31
Formation of Radical Zwitterions from Benzoic Acids
Formation of Radical Zwitterions from Methoxylated Benzoic Acids. 2. OH Adducts as Precursors P. O'Neill, S. Steenken, and D. Schulte-Frohllnde Institut fur Strahlenchemie im Max- Planck-Institut fur Kohlenforschung, D 4330 Mulheim a.d. Ruhr, West Germany (Received March 29, 1976) Publication costs assisted by Institut fur Strahlenchemie in Max-Planck-Institut fur Kohlenforschung
Using spectrophotometric and conductometric pulse radiolysis and in situ electron spin resonance techniques it was found that OH adducts, formed from mono-, di-, and trimethoxylated benzoic acids by attachment of OH to nonsubstitutedring positions, react with H+ (k = 108-109M-' s-l) to yield radical zwitterions via elimination of H20from the protonated OH adduct. With OH adducts from 2,4- and 2,6-dimethoxybenzoicacid and 2,4,5and 2,4,6-trimethoxybenzoicacid radical zwitterions are additionally produced by a pH independent process involving elimination of OH- ( k i= 4 X lo4s-l). The radical zwitterions react with H adducts with rate constants of (8-40) X 10' M-l 5-l. TABLE I: Spectral Characteristics and Rate Introduction Constants for Formation and Decay of OH Radical cations have been proposed as transient Adducts at pH 7-8 products from reactions in aqueous solution of the hyOH adduct k(OH + droxyl radical with a variety of In the case of 2k(OH benzene and its derivatives the available e v i d e n ~ e ~ ~ , ' , ~ h m a x , E , M-' substrate)? adduct), nm cm-' M-' s - ~ M-' s-' Substrate indicates that, in contrast to an earlier suggestion,' formation of one electron oxidation products proceeds via 2900 3.7 X l o 9 2.3 X 10' 330 2-MBA addition of OH followed by protonation of the OH adduct 4270 4.5 X l o 9 2.3 X 10' 3-MBA 350 and subsequent elimination of water. This mechanism is 370 3410 4.9 X l o 9 2.5 X 10' 4-MBA 2,3-DMBA 320 3400 6.8 X l o 9 2.1 X 10' possibly of a more general importance since it also applies 3390 8.3 X lo9 6.0 X l o 8 3,4-DMBA 310 to electron transfer reactions between OH and metal ions 370 3100 such as T1+l0and Ag+.ll As pointed out by Walling,3the 2,4-DMBA 350 4090 6.9 X l o 9 4.3 X 10' addition-protonation-dehydration mechanism is probably 2,6-DMBA 350 4320 4 . 5 X l o 9 3.9 X 10' preferred as compared to direct electron transfer since in 340b 4030 4.8 X l o 9 5.0 X 10' 3,5-DMBA the former case the heat of formation of water contributes 390 5500 2,3,4-TMBA 350 3050 6.8 X l o 9 1.5 X lo8 to the driving force of the overall reaction. 3,4,5-TMBA 390 6000 9.0 X lo9 1 . 6 X 10' In the majority of cases, radical cation formation from 2,4,5-TMBA 31OC 6400 4.8 X l o 9 5.0 X 10' benzene derivatives has been observed with benzenes 2,4,6-TMBA 350 4910 8.1X lo9 carrying electron-donating substituents. The present a Determined at pH - 7 using competition method with investigation was undertaken to elucidate the mechanism KSCN. Using a value19 for h o ~ +of ~7.5~X l~o 9-M-' of radical cation formation in the reaction of OH with s-' gives good agreement with the directly measured rate aromatics substituted by both electron-donating and constants. Shoulder. An additional absorption maxelectron-withdrawing groups. imum at h 420 nm was observed and is assigned to phenoxy1 radical.'' Experimental Section The pulse radiolysis and in situ radiolysis ESR systems anomalies in their reactions with OH. With 2,4,5-TMBA and 3,4-DMBA increases in absorption were observed after were as d e ~ c r i b e d . ~ JWith ~ the conductivity method, measurements were limited to 3.5 I pH I 11. The optical the pulse at 420 and 360-400 nm, respectively, extending density values used for plotting the spectra refer to a dose over 30-50 ps. These absorptions are not due to OH adducts, the formation of which is complete after -5 ps, of 3 krads and an optical path length of 1 cm. The substrates were obtained from Fluka, Merck, and Aldrich or to radical zwitterions, since the absorptions differ from and were used as received. those of the zwitterions as measured12in the T1+,Ag+,or Sz02- systems. A similar observation has previously been Results and Discussion reportedg concerning the reaction of OH with 1,4-diOptical Studies. Reaction of Substrates with OH. The methoxybenzene. The absorptions are assigned to pheoptical absorption spectra of the products of the reaction noxyl radicals produced15by oxidative demethoxylation of OH with 2-, 3-, or 4-methoxybenzoicacid (MBA) or 2,3-, by the OH radical. In the case of 2,4,6-TMBA the decay 2,4-, 2,6-, 3,4-, or 3,5-dimethoxybenzoic