daehan hwahak hwoejee (Journal of the Korean Chemical Society) Vol. 21, No. 6, 1977 Printed in Republid of Korea

니트로벤젠의 전해환원 반응 조건과 메카니즘

千井釣 •白 雲 基

서강대학교 이공대학 화학과

(1977. 7. 23 접수 )

Reaction Conditions and Mechanism of Electrolytic Reduction of

Jung-Kyoon Chon* and Woon-Kie Paik

Department of Chemistry, Sogang University, Seoul, Korea

(Received Jaly 23, 1977)

요 약 . 에탄올 -물 혼합용매에서 납전극을 사용하여 니트로벤젠 (©NOV 과 그 유도체의 전해 환 원반응을 조사하였다 . 산성용액에서는 퍼텐셜에 따라 ©NHOH 및 ^NH2 가 생성되었으며 니트로벤 젠 0NO)은 중간 체가 아닌것으로 보였다 . 염기성 용액에서는 ©NO가 생성되며 더 낮은 퍼텐셜에서 환원시키면 둥 짝지어진 화합물이 생성됨을 확인하였다 . 사용한 전해질 용액에서 ©NO 와 ©NHOH 사이에 화학적인 짝지음 반응 (coupling reaction)은 일어나지 않았다 . 각각의 반응에 대해 전류一전압관계와 pH 의존도 및 반응물질에대한 반응 차수로부터 반응 메카니 즘을 도출하였다 . ©NO 가 생성되는 반응은 치환기가 있을 때도 같은 메카니즘을 따르는 것으로 보 인다 .

ABSTRACT. Electrochemical reduction of nitrobenzene (©NO，and its derivatives on Pb electrode was studied by means of galvanostatic measurements and coulometric electrolysis in €thanol-water solvent. In acidic s사 utions phenylhydroxyl amine and were produced while nitrosobenzene and coupled products such as azo- and hydrazobenzene were produced in basic s시 utions. Nitrosobenzene (©NO) was not found to be an intermediate in the reduction reactions of ©NO2 in acidic solutions. No direct coupling between ©NO and ©NHOH was observed to occur in the electrolyte solutions used. Mechanisms of the production of and nitrosobenzene are deduced from Tafel slope, pH dependence and reaction order with respect to nitrobenzene. Mechanism for the reduction of substituted seems to be identical to that of nitrobenzene.

organic reactions due to its potential as synthetic INTRODUCTION and analytical tool1^6, one of the simplest Despite the growing interest in electrochemical reactions among the electroreduction of organic

*Present address: Korea Nuclear Fuel Development compounds does not seem to have received a full Institute, Seoul, Korea treatment of elucidating the reaction mechanisms

—404 — 니트로벤젠의 전해환원 반옹 조건과 메카니즘 405 and influence of experimental conditions such neutral aqueous solutions. However, the exact as electrode material and pH on the reaction mechanism remains unresolved. course. It is the electroreduction reactions of Fleischman, et aL17 proposed that nitroben­ nitrobenzene (^NO2) and related compounds. zene is reduced directly to phenylhydroxylamine In 1898 Harber 7>8 found that various reduction on mercury electrode in aqueous sulfuric acid products of nitrobenzene could be obtained on solutions, and nitrosobenzene is not an inter­ platinized platimm electrode and proposed a mediate in this reduction. But phenylhydroxyl- reaction scheme which seems to be generally amine reduced from nitrosobenzene were said to accepted at the present time, But it should be condense with remaining nitrosobenzene. noted that Harber1 s reduction scheme refers to In the present investigation kinetic measure­ the reaction on platinized platinum and applies ments of the electrochemical reduction of nitro­ to the particular potentials at which the reduc­ benzene and its derivatives on lead cathodes tion was carried out. were made in order to elucidate the mechanism Since these early studies by Harber, a number of the reduction reactions. of investigators9^18 have proposed reduction EXPERIMENTAL mechanism of nitrobenzene based on their polarographic experiments and other methods. Chemicals and Meterials. Reagent grade： Polarographic reduction of nitrobenzene to nitrobenzene (Eastman Kodak) was purified by phenylhydroxylamine in aquenous solutions n~13 means of vacuum distillation24. Phenylhydroxyl­ occurred with a 4-electron wave at all pH's, and amine25 was prepared by reduction of nitro­ below pH 4. 7 there appeared a second 2-electron benzene with zinc metal in the presence of am­ wave corresponding to reduction of the phenyl­ monium chloride. Nitrosobenzene26 was prepared hydroxylamine to aniline at more negative by oxidation of phenylhydroxylamine using potentials. . The crude products were However, in aqueous basic solutions (pH purified by recrystalization from ethyl alcohol. greater than 10) esr studies18^20 showed the pre­ Reagent grade tetraethylammonium chloride sence of a short-lived intermediate corresponding (Aldrich), sodium hydroxide (Baker), lithium to the nitrobenzene radical anion. In nonaqueous chloride (Baker), and sulfuric acid (Baker) were solvents21^23 such as acetonitrile and dimethyl­ used as supporting electrolyte without further formamide the nitrobenzene radical anion is purification. All electrolytic solutions were pre­ reported to be very stable. pared from reagent grade absolute ethyl alcohol The mechanism of the electrochemical and (Malinkrodt) and doubly distilled water (80 chemical coupling reaction between two nitroben­ percent by volume in alcohol) with tetraethyl­ zene derivatives to yield azoxy, azo- or hydra­ ammonium chloride (or lithium chloride) and zobenzene has also been studied14^18. As a result sodium hydroxide as the electrolyte for basic of these studies the coupling reactions were solutions and sulfuric acid for acidic solutions. either assumed to take place via a self-conden­ A smooth electroplated lead wire was used as sation reaction of a hypothetical intermediate, the working electrode. Lead was electroplated nitrosobenzene, in aqueous media, or via con­ on platinum wire or platinum mesh, on which densation of a nitrosobenzene molecule with a copper was previously electroplated, in a fluo­ phenylhydroxylamine molecule in acidic and borate bath27. These electrodes were chemically

