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Photoreactions of Nitrobenzene and Monosubstituted Nitrobenzenes with Hydrochloric Acid. Evidence Concerning the Reaction Mechanism’” Gene G . Wubbels” Ib and Robert L. Letsinger Contribution f r o m the Departments of Chemistry, Grinnell College, Grinnell, Iowa 501 12, and Northwestern University, Euanston, Illinois Received April 1, 1974
60201.
Abstract: Nitrobenzene and monosubstituted nitrobenzenes (substituents: 2-Br and -NO2; 3-Br, -NO2, -CHO, -OH, and -OCH3; 4-Br, -NOz, -C1, -CHO, and -OH) are found to react photochemically in concentrated aqueous
HCI. The principal reaction in most cases involves conversion of the nitro group to an amino group and replacement of three aryl hydrogens by chlorine, e.g., 2,4,6-trichloroaniline from nitrobenzene. 4-Chloronitrosobenzene is implicated as an intermediate in the nitrobenzene photoreaction since it is converted to the photoproducts when placed in the reaction medium in the dark. Irradiation of nitrobenzene in methanolic HCl (12 M ) gives N-(4chloropheny1)hydroxylamine as the major product. The quantum yield for disappearance of nitrobenzene at 3 13 nm in 12 M aqueous HC1 is 0.11. The efficiencydrops rapidly with decreasing HCl concentration; at 6 M HC1$ is 0.012. The efficiencies of disappearance of nitrobenzene, 3-bromonitrobenzene, and 4-nitrophenol in aqueous HCI-LiC1 solutions containing 12 M chloride ion are dependent upon the hydrogen ion concentration in the range 1-12 M . The dependence is attributed to acid-catalyzed tautomerization of a Meisenheimer-type adduct of HCI and nitro aromatic. Experiments with radical scavengers (2-propanol,phenol, and anisole) suggest that a chlorineatom intermediate is formed in the photoreaction. For the photoreactions in 12 M aqueous HCl, the quantum efficiencies are little affected by electron-withdrawing substituents. Electron-donating substituents cause marked decreases in efficiencies. The results are interpreted by a mechanism involving electron transfer from chloride ion to photoexcited nitro aromatic as the primary process.
N
itroanisoles react photochemically with nucleophiles such as pyridine,2 aliphatic amine^,^ hyd r o ~ i d e and , ~ ~cyanide5 ~ to give substitution products. Halide ions quench the excited states of nitroanisoles but d o not lead to substitution p r o d ~ c t s . ~ , ~ These reactions were studied using neutral or alkaline solutions. We speculated that interesting chemistry might result if the irradiations were carried out in acidic solutions. In such cases the course of reactions might be altered by protonation of excited nitro aromatics or of reaction intermediates. During a survey of photoreactions with mineral acids, we discovered a novel transformation of nitrobenzene in HCl.7 In this paper we report studies concerning the generality and mechanism of this photoreaction.
Results M ) in 12 M Irradiation of nitrobenzene (2 X aqueous HCl in a cuvette caused a shift of A,,, from 271 nm ( A = 1.4) to 291 nm ( A = 0.28). Chromatog(1) (a) We are grateful for financial support from the National Science Foundation (GP-5715) and the National Institutes of Health (Predoctoral Fellowship 5-Fl-GM, 771-03 to G. G. W.). Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work at Grinnell College. (b) Grinnell College. (2) (a) R. L. Letsinger, 0. B. Ramsay, and J. H. McCain, J . Amer. Chem. SOC.,87, 2945 (1965); (b) R. L. Letsinger and 0. B. Ramsay, ibid., 86, 1447(1964). (3) M. E. Kronenberg, A. van der Heyden, and E. Havinga, R e d . Trac. Chim. Pays-Bas, 86,254 (1967); 85,56 (1966). (4) S. deVries and E. Havinga, R e d . Tral;. Chim. Pays-Bas, 84, 601 (1965). (5) (a) R. L. Letsinger and J. H. McCain, J . Amer. Chem. SOC.,91, 6425 (1969); 88, 2884 (1966); (b) R. 0. deJongh and E. Havinga, R e d . Trac. Chim. Pays-Bas, 87, 1327 (1968). (6) For reviews of this work see (a) E. Havinga and M. E. Kronenberg, Pure Appl. Chem., 16, 137 (1968); (b) H. A. Morrison in “The Chemistry of the Nitro and Nitroso Groups,” Part I, H. Feuer, Ed., Interscience, New York, N. Y.,1969, Chapter 4. (7) For a prelimiaary report see R. L. Letsinger and G. G. Wubbels, J . Amer. Chem. SOC.,88,5041 (1966).
