2737
PHOTOCHEMICAL OXIDATION OF SUBSTITUTED AROMATIC AMINES A plot of Royolo-CaHe/RC8He vs. pressure is presented in Figure 2. The intercept gives ka/R = 0.40 and from the slope and intercept we get kg/ks = 0.078 Torr-'. Acknowledgment. This research was supported by Grant AP 00109, Research Grants Branch, National
Air Pollution Control Administration, Consumer Protection and Environmental Health Service, U. s. Health Service. R. S. also wishes to thank the Public Health Service, Division of Air Pollution, for a Postdoctoral Fellowship.
The Photochemical Oxidation of Some Substituted Aromatic Amines in Chloroform by E. A. Fitzgerald, Jr., P. Wuelfing, Jr., and H. H. Richtol* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New Y O T ~18181 (Received October 86, 1970) Publication coats borne completely by The Journal of Physical Chemistry
The solution phase photochemistry of N,N'-diphenyl-p-phenylenediamine (DPPD), N,N'-dimethyl-N,N'diphenyl-p-phenylenediamine (DMDPPD), N,N,N',N'-tetraphenyl-p-phenylenediamine (TPPD), and p hydroxydiphenylamine (HDPA) has been studied. Absorption of ultraviolet radiation by these compounds in chloroform or carbon tetrachloride yields permanent photooxidation. The products are the colored radical cations of the amines which are also produced by chemical and electrochemical oxidation with bromine. Irradiation of the amines in nonhalogenated solvents (e.g., ethanol, benzene) yields fluorescence emission only and no permanent oxidation. Oxidation quantum yields are greater than one for several of the amines, indicating that the oxidation can occur by a thermal mechanism after the primary photoprocess. The presence of dissolved oxygen increases the quantum yields significantly and identification of products resulting from the reaction of solvent radicals with oxygen suggests the thermal mechanism. Finally, the triplet states of the amines have been observed by flash photolysis experiments in degassed nonhalogenated solvents in which oxidation does not occur.
The purpose of this work is to investigate the mechanism of photooxidation of certain aromatic amines in fluid solution. Photooxidation of many similar compounds had been studied previously either in rigid solution where the radical cation product is isolated or by flash photolytic techniques in inert fluid solvents in which the reaction is not permanent and the radical cation decomposes very rapidly. Lewis and coworkers112were first to investigate photooxidation in rigid medium. They found that photooxidation occurred for aromatic compounds containing the amino, hydroxy, and mercapto substituents and the product was identical with the radical-cation produced by chemical oxidation. Other workers have made quantitative studies of photooxidation of aromatic amines in rigid solutions. Cadogan and Albrecht6r0 found evidence that the photooxidation of N,N,N',N'-tetramethylp-phenylenediamine (TMPD) in rigid solution occurred by a two-photon process via a triplet-state intermediate. Radical cations of similar amines have been observed7J as transient species after flash photolysis in fluid solution. Recently, Meyerg has investigated the photooxi-
dation of TR4PD in several halogenated solvents, including chloroform. The radical cations of the amines studied in this work, with the exception of tetraphenylp-phenylenediamine (TPPD), are sufficiently stable in chloroform to allow the product formation to be analyzed by conventional spectrophotometry at room temperature. The radical cation of TPPD decomposes after it is formed photochemically but the rate of de(1) G. N. Lewis and D. Lipkin, J . Amer. Chem. SOC.,64, 2801 (1942). (2) G. N. Lewis and J. Bigeleisen, ibid., 65, 2424 (1943). (3) H. Linschita, J. Rennert, and T. M. Korn, ibid., 76, 5839 (1964). (4) W. E. Meyer and A. C. Albreoht, J . Phys. Chem., 66, 1168 (1962). (5) K. D. Cadogan and A. C. Albrecht, J . Chem. Phys., 43, 2550 (1965). (6) K. D. Cadogan and A . C. Albrecht, J . Phys. Chem., 7 2 , 929 (1969). (7) 0. D. Dmitrievskii, Opt. Spektrosk., 19, 828 (1965). (8) H. Linschitz, M. Ottolenghi, and R. Bensasson, J . Amer. Chem. Soc., 89, 4592 (1967). (9) W. C. Meyer, J . Phys. Chem., 74, 2118 (1970).
