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Direct and alternating current polarography of nitrobenzene in aqueous solutions and in acetonitrile Kleinerman, Marcos Yusim 1957

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DIRECT AND ALTERNATING CURRENT POLAROGRAPHY OF NITROBENZENE IN AQUEOUS SOLUTIONS AND IN ACETONITRILE  by MARCOS YUSIM KLEINERMAN  B a c b i l l e r en Quimica Universidad Nacional Mayor de San Marcos, Lima, Peru,  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science i n the Department of CHEMISTRY  We accept this; thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA September,  1957  1953  (ii)  ABSTRACT The polarographic reduction of nitrobenzene i n aqueous buffered solutions, and that of nitrobenzene and o-nitrophenol t o n i t r i l e was investigated.  i n anhydrous ace-  Alternating current polarography was: used  i n addition to "conventional"' polarography f o r the work done with aqueous solutions.  Non-reducible surface-active agents, e. g. T r i t o n X-100,  s p l i t the d. c. waves of nitrobenzene i n basic and neutral solutions, i n two parts, the t o t a l current remaining nearly the same as i n absence of the surface-active agent.  The a. c. wave corresponding to the f i r s t  step  of the s p l i t d. c. wave becomes l a r g e r as the d. c. wave becomes smaller with increasing concentration of the surface-active agent.  A limiting  r a t i o of 1*3 i s obtained f o r the heights of the two r e s u l t i n g d. c. waves i n basic solution i n presence of s u f f i c i e n t amount of the surface-active agent, and the height of the a. c. wave corresponding to the f i r s t d. c. step reaches i t s maximum value.  In acid solution no s p l i t t i n g occurs, but  the d. c. wave i s shifted to more negative potentials.  The a. c. wave  does not become l a r g e r . The r e s u l t s are interpreted by postulating a mechanism f o r the reduction of nitrobenzene involving the i n i t i a l formation  of the semi-  quinone PhNO , whose s t a b i l i t y toward further reduction increases with the pH of the solution and with the concentration of the surface-active agent.  The l a t t e r hinders the reduction of the semi-quinone by forming  a f i l m at the mercury/solution  interface by adsorption.  (iii) Application of Delahay's theory of i r r e v e r s i b l e polarographic waves i s made, and some of i t s shortcomings are discussed. In anhydrous a c e t o n i t r i l e nitrobenzene and o-nitrophenol are reduced stepwise, and t h e i r behaviour resembles that of nitrobenzene and several other aromatic nitro-compounds i n aqueous basic solution i n presence of surface-active substances.  In p r e s e n t i n g the  this thesis in partial fulfilment  requirements f o r an advanced degree at the  of  University  of B r i t i s h Columbia, I agree t h a t the  L i b r a r y s h a l l make  it  study.  f r e e l y available f o r reference  and  I further  agree t h a t permission f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may  be granted by the  Department or by h i s r e p r e s e n t a t i v e .  Head of  my  I t i s understood  t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l gain  s h a l l not  be allowed without my  Department o f Chemistry The U n i v e r s i t y of B r i t i s h Vancouver S, Canada. Date  10th  September,  Columbia,  1957.  written  permission.  (iv)  TABLE OF CONTENTS  Page Introduction  1  A. I r r e v e r s i b l e processes  2  B. K i n e t i c c r i t e r i o n of polarographic r e v e r s i b i l i t y  6  C. Alternating current polarography  7  D. The polarographic reduction of aromatic n i t r o compounds  Experimental  11  15  A. Materials  15  B. Apparatus and procedure  17  Results and Discussion  23  A. Nitrobenzene i n aqueous solution  23  Bi Alternating current polarographic behaviour  26  C. Effects of surface-active agents  30  D. Nitre-compounds i n a c e t o n i t r i l e  47  Conclusions  51  References  53  LIST  OF  TABLES  Alternating current polarographic of N i * i n a solution 4 x 1 0 ° +  0.1  behaviour  M Ni(N0, )  z  in  M KC1  Irregular dependence of the alternating current on the height of the mercury column and the concentration of nitrobenzene i n presence and absence of T r i t o n  X-100  Values of the parameter  oiz^ f o r the reduction  of nitrobenzene at three d i f f e r e n t pH values  Polarographic reduction of nitrobenzene  Polarographic reduction of o-nitrophenol i n acetonitrile  (vi)  LIST OF FIGURES Page 1.  Plot of \ vs i / i j  5  2.  Preparation of moisture-free solutions f o r a c e t o n i t r i l e polarography  IS  3.  Apparatus f o r polarography i n a c e t o n i t r i l e  19  4.  Polarographic c e l l  20  5.  Polarogram of 3.92 x 10"* PhNOj,  24  6.  pH~- Ey relationship  25  7.  Influence of pH on tendency to maximum formation  27  8.  E l e c t r o c a p i l l a r y curves of Hg  28  9.  pH and a. c. height of PhN0  31  10.  t  a  E f f e c t of T r i t o n X-100 on the d. c. polarograms  32  of BhN.O* 11.  a. c. polarograms of the same solutions; as i n figure 10  33  12.  d. c. polarograms of PhNOj.  34  13.  a. c. polarograms of the same solutions as i n figure 12  36  14.  Polarograms of PhNO^  37  15.  i - t curves f o r i n d i v i d u a l Hg drops  38  16.  Polarogram of p - n i t r o a n i l i n e  41  17.  Polarogram of nitrosobenzene  44  18.  d. c. polarographic behaviour of o-nitrophenol and nitrobenzene i n a solution of 0.1 M C^H^NI i n acetonitrile  50  (vii)  LIST  OF  SYMBOLS'  A'  -  area  a. c.  -  alternating current  b  -  as subscript, means backward  -  concentration of substance 0 at the  0  0  electrode surface D  -  diffusion coefficient  d. c.  -  direct current  E  -  potential  -  half-wave potential  e  -  electron  F  -  the Faraday  f  -  as subscript, means forward  G  —  free energy of activation  H*  -  h  -  E,  /z  " enthalpy of activation Planck constants i f used as a subscript i t means heterogeneous  Ij  -  diffusion current constant  1  -  current  i  d  -  diffusion current  K  —  Boltzmann constant  kh  -  rate constant for a heterogeneous process at a potential of 0.0 volts vs. the normal hydrogen electrode  k  s > h  -  heterogeneous rate constant at the equilibrium potential of the redox system  (viii)  m  -  rate of flow of mercury through the capillary in mg sec  mv  -  m i l i volts  N  -  number of moles of substance 0  0  -  oxidized form of a substance  P  -  factor to convert homogeneous into  c  heterogeneous rate constants R  -  gas constant  R  -  reduced form of a substance  S*  -  entropy of activation  T  —  absolute temperature  t  -  timej drop time  x  -  distance from the electrode surface  z  -  number of charges involved i n an electrode process  z^  -  number of charges involved i n the rate-determining step of the overall process  <K  -  A  -  u&.  -  transfer coefficient  microamperes  (ix)  ACKNOWLEDGMENT  The author wishes to express h i s gratitude to Dr. H. M. Dagget, J r . , f o r h i s guidance throughout the progress of t h i s work. The author i s also indebted to the National Research Counc i l f o r the award of a Studentship.  1.  INTRODUCTION  There i s no u n i v e r s a l l y accepted d e f i n i t i o n of polarography. I t s inventor, Heyrovsky, defines i t ( l ) as "the science of studying the processes occurring around the dropping mercury electrode.  I t includes  not only the study of current-voltage curves, but also of other r e l a t i o n ships, such as the current-time curves f o r single drops, potential-time curves, e l e c t r o c a p i l l a r y phenomena and the streaming of e l e c t r o l y t e s , and i t s tools include, besides the polarograph, the microscope, the s t r i n g galvanometer and even the cathode-ray oscillograph ...".  