372
The Journal of Physical Chemistry, Vol. 82, No. '3, 1978
Communications to the Editor
encouragement during this work. References a n d Notes
0.03
0.06
CC14 ( m o l l l ) Figure 3. Stern-Volmer plot of the fluorescence quenching of diphenylamine (1.0 X M) in the presence of CCI4 in methanol (solid line) and in n-hexane (broken line).
k , is obtained and listed in Table I together with the fluorescence lifetime T~ of DPA in the absence of CC14. In these measurements the extent of the dynamic quenching was remarkable in comparison with that of static quenching. Also dynamic quenching occurred considerably in polar solvent such as methanol. It can be noted that the values of k , in Table I are typical for diffusion-controlled reactions in n-hexane and methanol. Though the transient species in the nanosecond time region cannot be measured directly with the time resolution of flash spectroscopy, it is clear from these results that the formation of transient species is excellently correlated with fluorescence quenching. The transient absorption spectra obtained here are considered to be due to the reaction intermediates via the initiation process of the reaction between DPA and CC4. As was mentioned already, DPA and C C 4 form the charge-transfer complex in the ground state.4 Moreover, our results suggest the existence of an exciplex formed between DPA in the excited state and CC4. From Mulliken's theory5 it is generalized that the donor in the excited state forms the charge-transfer complex more easily than that in the ground state. In addition, the fact that dynamic quenching becomes more distinct in polar solvents supports the existence of the exciplex. However the concentration of charge-transfer complex in the ground state is negligibly small compared with that of DPA, as the value of the equilibrium constants for the complex is 1.8 X M-l. Extinction coefficients for the complex and diphenylamine at their absorption maxima are 3.1 X lo4 and 1.8 X lo4 M-l cm, re~pectively.~ Therefore, the absorption of the charge-transfer complex in the ground state might be neglected in comparison with that of DPA, and this complex is considered to play only a minor role in the photochemical reaction. As for other aromatic amines such as N,N-dimethylaniline and N,N-diethylaniline the initiation reaction with CC14 is assumed to proceed by the same mechanism via a charge-transfer complex in the excited state for the following reasons. These aromatic amines also form charge-transfer complexes with C C 4 in the ground state, and their equilibrium constants are alike.314 Flash photolysis of these amines in C C 4 solvent12or y radiolysis in a CC14matrixg give rise to cation radicals of these amines, and their reaction products are ~ i m i l a r . ~ ~ ~ Acknowledgment. The authors gratefully thank Professor Kenichi Honda for his valuable suggestions and 0022-365417812082-0372$0 1.OO/O
K. G. Hancock and D. A. Dickinson, J. Org. Chem., 39, 331 (1974); C. J. Biaselle and J. G. Miller, J. Am. Chem. SOC.,96, 3813 (1974). D. P. Stevenson and G. M. Coppinger, J. Am. Chem. SOC.,84, 149 (1962); W. J. Lautenberger, E. N. Jones, and J. G. Miller, ibid., 90, 1110 (1968). K. Ikari, K. Maezawa, H. Yoshlda, and T. Tukada, Nlppon Shashin Gakkaishi,36, 12 (1973); K. M. C. Davis and M. F. Farmer, J. Chem. SOC. 8,28 (1967). T. Latowski and B. Zelent, Rocz. Chem., 48, 831 (1974); T. Latowski and E. Sikorska, bid., 42, 1063 (1968). L. E. Orgel and R. S. Mulliken, J. Am. Chem. SOC., 79, 4839 (1957); R. S. Mulliken, "Molecular Complexes", Wiley, New Yo&, N. Y., 1969. Kh. S. Bagdasar'yan, Kinet. Katal., 8, 1073 (1967); V. A. Kondrat'ev and Kh. S. Bagdasar'yan, Khim. Vys. Energ., 1, 197 (1967); 2, 10, 142 (1968). G. N. Lewis and D. Lipkin, J . Am. Chem. Soc., 64, 2801 (1942). R. H. Sprague, H. L. Fichter, Jr., and E. Wainer, Photog. Sci. Eng., 5, 98 (1961); J. Kosar, "Light Sensitive Systems", Wiley, New York, N.Y., 1965, pp 358-370. T. Shlda and W. H. Hamill, J . Chem. fhys., 44, 2369 (1966). Kh. S. Bagdasar'yan and 2. A. Sinitsyna, Dokl. Akad. Nauk SSSR, 175, 627 (1967); D. D. Dmitrievskii and A. N. Terenin, Izv. Akad. Nauk SSSR, Ser. Fiz., 29, 1271 (1965). T. Sawada and H. Kamada, Jpn. Anal., 22, 882 (1973). T. Iwasaki, T. Sawada, M. Okuyama, and H. Kamada, Meeting of the Society of Photographic Science and Technology of Japan, Tokyo, May 1975, paper no. 6-13.