Journal of the American Chemical Society 1 96:21
raphy on silica gel of the products of a preparative scale reaction (5 X Mnitrobenzene, external lamp) afforded 2,4,6-trichloroaniline (I, 44-6 1 %) and 2,4-dichloroaniline (11, -10 Gas chromatographic (gc)
z).
hv
I
C1 I1
C1
analysis of the products from subsequent runs revealed I (62 I1 (16 and 2,3,4,6-tetrachloroaniline (2.7 That the products are photochemically stable was shown by recovering I and I1 completely after prolonged irradiation in 12 M aqueous HCI. Investigation of the influence of various reaction conditions on the product yields was carried out using an immersion lamp. For a solution of nitrobenzene ( 5 X M ) in 12 M aqueous HCl, the yields of I and I1 were 58 and 23 %, respectively. These yields were not altered when the reaction was carried out in the presence of 1 ( 3 x 10-3 M ) . Analysis of the products at 63 conversion of nitrobenzene revealed a lower yield of I (40%), but no change in the yield of 11. Reducing the initial nitrobenzene concentration to 2 x M caused only a slight decrease (8 %) in the yield of I. M nitrobenzene) was When the reaction ( 5 x carried out in the presence of excess M ) phenol or anisole, the yield of I decreased by 5 %, while that of 11 increased by 5-10z. The small amounts of chlorophenols and chloroanisoles produced in these reactions were subjected to gc analysis. The yields, based on nitrobenzene, were: 2-chlorophenol, 0.5 %, and 4-
z), z),
October 16, 1974
z).
6699
chlorophenol, 0.7 %;* and 2-, 3-, and 4-chloroanisole, 1.35,0.0, and 2.20 %, respectively. The solubilities of many substituted nitrobenzenes in an entirely aqueous medium are too low to permit investigation on a preparative scale. The possibility of exploring the photochemistry of such compounds was opened by the observation that the photoreaction of nitrobenzene occurs cleanly in a solution of acetic acid: 12 M aqueous HC1 (1 : 4, v/v). The spectral changes and the product distribution for the reaction were similar t o those for the reaction in 12 M aqueous HCI. I and I1 account for about 80% of the nitrobenzene. Treatment of a photolysate (12 M HCI) with tin or stannous chloride caused slight increases (2-3 %) in the yields of I and 11. Since treatment with tin would convert phenylhydroxylamines and azo- and azoxybenzenes to aniline derivatives, we conclude that such incompletely reduced nitrogen compounds are not significant end products of the photoreaction. This conclusion is supported by thin layer and column chromatographic analyses which also failed to reveal such products. Nitrobenzene reacted very slowly ( 0.1 M ; i.e., increasing [H+] above 0.1 M caused no increase in efficiency. The basic behavior was attributed to species IV.22 In the present case, dependence of efficiency on [H+] occurs well beyond the acidity at which species IV would be completely protonated. This suggests involvement of an additional base which is insufficiently basic to be completely protonated in 12 M HCl. We attribute this behavior to adduct VI. 26 This interpretation may be placed on a quantitative basis by considering each of the four terms in eq 15 representing partitioning of reaction intermediates. Intermediate IV is essentially completely protonated at [H+] > 0.1 M . Since the effects of substituents on the basicity of IV should be relatively small, an assumption for each nitro aromatic that the second term in eq 15 is close to unity in the range of acid concentrations employed (1-12 M ) appears reasonable. For a specific nitro aromatic compound at constant 12 M [Cl-1, the values of the first and third terms are independent of [H+] and therefore constant. Thus, under these conditions, eq 15 reduces to eq 16, the inverse of which (eq
z
a
=
c-
kii
+ ki2[H+]
k12[H+1
(C
=
constant)
(16)
17) is a convenient working expression. The quotient 1 - 1 1 kll 1 a C Cklz[H+] of slope and intercept obtained from a plot of +-l us. [Hfl-' for each nitro compound represents kll/kll. The values of kll/k12for 3-bromonitrobenzene, nitrobenzene, and 4-nitrophenol are 9.8, 5.7, and 2.3, respectively. The narrow range of these values shows that substituent influences on the partitioning of intermediate VI are small. That the partitioning of intermediate VI could indeed account for the ratios observed may be seen as follows. The protonation process represented by eq 12 generates a cyclohexadienyl cation analogous to intermediates formed in electrophilic aromatic substitution reactions. On this basis, a+ substitutent constants would appear to be appropriate indicators of the influence of substituents on klz. According to a+ values, the bromo substituent is electron withdrawing at both the para and meta positions (a+ = 0.150 and 0.405, respectively), 27 whereas the hydroxyl substituent is electron donating at para and meta positions (a+ = -0.9227 and -0.13,28 respectively). Electron withdrawal by a substituent would be expected to decrease the value of k l ~ , whereas electron donation would increase kI2. If, as (26) Dependence of efficiency on [H+] through the entire concentration range would occur if k n were rate determining or if K e q for eq 12 were small and k13 were rate determining. Kinetic involvement of
(24) F. S . Brown and L. P. Hager, J . Amer. Chem. SOC., 89, 719 (1967). Direct comparison of the observed para:ortho ratio (1.6) with the ratios reported by Brown and Hager is not possible because of differences in reaction conditions. (25) R. Hurley and A . C. Testa, J . Amer. Chem. SOC.,90,1949 (1968).
