E. HAYON,T. IBATA,N. N. LICHTIN, AND M. SIMIC
2072
Electron and Hydrogen Atom Attachment to Aromatic Carbonyl Compounds in Aqueous Solution.
Absorption Spectra and Dissociation Constants
of Ketyl Radicals
by E. Hayon,"' T. Ibata,2N. N. Lichtin,Z and M. Simica Pioneering Research Laboratory, U.8. Army Natick Laboratories, Natick, Massachwretts 01760,and Chemistry Department, Boston University, Boston, Massachusetts 02316 (Received February 8, 1972) Publication costs assisted by the U.8. Army Natick Laboratories
The technique of pulse radiolysis-absorption spectroscopy has been used to study the addition of hydrated electrons and H atoms to a number of compounds incorporating both carbonyl and aromatic groups. The absorption spectra of the electron adducts and their oxygen-protonated conjugate acids have been determined for acetophenone, 9,10-anthraquinone-2-sulfonate,benzamide, benzil, benzoin, benzophenone, and fluorenone. The pK, values for all of these radicals except those derived from benzoin have been evaluated as have the bimolecular decay rates of the anion radicals. Bimolecular decay rates of the protonated electron adducts of acetophenone, 9,lO-anthraquinone-Z-sulfonate,and benzamide were also measured. Specific rates of addition of the hydrated electron to all the substrates for which resultant spectra were determined as well as anthrone, dibenzylketone, 1,4-naphthoquinone-2-sulfonate,1,2-naphthoquinone-4-sulfonate,and phenylacetone have been found to fall in the range (1.1-3.6) X 10'0 M-l sec-'. Spectra of the H-atom adducts and bimolecular specific rates of decay of these adducts have been determined for acetophenone, 9,lO-anthraquinone-2-sulfonate, benzamide, benzil, benzophenone, fluorenone, and 1,4-naphthoquinone-2-sulfonate. Systematic aspects of the dependence on structure of the reactivity toward addition of eaq- and H atoms, of the pK,'s of protonated electron adducts, and of the various spectra are discussed.
There is considerable interest in determining the specific role of quinones, ketones, and related compounds in biological reactions, and their importance in electron transport and oxidative phosphorylation mechanisms has been established for some of them.4 No detailed study has been carried out previously to investigate the reactions of solvated electrons and hydrogen atoms with aromatic carbonyl compounds and to differentiate the sites of attachment and the rates of reactions of these two reducing species. The absorption spectra of a number of ketyl radicals have been observed on flash photolysis of aromatic ketones in the presence of an H-atom donor (e.g., see ref 5-8). I n only a few cases, however, have the full spectra, extinction coefficients, and acid-base dissociation constants been determined. I n the present work solvated electrons and hydrogen atoms have been produced by pulse radiolysis of aqueous solutions and allowed to react with aromatic carbonyl compounds. The absorption spectra of the H atom and of the electron adducts, and of their protonated forms, derived from a number of aromatic carbonyl compounds have been determined. The reaction rate constants for addition of eaq- and H atoms and the pK, values of the protonated electron adducts have also been measured. The Journal of Physical Chemistry, Vol. 76,No. 16, 1072
Experimental Section The general method and experimental technique of pulse radiolysis used has already been d e s ~ r i b e d . ~ J ~ A double monochromator and a pulsed xenon lamp was used throughout this work. All solutions were thoroughly degassed by bubbling with argon gas. The OH radicals produced from the radiolysis of water were scavenged in presence of excess tert-butyl alcohol. The radical from tert-butyl alcohol (1) U. S. Army Natick Laboratories. (2) Chemistry Department, Boston University. Support under Grants No. R 01 RH 00394 and R 01 EC 0092 of the Environmental Control Administration of the U. S. Public Health Service is gratefully acknowledged. (3) National Academy of Science-National Research Council Research Associate at the Natick Laboratories. (4) R. A. Morton, Ed., "Biochemistry of Quinones," Academic Press, New York, N. Y . , 1965. (5) G. Porter and F. Wilkinson, Trans. Faraday Xoc., 57, 1686 (1961). (6) A. Beckett and G. Porter, ibid., 59, 2038 (1963). (7) J. A. Bell and H. Linschitz, J . Amer. Chem. Xoc., 85, 528 (1963). (8) A. Beckett, A. D. Osborne, and G. Porter, Trans. Faraday SOC., 60, 873 (1964). (9) M. Simic, P. Neta, and E. Hayon, J. Phys. Chem., 73, 3794 (1969). (10) J. P. Keene, E. D. Black, and E. Hayon, Rev. Sci. Instrum., 40, 1199 (1969); E. Hayon, J . Chem. Phys., 51, 4881 (1969).