The Journal of Physical Chemistry, Vol. 76, N o . 18, 1971
E. A. FITZGERALD, JR.,P. WUELFING,JR.,AND H. H. RICHTOL
2738
Degassed solutions were prepared by seven freezepump-thaw cycles on the vacuum rack. Actinometry and Light Sources. The incident light intensity was determined by potassium ferrioxalate actinometry. l7 The quantum yield studies were conducted at 366 mp by irradiation with a 1000-W Hanovia high-pressure mercury-xenon lamp. The 366-mp line was isolated with a Corning Xo. 5970 glass filter (half bandwidth of approximately 25 mp). The light source employed for the study of quantum yield variation with exciting wavelength was an Osram SBO 150-W xenon lamp. CHCls e- -+CHCL. C1The exciting wavelength was selected by focusing the beam through a Bausch and Lomb 250-mm grating This reaction is exothermic by 16 kcal/mol for chloromonochromator with slits set a t 1 mm (half bandwidth form.lo It is believed that the energy supplied by this reaction and the solvation energy of the ionic products 7 mp). The spectral cells containing the solutions to be photolyzed were contained in a thermostated cell in chloroform are sufficient to make the process enerholder and the temperature was maintained within getically feasible. Meyer9 has found evidence for a *2". charge-transfer complex in the photooxidation of TRIThe quantum yields determined in this work were P D in chloroform. The presence of chloroform does calculated as the integral quantum yields as defined by not alter the absorption spectra of the amines studied in Albrecht and I\ileyer.'* The products absorb in the this work and no evidence for charge-transfer complexes region of exciting wavelength and a correction for has been found. The dissociative electron attachment product absorbance is included in the above treatment. reactions of chloroform, carbon tetrachloride, and other Simplifying assumptions were made to the mathematihalogenated compounds have been studied by Hamill cal treatment and include the assumption that reactant and coworkers, 11,12 who found evidence for the haloabsorbance is essentially constant and does not vary methane free radical and halide ion after electron atwith time and the assumption that the rate of product tachment by y radiolysis. The quantum yield studies formation is linear with time over the initial short time and identification of coproducts of the radical cations in period of the exposure. Experimentally, the converthis work suggest a mechanism of photooxidation by a sion of the reactant was limited to 5% or less and soluprimary photoprocess and secondary thermal mechations were irradiated for a fixed time period in all expernism which includes dissociative electron attachment of iments. The rate of product formation was found to be the solvent. linear in the irradiation time period employed experiExperimental Section mentally. 'The above assumptions and experimental error in the quantum yield determinations lead to an Dimethyldiphenyl p - phenylenediamine General. error of *7ql, for the quantum yields reported. (DMDPPD) mas prepared from diphenyl-p-phenyleneLuminescence and Flash Photolysis Studies. The diamine (DPPD) (Eastman) by the methylation procedure of Picard.13 Tetraphenyl-p-phenylenediamine luminescence studies were performed on a modified equipped for Beckman DU quartz spectroph~tometer'~ (TPPD) mas prepared from DPPD by the method of recording emission spectra. Fluorescence quantum Fox. l 4 Hydroxydiphenylamine (HDPA) was Eastman yields were determined by comparison to 0.1 ppm quitechnical grade. The amines were purified by several nine sulfate standard solution. recrystallizations from ethanol, except for HDPA which was recrystallized twice from ligroin and twice (10) L. J . Forrestal and W. H. Hamill, J. Amer. Chem. Soc., 83, 1535 from water. The chloroform used was Fisher Reagent (1961). grade purified by the method of Gillo15 and distilled. (11) M. R. Ronayne, J. P. Guarino, and W. H . Hamill, ibid., 84, 4230 (1962). All other solvents were Fisher Spectranalyzed grade and (12) J. B. Gallivan and W. H . Hamill, Trans. FaTaday Soc., 61, 1 were used u-ithout further purification. (1966). The absorptivities of the radical cations were deter(13) J . Picard, J . Amer. Chem. SOC.,48, 2356 (1926). (14) C. J. Fox, Chem. Abstr., 60, 14051g (1964). mined by oxidation of successive amounts of amine with (15) J. Gillo, Ann. Chim. (Paris), 12, 281 (1939). coulometrically generated bromine and measurement of (16) E. A. Fitzgerald, P. Wuelfing, Jr., and H . H. Richtol, Anal. the absorbance at each increment of bromine generChem., 42, 229 (1970). ated.I6 This procedure yielded reproducible Beer's (17) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.,235, 518 law plots at the wavelength of maximum absorbance. (1950). (18) W. E. Meyer and A . C. Albrecht, J . Phys. Chem., 66, 1168 The absorptivities thus determined, the quantum yields (1962). of oxidation were then determined by spectrophoto(19) H. H. Richtol and F. H. Klappmeier, J . Chem. Phys., 44, 1519 metric analysis of the radical cations generated. (1966). composition is sufficiently slow a t 0" to allow its absorbance to be measured. The energy of ultraviolet light necessary to cause oxidation (ea. 3.5 eV) has been found to be significantly less than ionization potentials of similar amines (ca. 6 to 7 eV). The additional necessary energy in these systems is apparently provided by secondary thermal reactions. The key to the energetics of the photooxidation reaction is the dissociative electron attachment reaction which occurs with chloroform and other halogenated compounds.
