523
Hydroxyl Radical Reactions with Phenols and Anilines behavior of the negative ion in various solvents is under study. Acknowledgments. The authors express their gratitude to Dr. M. Hoshino for his helpful discussions throughout this work. They also express their gratitude to I. Takeshita, Y. Oikawa, and Y. Suzuki for the construction of the low-temperature irradiation apparatus.
References and Notes J. M. Warman, K.-D. Asmus, and R . H. Schuler, J. Phys. Chem., 73,931 (1969). T. Kimura, K. Fueki, and Z. Kuri, Bull. Chem. SOC.Jap., 43, 3090 11 970). W: E.’Wentworth, R. S. Bechker, and R. Tung, J. Phys. Chem., 71, 1652 (1967). W. E. Wentworth, R. George, and H. Keith, J. Chem. Phys., 51, 1791 (1969).
J. C. Steelhammer and W. E. Wentworth, J. Chem. Phys., 51, 1802 (1969). A. Kira, S. Arai, and M. Imamura, Rep. Inst. Phys. Chem. Res., 47, 139 (1971). J. T. Richards and J. K. Thomas, J. Chem. Phys., 54,298 (1970). M. 2 . Hoffman and G. Porter, Proc. Roy. SOC., Sei. A, 266, 46 (1962). S. Arai and L. M. Dorfman, J. Chem. Phys., 41, 2190 (1964); W. H. Hamill in “Radical Ions,” E. T. Kaiser and L. Kevan, Ed., interscience, New York, N. Y., 1968, p 321. K. Kimura and S. Nagakura, Mol. Phys., 9,117 (1965). G. Porter and B. Ward, J. Chim. Phys., 61, 1517 (1964). M. Hoshino, S.Arai, and M. imamura, submitted for publication. G. Porter and M. W. Windsor, Proc. Roy. Soc., Ser. A , 245, 238 (1958). J. B. Gallivan and W. H. Hamill, Trans. Faraday Soc., 61, 1960 (1965). T. Higashimura, A. Namiki, M. Noda, and H. Hase, J. Phys. Chem., 76, 3744 (1972). L. Grajcar and S. Leach, J. Chim. Phys., 61, 1523 (1964). C. Cossart-Magos and S. Leach, J. Chem. Phys., 56,1534 (1972).
Hydroxyl Radical Reactions with Phenols and Anilines as Studied by Electron Spin Resonance’ P. Neta and Richard W. Fessenden* Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon lnstitute of Science, Carnegie. Mellon University, Pittsburgh, Pennsylvania 15213 (Received May 7, 1973; Revised Manuscript Received October 1 7 , 1973) Publication costs assisted by Carnegie-Melon University and the U. S. Atomic Energy Commission
The radicals produced by reaction of OH with a number of carboxy-substituted phenols, aminophenols, and anilines have been studied by the in situ radiolysis esr technique. The radicals produced from the phenols are identified as of the phenoxyl type and are formed by addition of OH to the ring followed by elimination of water. In the case of 5-hydroxyisophthalic acid the intermediate hydroxycyclohexadienyl radical was of long enough lifetime to be observed. Also present in the esr spectra of the phenols were signals from corresponding ortho and para semiquinone radical anions. These latter radicals are believed to be produced in secondary reactions from dihydroxy compounds formed upon bimolecular reaction of the phenoxyl radicals. The aminophenols produced spectra with g factors of -2.0037 in contrast to the values of -2.0047 found for phenoxyl radicals. These spectra are assigned to aminophenoxyl radicals. A small amount of deamination to form the corresponding semiquinone radical was also found. The three anilines studied all gave spectra which could be ascribed to anilino radicals (RNH). Parameters of the = 12.94, uoH = 6.18, u m H = 2.01, unsubstituted radical CcHal\jH are g = 2.00331 and uN = 7.95, and u p H = 8.22 G.
