30,341. However, the formation of oxidized monomers with CrW) has not been reported previously. In contrast to 4MP the major product of the oxidation of 4-methoxyphenol (11) and 4-chlorophenol (111) was unsubstituted p-benzoquinone (P6)formed from hydrolysis and elimination of the chloro and methoxy substituent at the para position of the phenoxy cation (X2). Methoxyand chloro-substituted quinones (P8)also were detected. These products are signature compounds for two-electron
oxidations. Cr(VI) oxidation of 4-methoxyphenol and 4-chlorophenol also produced smaller amounts of ortholinked dimers (P2,P7)and traces of higher polymers (P3). The methoxy analog of Pummerer’s ketone (Pl) was detected only in trace amounts, and no ketone was identifiable in the oxidation products of 4-chlorophenol. In short reaction times 2,6-dimethoxyphenol (rv)was oxidized almost exclusively to dibenzoquinone P9). Dibenzoquinone produced a strong absorbance at Am= = 472 nm as has been reported for phenol oxidation by Mn(II1) (35). Oxidation products of 2,6-dimethylphenolO also showed predominantly para-para coupling: dibenzoquinone (P9) was the major identifiable product, accompanied by significant quantities of yet unidentified, potentially monomeric products. The principal oxidation products Of 2,4dimethylphenol (VI) were an oxidized monomer (2,4dimethyl-o-quinone (P121) and typical one-electron products, namely the dimethyl analogs of Pummerer’s ketone (P13)and the dimer of the parent (P14). Both of the preferred coupling positions (para and ortho) on 2,4,6-trimethylphenol MI) are substituted with a poor leaving group. The major products are monomers that suggest an initial two-electron oxidation to a quinone methide species (X3) with subsequent hydrolysis at the methyl carbon to form the stable compound P16 (alcohol substituted). Compounds Pl7 (aldehydesubstituted) and P18 (carboxylicacid substituted) result from facile oxidation of the newly formed alcohol group.
Discussion General Characteristics of the Reaction. The kinetic characteristics of Cr(VI) reduction by substituted phenols resemble in certain key ways Cr(VI) reduction by alcohols, aldehydes, and thiols. We therefore hypothesize that the mechanism of phenol oxidation is analogous to the mechanism already elucidated for the nonaromatic reductants: a preoxidation, equilibrium formation of a chromate ester, followed bya rate-limiting electron transfer with ester decomposition. In particular, the reaction mechanism must account for the following major observations reported here and in a previous paper (2): (A) Cr(VI)-phenol ester formation precedes the first electron transfer step and reaches a rapid equilibrium concentration. (B) The overall reaction is first-order with respect to the concentration of phenol and one or more monomeric Cr(VI) species. (C) The apparent reaction order with respect to [H+l approaches 2 at low pH, approaches 0 at pH 6, and varies depending on the phenol reductant. (D) Reaction products indicate both one-electron and two-electron pathways. Chromate Ester Formation. Formation of a chromate ester has been well-established in the Cr(VI) oxidation of alcohols (36-40)and aldehydes (41-44). Analogous C r O thiol esters have been observed in Cr(VI) oxidations of compounds with sulfhydryl moieties (45-50). Cr (VI) oxidation of these compounds begins with the rapid formation of a Cr-0-C ester or Cr-S-C thioester linkage. The chromate ester intermediate decomposes by an innersphere electron transfer. Stopped-flowand temperaturejump investigations of ester formation reported stability constant values of ca. lo3 M-’ for thiol esters (47-50) but only about 1-10 M-‘ for most Cr(VI)-oxy esters (51-55). The small absorbancechanges observed here for the Cr(VI)phenol esters are consistent with the reports of relatively weak stabilities of Cr(VI)-oxy esters.
Ester formation in this study was nearly instantaneous and therefore not rate limiting. With 4-chlorophenol at pH 2 and pH 5 and with 4-methylphenol at pH 5, the ester reached steady-state concentrationswithin the time it took to initiate the reaction and complete a spectrophotometric scan (typically 6.
