ELECTROCHEMICAL REDUCTION OF AROMATIC NITROCOMPOUNDS
3517
Electrochemical Reduction of Aromatic Nitro Compounds in the Presence of Proton Donors
by Steven H. Cadle, Paul R. Tice,’ and James Q. Chambers2 Department of Chemistry, University of Colorado, Boulder, Colorado 80606 (Receined April 10, 1967)
The effect of proton donors on the electrochemical reduction of aromatic nitro compounds in nonaqueous solvents has been studied. I n the presence of certain proton donors, the nitro compound is reduced by two independent pathways: a reversible oneelectron step and an irreversible process that yields a mixture of the corresponding phenylhydroxylamine and azoxybenxene. The irreversible process occurs at potentials more positive than the oneelectron reduction for the proton donors ptoluenesulfonic, trichloroacetic, o-phthalic, salicylic, benzoic, and phydroxybenzoic acids and N,N-diethylanilinium perchlorate. It is suggested that the reduction proceeds via a hydrogen-bonded complex formed between the nitro compound and the acid in the double layer.
Introduction The effect of proton donors on electrochemical processes in aprotie solvents has been investigated for the reduction of aromatic hydrocarbon^^-^ and carbonyl compound^.^ For these processes the role of the acid is usually to protonate the radical species resulting from the addition of one electron, thus making the subsequent additions of electrons easier. Accordingly, two oneelectron waves coalesce to one two-electron wave upon the addition of a proton source in the reduction of hyd r o c a r b o n ~ . ~In this paper the effect of proton donors in nonaqueous solvents on the reduction of aromatic nitro compounds is investigated. The one-electron reduction of nitro compounds has been well studied by electrochemical method^,^^^ by electron spin resonance (esr) spectroscopy in nonaqueous* and aqueousgmedia, and by visible absorption spectroscopy in nonaqueous1° and aqueous”J2 media. I n these solvenl, systems the anion radical of the nitro compound is found to be the species formed by the primary electrode process. In recent years, some of the details of the many-electron reduction process in neutral and alkaline media have been elucidated by ,14 and pulse radi~lysis’~ techniques. In these media the radical anion or its protonated form can either diffuse into the bulk solution and act as a reducing agent or can be further reduced in a series of heterogeneous reactions at the electrode surface.’3
When aromatic nitro compounds are reduced in the presence of a proton donor in nonaqueous solvents, independent reduction waves for both a reversible oneelectron and an irreversible many-electron process are observed. The irreversible reduction wave occurs a t potentials more positive than the one-electron wave and resembles descriptions of “preprotonation”lBand “prior (1) National Science Foundation Undergraduate Research Participation Fellow. (2) To whom correspondence should be addressed. (3) G. J. Hoijtink, J. van Schooten, E. de Boer, and W. I. Aalkersberg, Rec. Trav. Chim., 73, 355 (1954). (4) P. H. Given and M. E. Peover, J . Chem. Soc., 385 (1960). (5) K.S.V. Santhanam and A . J. Bard, J. Am. Chem. Soc., 88, 2669 (1966). (6) L. Holleck and H. J. Exner, Z . Elektrochem., 56, 46 (1952). (7) L. Holleck and D. Becher, J. Electroanal. Chem., 4, 321 (1962). (8) D. H. Geske and A. M. Maki, J . Am. Chem. SOC.,82, 2671 (1960). (9) L. H. Piette, P. Ludwig, and R. N. Adams, ibid., 84, 4212 (1962). (10) W. Kemula and R. Sioda, Nature, 197, 588 (1963). (11) B. Kastening, Electrochim. Acta, 9, 241 (1964). (12) J. Q. Chambers and R. N. Adams, Mot. Phgs., 9, 413 (1965). (13) R.Koopmann and H. Gerischer, Ber. Bunsengea. Physik. Chem., 70, 127 (1966). (14) B. Kastening, Z . Anal. Chem., 224, 196 (1967). (15) K. D. Asmus, A. Wigger, and A. Henglein, Ber. Bumenges. Physik. Chem., 70, 862 (1966). (16) 8. G. Mairanovskii, J . Electroanal. Chem., 4, 166 (1962).