Other authors r e s t r i c t the d e f i n i t i o n to the study of currentpotential curves produced at the dropping mercury electrode (henceforth referred to as d. m. e. ) i n the presence of depolarizers, while many others extend i t to include studies with s o l i d electrodes, and even related techniques, l i k e micro-coulometry.  In t h i s work the only microelectrode used was the d. m. e. Since not only current-potential curves w i l l be dealt with, Heyrovsky's d e f i n i t i o n can be used f o r a l l the aspects of t h i s work.  I t i s outside the province of t h i s t h e s i s to present the theory of polarography.  The most comprehensive  treatment published to date  i s that of Kolthoff and Lingane (2). An excellent introduction can be found i n the l i t t l e monograph by Muller (3), and a b r i e f condensation  2. of the fundamentals i n the M.Sc.  thesis of Jayadevappa (4).  Recent develop-  ments, however, make i t important to present at l e a s t a short treatment of the advances made during the l a s t decade.  A. I r r e v e r s i b l e processes  For a thermodynamically  reversible electrode process the equation  r e l a t i n g the current and the p o t e n t i a l of the d. m. e. at any point i n the polarographic wave i s the following:  E = E,,, +  RT  zF  in/ ij - i \  (A.1)  -f^T-)  Polarographic waves which do not obey equation ( l ) are said to be i r r e v e r s i b l e .  For many years a l l that was done about t h i s type of wave  was to notice t h e i r i r r e v e r s i b i l i t y , but since 1949 the problem has; been dealt with by the application of the theory of absolute reaction rates (5 - 7).  The treatment that follows i s that of Delahay (7).  Consider the cathodic process 0 +• ze  » R  I t w i l l be assumed that the process involves only one single rate-determining step, and that the e f f e c t of the backward process i s n e g l i g i b l e .  The r a t e  of the process i s expressed by  (A.2)  where dN„ dt"  i s the no. of moles of depolarizer reduced per u n i t time  Co  is; the concentration of substance 0 at the electrode surface  E  i s the potential of the d. m. e. with respect to the normal hydrogen electrode (NHE)  z _ l<K  i s the number of electrons involved i n the rate-determining step  k£  i s the rate constant of the process at 0.0 V with respect to the NHE  o<  is= the transfer coefficient, that i s , the fraction of the applied potential that causes the reaction to proceed in the direction of reduction.  The depolarizer reaches the electrode surface by diffusion; therefore dN. can be equated to the flux of substance 0* dt  >2i x:  (x, t)  exp  -cxziFE  (A.3)  RT  where D i s the diffusion coefficient of 0.  The current caused by the  process obeys the following relations i = zFA * (flux of 0) The other boundary condition i s that the sum of the fluxes of substances 0 and R at the electrode surface i s equal to zero*  a C. 2x  (x, t)  2C« 3x  (x, t)  4.  The i n i t i a l conditions are  C  where 0  o  (x, o)  a  r  Co  0\  ,  (x , o ) =  0  i s the bulk concentration of substance 0.  Also,  C  and  C„  as;  x.  (x:,t)  0  approaches  C  (x,t) approaches 0 * o°  The solution of equation I I that s a t i s f i e s the i n i t i a l and boundary conditions stated above was found to be  -_ 7T *  i  - >  exp ( A  k  where  and  k  exp  2  ) o^/c  ( A  )  t  - <* a;,, FE RT  Avplot of  i / i ^ . vs; ^  is; shown i n f i g u r e 1.  I t can be seen that p l o t s of i _  vs }\ give k  at any potem-  h  U t i a l i n the polarographic wave (Do* By p l o t t i n g l o g k  h  i s obtained from I l k o v i c ' s equation).  vs E one obtained, i n absence of k i n e t i c compli-  cations, a straight l i n e whose slope gives the value of  of  .  The f r e e energy of a c t i v a t i o n of the process can be c a l c u l a t e d by applying the following equationt  k* b  -  A  T ~h"  exp  exp RT  - <x Z^FEa. RT  5.  6. where  i s the Boltzmann constant h  i s the Planck constant  -AG*  i s the free energy of activation  The factor P results from the use of the rate constant k  h  for heter-  ogeneous process, whereas the factor A- T corresponds to a homogeneous; h reaction.  The value of P i s about 10  cm. E^, i s the value of the  potential at which the difference of potential at the interface i s equal to zero. -AH* and^S* can be determined by the usual way.  B; Kinetic criterion of polarographic reversibility It was assumed so far that the backward process occurs at a negligible extent.  I f this condition can not be f u l f i l l e d , equations  II and III become respectively dN, . dt ' *  and  D„  2 Co  C.kJ,,,  (x,t)  exp  -  f~- <* z FE I RT  (B.1)  a  k^  C (o,t) - k a  RT  M  C,(o,t)  (B.2)  where the subscripts f and b refer to the forward and backward processes respectively. The solution for these equations, taking into consideration the former i n i t i a l and boundary conditions, was found by Delahay (8) and compared with the equation for the reversible polarographic wave A 1. He found that both equations are equivalent when the rate constant k, . at  7.  the equilibrium potential of the system 0  is larger than 2 x r l O ^  +  ze  ^  R  cm sec '. -  The wave i s said to be totally irreversible when k ^ is smaller than 5  about 3 x 10  5  cm sec"' .  C. Alternating current polarography (i)  Electrolytic processes  The a. c. polarographic method has been described in detail by Breyer et al (9 -  11).  Briefly, the method can be described as follows. Let us suppose that we are obtaining a d. c. polarogram of a reversible system (say H 4-  e ^  +  * H(Hg)  ) in the conventional way.  an alternating voltage of small amplitude (AE direct applied voltage to the cell.  Now, let us superimpose = 5 - 50 mv) over the  When the latter corresponds to E'y  the total potential of the d. m. e. will vary from (E (E  +  A E ) and the system Ti*" + e  - <^E  ) to  vTl(-Hg) will tend to adjust  itself to the value corresponding to the instantaneous potential of the d. m. e.  This will cause periodic changes in the concentration of the  components of the system, and a resulting alternating current will flow whose magnitude depends on the nature of the system, the magnitude of the a. c. voltage, the concentration of the electrolyzed substance, the frequency of the a. c. voltage, and the non-faradaic properties of the Hg-solution interface.  8. Breyer's equations f o r the quantitative r e l a t i o n s between the variables mentioned above have not been s u f f i c i e n t l y tested and w i l l not be used i n t h i s work.  However, by keeping a l l variables except one con-  stant, i t i s possible to get valuable information about the behaviour of the system under study, as i t w i l l be apparent below.  Interpretation of a. c. waves due to i r r e v e r s i b l e polarographic processes i s a d i f f i c u l t problem.  Breyer et a l claim that i r r e v e r s i b l e  systems do not produce an a l t e r n a t i n g current. misleading:  This statement i s somewhat  the d. c. polarographic reduction of N i  + +  i n 0 . 1 M KCl  solution i s known to proceed i r r e v e r s i b l y , but the present author obtained a. c. waves i n the same solution (table l ) .  The magnitude of the  current, however, was much l e s s than the value corresponding  to a  r e v e r s i b l e electron transfer process f o r the same concentration of depolarizer.  Addition of small amounts of a non-reducible surface-active  agent, T r i t o n X - 1 0 0 (a water soluble  iso-octylphenoxypolyethoxyethanol)  lowers the value of the alternating current.  Further addition of T r i t o n  causes the a. c. wave to vanish.  Another substance whose polarographic reduction i s known to proceed i r r e v e r s i b l y i s 0  Z  i n acid solution.  In t h i s case no a. c. wave  i s observed. Since an e l e c t r o l y t i c alternating current r e s u l t s from periodic concentration changes at the mercury/solution i n t e r f a c e , i t must be concluded that i n the case of the electroreduction of N L .  ++  there i s some  9.  Table I  Alternating current polarographic behaviour of N i " i n + +  i n a solution 4 x 10"^ M Ni(NO,) i n 0.1 M KC1 z  Amplitude of the alternating voltage  Concentration of Triton X-100  50 mv.  Current ft  0.0000$  18.6/**.  O.OQQA%  13.0  0.0008$  11.0  0.0020$  4.8  0.0040$  0. 0  ft PbCNOj)^  i n an approximately equal concentration  i n the same supporting e l e c t r o l y t e produces a current of about 60/v* wave.  Pbgives  a r e v e r s i b l e d. c.  10. degree of r e v e r s i b i l i t y .  Therefore a non-theoretical l o g p l o t (equation  A l ) does not r u l e out the p o s s i b i l i t y of the occurrence o f a r e v e r s i b l e step i n an i r r e v e r s i b l e d. c. polarographic reduction.  This c r i t e r i o n  of r e v e r s i b i l i t y does not coincide with Delahay's d e f i n i t i o n of a d. c. r e v e r s i b l e polarographic wave.  I t can be i n f e r r e d from t h i s discussion that, i f two s i m i l a r systems, under s i m i l a r conditions of e l e c t r o l y s i s , are reduced with d i f f e r e n t degrees o f r e v e r s i b i l i t y , the higher a l t e r n a t i n g current w i l l be observed f o r the more r e v e r s i b l e one.  ( i i ) Adsorption-desorption  processes  I f a solution contains an e l e c t r o - i n a c t i v e surface-active substance, and the potential of the d. m. e. corresponds to the adsorptiondesorption p o t e n t i a l of t h i s substance, an a. c. wave i s observed. et a l (10) c a l l t h i s a "tensammetric wave".  Breyer  This type of process does  not give a d . c. wave.  The mathematical treatment of the a. c. polarographic wave has been omitted here.  The equations derived by Breyer e t a l have been  severely c r i t i c i z e d by several workers (12 - 14). I t has been pointed out (14) that the measurement of the a, c. c h a r a c t e r i s t i c s of the c e l l by means of a bridge gives more accurate information about the electrode process.  A mathematical treatment has been developed by Grahame (15).  (see also r e f . 7, p. 146 - 78)  11. D. The polarographic reduction of aromatic nitro-compounds  (i) Historical No systematic studies of the polarographic reduction of aromatic nitro-compounds that have been published so f a r have made use of the developments outlined above.  The f i r s t i n v e s t i g a t i o n s were re-  ported by Shikata and co-workers (16 - 18).  Their work was  concerned  mainly with the r e l a t i v e s t a b i l i t y towards reduction of a number of subs t i t u t e d nitro-compounds, and the c o r r e l a t i o n of the observed "reduction potentials"' with the nature and p o s i t i o n of the substituents.  The  reduction p o t e n t i a l s were obtained from the zero current intercept of a tangent to the polarographic wave, and d i d not correspond to the half-wave potentials, u s u a l l y determined i n modern work. Several other i n v e s t i g a t i o n s have been subsequently by several workers (19 - 26).  published  In almost a l l cases the nitro-compounds  were f i r s t dissolved i n ethanol to prepare the stock solutions then they were d i l u t e d with aqueous buffers. appreciable amounts of ethanol.  The f i n a l solutions contained  I t i s known that the presence of ethanol  s h i f t s the half-wave p o t e n t i a l s to more negative values, and at the same time introduces some uncertainty i n the pH measurements. were mainly concerned with the evaluation of E  's.  The studies  , I^and z f o r each corn-  pound studied and the e f f e c t of substituents on these quantities, although some attempts were made to determine the mechanism of the electrode process.  However, no agreement i s found among the d i f f e r e n t  authors. There are c o n f l i c t i n g reports from Pearson (19) and Page et a l (21) about the dependence of E, '2.  on the concentration of the depolarizer.  12. The former worker f i n d s E ,, independent of concentration, while Page et a l x  report a s h i f t of Ey  to more negative values with increasing concentration  of the nitro-compound.  Page et a l found that a p l o t of E ^ vs pH gives a s t r a i g h t (/  l i n e with a slope approaching 0.059 v o l t s per pH u n i t over a l i m i t e d range of pH.  They i n t e r p r e t t h i s behaviour as caused by a r e v e r s i b l e  potential-determining step involving one electrons RNO^  +  e  +  H  >  +  RN0,H  (Dl)  Pearson (19) f i n d s the same slope i n the range of pH from about 7.0 to 10.0,  but f i n d s a. slope about twice the former value i n the acid region.  In order to explain these f a c t s he postulates the following sequence of reactionss H"*"  +  e  H  (reversible)  (D2)  followed by RNOj,  +•  zH  9  product  (irreversible)  (D3)  i n neutral or basic solution, or by RNO,.  +  e  +  zH  >  H*—»product  (irreversible)  (D4)  i n acid solution. He f u r t h e r claims that t h i s sequence i s general f o r organic electroreductions i n v o l v i n g hydrogen.  I t w i l l be shown i n t h i s  thesis that t h i s mechanism should be discarded.  The most up-to-date systematic study of the polarography  of  aromatic nitro-compounds was undertaken by Holleck and Exner (24 - 26). They found that i n presence of surface-active substances the polarograms  13. of some nitro-compounds i n neutral and a l k a l i n e solutions are s p l i t i n two d i s t i n c t waves, the sum of t h e i r i n d i v i d u a l currents being equal to the l i m i t i n g current of the only wave obtained i n absence of the surfaceactive substance.  The e f f e c t i s more pronounced i n alkaline solution,  where the height of the f i r s t wave tends to a l i m i t i n g value corresponding to a mono-electronic reduction.  No s p l i t t i n g occurs i n acid solution,  but the half-wave potential i s shifted to more negative values. In order to explain these phenomena Holleck and Exoer (25) assume that two i n i t i a l processes expressed by equation DI and D5 compete during the reduction of aromatic nitro-compounds at the d. m. e. * RNO  RN0  i  2  +  +  e +  e  H  * RNOjH  +  >  (Dl)  RNOX  (D5)  The surface-active substance would hinder the reaction involving H, but i t would not i n t e r f e r e with a pure electron transfer.  Thus, i n acid  solution, reaction D l predominates and the whole wave i s s h i f t e d to more negative potentials.  In basic solution reaction D5 predominates,  f i r s t wave occurs at a potential not much d i f f e r e n t from the Ey  and the obtained  i n absence of surface-active substances.  The subsequent reaction,  however, involves either H*, H', or H^Oj  i t i s hindered, according to  Holleck and Exner, u n t i l the desorption potential of the adsorbed f i l m i s reached.  I t has to be mentioned that no similar studies on n i t r o -  benzene were reported by Holleck and Exner.  However, Heyrovsky and  Matyas: (27) reported a similar phenomenon with nitrobenzene i n basic solution using o s c i l l o g r a p h i c polarography and a streaming mercury electrode.  The polarography of aromatic nitro-compounds i n non-aqueous solvents has also been studied to some extent.  The solvents used i n -  cluded glacial acetic acid (28, 29), methanesulphonic acid (30), glycerol (31), acetonitrile (4).  These studies were limited to the  measurement of diffusion-current-constants and half-wave potentials, and the correlation of  with the structure of the depolarizer.  An  interesting feature i n the behaviour of nitro-compounds i n acetonitrile was that the reduction was stepwise, suggesting a mechanism similar to that of the nitro-compounds in neutral and basic aqueous buffers i n presence of surface-active agents. ftft  ft  ftft  4  Since nitrobenzene i s the simplest aromatic nitro-compound, i t was decided to study i t s polarographic behaviour by means of both d. c. and a. c. polarography, i n aqueous solutions end i n anhydrous acetonitrile.  The main purpose of this study was to obtain information  to establish the mechanism of the electroreduction of nitrobenzene. Some tests are also "made on other aromatic mono-nitro-compounds i n order to determine similarity of behaviour.  These tests, however, are to be  regarded only as preliminary, and more work should be done before the conclusions obtained for nitrobenzene are extended to other aromatic mono-nitro-compounds.  15.  EXPERIMENTAL  A. Materials ( i ) Buffer Britton and Robinson universal buffer was used f o r a l l work i n aqueous solution.  The concentration of sodium ion i n the buffer was  kept constant i n the whole pH range and equal to that of the a l k a l i n e buffer of pH 12 by the addition of the necessary amount of NaCl to the other solutions.  This was done because of the observation (32) that  i r r e v e r s i b l e waves are s h i f t e d to more negative potentials by the presence of cations.  A l l the chemicals used i n the preparation of the  buffer were of reagent grade.  pH values were measured with a Beckmann  pH meter, model G.  ( i i ) Nitrobenzene  Fischer's reagent grade nitrobenzene was d i s t i l l e d , and l e s s than one-third of the d i s t i l l a t e (the middle f r a c t i o n ) was. used to prepare the stock solution.  No ethanol was used since the s o l u b i l i t y of  nitrobenzene i n water i s about 10  M, and the range of concentrations  studied was from l e s s than 10" M to about 10"^ M. H  16. ( i i i ) Nitrosobenzene  I t was prepared according to d i r e c t i o n s i n the l i t e r a t u r e  (33),  and i t was r e c r y s t a l l i z e d twice from ethanol. ( i v ) o-Nitrophenol and p - n i t r o a n i l i n e These chemicals were of reagent grade and were used without f u r ther p u r i f i c a t i o n .  (v) A c e t o n i t r i l e I t was p u r i f i e d according to directions i n the l i t e r a t u r e  (33).  Usually several d i s t i l l a t i o n s were required before a "polarographically pure" product was obtained. ( v i ) Tetra-n-butylammonium iodide I t was prepared according to d i r e c t i o n s i n the l i t e r a t u r e (34). I t was r e c r y s t a l l i z e d several times from anhydrous ethyl acetate. was noticed that the residual-current-polarogram  It  of a 0.1 M solution of  the s a l t i n a c e t o n i t r i l e exhibited a small wave at about -1.8 vs the mercury pool anode which could not be suppressed by f u r t h e r p u r i f i c a t i o n . ( v i i ) Surface-active  substances  They included g e l a t i n , basic fuchsin, camphor and T r i t o n X-100. Their solutions i n the supporting  e l e c t r o l y t e were checked to be  "polarographically pure ", (fuchsin was used only at very small concen1  trations)  17. Bi Apparatus and procedure ( i ) Precautions regarding the use of a c e t o n i t r i l e as supporting electrolyte Since very small amounts of water a f f e c t the polarographic  behaviour  of organic compounds i n a c e t o n i t r i l e , the solvent was handled under a dry n i t r o gen atmosphere from i t s f i n a l d i s t i l l a t i o n  through the preparation of the  supporting e l e c t r o l y t e and stock solutions to the f i n a l recording of the polarogram.  The preparation of the supporting e l e c t r o l y t e and stock solutions i s  i l l u s t r a t e d by figure 2.  A portion of the solvent i s pumped with dry nitrogen  from i t s container A to f l a s k B, where i t dissolves the C^H^NI, and from there to the volumetric f l a s k C u n t i l the l a t t e r was f i l l e d up to the 500 (or 250) ml. mark. Various concentrations of the depolarizer could be prepared  directly  i n the polarographic c e l l by means of the apparatus described i n f i g u r e 3.  