8832
J . Phys. Chem. 1990, 94, 8832-8835
Determlnatton of Crttlcal Nlicelltzatlon Concentrations of Perfluorocarboxylates Using Ultraviolet Spectroscopy: Some Unusual Counterion Effects Pasupati Mukerjee,* Michael J . Gumkowski,+Chun C. Chan, and Ravi Sharmat School of Pharmacy, University of Wisconsin, Madison, Wisconsin 53706 (Received: April I O , 1990)
Long-chain perfluorocarboxylates have been found to have stronger electronic absorption bands in the 205-225-nm region than perfluoroacetates. The absorptivity increases significantly on micelle formation in some regions. This increase can be used to determine critical micellization concentrations (cmc) of perfluorocarboxylates in aqueous solutions and in the presence of high concentrations of chloride salts. The absorptivities are also high enough for quantitative analysis of the perfluorocarboxylates. Several cmc values are reported. The cmc of lithium perfluorooctanoate in 1 M LiCl shows a significant effect of the salting-out of the chain. The observed low cmc of tetraethylammonium perfluorooctanoate suggests that the contact hydrophobic interactions involved at the micelle surface are different from the ones that produce a tendency toward lack of chain mixing in mixed micelles of fluorocarbon and hydrocarbon surfactants. The absorptivity of the micelles of perfluoroheptanoic acid has been found to be much greater than that of the monomers. This suggests that a substantial fraction of the surface carboxylic acid groups in the micelles are undissociated. This explanation is in accord with the higher stability of the acid micelles compared to their sodium salts in such systems.
Introduction The electronic spectra of aliphatic carboxylic acids and their anions are known to produce absorption bands in the ultraviolet region which arise from n-r* and x-A* transitions of the chrom o p h o r e ~ . ' - ~ W e have found that long-chain perfluorocarboxylates such as perfluorooctanoates have much stronger bands in the 205-225-nm region than perfluoroacetate ions and that there are significant increases in molar absorptivity of perfluorooctanoates on micelle formation in aqueous solutions. Aliphatic carboxylates do not display such characteristics to this degree. W e have also found that the change in absorptivity on micelle formation is large enough to be used as a basis for a simple, noninvasive, spectrophotometric method for the determination of critical micellization concentrations (cmc) of perfluorooctanoate salts. W e present this method here and show that the method can be used in the presence of high concentrations of chloride salts. Several cmc values are reported which are of some interest with respect to a fairly large salting-out effect on the hydrophobic moiety and an unexpectedly large effect of substituting inorganic counterions with tetraethylammonium ion. We also report that absorbance measurements provide a simple, convenient, and precise method for the quantitative analysis of perfluorocarboxylates. Finally, we have found that the increase in absorbance on micelle formation for perfluoroheptanoic acid is much more pronounced than for the salts of perfluorooctanoic acid. We attribute this to the partial neutralization of the surface carboxyl groups in the micelles of perfluoroheptanoic acid which also produces a significant lowering of the cmc of the acid micelles when compared to their sodium salts. Materials and Methods Perfluorooctanoic acid (PCR Research Chemicals, Inc.), perfluoroheptanoic acid (Fluka Chemical Corp.), lithium hydroxide monohydrate (Aldrich, Gold Label, 99.99%). tetraethylammonium hydroxide (Aldrich, 20 wt % solution in water), lithium chloride (Johnson Matthey Inc., Puratronic, 99.996%), and sodium hydroxide (Mallinckrodt, Analytical Reagent) were used. Stock solutions of lithium, sodium, and tetraethylammonium salts of pcrfluorooctanoic acid (LiPFO, NaPFO, and (C,H,),NPFO. respectively) were prepared by adding to weighed amounts of vacuum-dried perfluorooctanoic acid appropriate amounts of bases in solution. The pH of the stock solutions was about 8. All sample solutions were prepared and diluted by weight. The
' Present address: Pfizer Central Research, Eastern Point Road, Croton. CT 06340. Present address: Research Laboratories, Eastman Kodak Co., Rochester. N Y 14650
0022-3654/90/2094-8832$02.50/0
TABLE I: Molar Absorptivities, and 25 O C substance
c
(L mol-' cm-I), at 205 nm
perfluoroacetate perfluorobutyrate
perfluorooctanoate (monomeric)
c
51 253 344
densities of the stock solutions containing surfactants were measured with a pycnometer and used to calculate the molar concentrations of mixtures with appropriate solvents assuming no change in volume on mixing. This procedure gave better relative precision and accuracy of concentrations than volumetric dilutions. Absorbances of solutions were measured in a Varian-220 spectrophotometer with the cell compartment thermostated at 25 "C, using cells of I - and 0.1-cm path lengths.