Introduction Hydroxyl radicals are known to add to aromatic rings to form hydroxycyclohexadienyl radicals and in a number of cases these radicals have been observed by esr (see e.g., ref 2-4). Pulse radiolysis experiments have shown that when the reacting compound contains an OH group water elimination follows the initial addition.5,6
H‘ OH This elimination is a first-order process, catalyzed by Hf
or by OH-, and its uncatalyzed rate constant is 5103 sec-l for phenol5 and 4.6 x lo4 sec-1 for hydroquinone.6 In accord with this mechanism, the esr spectra observed4 with several phenols in the Ti11r-H202 system showed the presence of the OH adducts in slightly acid solution and the phenoxyl radicals a t higher acidities. However, in alkaline solution only radicals of the benzosemiquinone type were ~ b s e r v e d The . ~ latter finding together with similar observations in the radiolysis of hydroxybenzoic acids7 and in the photolysis of phenol8 suggest the importance of secondary reactions in these steady-state experiments. The in situ radiolysis esr experiments reported here were undertaken in order to further investigate the reactions for various substituted phenols and were extended to hydroxyphenols, aminophenols, and anilines in an attempt to investigate the chemistry of those systems as well. The Journal of PhySiCal Chemistry, Vol. 78, No. 5 , 1974
524
Experimental Section The aromatic compounds were of the purest grade commonly available and were used without further purification. Most of them were obtained from Eastman Organic Chemicals and from Aldrich Chemical Co. and were 9899% pure. The two aminohydroxybenzoic acids and the aminohydroxybenzenesulfonic acid were obtained from Pfaltz and Bauer. Aqueous solutions containing from M of the organic compound were saturated with to N2O in order to convert eag- into OH. The details of the in situ radiolysis steady-state esr experiments are similar to those previously described.2
Esr Observations a n d Reaction Mechanisms Addition of OH to an aromatic ring can lead to a number of different intermediate hydroxycyclohexadienyl radicals depending upon the site of reaction. However, because the subsequent elimination reaction usually involves the OH group added in the initial step the final radical ( e + g . ,phenoxyl) will be the same for any position of reaction. Elimination of water following addition at the position of a substitutent other than an OH group can also produce the phenoxyl radical but elimination of the substituent may also take place, in effect, replacing that substituent by an OH group. The compounds studied can be divided into four types which can react with OH in somewhat different ways. Phenol and its carboxy derivatives can react as in reaction 1 to give phenoxyl radicals. The ortho and para dihydroxy compounds can react in a similar way to give semiquinone radicals. With the aminophenols the most probable type of radical is aminophenoxyl but addition at the NH2 position followed by loss of NH3 could produce a semiquinone radical. Finally, with anilines addition at any position but that of the NH2 group should give anilino radicals by water elimination while addition at the site of the NHz group could be followed by elimination of either H2O or NH3 to give, respectively, the anilino or phenoxyl radicals. In discussing the specific reaction mechanisms below the identity of the radicals as given in the tables will be assumed to be correct. In most cases a comparison of the hyperfine parameters for a given type of radical with substituents in various positions leaves little question as to their identities. Further discussion of the hyperfine constants will be given in a later section. Phenol. The esr spectra observed with irradiated aqueous solutions of phenol (10-3 M and saturated with N2O) at pH 9.3 and 12.2 were similar and consisted of the lines of three different radicals. The least intense set of lines (with hyperfine constants and g factor as shown in Table I) corresponds to the known spectrum of phenoxyl radical.9-11 The most intense lines correspond to the Obenzosemiquinone radical