+
+
-
T h e Journal of Physical C,hemistry,Vol. 76,N o . 18,1971
2739
PHOTOCHEMICAL OXIDATION OF SUBSTITUTED AROMATIC AMINES The flash photolysis experiments were conducted with an apparatus described by Strong, et aLZ0
Table I1 : Oxidation Quantum Yields as a Function of Amine Concentration in Air-Saturated Chloroform"
Results and Discussion Absorption and Fluorescence Spectra. Irradiation of the amines in chloroform in the wavelength range of 290 to 370 mp produces colored solutions. The visible spectra obtained matched the previously reported radical cation spectrum of DPPDa which was generated photochemically. The other radical cations have not been reported previously by photochemical generation but match those obtained electrochemically.16 The spectral data for the radical cations studied in this work are summarized in Table I.
Table I : Spectral D a t a for the Amine Radical Cations in Chloroform
Compd
Compd
Wavelength maximum, mrc
Absorptivity a t maximum, x 10-2
DPPL) DMDPPL) TPPD HDPA
700 675 725 450
10.8 9.4 6.0 1.46
380 400
405 380
Irradiation of the amines in the nonhalogenated solvents ethanol, benzene, acetone, or hexane produces no visible color and the ultraviolet spectra of the amines do not change after irradiation. There is apparently no efficient photochemical reaction in these solvents. Fluorescence is found in all these solvents and the fluorescence wavelength maximum is also given in Table I. Q u a n t u m Yield Studies. The quantum yields of oxidation of the amines in chloroform were studied as a function of amine concentration and the results are shown in Table 11. The oxidation quantum yields are greater than unity for DRIDPPD, DPPD, and HDPA and are virtually independent of amine concentration. The 25% decrease in quantum yield as concentration is increased tenfold for DMDPPD is not large compared with the experimental error of &7y0and there appears to be no large concentration dependence in this case. T P P D exhibits a definite increase in quantum yield as concentration is increased. The magnitudes of the quantum yields of DRIDPPD, DPPD, and HDPA indicate that oxidized product is formed in secondary thermal reactions. The concentration dependence for T P P D suggests that the amine is being oxidized in a secondary thermal step but also the fading of the TPPD radical cation color at room temperature is an indication of thermal instability of this species. All the other radical cations studied are stable and do not bleach.
Quantum yield
3.0 7.0 30
2.0 f 0 . 5 1 . 6 i0.4 1.4 f 0.2 1.9 f 0.2 1 . 7 i0 . 2 1.7rt 0.2 0.15 f 0.02 0.24 rt 0.02 0 . 3 7 i 0.04 0.55 f 0.06 1 . 2 =t0 . 2 1 . 3 i0 . 2 1.3 f 0 . 2
DMDPPD
3.0 7.0
HDPA
20
1.0 2.0 3.0 5.0 3.9 5.8 8.9
TPPDb
DPPD
a
Fluorescence maximum of parent amine, mp
Conon, M X 10'
T
=
Irradiated a t 0'.
250J hex 366 mfi.
The magnitudes of the oxidation quantum yields in chloroform have also been found to be dependent on oxygen concentration. Table I11 is a comparison of quantum yields in degassed and air-saturated chloroform solutions for T P P D arid HDPA, where this effect has been carefully studied.
Table 111: Oxidation Quantum Yield as a Function of the Presence of Oxygen"
Compd
TPPD*
HDPA
a
2'
=
Conon, M
3x 3x 3x 3x 2x 2x 2x 2x
10-4 10-4 10-4 10-4 10-8
10-3 10-3
10-3
250J hex 366 mfi.
b
Oxygen Concentration
Quantum yield
Degassed Degassed Air-saturated Air-satur ated Degassed Degassed Air-saturated Air-saturated
0.115=too.02 0.12 i 0.02 0.30 =k 0.03 0.32 rt 0.03 0.8 I O . 1 0.8 f0.1 1.7f 0.2 1.5 f0.2
2' = 0'.