Results and Discussion I n their vacuum spectra, aliphatic carboxylic acids show absorption maxima at about 200 nm with molar absorptivities ( t ) of about 50 L mol-' cm-'. These bands have been shown to be due to n-r* transitions.I4 At about 160 nm much stronger bands maxima with molar absorptivities of 2500-4000 L mol-' cm-' are observed. These can be attributed to x--x* t r a n ~ i t i 0 n s . I ~In~ solution solvent-induced shifts in the n-a* transitions of carboxylic acids have been With increasing chain length the molar absorptivity shows some increase between formic and butyric acids but very little at higher chain lengths up to stearic acid in the 200-250-nm range.6 For anions in solution the n-a* transitions appear to be weaker and are overshadowed by the tail of . ~ whereas the a-a* band in the near-ultraviolet r e g i ~ n . ~Thus, acetic acid in 0.01 M H 2 S 0 4has a peak around 205 nm, t = 38 L mol-' cm-I, the anion shows no peak around 205 nm and exhibits a higher t value of about 100 L mol-' cm-I. The electronic spectrum of trifluoroacetic acid has been described by Szyper and Z ~ m a n They . ~ found that the acid in 7 M H,S04 has a strong absorption below 195 nm ( t > IOO), a shoulder at 210 nm ( e = 30), and a weaker shoulder a t 225 nm. The anion has a higher absorbance at 195 nm and below, which (I) Jaffe. H. H.;Orchin, M. Theory and Applications of Ultrauiolet Spectroscopy; Wiley: New York, 1962. ( 2 ) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1975; Vol. 11. (3) Szyper, M.; Zuman, P. Anal. Chim. Acta 1976, 85, 357. (4) Timmons, C. J., Ed. UV Atlas for Organic Compounds; Plenum Press: New York, 1967; Vol. 5 . (5) Rusoff, I. I.; Platt, J . R.; Klevens, H. B.; Burr, G. 0. J. Am. Chem. Soc. 1945, 67, 673. (6) (a) Ralston, A. W. Fatty Acids and Their Deriuatioes; Wiley: New York. 1948. (b) Platt, J. R.; Rusoff, I.: Klevens, H. B. J . Chem. fhys. 1943, 11, 535.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 25. I990 8833
Determination of cmc of Perfluorocarboxylates is similar to the behavior of the r-r* transition in the case of hydrocarbon acids. The molar absorptivity at 210 nm of the anion is similar to that of the acid so that the shoulder at 210 nm, presumably due to a n-ir* transition, is about as weak as that for the acid. As the chain length increases, we found that the *-A* transition tail at 195 nm becomes significantly stronger for perfluorobutyrate. There is, however, a much greater increase in intensity of the n--T* transition at higher wavelengths, above 205 nm, so that the shoulder at 210 nm becomes more pronounced. This increase in intensity of the n-r* transition is even greater for perfluorooctanoates. Table I shows that the molar absorptivities at 205 nm of three acid anions containing 2,4, and 8 carbon atoms increase with chain length in the ratio 1:4.4:6.1. Thus, the chain length effect continues even when added CF2 groups are more than three bonds away from the chromophore. At higher wavelengths, above 2 10 nm, the absorptivities decrease rapidly. The relative increase in intensity with chain length is even greater in the 210-225-nm region where the contribution of the tail of the PA* transition is minimal. At 230 nm the molar absorptivities are 4.5, 16.2, 26.4, and 28.4 for perfluoroacetate, -butyrate, -heptanoate, and -octanoate, the chain length effect being similar