ELECTRON AND HYDROGEN ATOMATTACHMENT absorbse below -280 nm and has a low extinction coefficient. The presence of the alcohol increased the solubility of the aromatic ketones in water. I n all cases, the concentrations of ketone were adjusted to scavenge all the Bas- or H atoms produced under the experimental conditions employed. To determine the second absorption bands of the ketyl radicals or radical anions in the far-uv region, these solutions were diluted and normalized to the spectra obtained a t higher solute concentrations. Dosimetry was carried out9 using -0.1 M KCXS solution and an eso0 = 7.6 X lo3 M-l cm-l. The extinction coefficients were derived based on G(e,,-) = G(0H) = 2.8, and G(H) = 0.55. Considerable care was taken to minimize the photolysis of these aqueous solutions. Appropriate liquid and/or glass filters were used to filter the monitoring light from the xenon lamp. In addition, a synchronized shutter was used which opened for a total time duration of -5 msec. The aromatic ketones were obtained from Eastman Chemicals and from Matheson Coleman and Bell. I n all cases, the ketones were purified and recrystallized from alcohol.
Results Xpecific Rates of Addition of eaq- and H atoms. The specific rates of reaction of the hydrated electron with a variety of aromatic carbonyl compounds-ketones, quinones, diketone, acyloin, and amide-were determined by following the decay of eaq- a t 700 nm. Hydroxyl radicals were scavenged by 0.1 M tert-BuOH and the pH was maintained a t -9.2 by -2 mM sodium tetraborate. The rate constants obtained are presented in Table I. Although they are all close to the diff usion-cont rolled limit some significant variations of reactivity with structure are apparent. Both phenyl acetone and dibenzyl ketone (a,a'-diphenylacetone) react twice as fast as does acet0ne.l' Further acceleration is apparent when carbonyl is directly bonded to phenyl and there is no difference between the reactivity of acetophenone and benzophenone, both of which are five times as reactive as acetone. A small further increase in reactivity results when 0 and 0' positions of benzophenone are joined in anthrone and fluorenone. The reactivity of the benzoyl group is reduced by the vicinal CHOHC6H6 group in benzoin, while benzil is as reactive as fluorenone. The formally similar naphthaquinone is half as reactive as benzil. Particularly interesting is the high reactivity of benzamide which reacts about lo3 times as fast as acetamide. a Specific rates of reaction of some of the substrates with H atoms were determined from the rates of formation of the corresponding H-atom adducts observed at appropriate wavelengths. Hydroxyl radicals were scavenged by 1.0 M tert-butyl alcohol and the hydrated
2073 ~~
~~
Table I: Rates of Reaction of ea,- and H Atoms with Aromatic Carbonyl Compounds in Aqueous Solution Aromatic carbonyl compounds
k(eaq-
Acetophenone 9,lO-Anthraquinone2-sulfonate Anthrone Benzamide Benzil Benzoin Benzophenone Dibenzyl ketone Fluorenone 1,4-Naphthaquinone2-sulfonate l,%Naphthaquinone4-sulfonate Phenylacetone a
+ S),
M -1 sec -1
a
2 . 8 X 1010 2 . 2 x 1010
3.3 x 1.9 x (1.7 3.6 x 1.7 X 2.8 X 1.1 x 3.3 x 2.6 X
k(H
3.4 i= 0 . 6 x 109 4.1 =k 0 . 3 X lo9
... ...