protons in eq 12 is preferred over that possible in eq 14 since formation of VI11 is probably irreversible, Le., a competing path causing inefficiency is not feasible for VIII. (27) H. C. Brown and Y . Okamoto, J. Amer. Chem. Soc., 80, 4979 (1958). (28) G. Illuminati, J. Amer. Chem. Soc., 80,4947 (1958).
Wubbels, Letsinger 1 Photoreactions of Nitrobenzene and Monosubstituted Nitrobenzenes
6704
seems probable, the value of kll for the elimination of HCl represented by eq 11 is only slightly affected by substituents, the expected influence of substituents on klz would account qualitativelv for the observed variations in kll/kI2. Disappearance quantum yields for monosubstituted nitrobenzenes in 12 Maqueous HCI (Table I) are affected little or decreased slightly, when electron-withdrawing substituents (CHO, Br, or NOz) are present. Electrondonating groups (OH or OCH3) cause large decreases in the efficiency. Data are not presently available for assessing directly the relative effects of substituent changes on terms 1 and 3 in eq 15. Evidence suggests, however, that the rate constant ( k l ) for radiationless decay of the photoexcited nitro aromatic is decreased when a methoxy or hydroxy substituent is present. Thus, photoexcitation of nitrobenzene is believed to populate a short-lived (T sec) 91,7r* state,25 whereas photoexcitation of 3-nitroanisole populates a 37r,7r* state (T = 24-40 X ~ e c ) .By ~ ~analogy to spectroscopic findings for aromatic carbonyl comp o u n d ~ , electron-withdrawing ~~ substituents would have only slight effects on the lifetime and population of the n,n* state. Electron-donating substituents, by reducing the energy of the longer lived T,T* state and thus increasing its population at the expense of the n,n* state, would be expected to increase the excited-state lifetime. Since, in fact, hydroxyl and methoxyl groups decrease the efficiency of the photochemical reaction while increasing the excited-state lifetime, introduction of these substituents must cause either a marked decrease in k2 (which appears likely) or a decrease in the third term [ k l o / ( k , klo)]of eq 15. The data are consistent with the view that the photoreaction with HCI involves transfer of an electron from chloride ion to an electrophilic n , r * state of the nitro aromatic, the T,T* state being relatively unreactive. Alternative mechanisms ink olving ionic (nucleophilic or electrophilic) addition of the elements of HC1 to the excited nitro aromatic were rejected because of the positive evidence for radical intermediates. A mechanism involving protonation in the excited state' followed by electron transfer from chloride ion giving V would account for the radical scavenging results but is difficult to reconcile with the observed substituent effects and evidence obtained previously. 2 2 3 1 The effects of substituents on quantum yields for the reaction with HC1 are grossly similar to those observed for photoreduction of monosubstituted nitrobenzenes by 2-propanol, ? ? a process attributed to 3n,7r* states. Formation of 4-amino-3,5-dichlorobenzoicacid from 4-nitrobenzaldehyde in HCl suggests that 4-nitrosobenzoic acid is an intermediate.33 Formation of this intermediate was expected since it seemed likely that
-
+
(29) J. den Heljer, T. Spee, G. P. de Gunst, and J. Cornelisse, TetrahedronLett., 1261 (1973). (30) G. Porter and P. Suppan, Trans. Faraday Soc., 61, 1664 (1965); P. J. Wagner, A. E. Kemppainen, and H. N. Schott, J . Amer. Chem. SOC., ~~
95,6504 (1973).
(31) A . Cu and A. C. Testa, J . Amer. Chem. SOC.,96, 1963 (1974), have reported that, on flashing a solution of nitrobenzene in 50% 2propanol-water containing 6 M HCI, a transient absorption occurs at 440 nm, which they attribute to PhNOzH.. Testa has withdrawn his support for the proposallj that the reaction proceeds by protonation of excited nitrobenzene as the primary process. (32) S . Hashimoto and I