to that at 205 nm. The remarkable effect of the chain length on the intensity of the n-* transition and its persistence beyond several bond lengths are difficult to explain. They may be related to the electronwithdrawing capacity of the fluorine atoms. Fatty acid anions do not show this effect. Literature data’ and our measurements on potassium laurate below the cmc show that absorptivity changes little between the butyrate and the laurate system. We have recently found that the dissociation constants of the fluorocarbon acids, which are known to be strong acids,*p9increase significantly between perfluoroacetic acid and perfluorobutyric acid.lO*” This behavior is also different from that of fatty acids and is in line with the effect of fluorination persisting over several bond lengths. When absorbance measurements were made below and above the cmc of perfluorooctanoates, it was found that the apparent molar absorptivities increased above the cmc; Le., the absorbances were higher for micelles than for monomers in the wavelength region 205-2 1 5 nm. The spectral changes corresponded to a red shift as also a change in spectral shape upon formation of micelles. Because of the high cmc values 0.1-cm path lengths were used for LiPFO and NaPFO in water. For (C2H5),NPF0 I-cm cells were used below the cmc and 0.1-cm cells above the cmc. All of these measurements were made at 205 nm. We have used the method outlined here to determine the effect of LiCl on the cmc of LiPFO up to high concentrations of LiCl (2 M).” At high LiCl concentrations measurements need to be made at 210 nm in order to minimize the effect of the absorbance by chloride ions. We report here a representative measurement of the cmc of LiPFO in 1.012 M LiCI. Quantitative Analysis Below the cmc Beer’s law was obeyed by all the systems to a satisfactory degree, to well within 1%. We found, however, that the absorbance data ( A ) were somewhat better represented by a linear relationship A = a,
+ b,c
(1)
where a , and b , are constants and c is the molar concentration of the fluorocarbon system. With absorbance values in the range 0.1-1.8. 1 was obeyed with correlation coefficients in the range 0.999945-0.999985. The values of a , were both positive and negative but well below 0.01. These low values of a, are probably due to instrumental factors and not to a breakdown of Beer’s law. ( 7 ) Lcuthardt, F. Helr. Chim. Acta 1933, 16, 230. (8) Grunwald, E.: Haley. J . F., Jr. J . Phys. Chem. 1968, 72, 1944. (9) Covington. A. K.;Freeman, J. G.;Lilley, T. H. J . Phys. Chem. 1970, 7 4 , 3773. (IO) Mukcrjcc, P.; Gumkowski, M. J.; Chan, C. C.; Sharma, R.Unpub-
lished work. ( I I ) Mukcrjce. P.: Chan. C. C. Unpublished work.
0
0.02
0.04
0.06
0.00
moles/liter
+
Figure 1. Plots of A / I - (a, b,c) against c. A is the absorbance, 1 is the path length, c is the concentration in mol/L, and a, and b, are constants needed to fit the AI1 data below the cmc (wavelength 205 nm): A, NaPFO; 0, LiPFO. The bar represents an error of 0.01 in absorbance.
,
4
I
I
l,2
,
If
I
t
2p
,o,4
JI-
0.3
- 0.2 -0.1
- 0.0 0
2
4 6 8 (molrs/liter) x103
1
0
+
Figure 2. Plots of AI1 - ( a , b,c) against c with symbols defined in the legend of Figure 1: (A) (C,H&NPFO in water, 205 nm; (0)LiPFO in 1.012 M LiCI, 210 nm. Upper and lower bars represent errors of 0.01 in absorbance for the upper and lower curves.