Volume 71, Number 11
October 1967
3518
protonation"4 processes described in the literature. I n these mechanisms the substrate is viewed as being protonated in the electrical double layer prior to electron transfer. Given and Peover4 reported electrochemical waves of this kind for the reduction of benzophenone, benzaldehyde, and xanthone in dimethylformamide in the presence of benzoic acid, and Mairanovskii16 has suggested that this is a general mechanism for reductions in buffer solutions. Other workers have observed similar effects of proton donors on reduction waves of azobenzene17and nitro c o n i p ~ u n d s . ~ * ~ ~ ~ The many-electron pathway was found to depend markedly on the choice of proton donor. This paper reports on the effect of proton source, solvent, supporting electrolyte, and electrode material on the manyelectron process. Experimental Section Practical grade acetonitrile which had been in contact with calcium hydride for several days was purified by rapid distillation (once or twice) over phosphorus pentoxide followed by a slow distillation from calcium hydride. The tetraethylammonium perchlorate (TEAP) was prepared from the bromide (TEABr) and lithium perchlorate and recrystallized from water several times until the washings gave a negative test for bromide. The p-chloronitrobenzene (p-ClNB) and the TEABr were recrystallized from ethanol. The diethylanilinium perchlorate was prepared following directions in the 1iteraturee2O All other chemicals were reagent grade and used without further purification. The apparatus used to obtain the cyclic voltammograms has been described previously.z1, 2 2 The equipment for chronopotentioinetry was conventional. The constant potential coulometer was based on a design given by Uiiderkofler and ShainZ3and employed a Heath 3Iodel EUW-19A operational amplifier module. The current-time curves were displayed on a Xosley Model 1358 X-Y recorder using a sweep generatorz1 as a time base. The hanging-drop electrode was prepared by hanging three drops of mercury collected from a droppingmercury capillary on a mercury-plated platinum wire which had been force fitted through a bullet-shaped Teflon plug and sanded flush with the Teflon. The hanging-drop electrode had an area of 6.4 X cm2. The platinum working electrode was a commercial Beckman electrode. The reference electrode was a saturated calomel electrode which was separated from the working electrode conipartment by a salt bridge containing the solution being studied and a finePorosity sintered-glass frit. The Potentials reported in this work were found to be reproducible to A0.02 v T h e Journal of I'husical Chemistry
S. H. CADLE,P. R. TICE,AND J. Q . CHAMBERS
over a period of several months. The counterelectrode was a large platinum coil or foil immersed directly in the test solution. The solutions were well deaerated with prepurified nitrogen which had been saturated with the solvent. Occasionally air was purposely introduced into the solution. I n all cases oxygen could be eliminated as the cause of the many-electron waves anodic of the one-electron wave. The experiments were carried out at 24 f 2'. The coulometric analyses were performed on 25.0-ml aliquots of acetonitrile solutions which were 2.0 X M in p-ClNB and 0.10 M in TEABr. The acid concentration of these solutions varied from 2.0 X to 8.0 X M . The electrolysis vessel was a 50-ml beaker fitted with a rubber stopper which had been bored to allow insertion of the electrodes and a nitrogen inlet tube. A mercury-pool electrode served as the cathode and an Ag-AgBr electrode was the counterelectrode. The reference electrode arrangement described above was used. Nitrogen was bubbled through the cell during the electrolysis. Cyclic voltanimetry and dme polarography were used to select an electrolysis potential between the many- and the one-electron reduction waves. After every 1000 sec the electrolysis was stopped and a cyclic voltanimogram or polarogram was recorded. The electrolysis was continued in this fashion until the current had diminished to a small fraction of its original value and the nianyelectron wave had all but disappeared. The reduction products of some of the constant-potential electrolyses were isolated by addition of deaerated ether to precipitate the TEABr and evaporation of the supernatant to dryness under nitrogen. The products were identified by their ultraviolet and mass spectra. The procedure for the in situ electrolysis in a cavity of an esr spectrometer has been described.24 Results and Discussion In the absence of a proton donor, the reduction of aromatic nitro compounds proceeds by a simple reversible one-electron step to produce the radical anion. (17) J. H. Sharp, Ph.D. Thesis, University of New South Wales, Sidney, Australia, 1966. (18) W. Kemula and R. Sioda, BuZl. Acad. PoZon. Sci., Ser. Sci. Chim., 10, 107 (1962). (19) A. M. Hartley, private communication. (20) S. Bruckenstein and I. M. Kolthoff, J . A m . Chem. Soc., 78, 2974 (1956). (21) J. R. Alden, J. Q. Chambers, and R. N. Adams, J . Electroanal. 5 9 152 (1963). (22) C. A. Chambers and J. Q. Chambers, J . Am. Chem. Sac., 88, 2922 (1966). (23) W. L. Underkofler and I. Shain, A n a l . Chem., 35, 1778 (1963). (24) R. N. Adams, J . Electroanal. Chem., 8 , 151 (1964).