Vol. 21, No. 6, 1977 406 千井釣 •白雲基 polished in a solution of 80 % glacial acetic RESULTS add and 20 % hydrogen peroxide for a few seconds28. After the electrodes were withdrawn Polarization Curve and Reaction Order. from the bath, they were immediately degreased The steady state polarization (i^v) curves ob­ by washing with isopropyl alcohol followed by tained for the reduction of nitrobenzene in the washing with a stream of doubly distilled water. solutions of varying pH by galvanostatic Apparatus and Measurements. The cell method are presented in *Fig 1. The polarization used for the electrochemical reactions was a 3- curves with increasing current were close to compartment pyrex glass cell. The working those with decreasing current. electrode compartment was seperated from the The current (/) at a fixed potenti시 was ob­ counter electrode compartment by a fritted glass served to be independent of concentration of disc and connected to the reference electrode nitrobenzene (©NO?) in each solution down to compartment by a Luggin capillary. The general the lowest concentration used, that is, construction of the cell was similar to the one Elog 2/Slog (^NO2) pH = 0 (1) described elsewhere27. All electrochemical measurements were per­ formed with a Beckman Model Electroscan-30 electrochemistry system and an electrometer (Keithley 61OC). All the potentical data giver here are referred to the saturated calom니 elec­ trode (SCE). The electrolytes of pH 8.50 or greater were buffer mixtures of tetraethylammonium chloride and sodium hydroxide solutions whose pH's were measured with a glass electrodes. After desired period of electrolysis, the pro­ ducts were identified by TLC technique and spec­ trophotometry from an aliquot portion of sample removed quickly from the solution being electrolyzed. Kieselgel GF254 (Merck) and chloroform were used for the fixed phase and eluent, respectively, in the TLC. The spectro­ photometric identification was made by comparing the spectra obtained with a Beckman DK-2A spectrophotometer with the standard spectral -0.2 P.우 -0.6 -0.8 volts vs. SCE data in the literature31.

Stationary state current-voltage curves (Tafel Fig. 1. Steady-state P시 arization curve. The Tafel Plots) were obtained by applying constant slopes are indicated in mV. currents from the galvanostatically operating (I) 0. 03 M H2SO4, (II) 0.1M Et4NCl, (III) 9/10 XOaM Et4NCl+l/10X0. IM NaOH (pH=10.60); electrochemical control unit and reading the o： L0X10-W ©NO2, A : 1.0X10-3Af ^NO2j X : potential after the latter reached a steady value. 1. 0X10~2A/^NO2.