Absorbances calculated from the fitted a , and bl values of eq 1 agreed with the observed A values to within average errors of 0.346%. Thus, the spectrophotometric method provides a precise method for the quantitative analysis of perfluorocarboxylateswhich can be used in the presence of high concentrations of chloride salts. This analytical approach should be useful down to the M range of concentration using 10-cm cells. It provides a useful alternative to another method based on dye extraction published recentIy.I2 Determination of the crnc Absorbance data above the cmc can be represented very well also by using a linear relationship. A = a2 + b 2 ~ (2) The correlation coefficients were in the range 0.999 959-0.999 990. The b2 values were 12-18% higher than the b, values below the cmc, the a2 values being negative. After the usual precautions are taken in excluding measurements close to the c ~ c , I ~the - ’cmc ~ values can be determined graphically from the intersection of two straight lines of A vs c above or below the cmc. The intersection point of the lines represented by eq 1 and 2 can also be used to estimate the cmc. The reliability of the cmc is estimated to be in the range 1-2%. In order to display the change in slope on micelle formation in a magnified form, the absorbance data are shown as a deviation plot, Le., as A - ( a , b,c) against c in Figures 1 and 2 for four different systems. The ordinate values remain zero up to the cmc
+
(12) Sharma. R.; Pyter, R.; Mukerjee, P. Anal. Letr. 1989, 22, 999. ( I 3) Mukerjee, P. Ado. Colloid Interface Sci. 1967, I , 241. (14) Mukcrjee. P.; Mysels, K. J . Critical Micelle Concentrations of Aqueous Surfactant Systems. Null. Stand. Ref. Data Ser. (US., Narl. Bur. Stand.) 1971. NSRDS-NBS 36.
8834 The Journal of Physical Chemistry, Vol. 94, No. 25, I990
20. A
15-
ol!/
5l 0
01
02
03
,
,
,
04
05
06
~
C (moIes/liter) Figure 3. Plot of absorbance ( A ) against concentration c for perfluoroheptanoic acid a t 230 nm.
TABLE 11:
cmc Values at 25 O C
system N a P F O in water LiPFO in water LiPFO in 1.012 M LiCl ( C 2 H & N P F 0 in water pcrfluoroheptanoic acid
cmc. M 0.0306 0.0341 0.00277 0.007 19 0.0328
and then increase linearly with concentration above the cmc. The method appears to work successfully for inorganic and organic counterions and in the presence of high chloride concentrations. Figure 3 shows a plot of A vs c for perfluoroheptanoic acid at 230 nm. For this system the slope above the cmc is much higher than the one below, by a factor of 2.8, as compared to the 12-1 8% increase shown by the salts of perfluorooctanoic acid (Figures 1 and 2) at 205-210 nm.
Interpretation of cmc Values Five cmc values are reported in Table 11. The value for NaPFO is in good agreement with literature value^'^*'^ obtained by the conductance method. The value for (C2H5),NPF0 is in agreement with an independent estimate based on the fluorescence of toluidinonaphthalenesulfonic acid." It is interesting to note that the cmc of LiPFO is significantly higher than that of NaPFO. NaPFO was shown to have a higher cmc than KPF0.I' This counterion sequence is similar to that of dodecyl sulfates.'* The cmc of LiPFO in high concentrations of LiCL shows a rather pronounced effect of the salting-out of the chain." At low Li+ concentrations the slope of the log cmc vs log (Li'),,, line has a value of 0.55, which is very similar to the value of 0.54 for the sodium salt reported.I6 If one uses this slope, the cmc of LiPFO in 1.012 M LiCl is estimated to be 0.0053 M, which is almost twice the experimental value of 0.002 77 M (Table 11). This discrepancy can be ascribed to the increase in the activity of the monomer because of salting-out of the chair^'^,^^ A fuller report will be presented later." The cmc of (C2H5)4 N P F O appears to be anomalously low. When Na+ was replaced by (C2H5)4N+in dodecyl sulfates, the cmc was found to be lowered by a factor of 2.16.18 This was ascribed to the effect of hydrophobic interactions of (C2H5)4N+ with the cxposed hydrocarbon groups at the micelle-water interface. I n view of the now well-known mutual antipathy of fluorocarbon and hydrocarbon groups in many interfacial and micellar systems giving rise to severe nonideality the ( 1 5 ) Sugihara, G.;Mukerjee, P. J. Phys. Chem. 3981, 85. 1612. (16) Mukerjee, P.;Korematsu. K.: Okawauchi, M.; Sugihara. G.J. Phys. Chem. 1985. 89, 5308. (17) Shinoda, K.: Katsura. K. J . Phys. Chem. 1964, 68, 1568. (18) Mukerjee, P.; Mysels, K. J.; Kapauan, P. J. Phys. Chem. 1967, 71, 4166. (19) Mukerjee, P. J. Phys. Chem. 1965, 69, 4038. (20) Mukerjee, P.;Mysels, K. J. A C S S y m p . Ser. 1975, No. 9, 239. (21) Mukcrjcc, P.:Yang, Y. S. J . Phys. Chem. 1976. 80,1388.