10'0 1010
x
+ S),
M - 1 sec-1 b
101o)c
1010 1010 1Olo 1010 10'0 1010
1 . 0 =k 0 . 2
x
1010
5 . 6 =t0.3
x
109
x
109
... ...
5 . 4 =k 0 . 6
6 . 8 i= 0 . 4 X 109
1 . 7 X 1O1O 1.2
x
Rate constant values & l O % .
1010
...
Determined from the rate
of formation of the H-atom adduct, values ~ t 3 0 7 ~ . From ref 11.
electrons were converted into H atoms by maintaining a pH of 1.0. The resulting second-order rate constants eaq-
+ H30+ +H
2.3 X 1O1O M-l sec-' (ref 11) (1) are listed in Table I. These specific rates are about kl
=
half an order of magnitude lower than the specific rates of addition of the hydrated electrons. Their values, like those of ea,-, vary over a factor of -3, but the differences do not parallel those observed with ea,- and are apparently due to different factors.
Spectra and pK, Values of Ketyl Radicals Benzophenone. The transient optical absorption spectrum of the benzophenone ketyl radical is shown in Figure 1. Absorption maxima at 545 and 332 nm were found in good agreement with results obtained by flash photolysis6 (545 and 330 nm). Only the band in the visible region has been reportedl3?l4in previous pulse radiolysis work. The weak band with A, -390 nm observed at pH 4.9 in Figure 1 is due to the H-atom adducts to benzophenone (see Figure 2). These H atoms are produced in the radiolysis of water with a G(H) = 0.55. I n alkaline solutions, a considerable shift in the absorption spectrum of the ketyl radical anion can be seen (11) M. Anbar and P. Neta, I n t . (1967)
J. Appl. Radiat. Isotopes,
18, 493
I
(12) E. Hayon, T. Ibata, N. N. Lichtin, and M. Simic, J. Amer. Chem. Soe., 93, 5388 (1971). (13) G . E. Adams, J. H. Baxendale, and J. W. Boag, Proc. Roy. Soc., Ser. A , 277, 549 (1964). (14) E. J. Land, ibid., 305, 457 (1968).
The Journal of Physical Chemistry, Vol. 76,N o . 16, 1972
E. HAYON, T. IBATA, N. N . LICHTIN,AND M. SIMIC
2074
0.4,
f 0
0.3
0.2
0.5
0
300
400
800
500
700
X , nm
250
300
350
X.
Figure 1. Optical absorption bands of the benzophenone ketyl radical ( 0 , p H 4.9) and radical anion (0,pH 12.3). Insert: absorbance a t 340 and 610 nm as a function of pH.
4000
t
a
t
400
450
5
nm
Figure 3. Optical absorption bands of the acetophenone ketyl radical (0,pH 5.6) and radical anion (0,pH 12.2). Insert: absorbance a t 326, 370, and 442 nm as a function of pH.
The following reactions are considered
0
I1
0-
I
esq- f CeHsCCaHs +CsHsCCsHs 0-
(2)
OH
4000
-E I
2000
=* u)
1x10‘
250
350
450
300
X,
400
500
0
nm
Figure 2. Optical absorption spectra of the H-atom adducts to some aromatic ketones. Spectra obtained in aqueous 1-2 M tert-butyl alcohol solutions, at pH 1.0.