ELECTROCHEMICAL REDVCTION OF AROMATIC NITROCOMPOUNDS
A further reduction of the radical anion occurs a t more negative potentials ( - 1 . 3 to -1.4 v us. sce) in which protons are furnished by the solvent and which was not studied in this work. When a proton donor is added to nonaqueous solvents (e.y., acetonitrile, dimethylformamide) containing a supporting electrolyte and the nitro compound, a wave appears a t potentials more positive than the one-electron process. In the experiment illustrated in Figures 1 and 2, the nitro compound was p-chloronitrobenzene (p-ClSB), the proton donor was o-phthalic acid, the supporting electrolyte was 0.1 M TEAP, and the solvent was acetonitrile. The new wave can be seen at ca. -0.60 v vs. sce. (The phthalic acid was electroinactive in the potential region more positive than c a . - 1.3 v us. sce.) As the concentration of the o-phthalic acid was increased, the height of this wave increased linearly and the height of the oneelectron nitro wave decreased. The height of this wave increased to that of a three- or four-electron wave and simultaneously the one-electron wave at - 1.1 v disappeared from the cyclic voltammograms (Figures 1 and 2 ) . The most striking feature of the many-electron wave is that it is influenced markedly by the proton donor chosen. The half-peak potential (E,l2) for a peak voltammogram or the quarter-wave potential (ET/4) for a chronopotentiogram varied by ca. 0.5 v depending on the acid used. The most positive half-peak potentials were observed for trichloroacetic acid and p-toluenesulfonic acid. (The behavior of these acids is complicated by the fact that they are electroactive a t potentials more positive than the one-electron process at - 1.1v. This is in accord with the work of V1cekZ5and Kolthoff,26who found that the overpotential for the evolution of hj,drogen on mercury in acetonitrile was much less than in mater. The many-electron nitro reduction waves For these two acids occurred ca. 0.1-0.2 v more positive than the currents observed in the presence of the acids alone.) The most negative wave before the one-electron wave was observed in the presence of benzoic acid. I n this case the many-electron wave was merged with the one-electron wave. ?io waves at potentials more positive than the one-electron wave were found for phenol, hydroquinones, or water (at low concentrations). The data are summarized in Table I. A many-electzon wave a t potentials more positive than the one-electron wave was also observed for different nitro cclinpounds, in the solvents dimethylforniamide, dimethyl sulfoxide, and propylene carbonate, and in the presence of a TEABr-supporting electrolyte. The wave was obtained using both platinum and niercury working electrodes although it was 0.10.2 v more negative on platinum and more drawn-out.
- 20
3519
V I
- 0.4
I
I
I
I
- 0.8
-0.6
I
1
1
I
- 1.0
- 1.2
Volts us. sce.
--
Figure 1. Effect of a proton donor on the p-ClNB reduction. Cyclic voltammogram of 4.18 mM p-ClNB, 1.76 mM o-phthalic acid, and 0.1 M TEAP in acetonitrile. Sweep rate, 60 mv/sec; electrode area, 6.4 X IO-* om2.
100
I t
4
20
-
0
I
- 0.4
- 0.6
I
I
-0.8 vO1b US.
I
I
- 1.0
I
I
I
4
- 1.2
BCe.
Figure 2. Effect of a proton donor on the p-ClNB reduction. Same as Figure 1 except 6.95 mM o-phthalic acid was used.
These data are given in Table IT. It is clear that the process involved in these reduction waves is a general one. The reduction of the p-chloronitrobenzene-phthalic acid-acetonitrile system using either TEAP or TEABr as a supporting electrolyte was chosen from those of Tables I and I1 for a more detailed study. This choice was made because the chemicals are easily obtained in a pure state and the use of o-phthaJic acid as a donor gave a many-electron wave well separated from the one-electron wave. The parameters which characterize typical singlesweep peak voltanimograins of the many-electron wave are given in Table 111. These data do not describe the process in a simple manner. At low ratios ( R ) of acid (25) A. A. Vloek, Collection Czech. Chem. Commun., 20, 636 (1955). (26) J. F. Coetzee and I. M. Kolthoff, J. Am. Chem. Soc., 79, 6110 (1957).