Mukerjee et al. effect of replacing Na+ by (C2H5)4N+in PFO- systems would be expected to be less than for dodecyl sulfates since the hydrocarbon groups of many (C2H5)4N+counterions would be in contact with fluorocarbon groups at the micelle-water interface. Experimentally, however, the effect is much larger, by a factor of 4.2 (Table 11). Thus, relative to Na+ or Li+ counterions, (C2H5)4N+counterions stabilize perfluorooctanoate micelles significantly more than dodecyl sulfate micelles. This antiintuitive result suggestes that a distinction should be made between contact type hydrophobic interactions and those where chain mixing is involved as in mixed or fluid interface^.^^,^^ The (C2H5)4N+counterion at the micelle surface should show primarily a contact type interaction. Compared to the case of mixed micelles where chain mixing is involved, fluorocarbon-hydrocarbon nonideality effects may be less important for contact type interactions which may be influenced to a greater extent by water structure effects. It has been shown that the role of water structure effects is more pronounced for fluorocarbon chains than for hydrocarbons.I6 The greater effectivenessof the (C2H5)4N+counterions in conferring stability to the fluorocarbon micelles is also likely to involve a greater ability of the hydrophobic counterions to reduce the micelle-water interfacial tensions of the fluorocarbon micelles. Fluorocarbon-water interfacial tensions are significantly greater than hydrocarbon-water tension^.^^^^^ It is to be noted, however, that sodium octyl and decyl sulfates reduce the interfacial tension of the perfluorohexane/water interface much less than that of the hexane/water interface.22 Some years ago some literature data on the cmc of perfluorocarbon acidz6were corrected by reinterpreting the relevant electrical conductance data. The revised values were reported in a critical ~ompi1ation.l~ From these values at a number of temperatures the estimated cmc values of perfluorooctanoic and perfluorohexanoic acids at 25 OC are 0.0096 and 0.104 M, respectively. These estimates, reliable to within 3-4%, can be used to estimate the cmc value of perfluoroheptanoic acid at 25 O C , using the well-known rule of the linear variation of In cmc with chain length.i3.27 This estimated cmc value of 0.0316 M is in excellent agreement with the value of 0.0328 M found by the present spectrophotometric method. This provides some additional validity to the substantial upward revisions of the original cmc'sZ6 of perfluorooctanoic and hexanoic acids by 60-1 2O%.I4 Finally, the pronounced effect of micelle formation on the absorptivity of perfluoroheptanoic acid (Figure 3) is of considerable interest. Perfluoro acids are known to be strong acids although there is considerable controversy about their dissociation constants.&-I0 For perfluoroacetic acid dissociations constants have been reported to be in the range 0.48-5 M-'9 Szyper and Zuman3 reported that the absorbance of perfluoroacetic acid at 230 nm in 7.0 M H2S04 is about 2.2 times higher than in 0.01 M H2SO4. Similar variations in molar absorptivity between the undissociated acid and the anion have been observed in our studies on the dissociation constants of perfluoroacetic and perfluorobutyric acids.I0 The data in Figure 3 indicate that the apparent absorptivity of micellar perfluoroheptanoic acid is about 2.8 times higher than that of the monomer. This must be ascribed to a major fraction of the surface carboxyl groups in the micelle being associated. Since only about 2-3% of the monomers are expected to be undissociated at the cmc, this effect is caused by some factors that are important for micelles but not for monomers. The increase in the concentration of the hydrogen counterion at the micelle surface arising out of the local electrostatic potential due to the remaining charged groups is undoubtedly of importance. The magnitude of the effect suggests, however, that an increase in pK values of acidic groups at the micellar interface caused by the (22) Mukerjee, P.; Handa, T. J. Phys. Chem. 1981.85, 2298. (23) Handa, T.;Mukerjee, P. J. Phys. Chem. 1981, 85, 3916. (24) For many recent references, see: Asakawa. T.; Mouri, M.; Miyagishi, S.; Nishida, M. Langmuir 1989, 5, 343. (25) Nishikido, N.; Mahler, W.; Mukerjee, P. Langmuir 1989, 5, 227. (26) Klevens, H. B.: Vergnolle, J . Proc., In(. Congr. Surf. Act. 1957, 2, 395.