acid (DMBA) or of the OH adduct is mixed order and a further absorption, 2,3,4-, 2,4,5-, 2,4,6-, or 3,4,5-trimethoxybenzoic acid which is assigned to the radical zwitterion, "grows in" at (TMBA), measured using NzO saturated 0.1 mM solutions A,, 580 nm. In the pH range 7-10 its rate of formation at pH -7, have absorption bands at X ~300-400 nm follows first-order kinetics ( K i= 4 X lo4 s-l) and is inde(Table I), in agreement with OH adduct spectra9J3J4of pendent of 2,4,6-TMBA concentration (0.1-1 mM). This related aromatics. The rate constants for bimolecular excludes production of the zwitterion by direct electron decay of the OH adducts (Table I) are similar to that transfer between OH and 2,4,6-TMBA. The yield of reported14 for the OH adduct of benzoate. zwitterion observed at pH 9 is -20% of G(0H). In comparison with the other methoxylated benzoic Reaction of OH Adducts with H+. When the pH of the acids, 3,4-DMBA and 2,4,5- and 2,4,6-TMBA show some substrate solutions was decreased to 7 have ~ - are represented as decreases instead been multiplied by p ~ + / p oand of increases. The changes are constant in the pH region 7-10. Both dependences were determined using -600 rads11 /*s pulse.
sorptions, which were absent in neutral solutions with the exception of those of 2,4,6-TMBA, were observed at X -300 nm and X 2400 nm (Figure 1). These new absorptions “grow in” while those due to the OH adducts decay with an enhanced initial rate as compared to the situation at pH 7. From the agreement of the absorption spectra at X 1400 nm with those observed12in the TP, Ag+, and Sz02‘ systems it is concluded that the absorptions are due to radical zwitterions, the presence of which was confirmed by the in situ ESR method using N2O saturated 1mM solutions at pH -3. Under these conditions, with the exception of that from 2,6-DMBA, the stationary concentrations of the zwitterions are however considerably lower thanlZin the T1+ or Ag+ systems. With 2,4,6-, 2,3,4-, and 3,4,5-TMBA and 2,4- and 2,6DMBA the optical absorption spectra of the OH adducts and those of the zwitterions are sufficiently separated to compare the initial rates of decay of the former with the rates of formation of the latter. Within the limits of experimental error, these rates were found to be the same. The rates of production and the yields of radical zwitterion increase with increasing [H+] in the pH range -5 to -2 (Figures 2 and 3) indicating that the OH adducts are converted to radical zwitterions by reaction with protons. The rate constants for reaction of the OH adducts with H+, as determined in the pH range 5-4, are 108-109 M-’ s-l. This value is comparable to thoseg observed with methoxylated benzenes. At pH 5 2 the rates of formation of radical zwitterions approach those for addition of OH to the aromatic ring. At pH 1 the yields of radical zwitterion are 290% of G(0H) for 2,4-, 2,6-, and 3,5-DMBA and 2,4,6-TMBA, which are all characterized by methoxyl groups in meta
positions to each other, whereas for the remaining di- and trimethoxylated substrates, which carry methoxyl groups in ortho or para positions relative to one another, the yields are estimated to be 160% of G(0H). Completely analogous results have been reportedQ using di- and trimethoxylated benzenes; with these the maximum yields of radical cation, produced by reaction of protons with the OH adducts, also depend on whether the methoxyl groups are in an ortho/para or in a meta position relative to each other! The differences in yields are explained by selective addition of OH to ring positions followed by either radical zwitterion formation or elimination of methan01.l~With the exception of 2,6-DMBA and 2,4,6-TMBA the H+ concentration needed to produce 50% of the maximum obtainable yield of radical zwitterion is -10 times higher than thatgnecessary with the correspondingmethoxylated benzenes. The exceptional behavior of 2,6-DMBA and 2,4,6-TMBA may be related to the noncoplanarity16of the carboxyl group with the ring by which the electronwithdrawing effect of the carboxyl group, which should lower the basicity of the OH adducts, becomes less effective. Due to formation of radical zwitterions, the pKa values of the OH adducts are difficult to determine. However, on the basis of -410-20-nm spectral shifts of the OH adduct absorptions on lowering the pH, the pKa values are estimated to be in the same region as the pKa values of the substrates. The pKa of the OH adduct of benzoic acid is 0.2 units above that of benzoic acid it~e1f.l~ Decay of Radical Zwitterions. The rates of decay of the radical zwitterions formed in acid solutions depend on pH and on the individual methoxylated benzoic acid. At pH >5 with all the substrates the rates tend toward first order
2
L
6
6
pH
-
The Journal of Physical Chemistv, Vol. 8 1, No. 1, 1977
33
Formation of Radical Zwitterions from Benzoic Acids
Scheme I PRODUCTS
I
+ cyclohexadienyl radicals
Y 4 A”
I
t
t cyclahexa-
OH
dienyl radicals
0 ,
disnyl radicals
T
radical zwitterions
PRODUCTS
PRODUCTS
0 +
OH@
and are different from the decay rates of the OH adducts. Due to overlap of the OH adduct spectra with those of the radical zwitterions and due to differences in yields of zwitterion (Figure 3) from individual methoxylated benzoic acids, below pH 5 the kinetic behavior of the majority of the transients is complicated. The kinetic behavior can however be explained by reaction of radical zwitterions with OH or H adducts. The rate constants for the former reaction were determined in the pH range 3-5 and are in system. agreement with those12 measured in the S20E2The rate constants for reaction of zwitterions with H adducts were measured at pH 1-2 and were found to be in the region (8-40) lo8 M-l s-l. With 2,6-DMBA and 2,4,6-TMBA the rate of decay of the zwitterions at pH 3 is second order and the respective rate constants are the same as those12 measured in the Tl+ or Ag+ systems. Conductometric Studies. In order to support the conclusions drawn from ESR and optical measurements concerning formation of radical zwitterions by reaction of OH adducts with H+, conductivity experiments were performed in the pH range 3.5-9 using N 2 0 saturated solutions containing 0.1 mM substrate. At pH 5-7 a decrease in conductivity within the first 100 ps after the pulse was observed with all the substrates. This decrease is explained by assuming that H+(p = 34 X V-l cm2 s-l) is replaced by nonconducting radical zwitterions. In order to support this concept, the extinction coefficients of the radical zwitterions from 2,6-DMBA and 3,4,5- and 2,4,5-TMBA were determined, for solutions at pH -6, by combining G(radical zwitterion) measured by conductivity and G(radica1 zwitterion) X E obtained optically. The
extinction coefficients thus calculated are in agreement to within &lo% with those12determined using the persulfate system. The dependence on pH of G(maximum conductivity decrease) at pH pH > pK,(zwitterion) I 2.12 At pH 4-7, the rate of decrease of conductivity is the same as the rate of formation of the radical zwitterions, observed optically at the same pH value, demonstrating that with both methods the same process is measured. The conductivity decrease is followed by a return of conductivity on the millisecond timescale, but not to the original value prior to the pulse as was observedg using methoxylated benzenes. At pH >5 the rate of return of conductivity follows first-order kinetics with the same rate constants as those for decay of the radical zwitterions measured optically. With 2,4- and 2,6-DMB and 2,4,5- and 2,4,6-TMBA at pH >7, an increase of conductivity was observed extending over the first 100 ps after the pulse. The reaction responsible for this conductivity increase is much slower than The Journal of Physical Chemistv, Vol. 8 1, No. 1, 1977
34
that between OH and substrate, in agreement with the kinetic observations obtained optically. The conductivity increase is assumed to be due to reaction of a fraction of the OH adducts to yield OH- and radical zwitterion (reaction 11). Using p = 1.8 X V-l cm2 s-l for OH-, G(radical zwitterion) at pH 7-9 is equal to -5% of G(0H) for 2,4- and 2,6-DMBA and 2,4,5-TMBA and 21% for 2,4,6-TMBA. Reaction Scheme for Formatiopof Radical Zwitterions. Reaction scheme 1,which is similar to those previously proposed for methoxylated benzenesg and other arom a t i c ~ summarizes ,~*~ the experimental observations using 2,4,6-TMBA as an example. The initial attack of OH radicals is by addition to the aromatic ring to produce hydroxycyclohexadienyl radicals which were observed optically. Only those OH adducts are assumed to yield radical zwitterions where OH is not attached15 to a ring position which carries a methoxyl group. The main path leading to radical zwitterion involves steps A(A')-D. This reaction sequence accounts for >95% of the total zwitterion yield for all solutes with the exception of 2,4- and 2,6-DMBA and 2,4,5-TMBA, for which the contribution from steps A(A')-D is -95% and of 2,4,6-TMBA, for which steps A(A')-D contribute 80% to the total zwitterion yield. A t pH