(Figure l), with, , ,A at 615 and 339 nm, as compared t o 630 and 339 nm obtained by flash photolysis.6 The narrow intense absorption bands in the uv region have not been previously presented. To show that these bands are due to the ketyl radical and radical anions of benzophenone, corresponding t o the mellknown absorption bands in the visible region, the acidbase equilibrium of the ketyl radical was investigated at 610 and at 340 nm (see inert Figure 1). From these “titration” curves, a pKa = 9.25 5 0.1 was obtained at both 610 and 340 nm, in agreement with the reported6 value of pK, = 9.2. The Journal of Physical Chemistry, Vol. ‘76,hTo. 16,1079
and under certain experimental conditions, the fast (k3 2 1O1O M-‘ sec-l) rate of protonation of the ketyl radical anion could be observed. Various values for the extinction coefficient of (CsH5)&OH have been determined and are summarized by Land.14 The value obtained in this work = 5.5 X lo3 M-l cm-’ should be compared with the recent14 value of e538 = 3.2 X lo3 M-l cm-’. The extinction coefficients of the other bands and species, as well as the decay kinetics of these radicals, are given in Table 11. Acetophenone. The absorption spectra of the ketyl radical and radical anion of acetophenone are shown in Figure 3. In addition t o the bands8?l5in the visible region, very intense bands are observed in the near-uv -370 region of the spectrum. The band with A, nm obtained at pH 5.6 (Figure 3) is that of the Hatom adduct to acetophenone (see Figure 2); the second band of the H-atom adduct can also be seen to contribute to the observed spectrum of the acetophenone ketyl radical, In addition to the intense band with Amax 273 nm, a relatively weak band with Xmax 2410 nm can be seen (Figure 3). The acetophenone ketyl radical anion has bands at 445 and 314 nm. These bands are in fair agreement with those reported from flash photolysis studies. by Beckett, el d.,* (15) H. Lute and L. Lindquist, Proc. Chem. Soc., 493 (1971).
ELECTRON AND HYDROGEN ATOMATTACHMENT
2075
Table 11: Absorption Maxima, Extinction Coefficients, Decay Kinetics, and pK, Values of Ketyl Radicals and Radical Anions of Some Aromatic Carbonyl Compounds
-----
Aromatic cai:bonyl compound
nm
273
Acetophenone Benzophenone Benzamide Fluorenone
Benail Benzoin 9,lO-Anthraquinone2-sulfonate!
----
Ketyl radical----
Amax,
2k,
f,
M-1
2.9
om-'
x
Amax.
...
104
332 545 295 430 347 362 520 372 495 310
2.9 x 5.5 x 2.3 x 4.5 x 1.6 x 1.3 x 1.1 x 8.8 X 2.0 x 9.0 x
104 103 104 103 104 104 103 lo3 103 108
1.8 x 2.0 x 4.4 x 5.2 x 3.8 x 4.4 x
387
1.4
x
104
1.6
nm
P K ~
M - 1 sec-1
109 109 109 109 109 109
9.9 f 0 . 2 10.1 zt 0.2 9.28 10 . 1 9.25 =t0 . 1 7.7 1 0 . 2
... ...
6 . 3 3Z0.2
...
...
...
5.5 zt0.2
...
..*
1.1 x 109
x
109
...
...
... 3.9 f 0 . 2
5
Ketyl radical anion----2k, sec-1
€,
,v-1 om-'
M-1
314 445 339 615 320 440 360 450
2.9 x 4.1 x 2.0 x 6.1 x 2.9 x 5.6 x 1.1 x 6.2 x
104 103 104 103 104 103 104 103
362 545 312 460 398 465 496
6.4 3.6 2.5 2.9 9.8 8.0 9.5
x x x x x x x
103 103 104 103 103 103 103
9.7
... x 108 ...
a 8.1 X 108 7 . 3 x 108
... ... a a
... ... 1 . 6 x 109 ... 1.3
x
109
Complex decay kinetics.
No extinction coefficients appear to have been obtained in earlier investigations. These values for the acetophenone ketyl radicals and radical anions are given in Table 11. The acid-base properties of the acetophenone ltetyl radical were followed at 442 and at 326 nm to show that both bands are due to the ketyl radical. A pKa = 10.1 :I0.2 = at 442 nm and a pK, = 9.9 f 0.2 at 326 nm were obtained. These values are considerably lower than the pK, = 10.9 found by Beckett, et aL8 No explanation is apparent to account for this significant difference. The change in absorbance at 370 nm was followed as a function of pH (see insert Figure 3) to show that this band is not due to the ketyl radical but to the Hatom adduct to acetophenone. The small decrease T ~ increase in pH is probably due to the shift in O D ~with with pH of the overlap of the spectra of the ketyl radicals, as well as to the partial conversion in alkaline solution of H atom into eaq-. H
+ OH- +eaq-
(4)
Benxamide. The electron adducts of benzamide and its conjugated acid have apparently not been observed previously. The ketyl radical has an absorption spectrum with maxima at 430 and 295 nm; see Figure 4. I n alkaline solutions when the anion radical is present, the absorption bands are shifted to 440 and 320 nm, respectively. The acid dissociation constant of the ketyl radical of beneamide has a pK, = 7.7 f 0.2. It is interesting to note that this pK, value is significantly lower than the pK, of the ketyl radical of CeH5COCH3. The H-atorn adduct to benzamide has a maximum at -350 nm (Figure 2), and its contribution to the ob-
O''
3b0
'
360
400
450
!d
5'00
X. nm Figure 4. Optical absorption bands of the beneamide ketyl radical ( 0 ,pH 5.5) and radical anion (0,pH 9.5). Insert: absorbance at 320 nm as a function of pH.