(27) Mukerjee, P. Eer. Bunsen-Ges. Phys. Chem. 1978, 82. 931.
J . Phys. Chem. 1990, 94, 8835-8839
8835
reduced effective polarity of the interfacial microenvironmentZ8 is also a significant factor in this case in increasing the association of hydrogen ions with the surface carboxyl groups. These points have been made before in connection with the low conductivity above the cmc of perfluorooctanoic acid.I5 The neutralization of the surface charges of the micelles of perfluoro acids and the resultant reduction of electrostatic de-
stabilization of micelles are also likely to be responsible for an increased stability of the fluorocarbon acid micelles as compared to their salts. Literature datal4 on the strong acid, dodecylsulfonic acid, show that its cmc is lower by a factor of only about 1.15 than the cmc of the sodium salt over a wide temperature range. I n contrast, the cmc of perfluorooctanoic acid, 0.0096 M at 25 "C, is lower than that of the sodium salt, 0.0306 M (Table 11), by a much larger factor of 3.2.
(28). (a) Mukerjee, P.; Banerjee, K. J . Phys. Chem. 1964, 68, 3567. (b) Mukerjee, P.; Ray, A. Ibid,1966, 70, 2144. (c) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. Micellization, Solubilization, Microemulsions, [Proc. Int. Symp.1 1977, I , 241. (d) Ramachandran, C.; Pyter, R. A,; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198.
Acknowledgment. This material is based on work supported by the National Science Foundation under Grant CPE 8216450. Registry No. LiPFO, 17125-58-5; NaPFO, 335-95-5; (C2HS).,NPFO, 98241-25-9; LiCI, 7447-41-8; perfluoroheptanoic acid, 375-85-9.
Direct Spectroscopic Detection of Sulfonyloxyl Radicals and First Measurements of Their Absolute Reactivities" Hans-Gert Korth,* Institut fur Organische Chemie, Universitat Essen. 0 - 4 3 0 0 Essen. Federal Republic of Germany
Anthony C . Neville,lb and Janusz Lusztyk Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1 A OR6 (Rereiaed: April 25, 1990: In Final Form: June 18, 1990)
Two sulfonyloxyl radicals, CH,S(=O),O*, 2a, and 3-CF3C6H4S(=O),O', 2b, have been generated by 308-nm laser flash photolysis (LFP) of their parent symmetrical peroxides in CH3CN solution, in which they have lifetimes of 7-20 ps. Both 450 nm. This absorption can radicals exhibit a broad, structureless absorption similar to that known for SO4'- with A, be bleached for 2a but not for 2b by firing a second laser at 480 nm, presumably reflecting a photoinduced cleavage of the 1.6 X H3C-S03' bond. Radicals 2a and 2b react with the acetonitrile solvent by abstraction of a hydrogen atom, kH I Os M-' s-', k H / k D 2.0. Bimolecular rate constants for attack of these radicals on cyclohexane (viz., 1.9 X lo8 and 6.5 X IO8 M-l s-' for 2s and 2b, respectively) and chloroform (viz. ca., 3 X lo5 M-I S-I for both) demonstrate that they are more reactive than almost all other oxygen-centered radicals. Product studies demonstrate that both the photodecomposition and the thermal decomposition of the parent peroxides yield the corresponding sulfonyloxyl radicals, a result that contrasts with that we have previously obtained for the decomposition of [Ph,P(=O)O],, which yields radicals on photolysis but few if any radicals on thermolysis. Semiempirical AMI /PM3-UHF calculations on 2a are also reported.