It can be seen from the data in Table I11 that an approximately twofold increase in oxidation quantum yields is found in the presence of oxygen. This indicates that oxygen is entering into secondary thermal reactions that produce amine radical cations. Somewhat similar results are obtained in CC14 solvent, but due to the radical cation salt being insoluble, this system was not investigated very thoroughly. Irradiation of solutions of the amines in ethanol containing low concentrations of added chloroform produces both fluorescence and oxidation. Figure 1 shows that, as the concentration of added chloroform is in(20) R. L. S t r o n g and J. Perano, J . Amer. Chem. Soc., 89, 2535 (1967).
The Journal of Physical Chemistry, Vol. 76, No. 18, 1971
E. A. FITZOERALD, JR.,P. WUELFING,JR.,AND H. H. RICHTOL
2740
was added to the chloroform solution of amine and irradiation was conducted. It is believed that the ethyl formate found results from the “trapping” of formyl chloride intermediate by its esterification with ethanol. Mechanism of the Photooxidation. The following mechanism is proposed on the basis of the above results hv
A
---f
(1)
’A*
’A* +A
+ hvf
1A* -+SA* ---f A
+ CHCL +A . + C1- + CHClz. CHClz. + 02 +CHClzOz. CHClzOz. +CHClO + C10. CHClO HC1 + CO C10. + CHCL +COClz + HC1 + C1. C1. + A + A . + + C1-
‘A* Figure 1. The effect of added chloroform on the fluorescence and quantum yields of D M D P P D in air-saturated ethanol: [DMDPPD] = 10-3 M; Xex 366 m r ; ---, oxidation quantum yield; - - - -, fluorescence quantum yield.
-
creased, the quantum yield of oxidation increases and the quantum yield of fluorescence decreases. These data suggest that the two processes are occurring from the same excited state. Thus, the excited state from which oxidation occurs is most probably the first excited singlet state. The participation of the singlet state in the oxidation mechanism is also suggested by the oxygen effect. If the triplet state were involved in the oxidation, one would expect a marked decrease or complete quenching of reaction in the presence of oxygen. Identification of Coproducts of the Radical Cations. Several other products have been found after irradiation of air-saturated solutions of the amines in chloroform. Hydrogen ion is produced after irradiation. Table IV shows the results of titration of the chloroform solutions with sodium ethoxide after irradiation.
Table IV : Results of Sodium Ethoxide Titration after Irradiation
-t
-3
(2) (3)
(4) (5)
(6)
(7) (8)
(9)
= amine, lA* = excited singlet state of amine, amine radical cation, and hvr = quantum of fluorescent light. Steps 1-3 are standard and need no further elaboration. Step 4 is the oxidation of the excited singlet state of the amine accompanied by dissociative electron attachment of the solvent. Similarly, Bowen and Rhohatgi2‘ found that photooxidation of anthracene in the presence of oxygen in chloroform solution appeared to proceed via the singlet state. The decomposition of the dichloromethyl radical via a peroxy radical intermediate in steps 5-8 includes steps which have been found to be operative in similar systems.22-26 The ethyl formate produced in the presence of ethanol suggests these intermediate steps. The oxidation of amine by chlorine radical in step 9 accounts for the quantum yields of DPPD, DMDPPD, and HDPA which are greater than one and also for the oxidation quantum yield being approximately twice as great in the presence of oxygen (steps 5-9) According to the mechanism, the presence of oxygen should double the oxidation quantum yield and 1mol of HC1 should be found for every mole of amine oxidized to the radical cation. This is reflected in the experimental results. Other mechanisms have been proposed for the oxidation of DPPD8 and TiIIlPD5’6but these have not been
where A
A.+
=
I
Compd
DMDPPD TPPD HDPA
Mequiv of amine oxidized
Mequiv of H + found
0.09 i:0.01 0.09 zt 0.01 0.026i. 0.03 0.026 i.0.03 0.11 rk 0.01 0.11 f 0.01
0.065 zk 0.01 0.065 rt 0.01 0.033i.0.006 0 . 0 3 0 2 ~0.006 0.073 zk 0.015 0.078 rt 0.015
The data of Table IV indicate that approximately 1 mol of H + is formed for each mole of amine oxidized. Phosgene has also been found after photooxidation by its characteristic condensation reaction with benzidine. Chloride ion was detected by precipitation with silver nitrate. Also, ethyl formate was identified by gas chromatographic and infrared analysis after 1% ethanol The Journal of Physical Chemistry, Vol. 76,N o . 18, 1971
(21) E. J. Bowen and K. K. Rhohatgi, Discuss. Faraday Sac., 14, 146 (1953). (22) H.A. Staab, Angew. Chem., 75, 1203 (1963). (23) W. Brenschede and H. J. Schumacher, 2 . Phys. Chem., Abt. A , 177, 245 (1936). (24) H.J. Schumacher and K. Wolff, {bid., Abt. E , 26, 453 (1934). (25) K.B.Krauskopf and G. K. Rollefson, J . A m e r . Chem. SOC., 56, 2542 (1934). (25) J. W.T.Spinks, Chem. Rev., 26, 129 (1940).