served spectra of the ketyl radicals in that wavelength region can be seen in Figure 4. The extinction coefficients and decay kinetics of the benzamide ketyl radicals are given in Table 11. Benxil and Benzoin. Figure 5 shows the absorption spectra of the ketyl radical and radical anion of benzil in aqueous solution. The ketyl radical has absorption maxima at 495 and 372 nm (compared t o 490 and 365 nm obtained8 by flash photolysis). The ltetyl radical anion has maxima at 545 and 362 nm (compared8 to 550 and 330-380 nm). Another absorption band below -310 nm could be seen for both the neutral and the alkaline forms, but could not be accurately measured due to the strong absorption of benzil itself in that wavelength region. The pK, of the benzil ketyl radical x-as determined 0.2 at 540 nm (insert, Figure 5) and a pK, = 5.5 The Journal of Physical Chemistry, Vol. 76, No. 16, 1.978
E. HAYON,T. IBATA,N. N. LICHTIN, AND M. SIMIC
207G
1.5
-
3
1.0I
I
.z *-
4
0.5-
X W
01
350
'
I
400
'
I
450
'
I
500
'
550
'
X , nm 1
300
s
t
500
400
A,
600
Figure 7 . Optical absorption bands of the 9,10-anthraquinone-2-s~1fonate ketyl radical (0,p H 3.5) and radical anion (0,p H 8.3). Insert: absorbance a t 500 nm as a function of pH.
nm
Figure 5 . Optical absorption bands of the b e n d ketyl radical (0,p H 4.2) and radical anion (0,p H 8.2). Insert: absorbance a t 540 nm as a function of pH.
1.5
0.3
*
-B
'0
-.
0.2
I
a
h
6
1.0
-I5
0.10
I
u- 1x10'
"0.I P
0.5
ti 0.05
0
0
300
350
400
450
500
A, nm
Figure 6. Optical absorpt.ion bands of the benzoin ketyl radical ( 0 , pH 4.9) and radical anion (0,pH 12.6).
was derived. This compared with a pK, = 5.9 reported by Beckett, et aL8 The absorption spectra of the ketyl radicals of benzoin are shown in Figure 6. Owing to the instability of the species produced, it was not possible to derive the dissociation constant of this ketyl radical. The spectrum determined at pH 12.6 is presumed to be that of the ketyl radical anion. Anthraquinone. The absorption spectra of the semiquinone radical and radical anion of 9,lO-anthraquinone2-sulfonate (A) are shown in Figure 7. The spectrum of the radical anion A- has maxima at 496, 465, and 398 nm, and these are close to 500 and 390 nm reported in both pulse radiolysisl6 and flash photolysis17 studies. at 387 The radical AH has one main band with A, nm. The H-atom adduct to A has maxima at 390 and -425 nm (Figure 2) and can be seen t o contribute to the absorption spectra of thc semiquinone radicals shown in Figure 7. The Journal of Physical Chemistry, Val. 76, No. 16,1978
0
0
300
360
400
450
500
A.nm
Figure 8. Optical absorption bands of the fluorenone ketyl radical (0, p H 4.3) and radical anion (0,pH 12.2). Insert: absorbance a t 450 nm as a function of pH.