anion (VIII, Table 11) and the third set of lines to the p-benzosemiquinone radical anion (XV, Table 11). The peak heights of the unit intensity lines of the ortho and para benzosemiquinone ions were 6 and 1.5 times those of the phenoxyl radicals. However, the semiquinone radicals are relatively long lived and under conditions of quantitative production (ie.,from hydroquinone) give signals about two orders of magnitude higher. From this fact it must be concluded that in the phenol system the yield of serniquinones is relatively small and can be attributed to secondary reactions. In support of The Journal of Physical Chemistry, Voi. 78, No. 5 , 1974
P. Neta and Richard W. Fessenden
this interpretation, it was found that the intensities of their spectra decreased a t higher flow rates. The secondary reactions leading to the formation of the semiquinones are most probably the disproportionation of the phenoxyl radicals to produce hydroquinone, catechol, and phenol followed by reaction of OH with these dihydroxy compounds. To explain the production of the dihydroxy compounds it is suggested that disproportionation takes place by electron transfer (reaction 2) to form a neg-
bH ative ion (phenoxide) and a positive ion which can then be neutralized by reaction with H + and OH-, respectively, or by reaction with HzO. The positive ion can thus yield dihydroxybenzenes, most likely hydroquinone and cathecol as main products. Disproportionation of radicals by electron transfer has also been suggested to explain recent results with several uracil derivatives.12,13 Similar observations have been made on phenol in a photolytic experiment8 where multiply oxygenated radicals were also found. These radicals were ascribed to secondary reactions similar to the above, but a somewhat different reaction was proposed to account for formation of the hydroquinone. In the Ti111-Hz02-phenol system a t neutral or basic pH the conversion of the initial OH adducts to semiquinone radicals is even more efficient with only the latter spectra ~ b s e r v e d .Although ~ a direct conversion by H202 of the hydroxycyclohexadienyl radicals to semiquinone has been proposed4 it is more likely that an indirect path is involved via the dihydroxy compounds as suggested here. Both the metal ions and the H2Oz could facilitate oxidation of the primary radicals and increase the importance of the secondary products. The radiolytic and photolytic8 experiments show that such oxidants are not necessary for the observation of these secondary products. Salicylic Acid. The o-carboxyphenoxyl radical has been observed with irradiated solutions of salicylic acid at all pH values. The hyperfine constants and g factor changed slightly around pH 3 (radicals I1 and 111, Table I). This region must be the p K for the carboxyl group although the carboxyl proton splitting was not observed at the low pH region. Secondary radicals of the semiquinone type have also been observed, As in the case of phenol these radicals are produced by oxidation of the dihydroxy compounds formed by hydroxylation of the initial compound at positions ortho and para to the hydroxyl group. This pattern of secondary reaction is maintained for the other phenols to follow. The 3-carboxy-1,2-semiquinoneradical was present as a monoanion at pH 3-4 (X, Table 11), its esr lines were weak at pH 7 , and at pH 9-12 it was in the dianion form (IX, Table 11). The 2-carboxy-1,4-semiquinone
525
Hydroxyl Radical Reactions with Phenols and Anilines TABLE I: Esr Parameters of Phenoxy1 Radicalsa Radical
I
6
aOn
a mn
6.61 (2)
1.85 (2)
6.39
ann
P
factor
10.22
2.00461
1.84; 1.78
9.98
2 .00476
7.11
2.09; 1.23
10.20
2.00459
6.82; 6.56
1.83
9.99
2.00472
6.53 (2)
1.93 (2)
CO1-
I1
41
PK
.3
&
CO,H
111
0
IV
b
c0:-
VI - O!C
2 ,00477
6.75 (2)
9 .80
2.00481
6.87 (2)
9.72
2 .00494
c0.-
11 . pK:r
3-1,
Q
VI1 H0.C
kH
a Hyperfine constants are given in gauss and am accurate to 10.03 G . The number of nuclei displaying the splitting is given in parentheses if different than one. Theg factors are determined relative to the peak from the silica cell and are accurate to f0.00005.