-
-
-
Transient, highly reactive, oxygen-centered radicals of the general structure Y,X(O)O*,1, such as aryl-, ethenyl-, and (ethynylcarbonyl)oxyI radicalsz3 (ArC(O)O', RCH=CHC(O)O', and RC=CC(O)O'), (alkoxycarbony1)oxyl radicals4 (ROC(O)O'), and (phosphinoy1)oxyl radicals5 (Ph2P(0)O') are characterized by broad optical absorptions in the visible and, for the carbonyloxyls, by unique EPR spectra.3*6 We have interpreted the high reactivity (relative to tert-butoxyl, for example) of YnX(0)O' radicals toward organic substrates (both hydrogen abstractions and additions) as a consequence of their electrophilic character (Le., electron affinity) which can formally be represented by contribution made to the radical's ground state by the two charge-separated canonical structures shown here:
1
( 1 ) (a) issued as NRCC No. 32292. (b) NRCC Research Associate 1989-1990. (2) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J . Am. Chem. SOC.1988, IIO, 2817-2885, 2886-2893. (3) Korth, H. G.; Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J . Am. Chem. SOC. 1988, IIO, 5929-593 I . (4) Chateauneuf, J.; Lusztyk, J.; Maillard, B.; Ingold, K. U. J . Am. Chem. SOC.1988, IIO, 6121-613 I . ( 5 ) Korth, H. G.; Lusztyk, J.; Ingold, K. U. J . Org. Chem. 1990, 55, 624-631; 1990, 55, 3966. (6) Korth, H. G.; Muller, W.; Lusztyk, J.; Ingold, K. U. Angew. Chem., In(. Ed. Engl. 1989, 28, 183-185.
0022-3654/90/2094-8835$02.50/0 I
,
According to this hypothesis,sulfonyloxyl radicals, R S ( = O ) Q , 2, would also be expected to be highly electrophilic, oxygencentered radicals.' They should be formed by photolysis of bissulfonyl peroxides, 3, compounds that have frequently been used 0
0
3a, R = CH3 3b, R = 3-CF&H4
0
2a, R = CH3 2b, R = 36F3CbH4
as polymerization initiators and as sulfonylating reagentsg Despite the use of these peroxides for such reactions almost nothing is known about the structure and reactivity of the presumed sulfonyloxyl radicals. We report herein on the generation and (7) In contrast to carbonyloxyls the RS02' radicals have been shown by EPR spectroscopy* to have ca. 40% of their unpaired spin density located on the sulfur atom. These radicals are therefore best referred to as sulfonyl radicals and represented as R$(O)O and not as sulfinyloxyl radicals, RS(0)o'. Sulfonyl radicals are not, therefore directly comparable to the Y,X(0)O'family of radicals that we have been investigating, all of which have X in its highest (or only) valence state and hence have the unpaired electron very largely located on oxygen. (8) Freeman, F.; Keindl, M. C. Sulfur Rep. 1985, 4 , 213-305. Chatgilialoglu, C. In The Chemistry of Sulphones and Sulphoxides; Patai, S., Rappaport, Z., Stirling, C. J. M., Eds.; Wiley: New York, 1988; Chapter 25, DD .. 1089-1 113. (9) Hoffman, R. V. In The Chemistry of Sulphones and Sulphoxides; Patai, S . , Rappaport, Z., Stirling, C. J. M., Eds.; Wiley: New York, 1988; Chapter 9, pp 259-211 and references therein.
0 1990 American Chemical Societv