0.06 h
T; v) 7
1
0.05
4-methyl phenol
PI
SCHEME 2 U
R
ko
O
O
+
'
Cr(V)
(16)
0
r
C
1
H+ 0
R
0
0.04
0.08
0.12
0.16
The rate parameters for eq 11 given in Table 1 were obtained for excess phenol. Rate data for excess CrW) reactions were used to re-evaluate the rate law and thereby test its predictive capability for a full range of conditions. The rate of oxidation of 4MP was measured at four pH values in the range pH 1-2. Under these conditions, the rate law is of the form -d[ArOH] / d t =
{k2[H+12+ k,[H+l
+ ko}Ke[HCrO,-][ArOH] (12)
which is analogous to eq 8. The bichromate-normalized second-order rate constant, k ~ ~can ~ be 0 expressed ~ , by
in which case eq 15 obtains
Within the pH range examined (pH 1-21, the protonindependent pathway is expected to contribute insignificantly to the net reaction, and so the ko rate component in eq 15 is difficult to fit accurately to the rate data. To get a more realistic fit of eq 15, we used a value of koK, = 3.39 x determined previously from the excess 4-methylphenol experiments. The products klK, and k2K, were found from the plot of kexp/[HCrO4-1vs [H+l(Figure6, Table 1). Comparison of the rate constants obtained from the two data sets are within 7% for the two-proton pathway and within a factor of 2 for the single-proton pathway. The apparent reaction order with respect to [H+]for 4MP in this pH range is 1.7,showing the predominance of the twoproton pathway. The good agreement between the values of the two rate constants for the two-proton pathway (k2KJ lends support to the proposed mechanism. A tentative mechanism scheme for the electron transfer decomposition of the ester involvingthree parallel reaction pathways is depicted in Scheme 2. The proton independent pathway (eq 16) is a unimolecular decomposition of the ester, possibly via a homolytic cleavage of the Cr-0 bond, leading to C r O and phenoxy radical products. This pathway should dominate at less acidic conditions (>pH 5). The proton dependent pathways (eqs 17 and 18) are rapid, successive protonations of a Cr(VI) hydroxy ligand
O
O
'
+ Cr(V)
0
r
0
1'
0
I'
0
J
IH+1 (M)
FIGURE 6. Plot of k,,d[HCrOs-l vs [H+] for the oxidation of 4-methylphenol by excess Cr(VI) at pH 1.0 and 25 "C. Curve shows the least-squares fit of eq 15.
1'
0
2H+
R e o ++ 0
R e O - ; r - O -II
-
R
O
O
'
(17)
Cr(1V) (18)
+ Cr(IV)
(19)
0
with subsequent electron transfer and Cr-0 bond cleavage. Each successiveproton addition makes the Cr center a better electron acceptor. A single-proton pathway (eq 17)exhibits a first-order rate dependence on [H+land could dominate at moderately acidic conditions (e.g., pH 2-5) potentially yielding C r O and a phenoxy radical as before. Similarly, the two-proton pathway (eq 18) shows second-order rate dependence and may dominate at low pH (pH < 2). For some phenols, such as those with resonance-electrondonating substitutents in the ortho or para positions, heterolytic cleavage of the Cr-0 bond with a two-electron transfer might be expected. The assignment of homolytic or heterolytic cleavage of the Cr-0 ester bond is conjectural but is fully consistent with (a) our product studies, (b) our investigations of C r O and Cr(IV) intermediates, and (c) the behavior of similar reaction systems reported in the literature. For our systems, the organic products from reactions at pH 1and pH 2 show evidence of both one- and two-electron pathways. Whether these products stem from the initial reduction of CrW) or a subsequent reaction with an intermediate Cr species is uncertain, but the formation of oxidized monomers is good evidence of a two-electron pathway. Similarly, coupled products suggest phenoxy radical polymerization resulting from one-electron products. Products resulting from oxidation of 4MP were predominantly one-electron products, although some evidence suggests limited contribution from a two-equivalent pathway as