Department of Industrial Chemistry Faculty of Engineering The University of Tokyo Hongo, Bunkyo-ku, Tokyo
Tamotsu Iwasakl" Tsuguo Sawada Masayoshl Okuyama Hltoshl Kamada
Received July 18, 1977
Selectivity of Addition of the Hydroxyl Radlcal to Ring Positions of Pyridlne and Pyridine Mono- and Dicarboxylic Acids. An Electron Spin Resonance Investigatlon Publication costs assisted by the Institut fur Strahlenchemle
Sir: The electrophilic' OH radical has been shown to add in a selective way to ring positions of benzene derivatives containing electron-donating substituents.*-' However, with respect to addition to benzenes carrying electronwithdrawing groups OH seems to be unsele~tive.~-'~ The present study was performed in order to investigate the specificity of OH addition to pyridine and pyridinecarboxylates as examples of aromatics characterized by a built-in electron-withdrawing group. Pyridine and the pyridine monocarboxylates have previously been studied with the pulse radiolysis method'lJ2 and, concerning pyridine monocarboxylates, with the ESR te~hnique.'~ It was c o n ~ l u d e d that ~ ~ -OH ~ ~ radicals add to the heterocyclic ring. However, from the data reported13 no unambiguous conclusions can be drawn concerning the selectivity of OH attack. Reaction of pyridine and of mono- and dicarboxypyridines with OH was carried out by irradiating with 2.8-MeV electrons N 2 0 saturated aqueous solutions containing 5 mM substrate, using the in situ radiolysis ESR technique.14 With all substrates except pyridine3,5-dicarboxylate the radicals observed (Table I) are produced by addition of OH to the least electron deficient positions of the pyridine ring, i.e., positions 3 or 5. From the absence of additional lines in the spectra and from the signal-to-noise (S/N) ratios it is estimated that with pyridine (where S / N was low) and the carboxypyridines 0 1978 Amerlcan Chemical Soclety
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 373
Communications to the Editor
-
TABLE I: Coupling Constantsu (G) and g Factorsbof Radicals Formed on Reaction of OH with Pyridine and Pyridinecarboxylic AcidsC at pH 6-10d and 5 C 9.82h.73
7.65
8.68/8.46
H
H
H
H
3.52
3.44
2.00219
2.00239
2.00255
2.00214
9.72/8.85
9.05B.98
L
3.25
3.18
2.00334
2.OO 258
7.60 2.37
-0,C
OH H 29.37 0.52
H&
N
CO;
'O H 31.95 -02c&OH 13.40
H
2.00274
2.00245
7.41
2.94/1 . I 7
H
H
0.58
N
H
9.19
3.07
3.25
3.40
4.07
2.00270
2.00235
2.00237
2.00335
The coupling constants In those cases where two coupling constants are given for a proton, the sign I represents or. respectively. The g factors are corrected for second-order effects, The and g factors are accurate to 30 mG and 5 X At pH 11-13 the splitting of the OH radicals are derived from the substrates by OH addition to the positions indicated. proton is invisible due to base-catalyzed exchange (see text). a
I
1
Gain x 08
Figure 1. Radicals observed on reaction of OH with 5 mM pyridine-2,4-dicarboxylate at pH 8 and -5
(where S / N was large) the percentage of OH addition to these positions corresponds to 180 and 290%, respectively.15J6 The OH radical is thus seen to exhibit a remarkable selectivity which is in support of the electrophilic character of OH as derived from kinetic studies of OH reactions with substituted benzenes1 and pyridines.17 If either position 3 or position 5 is substituted by a carboxyl group, OH addition takes place a t the unsubstituted position. The tendency of OH to avoid adding to a carbon carrying a carboxyl group is analogous to that observed8J8J9 in the case of carboxybenzenes.20 In the case of pyridine-3,5-dicarboxylate, where both position 3 and position 5 are blocked, 290% of the OH radicals15 add to position 2 (= 6). In the case of pyridine-2-carboxylate and pyridine2,4-dicarboxylate (Figure 1)the stationary concentration of the radicals formed by addition of OH to position 5 is larger by a factor 1.5 than that of the radicals produced by addition to position 3. This preference15 for addition of OH to position 5 over that to position 3 may be due to
OC.