radical has been observed only in its dianion form at pH 9-12 (XVI, Table 11). At pH 12 a fourth radical was also found and identified to be the noncarboxylated o-semiquinone, resulting apparently from the small contribution of ~ in the the decarboxylation reaction by OH r a d i ~ a 1 s . lAs case of phenol the line intensities of the semiquinones decreased with increase in flow rate. m-hydroxy benzoic Acid. The 3-carboxyphenoxyl radical was observed at pH 6 and 12 (Table I). At pH 6 one additional radical was present with similar esr line intensities and was identified as the 4-carboxy-1,2-benzosemiquinone (XI, Table 11). Its line intensities increased about tenfold at pH 12 and a third radical with weaker lines was found which was identified as the 3-carboxy-1,2-semiquinone (IX). These two semiquinones are produced following the hydroxylation of m-hydroxybenzoate a t the two positions ortho to the OH group. Hydroxylation at the para position should result in radical XVI but this radical was not observed. p-Hydroxybenzoic Acid. The phenoxyl radical V (Table I) was observed at pH 4.5-12. Only one semiquinone (XI) is expected to be formed from this compound and it was observed at pH 6-12. 5-Hydroxyisophthalic Acid. The irradiation of aqueous solution of this compound at pH 12 gave rise to an esr spectrum composed of lines of three different radicals with relative unit line intensities of 1:2.5:30. The least intense spectrum was that of the phenoxyl radical VI (Table
I). The most intense was the dicarboxy-o-semiquinone XI1 (Table 11) and the third radical was the dicarboxy-p-semiquinone XVIII (Table 11). At pH 6.6 the phenoxyl radical spectrum did not change. However, the o-semiquinone appeared in the protonated form XI11 with line intensities -50 times lower, and the p-semiquinone was not clearly observed. At this p H many additional lines were observed which can be assigned to the hydroxycyclohexadienyl radical
with a 23.03-G proton splitting characteristic of the H on the carbon where OH added. Four other proton splittings were observed, 2.10 and 1.64 G due to the ring protons meta to the addition site, and 0.68 and 0.46 G due to the OH protons. The g factor is 2.00313. This radical represents the only case in the present study where the initial adduct was observed. Apparently the two carboxyl groups stabilize the radical against elimination of water. This adduct was not seen either at the higher or a t the lower pH values because of the acid- and base-catalyzed water elimination. At pH 3 the phenoxyl radical showed slightly different parameters and was most probably protonated (VII, Table The Journal ot Physical Chemistry, Voi. 78, No. 5 , 1974
526
P. Neta and Richard W . Fessenden
TABLE 11: Esr Parameters of Benzosemiquinone Radicalsa Rad ica 1
a?'
.onH
abH
aeH
3.66
3.66
0.76
2 .00455
3.92
3.45
0.67
2 .00457
6.59
1.55
2.89
3.26
0.73
2 ,00471
2 ,00471
.,'I
g factor
Ortho
0 0.76
VI11 (?
IX
&O-
w-
X
0.57
2,00438
0
1.25
XI c0.-
i,
XI1
3 .OO
1.22
XI11
5.94
3.26
0.37
2 ,00456
XIV
5.47
3.77
0.25
2,00463
Radical
a" (ortho and meta)
ao#
g factor
Para 0
xv
Q
2.38 (4)
2 .00455
2.57; 2.20; 2.01
2 ,00464
0
0
XVI 0-
It 0
4.90; 2.51; 1. 3 5 b
XVII
0.81
2.00455
OII
XVIII
1 . 8 6 (2)
2.00474
'See footnote to Table I. I t is not clear whether the radical is 3-carboxy-4-hydroxyphenoxyl as indicated, with 4.90- and 2.51-G aplittinga by the ortho protons and 1.35 G by the meta proton, or 2.carboxy-4-hydroxyphenoxylwith 4.90-G splitting by an ortho proton and 2.61 and 1.36 G by the meta protons.
The Journal of Physical Chemistry. Vol. 78, No. 5 , 1974
527
Hydroxyl Radical Reactions with Phenols and Anilines
TABLE 111: Esr P a r a m e t e r s of Aminophenoxyl Radicals" Radical
asH
aP
abH
aP
aNaP
aN
g factor
I2 and a different radical was found at pH 14. This radical was identified as IX and its line intensities were -200 times smaller than those observed using 2,3-dihydroxybenzoic acid. It appears that the NH2 group in the original compound is replaced by an OH group with a very low yield and the resulting radical only becomes observable when the spectrum of the main radical XX disappears. The replacement of NH2 by OH to form radical IX can only take place uia OH addition to the 2 position.
(Table IV) was observed at pH 7-13.3 and again the line intensity increased in going from pH 7 to 9 to 11. No other radical was detected. p-Aminobenzoic Acid. The p-carboxyanilino radical XXVII (Table IV) was present in the irradiated solution between pH 6 and 13. With this compound a secondary radical, with a somewhat similar line intensity, XXI (Table 111) was also detected. The formation of this aminophenoxyl radical suggests that the anilino radicals, similarly to phenoxyl radicals, react with each other by electron transfer resulting in hydroxylation.
co;
c0,-
1
3
&.