The dissociation constant of the semiquinone AH was determined by following the change of absorbance a t 500 nm with pH. A value of pK, = 3.9 f 0.2 was obtained, which is higher than the value of 3.25 reported by Hulme, et U Z . ' ~ A value of pK, = 4.0 has been obtained18 for the semiquinone radical of benzoquinone. The extinction coefficients given in Table I1 for the semiquinone radicals of A are in good agreement with those reportedI6recently. Fluorenone. The full absorption spectra and dissociation constant of the ketyl radicals of fluorenone (16) B. E. Hulme, E. J. Land, and G. 0. Phillips, Chem. Commun., 518 (1969). (17) N. K. Bridge and G. Porter, Proc. Roy. Soc., Ser. A , 244, 277 (1958); G. 0. Phillips, N. W-.Worthington, J. F. McKellar, and R. R. Sharpe, Chem. Commun., 835 (1967). (18) G. E. Adams and B. D. Michael, Trans. FaradaV Soc., 63, 1175 (1967).
ELECTRON ANI) HYDROGEN ATOMATTACHMENT
2077
apparently have not been determined.Ig The ketyl radical of fluorenone has absorption maxima at 520, 362, and 347 nm (see Figure 8), with extinction coefficients ranging from 1.1 X lo3 M-l cm-l at 520 nm to 1.6 X lo4 M-' cm-l at 347 nm. The ketyl radical anion has a relatively sharp band at 450 nm, another broader band at 360 nm, and a third band hidden under the 450-nm band at higher wavelengths. Since the Hatom adduct of fluorenone has bands at 500, 348, and 270 nm which are relatively strong (Figure 2), the contribution due to this radical has been corrected in Figure 8. Hence the spectra of the ketyl radicals shown in Figure 8 do not include a contribution from the H-atom adduct. The acid-base property of this ketyl radical was monitored a t 450 nm, and a pK, = 6.3 f 0.2 was obtained. H - A t o m Adducts to Aromatic Ketones. Figure 2 combines the optical absorption spectra of the H-atom adducts of benzamide, acetophenone, benzophenone, anthraquinone, benzil, and fluorenone. Significant differences are observed in the number of bands, absorption spectra, and extinction coefficients of these radicals (see Table 111). Second bands at lower waveTable I11 : Absorption Maxima, Extinction Coefficients, and Decay Kinetics of H-Atom Adducts of Some Aromatic Carbonyl Compounds -H-Atom
adducta-----
Aromatic carbonyl
Amax,
compound
nm
M-I om-1
285 370 315 390 350 27 0 348
390
x 103 7 . 3 x 103 3 . 6 x 103 7 . 2 x 103 4.0 x 103 2 . 0 x 104 6 . 5 X 108 2 . 0 x 103 4 . 6 x 103 4 . 8 x 103
380
9.4
Acetophenone Benzophenone Benzamide Fluorenone
Benzil 9,lO-Anthraquinone2-sulfonate 1,4-N apht ho quinone2-sulfonate
500 385
emax,
*
2k,
M-1
...
3.8
x
103
a Determined in aqueous solution at pH 1.0. Iltl5%.
sec-1 b
x
4.1
109
... 3 . 2 x 109 2.0 x 109 2 . 6 X lo8 I
.
.