(partial) steric shielding of these positions by the adjacent carboxyl group(s). The ESR spectra of the OH adducts do not change in the pH region 6-10. At pH -11 the ESR lines broaden whereas at pH 212 they are narrow again but their number is reduced by a factor of 2, caused by disappearance of the coupling due to the OH proton. Since the coupling constants of the remaining nuclei and the g factors of the radicals do not change in the pH range 6-13, the chemical nature of the species is concluded to remain the same in this region. The reversible spectral changes are suggested to result from base-catalyzed exchange of the OH proton,z1 as observed in analogous cases.19,22,23 This conclusion is in agreement with the results of pulse radiolysis studies concerning the effect of ionic strength on the lifetimes of the OH adducts from pyridine-2,6- and -3,5-dicarboxylate at pH 13. From these experiments it follows that the OH adducts are present as dianions at pH 13 and not as trianions. The following generalizations can be drawn concerning
374
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
the ESR coupling constants of the OH adducts: a(14N)is in the range 2.9-4.1 G, a(ortho-IH) = 7.4-9.8 G, a(rneta-IH) = 1.2-2.8 G, and ~ ( p a r a - l H=) ~12.5-13.8 ~ G.25 From a comparison of the coupling constants of the OH adduct of pyridine with those of the OH adducts of the pyridinecarboxylates it is evident that the carboxyl group exerts only a small influence on the distribution of the unpaired spin within the ring. However, in those cases where the hydroxymethylene group, CHOH, is adjacent to a carboxyl group, the coupling constant for the methylene proton is smaller by 2.2-5.4 GZ6than if these groups are not adjacent, whereas the coupling constants for the rest of the nuclei are practically independent of the relative positions of CHOH and carboxyl. The situation is very similar to that observedlg with the OH adducts of benzenecarboxylates. An additional analogy exists with respect to OH adducts of carboxypyridines and carboxyben~enesl~ on the one hand and H adducts of these compounds19~27 on the other; with the OH adducts the coupling of the methylene proton is smaller by -10 G than in the case of the H adducts, although the coupling constants of the remaining nuclei are approximately the same in the two cases.
References and Notes M. Anbar, D. Meyerstein, and P. Neta, J . Pbys. Cbem., 70, 2660
(1966). J. H. Fendler and G. L. Gasowski, J . Org. Chem., 33,2755 (1968). M. K. Eberhardt and M. I. Martinez, J. Pbys. Chem., 79, 1917 (1975). S. Steenken and P. O'Neill, J. Pbys. Chem., 81, 505 (1977). P. O'Neill and S. Steenken, Ber. Bunsenges. Pbys. Chem., 81, 550
(1977). P. O'Neiil, D. Schulte-Frohlinde, and S. Steenken, J . Cbem. Soc., Faraday Discuss, 63, 141 (1977). M. K. Eberhardt, J. Pbys. Chem., 81, 1051 (1977). G. W. Klein, K. Bhatia, V. Madhavan, and R. H. Schuler, J. Pbys. Chem., 79, 1767 (1975). M. K. Eberhardt, J. Phys. Chem., 79, 1913 (1975).
Communications to the Editor Concerning the difficulties involved in interpreting observed product distributions in terms of yields of radical Intermediates see K. Bhatia, J. Phys. Cbem., 79, 1032 (1975). B. Cercek and M. Ebert, Trans. Faraday Soc., 63, 1687 (1967). M. Simic and M. Ebert, Inf. J . Radiaf. Phys. Chem., 3,259 (1971). T. Shiga, T. Kishimoto, and E. Tomb, J. Phys. Chem., 77,330 (1973). K. Eiben and R. W. Fessenden, J . Phys. Cbem., 75, 1186 (1971). On calculating percentages of OH additlon to various positions it Is assumed that the line widths and lifetimes of hypothetical additional radicals are the same as those of the radicals detected. No evidence for the reported" addition of OH to the ring nitrogen was obtained. 0. S. Saveleva, L. G. Shevchuk, and N. A. Vysotskaya, J. Org. Chem. USSR, 9, 759 (1973). G. FiltJy and K. Gunther. J . Pbys. Chem., 78, 1521 (1974). K. Eiben and R. H. Schuler, J. Chem. Phys., 62, 3093 (1975). I n the case of 1,2,4,54etracarboxybenzene a radlcal formed by addition to C, has, however, been observed (see ref 18 and footnote 32 in ref 18). The rate constant for OH' catalyzed exchange of the methylene OH 108-108 proton is greater by a factor 10 than that ooserved ( k M-' s-'; see note 12 in ref 19)in the case of OH adducts of carboxybenzenes. G. P. Laroff and R. W. Fessenden, J. Pbys. Chem., 77, 1283 (1973). A. Samuni and P. Neta, J. Phys. Chem., 77, 1629 (1973). Ortho, meta, and para define positions relative to the methylene group, CHOH. Concerning the OH adducts of pyridine monocarboxylates, only some of the coupling constants reported in ref 13 are in agreement with those presented in Table I. It is, however, believed that the data in Table I represent the correct values of the coupling constants, since interpretation of the spectra was straightforward due to their considerably better resolution as compared to that reported in ref
-
13.
-
-
a(CH0H) is reduced by 10.5G if two carboxyl groups are adjacent to CHOH. H. Zeldes and R. Livingston, Radiat. Res., 58, 338 (1974);82, 28
(1975). Institut fur Sfrahlenchemie im Max-Planck-Institut fur Kohlenforschung Stiffstrasse 34-36 0-4330 Mulheim, West Germany Received October 26, 1977
S. Steenken" P. O'Neill