Journal of the Korean Chemical Society 니트로벤젠의 전해환원 반옹 조건과 메카니즘 407 solutions the Tafel slope for the reduction of The Tafel slopes, are varied nitrosobenzene was obtained to be 60 mV, and as indicated in mV in the figure. was similar to that for the second step in re­ pH Depenence. Figs. 1 and 2 show that the duction of nitrobenzene. In acidic solutions the polarization curves are varied in various pH Tafel slope is 110 mV for phenylhydroxylamine ranges. As presented in Fig. 2 the pH depen­ and 280 mV for nitrosobenzene. dence of current at constant potential, (Slog i/ Analyses of Products. Products obtained QpH) e, was observed to be 一 2 in basic solution at various potential and pH were analyzed by and was zero in acidic solution. The pH?s of TLC technique and spectrophotometric method. the electrolyte solutoins were measured before Unambiguous identification was mostly possible and after each experiment and were found to by mashing positions of 바 le TLC spots against be unaltered by the electrolysis for those solu­ those of known reference materials. Con­ tions presented in Fig. 2. firmation was also made by comparing the Reduction Kinetics of Nitrobenzene Deri­ uv spectra with 바此 standard spectra in the 〜 vatives. The steady state current-voltage (i v) literature31. curves of nitrobenzene and the chemically The uv absorption at the 사 laracteristic wave synthesized nitrosobenzene and phenylhydroxyl- length for each compound (^NO2：260 nm, <^NO： amine are shown in Fig. 3 and 4. In basic

l0( 房

、 1^- 0,

.0

!c

右i i c, 0.C3GK 터二 与 마 L&5Q 유 *0, 10.60 s 0.3 6 M L 10° 浏 -O.s- ~0.6 volts vs. SCH Fig. 3. Steady state polarization curves for the der­ ivatives of nitrobenzene in acidic solution of pH 1. 90. Fig. 2. Steady state polarization curves of 1Q^3M O : 10一頌 ©NQ?, △ : 10-4M ©NO, x : 10'4M q5NC)2 solutions. 6NH0H.

Tol. 21, No. 6, 1977 408 千井 钧•白雲基

Table 1. Yie너 '*' of products in reduction of ©NO2 in 0. 036 M H2SO4, %.

Potential Product —800mV -600mV -500mV

时 H2OH* 26 46 65

©N+HW* 62 40 17 + Percentage to total products and reactant. 쓰 ** In strongly acidic solutions, ©NHOH and exist in the form of ©N+HzOH and ^N^Hs31.

Table 2. Yield for production of nitrosobenzene with current Density, 15. 4 /zA/cm2

pH 11. 55 10.60 0.65

Yield (%) 41 58 71

Vink, et al,32 and Peterson, et al.33 found that nitrobenzene is directly reduced to nitroso­ benzene by photochemical method. However, in this experiment nitrosobenzene was produced in basic solutions in the Tafel region. Coupling products such as and hydrazobenzene Fig. 4. Steady state polarization curves for the were formed with hydrogen evolution at poten­ derivatives of nitrobenzene in basic solution of pH 10. 60. (I) 10-4M 0NO2, (II) 10-4M ©NO. tials more negative than — 1. 5 volts. In basic solutions the products were determined 305 nm, ©NHOH: 237 nm, ^NH2： 286 nm) was when the total charge passed was 4 faradays- measured to determine the concentration. Yields for one mole of nitrobenzene. The yields for of the product compounds (as the mole ratio the production of nitrosobenzene at various pH of the products to nitrobenzene) were calculated values with the current density of 15.4 “A/cm‘ from the spectrophotometrically determined con­ are shown in Table 2. The higher yield was centrations. obtained with the lower current density. The In acidic solutions reduction products of yield of product ©NO was larger at lower pH- nitrobenzene were phenylhydroxylamine and DISCUSSIONS aniline； no nitrosobenzene was found in the solution. When solutions of nitrosobenzene or Reaction Mechanism in Acidic Solution. phenylhydroxylamine was reduced aniline was The Tafel slope for the reduction of nitroben­ produced in 놔 le Tafel region of each solution zene, 130 ±10 mV as shown in Fig. 1, suggests (see Fig. 3). a single electron transfer in the rate determining Under the chemical27 or electrochemical1^10*18 step. In Fig. 2 the pH-dependence seems to be reaction conditions that have been reported in absent, therefore, protons are not involved in the literature, nitrosobenzene could not be obta­ the rate determining step. A full coverage of ined by direct reduction of nitrobenzene. the electrode by adsorbed species can be reason­

Journal of the Korean Chemical Society 니 트로벤젠의 전해환원 반응 조건과 메카니즘 409 ably assumed. (eqn(l)), the electrode reaction can be reasonably The following mechanism is assumed: assumed to take place through a strongly adsorbed species (^NO2)a(ls ^NO2 Sn + 2H+=0N(OH)22七 。In If the unprotonated species acquire one or two =©N(OH)2 자 ad, ⑵ electrons, the reduced species can be immedi­ ©N(OH)22+赤 +厂 쁘七 应+0赤 +压0 (3) ately protonated. Tafel slope, 30 mV as shown ©N+C韓 +3厂+3H+竺 您+玦 0瓦 击 (4) in Fig. 1 (curve III) suggests a reversible two 评+ 也 0瓦 & =酒 + 压0&血 electron transfer. The following mechanism is thus assumed - Sadek, et al. 34 found that the pKa of ©NQH/+ is 5.6 by polarographic experiments. Thus, {©NO?} ads + 2厂 + 2H2O = {^N(OH)2}ads+2OH- (6) if pH of the solution is lower than 2.8, ^NO2H22+ is probably dominant over ©NO》 (^N(OH)2}ads 0NO}心 +压0 (7) Writing the rate of step (3), — k30 渺 N이 Ms 羊 二二土 0NO}s°ln exp{—a3FE/RT}, the current due to the {©NO}/ further reduction一 (^NH0H}ads sequence of the above steps becomes (8) The reactions of (7) and (8) are consecutive j=4F扇 。exp{--噹£ } (5) steps. Since the combined steps (6) and (7) where 0 is the coverage by ^N(OH)22+,旳 is are more strongly potential-dependent than step the transfer coefficient of reaction (3), and F (8), it may be assumed that at more negative is the Faraday constant. If 0=1 and a3 is con­ potentials the production of ©NO is faster than, sidered to be about . as usual, the Tafel slope its further reduction while at less negative from equation(5) will be 120 mV, which is potentials ©NO is immediately converted to close to the observed 130 ±10 mV. ^NHOH as soon as it is produced. From Fig. 3 and Table 1 the reduction of In Fig. 4, curve (I ) corresponds to reactions phenylhydroxylamine seems to go to aniline by (6) and (7)； curve (II) to reaction (8). It is an irreversible one electron transfer as the rate to be noted that the two curves merge at more determining step. negative potentials. At less negative potentials The fact that no nitrosobenzene was found in than the crossed point between curve ( I ) and the solution after the electrolysis of nitrobenzene (II) in Fig. 4 phenylhydroxylamine was the solution and that the rate of reduction of nitro­ main product. sobenzene is slower than that of nitrobenzene The following relations are obtained from (Fig. 3) indicates that nitrosobenzene is not an equations (6) and (7): intermediate in the reduction of nitrobenzene in 緡 N (OH) 2 = K勤06 (0^-)2 SP {I 을*} an acidic solution. Fleischman, et al.17 also proposed that the intermediate is ©N+O as in (9> the present reaction scheme. ，&=扇 饥 N(OH)2

Reaction Mechanism in Basic Solutions. =长决7&n眼 exp{- -rt } (10), In a basic solution unprotonated species ^NO2 is probably dominant over the protonated species. where *)涉抑 2 and %n(oh)2 are the coverages by Since the rate is independent of ©NQ concentration (0NC)2)ads and (©N (OH) 2) ads, respectively..

Vol. 21, No. 6, 1977 410 千井鈞 •白雲基 :Results of adsorption studies35^37 on various 1 G j_ FE \ 。菸oh — (oh」)exp] 匸j metal electrodes by Bockris, et al. indicate that (16) the adsorption of organic molecules is more favorable than water near the potential of zero The rates are then, ■charge. If %02-l and ^n(oh)2-0, equation ■R】3=Ki法 13%no(0口一 )exp ｛- ｛ 匕 「｝ (17) (10) is 死 4=庵 (財 oh)2 (18) R尸瓦知 力吉 亍 料 ｛ -■帯糾 At moderate potential it may be assumed that Thus, the current for the production of ©NO is %no=L and ^nOh—0, and unless supply of proton donor is limited Thus, the i=2FK6知 -言二y exp ｛ 一 ~쓰另 ｝ (11) current for the production of ©NHOH is