V o l u m e 71, Number 11
October 1967
S. H. CADLE,P. R. TICE, AND J. Q. CHAMBERS
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Table I: Reduction of p-Chloronitrobenzene in the Presence of Different Proton Donors"
_-----Proton donor
Er/a
p-Toluenesulfonic acid Trichloroacetic acid N,N-Diethylanilinium perchlorate o-Phthalic acid o-Phthalic acidd Salicylic acid p-Hydroxybenzoic acid Benzoic acid
One-electron reduction
Many-electron reducoitn-
EPb
Epilb
EPb
-O.4gc
-1.11
-0.56
-0.65
-1.14
-0.58
-0.68 -0.848
-1.16 -1.13
-0.87 -1.05ff
-1.12
...
-0.54 (i = 90.0 pa)'
...
-0.65 (i = 95.7 pa)
-0.81 CU.
f
The solutions were 4 mM in p-CINB, 10 mM in proton donor, and 0.1 M in TEAP, and the solvent was acetonitrile unless noted cm2was employed. Units: volts us. sce. The sweep rate was 60 mv/sec. These waves otherwise. An HDE of area 6.4 X The concentration of acid was 8 mM. e Split wave. The one-electron wave occur near a poorly defined background process. The wave was merged with the one-electron wave. was distorted.
'
ff
Table I1 : Reduction of Nitrobenzene Derivatives in the Presence of +Phthalic Acid in Different
Electrode
Solventelectrolyte
Nitro compound
--Many-electron EPP
AN-0.1 M TEAP AN-0.1 M TEAP AN-0.1 M TEAP AN-0.1 M TEAP DMSO-0.1 '%1 TEAP PC-0.1 ill TEAP DlIF-0.1 Ai' TEAP AX-0.1 M TEAP AN-0.1 M TEABr
p-Nitrobenzaldehyde p-Nitroaniline Nitrobenzene p-Nitroanisole p-C1NB p-ClNB' p-ClNB p-CINB P-CINB~
reductione-E,
-0.67 -0.77 -0.76 -0.85 -0.75 -0.86 -0.82 -0.93 -0.81 -0.88 -0.515 -0,605 ( E r / 4= -0.88 v at 106 pa) -0.69 -0.88 -0.76 -0.88
One-electron reduction' E,
-0.97 -1.42 -1.19
... -1.06 -1.05
... -1.17 -1.07
a Abbreviations: p-ClNB, p-chloronitrobenzene; DMSO, dimethyl sulfoxide; D l I F , dimethylformamide; AN, acetonitrile; TEAP, tetraethylammonium perchlorate; TEABr, tetraethylammonium bromide; PC, propylene carbonate. The solutions were 4 mM in nitro compound, 10 mM in phthalic acid, and 0.1 M in supporting electrolyte unless noted otherwise. The sweep rate was The concentration of o-phthalic acid was 2.4 mM and the concentration of p-ClNB was 2 mM. The concentration 60 mv/sec. Units: volts us. sce. of o-phthalic acid was 8 mill and the concentration of p-ClNB was 2 mM.
(HA) to nitro compound the value of Ep,P became more negative by ca. 45 mv per decade increase in the acid concentration. At large values of R ([p-ClNB] = 0.25 mM, [HA] > 1mM), the value of E p i 2became constant or slightly more positive as the acid concentration was increased. The width of the wave was not constant, but became broader as the acid concentration increased and the ans,values calculated from the equation2' ana =
4s
E,
- EPl2
decreased from 0.57 to 0.37 in the experiment of Table 111. The value of i,/V'/z was constant within the limits expected for spherical difhsion for sweep rates The Journal of Physical Chemistry
from 24 to 2000 mv/sec. The many-electron wave did not display any reversible character at sweep rates up to ca. 10 v/sec. Analogous behavior was found from the chronopotentiometric studies. The value of the chronopotentiometric constant, irl", for the many-electron wave was directly proportional to the p-ClNB concentration a t high ratios ( R ) and directly proportional to Figure 3 the HA concentration a t low R (