well. Oxidation of 4-methoxyphenol, which has a strong resonance electrondonating substituent (+MI in the para position resulted in predominantlytwo-equivalent products. The ability of +M substituents to affect a two-electron pathway may be a result of enhanced resonance stabilization of the electron deficient phenoxy cation (as in eq 18) or by invoking the resonance structure in the electron-transfer step itself (eq 19). In acidic solutions, aliphatic alcohols have been shown to react predominantly via a two-equivalent pathway with Cr(VI) reduced directly to Cr(IV) (3,4, 15, 26, 59). On the other hand, thiols in the range of pH 2-7 tend toward an initial one-electron transfer: a homolytic ester cleavage producing C r O and a sulfhydryl radical (46-50). VOL. 29. NO. 8, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY m 1939
TABLE 2
Second-Order Rate Constants and QSAR Parameters for Cr(VI) Reductions by Substituted Phenols compd
phenol derivative
1 2
phenol 4-methyl 2,4-methyl 3,4-dirnethyl 2,g-dimethyl 2,4,6-trimethyl 4-rn et h oxy 2,6-dimethoxy 3,4-dimethoxy 3,5-dimethoxy 2-methoxy-4-aldehyde 2-methoxy-4-methyl 4-chloro 4-nitro
3 4 5 6
7 8 9 10 11 12 13 14 a
(M-'S-')
xu+ *
ZE'
2.63 f 0.06 x 1.17 i 0.01 10-3 9.37 rt 0.08 10-3 2.84 f 0.05 x 1.23 f 0.01 10-3 5.30 f 0.04 x 2.18 f 0.04 x lo-' 4.35 f 0.18 x lo-' 3.75 f 0.055 2.54 f 0.02 10-4 1.12 0.01 10-4 3.84 f 0.02 x lo-' 2.26 f 0.03 x unreactive < IO-'
0.00 -0.31 -0.62 -0.38 -0.62 -0.93 -0.78 - 1.56 -0.92' -0.28' -0.05 -0.22 0.1 1 0.79
0 0 - 1.24 0 -2.48 -2.48 0 -1.10 0 0 -0.55 -0.55
& A ~ Hf SD
b
0 0
xP 0 0 -0.04 0 -0.08 -0.08 0 0.52 0
0 0.26 0.26 0 0
Values from ref 65 unless otherwise noted. Values from ref 68. Values from ref 63.
Role of C r O and Cr(W Intermediates. The C r O and Cr(IV) investigations provide additional insight into the initial redox step. At pH 1 and pH 2 , there was no direct evidence for a C r O reaction intermediate, suggesting that C r O , if produced, is consumed so rapidly that a measurable concentration does not develop. The lackof a Mn(I1)scavengingeffect on CrW) reduction rates or reaction stoichiometries with excess of 4MP or 26DMX suggests that Cr(IV), if produced, does not react with a second Cr(VI)molecule as suggested previously (15, 401, but instead disproportionates to C r O and Cr(II1). Alternatively, Cr(IV)may react with a second molecule of 4MP when the reductant is in great excess. Measuring the rate of phenol disappearance in the presence of excess Cr(VI) is not a feasible alternative for examining Mn(1I) scavengingeffects. Cr(IV)oxidizes Mn(I1)to Mn(II1)which in turn accepts an electron equivalent from the phenol (34, 60) and hence no change in the phenol oxidation rate is observed. The absence of a Mn(I1) effect on reaction rates in our Cr(VI)/phenol studies agrees with a previous study of Cr(VI) oxidations of 4MP and creosol (34). Tanaka and co-workers found that, despite the apparent scavenging of Cr(IV)by Mn(I1)(based on substantial increases in dimeric products yields), Mn(I1) had no effect on rates of Cr(VI) reduction. General Trends in Reactivity. The reactivity of the CrW)/phenol system depends on both the formation of a Cr(VI) ester and the subsequent electron-transfer step. Accordingly,the reaction rate should depend on the stability of the Cr(VI) ester (Le.,on Ke) and on the sum of the rates of the electron transfer reactions as reflected by kd in eq 6. We anticipate that substitution of the phenol structure affects these reaction processes through resonance, field, and steric effects,but it is likelythat these substituent effects influence esterification and electron transfer to differing extents. The effect of a substituent R on the esterification equilibrium can be inferred from its effect on the relative thermodynamic stabilities of the phenol reactant and the ester product. The strengths of the RArO-H and the RAroCrO:
