Reaction of OH Radicals with 2- and 4-Pyridones
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979 2407
positions. Thus generalization of the conclusions to equations for the angular dependence over all dihedral angles must be made with caution.
The treatment in the previous paragraph is based on the premise that though alanine provides a nonrepresentative J,,, the value for K,, may be taken as typical of other amino acids, An argument supporting this assumption is that the observed value of 2K,, = 8.4 for alanine nearly falls on the straight line for amino acids at t = 1/3 in ref 3. Equation 15 emphasizes again the strong linkage between JT and JG, on one hand, and KT and KG,on the other. Conformational analysis by both proton-proton and carbon-proton vicinal coupling constants should employ a consistent set of JT, JG and KT, KG parameters. It is still necessary to supply one more item of information before assigning values to the four coupling constant parameters contained in R. From examination of observed coupling constants in amino acids, the minimum value of JT> 13.0 Hz, and the sum JT JG approaches 15.8 H z . ~A set of values consistent with all equations and recommended for use is KT = 10.0, KG = 1.2, JT = 13.3, and JG = 2.4 Hz. Consideration of the standard deviations in the slope and intercept of Figure 2 suggests standard uncertainties of f0.2 in KT and KG, and of f0.3 in JT and JG. These recommended values of the four coupling constant parameters apply to the dihedral angles actually occurring in amino acids and may not correspond exactly to 60" (300') and 180' dihedral angles for gauche and anti
Experimental Section For 1 M alanine in neutral aqueous solution the carboxylate 13C NMR spectrum consists of a pair of overlapping quartets. The carboxylate 13Ccoupling constants were found to be 4.2 Hz for the three-bond coupling to the p protons and 5.0 Hz for the two-bond coupling to the (Y proton. The spectrum was obtained on a JEOL PS100/EC 100 Fourier transform spectrometer. A spectral width of 200 Hz was used with 4K data points in the frequency domain and 10 000 accumulations were signal averaged.
+
References and Notes J. J. M. Rowe, J. Hinton, and K. L. Rowe, Chem. Rev., 70, 1 (1970). R. 6. Martin and R. Mathur, J . Am. Chem. SOC.,87, 1065 (1965). W. G. Espersen and R. B. Martin, J. fhys. Chem., 80, 741 (1976). J. P. Casey and R. 6. Martin, J . Am. Chem. SOC.,94, 6141 (1972). K. G. R. Pachler, Spectrochim. Acta, 20, 581 (1964). V. F. Bystrov, Prog. NMR Spectrosc., 10, 41 (1976). J. Feeney, J. Magn. Reson., 21, 473 (1976). (8) A. J. Fischman, H. R. Wyssbrod, W. C. Agosta, and D. Cowburn, J . Am. Chem. Soc., 100, 54 (1978). (9) T. Akimoto, M. Tsuboi, M. Kainosho, F. Tamura, A. Nakamura, S. Muraishi, and T. Kajiura, Bull. Chem. SOC.Jpn., 44, 2577 (1971).
(1) (2) (3) (4) (5) (6) (7)
Reaction of OH Radicals with 2- and 4-Pyridones in Aqueous Solution. An Electron Spin Resonance and Pulse Radiolysis Study S. Steenken" and P. O'Nelll' Institut fur Strahlenchemie im Max-Planck-lnstitut fur Kohlenforschung, 0-4330 Mulheim, West Germany (Received April 3, 1979) Publication costs assisted by the Institut fur Strahlenchemie
OH radicals react with 2- and 4-pyridone (h = 4.4 X lo9 and 3.6 X lo9 M-' s-l , respectively) and with 2- and 4-pyridonecarboxylic acids by preferential attachment to the 3 or 5 position of the pyridone ring. With the exception of 4-pyridone-2,6-dicarboxylic acid, the OH adducts thus formed undergo keto-enol tautomerization to yield OH adducts of the pyridol type ( h Z lo3 s-'). A t pH I 10 the OH adducts deprotonate to yield the corresponding radical anions. The ionized OH adduct of 4-pyridone eliminates OH- (h = 1.8 X lo4s-l) to yield the pyridin-4-oxy1radical which is able to oxidize ascorbate. The pyridin-4-oxy1radical can also be obtained from the OH adduct of 4-pyridone by H+ assisted dehydration (h = 2.5 x lo6 M-' s-l). Corresponding reactions involving the OH adduct of 2-pyridone were not observed.
Introduction Heterocyclics containing double bonds have been to be very reactive toward the radicals produced in the radiolysis of water, i.e., H, ea;,, and OH. These species react with the heterocyclics typically by addition. In the case of OH, a distinct selectivity with respect to the sites of attachment is frequently o b ~ e r v e d . ~This - ~ selectivity appears to result from and reflect the electrophilicity10 of OH. Pyridones, which may be considered as model compounds for biologically relevant heterocyclics like, e.g., uracil, in nonradical electrophilic substitution show a reactivity similar to that of benzene and the distribution of electron density at the ring positions'' is such that electrophilic reagents preferentially attack positions 3 and 5.12 It was considered of interest to investigate whether in the reaction of pyridones with the OH radical an analogous selectivity is exhibited. Furthermore, the pyridones may also exist as pyridols (eq A). In the case of 2-pyridone and 4-pyridone, the pyridone/pyridol ratio in aqueous solution is 340:l and 0022-3654/79/2083-2407$0 1.OO/O
R
?-
2200:1, respectively.12 Since the pyridols may be compared with phenol, the OH adducts of pyridones could, in principle, undergo elimination of water to yield pyridinoxyl radicals, analogous to the reaction observed13-15in the case of OH adducts of phenol.
Experimental Section The 3-MeV van de Graaff accelerator and the optical and conductivity systems have been described.16 Solutions, saturated with NzO in order to convert e,; into OH, were irradiated at 20 f 2 'C with electron pulses of 1-ps duration. The pH was adjusted with HC104 or NaOH. Dosimetry was performed with either 10 mM FeS04 in 0.8 N H2S04or C(N0z)4.17The optical density (OD) values have been normalized to 3 krd and refer to a 1-cm pathlength. 0 1979 American Chemical Society
2408
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
S. Steenken and P. O'Neill
TABLE I: Structure and ESR Parametersa of Radicals Produced by Reaction of OH with Pyridones a t
2
2'
12.17
5, c, d, e
3b
11.18
3.28
U
14.12
H
-ox. - ' -A ,o-
2.00280
11.69 H
0'
10.94
1.63
10 14.49
'&OH
02c
0.97
12
~0.06
00.83
H&H
017.56
OH 0.36
0.38 *YO.91
cO;
co;
-02c 2.00
HI .oo
2.00451
11
2.49
9
'p2 2.00287
-20.7
2.00287
2.53 H
6.52
2.72
I ,61
8b
10.94 H
13.63
HO
2.00468
7 I c, d, e
IH,H,
22.57 H,
11.69
0.85
2.00256
- 5 "C
2.00349
2.00345
15'
14 0.27
Ho
I4'l6
"
2.39
Ho.58
2.002 9 6
2.00339
2.00336
22.30
3.60/4.48
5.23
4.48/3.60
o,54
0.10
2.00489
a The coupling constants are in gauss, The g factors are corrected for second-order effects, Where two coupling constants are given for a proton, the sign/represents or. Secondary radical. A t pH 11 the splitting of the methylene OH proton is invisible due t o base-catalyzed exchange. The coupling constants found for the OH adduct of 2-pyridone can be interpreted to be due t o either 5 or 7 (see text). e As a result of polarization the low-field lines due t o coupling of the methylene proton were not visible. This coupling and the g factor of the radical could therefore not be measured. The methylene coupling given in the table was calculated by assuming that the difference in g factors between 5 ( 7 ) and 5' ( 7 ' ) is equal to that between 2 and 2', Le., that the g factor of 5' ( 7 ' ) is 2.00331.
The in situ radiolysis ESR experiments were carried out by using the method described by Eiben and Fessenden.ls The concentration of substrate in the N20-saturated solutions was 1-5 mM. The g factors and coupling constants of the radicals (accurate to 30 mG and 5 X respectively) were determined from simultaneous measurements of field and microwave frequency, taking account of the difference in magnetic field between the ESR cell and NMR probe positions. The substrates were obtained from Fluka and Aldrich and were used as received.
Results and Discussion 1. ESR Studies. 5-Carboxypyridone-2. On reaction of 5-carboxypyridone-2 (6-hydroxynicotinic acid) with OH radicals at pH 5-10, we observed an ESR spectrum resulting from the interaction of the unpaired spin with five nonequivalent protons and one nitrogen. The coupling parameters taken from this spectrum (Table I) are interpreted in terms of radical 2, formed by addition of OH to C3 followed by keto-enol tautomerization: B
A
1 1
I ;
2
2'
A reaction analogous to step 1B has previously been proposed in the case of the OH adduct of a pyrone derivative.lg The identification of radical 2 and the assignment of the coupling constants to individual protons (Table I) is based on a comparisoq of the coupling parameters of 2 with those of OH and e- adducts of carboxypyridines. In the case of the OH adducts the coupling constants of the ring protons depend in a characteristic way on the positions of the protons relative to the hydroxymethylene group, CHOH.g On this basis, the 12.17- and 13.60-G splitting5 (Table I) are due to protons ortho and para, respectively, to CHOH. An OH adduct of this type can only be formed by addition of OH to C3. The assignment of the radical observed to structure 2 rather than 1 is based on a comparison of the coupling constants of the exchangeable protons of the radical with of the NH protons of protonated eadducts of carboxypyridines. In these radicals the coupling constant for the NH proton is of a similar magnitude as that for the nitrogen, whereas in radical 2 the coupling constants of the exchangeable protons are considerably smaller than a(N). No lines were seen from additional radicals, possibly formed by attachment of OH to positions other than Cs. From the signal-to-noise ratio observed for 2 and assuming equal line widths and chemical lifetimes for any additional radicals, it is concluded that 280% of the OH radicals add to c3. In the pH region 11-12 the lines due to 2 broaden and are difficult to detect. At pH 13 a new spectrum is observed which is characterized by smaller couplings (Table
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979 2409
Reaction of OH Radicals with 2- and 4-Pyridones
Figure 1. Second derivative ESR spectra observed on radiolysis of a N,O-saturated aqueous solution containing 5 mM chelidamic acid at pH 12 and -5 O C . The lines denoted by I are from an impurity or secondary radical. Q denotes the quartz signal. The spectrum of the H adduct shows second-order splitting of the central lines.
I) than those of 2 and by the absence of the two smallest splittings. This spectrum is interpreted to be due to 2’, formed from 2 by deprotonation from C2-OH (step 1C). This assignment is based on the following arguments: (a) If an OH group is attached to a conjugated system, ionization of OH generally results in a reduction of the unpaired spin density in the remaining part of the system, due to increased localization of the unpaired spin at the ionized oxygen. Of the two OH groups present in 2, the OH group at Cz is expected to be the one undergoing deprotonation, due to the phenolic character of C2-OH. (b) On the basis of a comparison of 2 with OH adducts of pyridine and carbo~ypyridines,~ the OH group at C3 is not expected to undergo ionization at pH 13. The absence at pH 13 of the C3-OH coupling is therefore suggested to result from base-catalyzed exchange of the OH proton, as observed in analogous cases.7~27,28 In the pH range 10-13 relatively strong lines from an additional radical were observed, whose ESR spectrum results from the interaction of the lone electron with two nonequivalent protons and one nitrogen atom. On the basis of the g factor of the species (2.00468), which is characteristic of semiquinone type radical^,^^^^ the radical is identified as 3 (Table I). From the fact that the amplitude of the lines due to 3 increases with decreasing flow rate of the solution containing 5-carboxypyridone-2 it is concluded that 3 is a secondary radical. It is probably produced via 5,6-dihydroxynicotinic acid, formed from 2 (or 2’) by disproportionation. 2-Pyridone. On reaction of OH radicals with 2-pyridone at pH 3-10, we observed lines from essentially only one radical. The coupling constants derived from the spectrum (Table I) are interpreted as due to one OH proton, one nitrogen in a pyridol type ring, one methylene proton, one proton meta to the CHOH group, and either (a) two protons ortho to CHOH or (b) one proton para and one proton ortho to CHOH, i.e., to either31 radical 5 or 7 , formed by addition of OH to C3 or C5 followed by ketoenol tautomerization (eq 2). From the fact that lines
0.7 H
4
5
6
7
7’
assignable to 4 to 6 were not observed it is concluded that the rate constant for keto-enol tautomerization is >lo3 s-l.
From the signal-to-noise ratio observed for 5 (or 7 ) and assuming equal line widths and chemical lifetimes for any additional radicals it is estimated that 280% of the OH radicals add to C3 or to C5.32 A t pH 10 the lines of 5 (or 7 ) are broadened due to the onset of OH- catalyzed exchange of the methylene OH proton. At pH 11 the lines are sharp again but their number is half of that at pH ell. This is caused by the absence of the methylene OH splitting. Since the coupling constants of the remaining nuclei at pH 11 are identical with those measured at pH ell, the reversible disappearance of the methylene OH splitting must be due to rapid exchange of the OH proton and not to ionization of this OH group or of C2-OH. The pK, of 5 (or 7 ) is therefore concluded to be >11. At pH >11the spectrum of 5 (or 7 ) became progressively less intense and more polarized. At pH 13 new lines, due to 5’ (or 7’1, were visible but their intensity was insufficient for a complete interpretation (see Table I, footnote e). At pH 211 lines from the 2,3-dihydroxypyridine semiquinone radical anion 8 (Table I) were additionally observed. From the fact that the stationary concentration of this radical decreased with increasing flow rate of the solution containing 2-pyridone it is concluded that 8 is a secondary radical produced via 7 . 8 could also be obtained by reaction of OH with 2,3-dihydroxypyridine a t pH 29, in which case 8 is the main primary reaction product. 2,6-Dicarboxypyridone-4 (Chelidamic Acid). On reaction of OH with 5 mM chelidamic acid at pH 16, we observed very intense lines (Figure 1). In the pH region 6-10 the spectrum, which is assigned to radical 9, is pH independent. At pH 10-10.5 lines from a new radical (lo), which is similar to 9, appear while the intensity of the lines due to 9 reversibly decreases. On the basis of this behavior and since the spectrum of 10 contains one doublet splitting less than that of 9, 10 is identified as the conjugate base of 9. 0
0-
0-
9
10
The assignment of the zwitterionic structure to 9 is based on a comparison of the coupling constants of 9 and 10 on one hand with those of 2 and 2/ or 5 ( 7 ) and 5’ (7’) on the other. In the case of 9/10 the pattern of change in the coupling constants on protonation-deprotonation is distinctly different from that of 2/2’ or 5 (7)/5’ (7’). This indicates that with 9/10 the changes in the electronic character of the molecule are of a different nature than those in the case of 2/2’ or 5 ( 7 ) / 5 ’ ( 7 9 . Since on de-
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The Journal of Physical Chemistry, Vol. 83, No. 18, 7979
S. Steenken and P. O’Neill
protonation a pyridone is converted into a pyridol, the At pH >9 the amplitudes of the lines due to 14 de(triply) deprotonated OH adduct of chelidamic acid (10) creased but no lines assignable to an anion of 14 were is necessarily of the pyridol type. The changes in the observed. At pH 10-11lines from a radical (15) were seen coupling constants observed on protonation of 10 to yield which contained one ring proton less than 14. The fact 9 are interpreted as resulting from conversion from a that the stationary concentration of 15 increased with pyridol to a pyridone structure. In support of this sugdecreasing flow rate (i.e., with increasing dose rate) ingestion is the fact that with 9 a(N) is of comparable dicates that 15 is a secondary radical. 15 is tentatively magnitude as a(H)”. This seems to be characteristic of identified as the pyridine-3,4-semiquinoneanion (see Table pyridine-derived radicalswz6 containing NH as a structural I). element. From a comparison of the coupling constants of As shown later (see section 2b), at pH >9 14 is converted 10 with thoseg of l-aza-2,6-dicarboxy-3-hydroxycyclo- to the pyridin-4-oxy1 radical. Although lines from this hexadienyl radical, produced by addition of OH to C3 of radical could not be seen, its existence could be demon2,6-dicarboxypyridine, it is seen that replacement of H at strated indirectly by the observation of the ascorbate C4 by 0- results in a considerable decrease of the coupling radical on irradiation of a solution at pH 11containing 5 constants of all nuclei. With respect to the benzene mM 4-pyridone and 0.1-0.5 mM ascorbate. By analogy system, an analogous conclusion can be drawn from a with phenoxyl radicals, which have been shown%to oxidize comparison of the coupling constants of the 1,3-diascorbate specifically, we suggest that the observation of carboxy-6-hydroxycyclohexadienylradical2@with thosez9 the ascorbate radical is an indication of the formation, at of the 1,3-dicarboxy-5,6-dihydroxycyclohexadienyl radical. pH 11, of the pyridin-4-oxy1 radical. In the pH region 6-10 weak lines from an additional 2. Pulse Radiolysis Studies. 4-Pyridone. ( a ) p H 57. radical containing two equivalent protons were observed. The optical absorption spectrum, measured 10 p s after the The intensity of the lines of this radical is 110% of that pulse on irradiation of a 0.5 mM solution of 4-pyridone of 9. The radical is identified as the H adduct of chelisaturated with N 2 0 at pH 6.8, is shown in Figure 2. On damic acid (11, see Table I). The coupling constants of the basis of the ESR results described above, the optical the H adduct are independent of pH in the region 6-10 absorption spectrum is assigned to the l-aza-3,4-dibut change at pH 10-11as a result of deprotonation of 11 hydroxycyclohexadienyl radical 14. The shape of the to yield 12. Whereas 12 necessarily has a pyridol type absorption spectrum of 14 did not change within 100 ps after the pulse. From this a rate constant of >lo5 s-l can structure, from the coupling constants of 11 it is not clear whether 11 is of the pyridone or pyridol type. From a be calculated for conversion of 13 into 14, provided the comparison of the coupling constants of the H adduct 12 optical absorption spectra of 13 and 14 are different. The with those of the corresponding OH adduct 10 it is seen rate constant for reaction of OH with 4-pyridone a t pH that substitution in the methylene group of H by OH 6-7 was determined to be 3.6 X lo9 M-’ s-l, using the results in a drastic decrease of the coupling constants of competition method with SCN- and taking the value34 ~(OH+SCN-)= 7.5 x 109 M-I s-1*35 all nuclei. This effect is different from that observed in the case of H and OH adducts of carboxybenzenesz@and At pH -7 the decay of 14, monitored at 400 nm, follows second-order kinetics with a rate constant (2k) of (1f 0.1) c a r b o x y p y r i d i n e ~where , ~ ~ ~ ~replacement ~~~ of H by OH in the methylene group results essentially in a reduction of x 109 M-I s - ,~based on t400nm = 8180 M-l cm-l and asG(OH) = G(14) = 5.5. As the absorption of the only the coupling constant of the methylene p r o t ~ n . ~ ~ ~suming @ OH adduct decays an absorption with ,,,A = 320 nm At pH 13 the number of ESR lines of 10 is reduced by “grows in”; this absorption is assigned to product(s) formed a factor of 2 as compared to that at pH 11-12. This is due on reaction of 14 with each other. to disappearance of the splitting due to the OH proton of On decreasing the pH to 9 the decay kinetics of the OH adduct of 4pyridone change from second to first order on increasing the pH. At pH >11, the rate constant for decay of 14” is (1.8 f 0.3) x lo4 s-’ and is independent of [OH-] (1-10 mM) and of [4-pyridone] (0.1-0.5 mM). These results are interpreted in terms of OH- elimination from 14” to yield the pyridin-4-oxy1 radical as shown in reaction 6. The optical absorption spectrum of pyridin-4-oxyl, measured at pH >11,is similar to that measured at pH 1. Since the 1500 extinction coefficient of pyridin-4-oxy1 is low M-I cm-l) and since its absorption spectrum is broad and relatively uncharacteristic, additional evidence for the formation of pyridin-4-oxy1 was obtained by pulse irradiating a N20-saturated solution containing 5 mM 4pyridone and 0.5 mM ascorbate at pH 11.2 and monitoring the absorption at 300-400 nm, where the ascorbate radical absorb^.^^^^^ Under these conditions, the ascorbate radical was found to be formed within 400 1.1safter the pulse. The ascorbate radical is suggested to be produced by oxidation of ascorbate by pyridin-4-oxyl. This reaction is analogous to that33 involving phenoxy1 radicals. Using €(ascorbate = 3600 M-’ we determined G(pyridin-4-oxyl) to be 4.9 f 0.4. The rate of decay of 14”, monitored at 400 nm, was found to be independent of ascorbate concentration (0.2-0.5 mM), confirming assignment of the initial absorption to 14”. Due to strong overlap of the spectrum of the ascorbate radical with that of 14”, it was not possible to measure the rate constant for formation of the ascorbate radical. The dissociation and elimination steps in reaction 6 result in changes of the conductance of 4-pyridone solu-
-
Flgure 3. Optical absorption spectrum measured 10 w s after a 1-bus pulse in a N,O-saturated aqueous solution containing 0.5 mM 2-pyridone at (a) pH 7 (X) and (b) pH 13 (0).
tions on reaction with OH. At pH >9 a decrease of conductance, which occurs within 10 ps after the pulse, was observed followed by an increase of conductance occurring within 400 ys, which is on the same time scale as observed optically for the decay of 14”. The increase of conductance falls short of reaching the value prior t o the pulse by -20%. The changes in conductance are interpreted in terms of production of Ht (step 6A) which reacts with OHto yield a reduction in conductance. The subsequent increase of conductance is due to production of OH- (step 6B). On the millisecond time scale further changes of conductance occur which are suggested to be due to formation of products with pK values different from that of 4-pyridone. The amount of the initial (510 ps) decrease in conductance increases on increasing pH; however, the changes are difficult to analyze accurately in terms of yields, due to the buffering effect of 4-pyridone (pKa = 11.09). The buffering effect is apparent from the observation that the amount of the initial decrease goes through a maximum at pH -10.6. In summary, from the optical and conductivity results obtained at pH >9 it is concluded that the pKa of 14 is 10.0 and that OH- is eliminated from 14” to produce the pyridin-4-oxy1 radical which may be scavenged by ascorbate. It is of interest to note that the rate constant for OH- elimination from 14” is at least 2 orders of magnitude smaller than that15 for OH- elimination from ionized OH adducts of phenols. The lower rate constant in the case of the pyridine system as compared to the benzene system is suggested to result from and reflect the electronwithdrawing effect of the ring nitrogen. It has recently been shown38that the rate constants for OH- elimination from OH adducts of phenolates decrease with increasing electron-withdrawing power of the substituent; this effect is analogous to that described above. It is also noteworthy that 10, the ionized OH adduct of 2,6-dicarboxypyridone-4, does not eliminate OH- (see section 1). This is suggested . t o be due to the additional electron-withdrawing effect of the 2 carboxyl groups in 10 as compared to 14”. An analogous situation existsz9with respect to the OH adduct of phenolate as compared to the OH adduct of 3,5-dicarboxyphenolate; in this case the carboxyl groups also result in a decrease of the rate of OH- elimination. 2-Pyridone. The optical absorption spectrum measured 10 ps after the pulse on irradiation at pH 6-7 of a 0.5 mM solution of 2-pyridone saturated with N 2 0 is shown in Figure 3. On the basis of the ESR measurements, the spectrum is assigned to radical 5 (or 7). From the absence of spectral changes 510 ys after the pulse, the rate constant for keto-enol tautomerization (4 (6) 5 (7)) can be estimated to be Z105s-l, provided the absorption spectra of the pyridone and pyridol forms of the OH adduct are
-
2412
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
different. At A,, = 350 nm, 4 5 or 7) = 1460 M-l cm-l, assuming G(5 or 7) = G(0H) = 5.5 and neglecting the contribution ( < I O % ) from H adducts. The rate constant for reaction of OH with 2-pyridone at pH 6-7 is (4.4 f 0.5) X lo9 M-ls-l, based on k(0HtSCN-) = 7.5 X lo9 M-l s-l, The former value is in agreement with that39previously measured. In order to obtain further information on the question of whether the OH adduct of 2-pyridone is in the keto or enol state, the optical absorption spectrum of the OH adduct of 2-methoxypyridine, which can only exist in the pyridol state, was measured. The spectrum, which has a maximum at 340 nm and talk = 1990 M-l cm-l, is similar to that of 5 (7). The OH adduct of 2-methoxypyridine decays by second-order kinetics with 2k = (1.2 f 0.2) X 109 M-I s-1 At pH 6I7, the OH adduct of 2-pyridone, monitored at 350 nm, decays by second-order kinetics with a rate constant (2k) of (1.1f 0.2) X lo9 M-l s-l. The absorption spectrum and the decay rate constant of the OH adduct are independent of [H+] in the pH range 3-10. At pH 2, the absorption spectrum, after correction for the contribution due to H adduct, is still the same as that measured at pH 6-7, and from the decay kinetics there is no evidence for an H+-assisted reaction like those observed in the case of the OH adduct of 4-pyridone or of OH adducts13-15of phenols. From the invariance with pH of the absorption s ectrum of the OH adduct it is concluded that the pK f#r protonation of the OH adduct is C1. The absence of doncluctivity changes in the pH range 4-10 on reaction of OH with 2-pyridone is further confirmation that 5 (7)is not charged in that p H region. At pH 10-13, the optical absorption at 370-400 nm increases with increasing pH (Figure 3), which is suggested to result from deprotonation of 5 (7)to yield 5' (7') (eq 2). From the dependence of the OD at 390 nm on pH, the pKa of 5 (7)is estimated to be 11.7 f 0.3, which is similar to the pKa of 2-pyridone (11.6). The deprotonated OH adduct of 2-pyridone, 5' (79,monitored at 390 nm, was found to decay by second-order kinetics with a rate constant (212) of (8 f 1) X lo8 M-l s-l.
Conclusion It has been shown that OH radicals react with 2- and 4-pyridones by preferential addition to the ring positions of highest electron density. With the exception of that of chelidamic acid, the OH adducts thus formed undergo keto-enol tautomerization to yield pyridol type radicals which deprotonate at pH >10 to yield the corresponding radical anions. The deprotonated OH adduct of 4pyridone is the only species that undergoes elimination of OH- to yield a pyridinoxyl radical. The rate constant for
S, Steenken and P. O'Neill
this process is slow as compared to that involving the OH adduct of phenolate. This is suggested to be due t o the electron-withdrawing effect of the ring nitrogen.
Acknowledgment. We thank Schmidt for technical assistance.
H, Selbach and H.
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