OH adducts at other positions
1"- lH+ (or H,O)
c0,-
-A&NK
"2
(or Hp)
2"
0.
xx
rx
3-Amino-4-hydroxybenzenesulfonic Acid. With this compound only the phenoxyl radical (XXII, Table 111) was observed. The spectrum became weaker at pH >11 and no lines were observed a t pH 14. p-Aminophenol. The p-aminophenoxyl radical (XXIII, Table 111) was observed at pH 3.5-11. The intensity of the spectrum decreased at pH l o . The disappearance of the spectrum at pH -3 could be a result of protonation of NH2 to NH3+ causing line broadening, and its disappearance at pH >11 is a result of exchange of the NH2 protons as suggested above. At pH 11-14 very weak lines were observed which can be assigned to the p-benzosemiquinone radical and, therefore, suggest a low yield of deamination similar to that outlined in reaction 3. 5-Amino-2-hydroxybenzoic Acid. The results here were similar to those with the previous amino compounds. The phenoxyl radical XXIV (Table 111) was observed in neutral solutions, its spectrum disappeared in alkaline solutions, and at pH 12 only a weak spectrum of the semiquinone XVI was observed. Aniline. The hydroxycyclohexadienyl radical produced from aniline can eliminate water only through loss of one of the NH2 hydrogens to produce an anilino radical XXV. This mechanism has also been suggested on the basis of recent pulse radiolysis r e ~ u 1 t s . l ~
"*I ' H O 'H
xxv
The esr spectrum observed a t pH 6-12 is assigned to this radical (Table IV). Neither the initial adduct nor secondary radicals have been observed in this case. The intensity of the spectrum was weaker in neutral than in alkaline solutions probably because of the slowness of water elimination in the neutral region. Anthranilic Acid. The o-carboxyanilino radical XXVI The Journalof Physical Chemistry, Vol. 78, No. 5, 7974
cozEsr Parameters The esr parameters of the phenoxyl radical agree with those previously reported.1° The other radicals in Table I have not been reported previously. In each case, however, the parameters for the particular positions differ little from those of the unsubstituted radical, confirming the assignments. The carboxyl group is seen to have very little effect on the spin distribution. The hyperfine constants for the ortho and para benzosemiquinone radical anions also agree with those reported for aqueous s o l ~ t i o n and ~ ~ those ~ 6 ~ of ~ the ~ derivatives follow with little change in value. The hyperfine values for the neutral semiquinone radicals X, XIII, XIV, XVII present more of a problem in that protonation can occur on either of the oxygens which, in these substituted radicals, are not equivalent. The positions of protonation as given in Table I1 were chosen to give better agreement with the values for the unsubstituted radicals. The structure assigned to radical XI11 correlates the splittings of 3.26 and 5.94 G with those of 4.30 and 8.72 G for the ortho and para protons16 in the unsubstituted radical. The alternative assignment is much less satisfactory in that the observed values would then have to be correlated with the values for the meta protons of 1.92 G.I6 The structures of radicals X and XIV are assigned by comparison with radical XIII. The radicals obtained from the aminophenols clearly display hyperfine constants of an NH2 group with equivalent protons and thus an aminophenoxyl radical is indicated. The only other possibility is a radical of the type C6H&Hz+ but the existence of the anilino radicals of Table IV in the form CsH5NH at similar pH values rules out this possibility. The most striking effect of the amino group in the aminophenoxyl radicals of Table I11 is to decrease the g factor from -2.0047 to -2.0037. This effect indicates a rather large transfer of spin density from the oxygen to the NH2 group where the perturbation of the g factor should be less. The ring proton splittings are also
529
Association in Solutions of Ethanol and 2,2,2-Trifluoroethanol more comparable with the values for the neutral semiquinone radicals than with those for phenoxyl. The assignment of hyperfine constants to particular protons is made by internal comparisons among the various radicals and by comparison with the neutral semiquinones. In particular the similar splittings for XIX and XX and the small value of 0.49 G for XXII clearly establish that assignment of one splitting of