「With T=298°K, equation (11) is rewritten in £=가雁如 3-（0"一） exp {•一 — 흐牛 } the logarithmic form This equation is rewritten in the logarithmic log z—log (const. ) — 2pH— 2 f 0. 0592 form, The Tafel slope and pH dependence of current, E log ?=log (const.) -pH一 -弓 §2" (31ogz/3pH)£, will be 30 mV and —2 respec­ tively. The observed values in Fig. 2 (lower This equation predicts Tafel slope of 59. 2 mV parts of the three curves) are in good agreement and (Slog z/3pH) = — 1. The observed values in with this prediction. Fig. 2 (upper parts of the three curves) are in The reducion of nitrosobenzene to phenylhy- agreement with this pre처 ction. The coupling droxylamine is assumed to take place through reaction (14) occurs at much higher potential beyond the range of Fig. 2, along with the the following steps. hydrogen evolution reaction. Therefore, the (0NO) sd3+厂+H2O?=^ (©NOH) *a +OH- coupling mechanism could not be confirmed in (12) 나 lis experiment. However, contrary to the (©NOH) ads+压。쓰 ‘ 0NHOH) +ads + OH- mechanism proposed by Fleischman, direct condensation of ©NO with ©NHOH was not (13) (^NOH)ads+(^NOH)ads observed, either during the reduction of ©NO at potentials in the Tafel region or in the mix­ O ture of ©NO and ©NHOH in basic solutions ---->0 —N=N —©)ads + H2。 (14) with pH<12. (航 沽泗捻+厂纸⑷阳啊独 (15) Effect of Substituents. Some preliminary The reactions (13) and (14) are competitive kinetic measurements for the electrochemical but reaction (14) is considered to be prevalent reduction of substituted nitrobenzenes were at higher coverage of (^NOH)atjs while at made and further studies are under way. lov； coverage reaction (13) will be favored. The substituted nitrobenzenes are found to be Th년 relation between the relative coverage by reduced in a similar manner in basic solutions ©KO(%no) and by。必 Mh will be to that for the unsubstituted reactant, but at Journal of the Korean Chemical Society 니트로벤젠의 전해환원 반응 조건과 메카니즘 411

-different rates. The effect of the substituants 8. F. Harber and C. Schmidt, Z. Phys. Chem. 32, seemed to generally follow the Hammett relation 193 (1900). 9. P. Zuman and C. L. Perrin, "Organic Polarogna- with a negative p. The negative p indicates phy", Interscience, New York, 1969, that substituted nitrobenzenes also follow the 10. A. J. Fry, “Synthetic Organic Electrochemistry” mechanism for the unsubstituted nitrobenzene P. 224〜236, Harper and New York, 1972. (reactions (6) and (7)) in basic solution. 11. S. K. Vijayalkshamma and R. S. Sub也 hmanya, The mechanism can be rewritten in a gener­ J. Electroanal. Chem., 23, 99 (1969). alized form for the substituted nitrobenzenes, 12. H. Sadek and B. A. Abd-El-Naby, Electrochim. Acta. 17, 2065 (1972). 13. Kastening, Electrochim. Acta. 9, 24 (1964). 14. L. Chuang, I. Fried and P. J. Elving, Anal. Chem, 36, 2426 (1964). 15. S. K・ Vijayalkshamma and R. S. Subrahmany, Electrochim. Acta, 17, 471 (1972), 16. W. H. Smith and A. J. Bard, J. Amer. Chem. Soc., 97, 5203 (1975). If reaction (20) is a rate-determining dehy­ 17. G. H. Fleischman, I. N. Petrov and W. F. K. dration step it will have a negative p value in Wynne-Jones, "Proc. 1st. Australian Conference the the Hammett equation (log(—§—)=p

Vol, 21, No. 6, 1977 412 千井韵 •白雲基

29. J. Chon and W. Paik, J, Korean Chem. Soc., 33. W. C. Peterson and R. L. Letsinger, Tetrahedron: 18, 391 (1974) Letters, 2197 (1971). 30. H. E. Ungnade, MOrganic Electronic Spectral 34. S. Sadek and B. A. Abd-El-Naby, Electrochim. Data”，Vol. II, P. 52〜56, Interscience, New Acta, 17, 511 (1972). York, 1953〜1955. 35. J. O'M. Bockris and D. A. J. Swinkels, J. 31. M. Heyrovsky, S. Vavricka, L. Holleck and Electrochem. Soc., Ill, 736 (1964). B. Kastening, J. Electrcanal. Chem., 26, 399 36. J. O'M. Boekris, M. Green and D. A. J. Swinkels,. (1970). J. Electrochem. Soc., Ill, 743 (1964). 32. J. A. J. Vink, J. Comelisse and E. Havinga, 37. E. Blomgren, J. O'M. Bockris and C. Jes사以 J. Royal Neth. Chem. Soc., 90, 1333 (1971). J. Phys, Chem., 65, 2000 (1961).

Journal of the Korean Chemical Society