274 1
THEPHOTOCHEMISTRY OF THE XYLENES performed in the presence of CHCL. I n the presence of CHCls, the above proposed mechanism is so efficient that all other mechanisms are unimportant. Flash Photolysis Studies. Flash photolysis of degassed solutions of the amines in nonhalogenated solvents yielded transient species which had absorption maxima between 590 and 660 mp. The absorbance of the transients decayed by first-order kinetics. No transient spectra were observed when air was allowed to enter the photolysis cell before flashing. Table V shows ~~~
~
Table V : Flash Photolytic Data for the Amines in Degassed Solvents Xmsx of transient,
ki,
Amine
Solvent
mp
sec-1 X
HDPA TPPD DMDPPD
Benzene Benzene
590 660 640
1 . 0 3z 0.1 4.8 =k 0.5 11.03z 1.0
Ethanol
the absorption maxima and first-order decay constants for the three transients studied. The above data indicate that the transient species observed is the triplet state. Phosphorescence emission of the amines in rigid solution has been observed with the emission maxima a t approximately 500 mp in all cases. Linschitzs and coworkers have observed a similar transient absorption after flash photolysis of DPPD in rigid solvent and attributed this to the triplet state. They obtained an intermediate with an absorption maximum at 610 mp and a first-order rate constant of decay in the range of 10 sec-' a t -160" in rigid EPA. The triplet state apparently is not involved in the photooxidation in chloroform.
Acknowledgment. E. A. F. gratefully acknowledges the support of a National Aeronautics and Space Administration Fellowship. This work was supported by the National Science Foundation under Grant No. GP5268.
The Photochemistry of the Xylenes. A Discussion of Method' by W. Albert Noyes, Jr.,* and D. A. Harter Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60480 (Received March 29, 1971) Publication costs assisted by the Argonne National Laboratory
Quantum yields of isomerization of the three xylenes increase with decrease in incident wavelength to about 240 nm. These quantum yields are not identical with formation yields for intermediates such as dimethyl benavalenes and dimethyl prismanes. The latter are destroyed by a variety of processes and their primary yields may be severalfold greater than the yields of ultimate isomers. Yields generally increase with temperature and decrease with increase in total pressure. Yields at long wavelengths and high total pressures approximate those in the liquid phase. Toward the long wave limit of the absorption region (260-270 nm) the sums of fluorescent and of triplet-state yields approximate unity. The latter have been determined by the Cundall method. A critical survey of triplet-state methods is made. Valid objections may be raised to all of them. The use of biacetyl is not in general to be recommended. In all probability the emitter in the case of biacetyl-aromatic mixtures is a complex and apparent yields are often unacceptability high. Oxygen quenches the fluorescences of the three xylenes effectively. Both fluorescence and crossover to the triplet state diminish as incident wavelengths decrease, thus indicating that other processes become of increasing importance at high vibrational levels of the excited singlet state. A fairly consistent picture of the photochemical behaviors of the xylenes can be given. p-Xylene appears to have considerably greater photochemical stability than the other xylenes and has the highest fluorescent yields. The photochemistry of simple aromatic compounds has received so much attention during recent years
identified are : benzvalene, prismane, Dewar benzene, and fulvene. Of these, Dewar benzene seems not to be
that no attempt will be made to cite references to more than those papers of immediate interest to the problems at hand. It is now recognized that benzene and simple benzene derivatives isomerize under the influence of ultraviolet light in the region 240-270 nm.2 The isomers so far
(1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) K. E. Wilzbach and L. Kaplan, J. Amer. Chem. Soc., 86, 2307 (1964): (b) L. Kaplan, K. E. Wilzbach, W. Brown, and 9.S. Yang, {bid., 87, 675 (1965); (0) K. E. Wilzbach and L. Keplan, ibid., 87, 4004 (1965); (d) I. E. Den Besten, I. Kaplan, and K. E. Wilzbach, ibid., 90, 5868 (1968).
The Journal of Physical Chemistry, VoE. 76, No. 18, 1871