1.3 1.9
x x
109 109
1.2
x
io@
Values to
lengths and with lower extinction coefficients can be seen for benzamide, acetophenone, and benzophenone. Presumably in this case, as with the other aromatic ketones, the addition of H atoms takes place at more than one site, It is interesting to note that this addition reaction is not specific, whereas the attachment of electrons to aromatic ketones occurs almost exclusively with the carbonyl group. A similar selectivity has recently been observed for 2-, 3-, and 4-benzoylpyridines.20
Discussion Relative Reactivity of the Reactions of ea,- and H . I n spite of the fact that the specific rate of reaction of the electron with the various aromatic carbonyl compounds is in every case greater than 1O'O M-' sec-l, systematic variation of reactivity with structure is apparent. It was previously12p21pointed out that a correlation between structure and reactivity in the very fast reactions of OH radicals with amides occurs but the reactions are about one-tenth as great as the present ones. Dependence of the reactivity of the various carbonyl compounds on their molecular structure appears to correlate the intramolecular properties not, for instance, with differences in solvation. Specific rates of reaction with H atoms also vary with structure but the reactivity sequence does not parallel that followed in reactions of ea,-, Clearly different factors determine the sequences for the two radicals. A large effect of structure on reactivity is observed in the reaction of benzamide with ea,- for which L(e,,amide) is between lo2 and lo3times as large as it is for a number of aliphatic amides.12 This difference contrasts with the ca. fivefold difference in reactivity between acetone and acetophenone, although in both cases reactivity is increased by conjugation of carbonyl with the aromatic ring. The specific rate of reaction of benzene with eaq- is similar'l to those of the aliphatic amides.12 Theoretical inspection of these differences would require quantum mechanical calculations. Such calculations have been reported for benzoic acidz2and have been carried out for benzamideeZ3 Both EHMO and CNDO calculations show that the lowest unoccupied molecular orbital of benzamide is characterized by high p character at all carbon atoms, oxygen, and nitrogen, indicating extensive delocalization. Both calculations indicate a large net positive charge at the carbonyl carbon. According to the CNDO calculations addition of an electron to the lowest unfilled orbital has little effect on its composition in terms of atomic orbitals. Similar features pertain to benzoic acid and bcnzoate. Just how they relate t o the high specific rates of reaction with eaq- is not clear as yet. The initial site of interaction of ea,- appears to be with the carbonyl groups of these aromatic compounds. Conjugation of the odd electron with the rest of the molecule follows, as has been observed by esr studies (see ref 19, and other references therein). Similarly, eaq- initially attaches itself to the >CO
+
(19) N. Hirota and 9 . I. Weissman, J . Amer. Chem. Soc., 86, 2538 (1964); H. V. Carter, B. J. McClelland, and E. TVarhurst, Trans. Faraday Soc., 56, 455 (1960). (20) D. A. Nelson and E. Hayon, unpublished data. (21) N . N. Lichtin, Israel J . Chem., 9, 397 (1971). (22) M.Simic and M. 2. Hoffman, J . Phys. Chem., 76, 1398 (1972). (23) Private communication from Dr. Richard H. Mann. The Journal of Physical Chemistry, Vol. 7 6 , N o . 16,1972
2078 groups in substituted pyrimidinesZ4 (e.g., uracil, thymine, cytosine, orotic acid, etc.). I t is suggested that the resulting odd electron also interacts strongly with the 5,6 double bond in pyrimidines. Keto-enol tautomerism further enhances such a conjugation. Dependence of p K , Structure o n Protonated Electron Adducts. I n all cases the negatively charged electron ‘adducts are much more basic than the corresponding neutral molecules. The similarity of the equilibrium constants of the radicals derived from acetophenone and benzophenone suggests that the dominant influence is similar with changes in delocalization energy consequent upon protonation of the electron adduct. The difference between them is similar in magnitude to the difference in acidity of acetic and benzoic acids. The neutral radical derived from fluorenone, which differs from benzophenone by linkage of ortho positions of the two phenyl groups, has a dissociation constant -lo3 times as great as that of benzophenone. This large difference can be associated with the cyclopentadiene nakure of the central ring of
Tho Journal of Physical Chemistry, Vol. 76, N o . 16,1972
E. HAYON, T. IBATA,N. N . LICHTIN, AND M. SIMIC fluorenone. I n the anion radical this ring has an electron distribution analogous t o that of the cyclopentadienyl anion, a species which is unusually stable becase the aromatic sextet is filled. Protonation on oxygen removes an ,electron from the central ring with concomitant loss of stabilization. The even greater OH I
acidities of the neutral radicals derived from benzil and 9,10-anthraquinone-2-sulfonate can be related to the greater loss of resonance stabilization resulting when the symmetrical anion radicals are converted into unsymmetrical species by protonation. Qualitative consideration of the “inductive effect” of the amino group suggests that the neutral radical derived from acetamide should be more acidic than the radicals derived from acetophenone or benzophenone. (24) E. Hayon, J. Chem. Phys., 51, 4881 (1969).