Dry  nitrogen could be used either to deaereate the solutions contained i n f l a s k s A and/or B, or, by proper use of the stopcocks, to pump the solutions to the burettes, from which a known volume of either solution could be delivered to the c e l l .  About 100 ml (measured to 0.1 ml) of the previously deaereated  sol-  ution of the supporting e l e c t r o l y t e , contained, say, i n f l a s k A, were transferred to the dry polarographic c e l l v i a the' burette A , 1  mercury was added to make the mercury pool anode. gram was then obtained.  and  triple-distilled  A residual current polaro-  Then an accurately measured amount of the stock solu-  tion containing the same concentration of supporting e l e c t r o l y t e was into the c e l l from f l a s k B, v i a the burette B , 1  geneous by bubbling nitrogen through i t .  introduced  and the mixture was made homo-  Thus i t was possible to work with  increasing concentrations of depolarizer by the addition of several successive amounts of stock solution.  The burettes were provided with Teflon plugs i n  order to avoid contact of a c e t o n i t r i l e with stopcock  grease.  The polarographic c e l l i s shown i n figure 4.  Fig,  2  P r e p a r a t i o n of m o i s t u r e - f r e e s o l u t i o n s f o r acetoni*-ri l e polarography  H 00  Fig*  3  Apparatus for polarography i n a c e t o n i t r i l e .  21.-  The reference electrode used for polarography in acetonitrile was the mercury pool i n presence of 0.1 M C^H<.NI. Its potential has been calculated by Wawzonek (35) to be about -0.78v vs SCE,  An external refer-  ence electrode has the disadvantage of introducing an unknown liquidjunction potential with the acetonitrile solution i f i t i s aqueous, or with the normal hydrogen electrode i f i t i s non-aqueous. A' saturated calomel electrode was used for aqueous polarography. It was kept out of the electrolytic circuit in order to minimize the ohmic potential drop i n the c e l l .  A mercury pool was the "active"' anode.  Two capillaries, I and II were used at different stages of this work. Capillary I was used with a head of mercury of 50.56 cm. I t had a drop time of 4.43 seconds and a rate of flow m = 1.463 mg/sec when dropping in 0.1 M KCl with the circuit open. tonitrile.  It was used for a l l the work i n ace-  Capillary II was used with a head of mercury of 33.4 cm.  When dropping i n 0.1 M KCl (open circuit) i t had a. drop time of 4,3 seconds and a rate of flow m -=1.911 mg/sec. The resistance of the cell was kept low by maintaining a distance between the mercury pool and the d. m. e. of less than 1 cm. A l l the results obtained i n this work were obtained with the polarographic c e l l in a constant-temperature bath, maintained at 25.0 t 0.1 C. ( i i ) Instrumentation A Sargent Polarograph model XXI was used for a l l the runs. I t was modified according to Miller (36) i n order to make possible the  22. recording of a. c. polarograms, i n addition to the d. c. ones.  The  voltage scale of the polarograph was c a l i b r a t e d by means of a Leeds & Northrup Student's Potentiometer.  For accurate measurement of half-wave rectc.  potentials a slow voltage scanning (0.0375 v/min) was used i n order to minimize the recorder l a g .  The runs were stopped at two or three points  i n the v i c i n i t y of the Ej, , and the corresponding potentials were measured with the potentiometer.  E  was then found by interpolation.  The amplitude of the alternating voltage used f o r the obtention of a. c. polarograms was 50 mv.  The frequency of the alternating voltage  was 60 cycles per second.  Unless otherwise specified, current values i w i l l be expressed i n microamperes (yUau), and p o t e n t i a l values E w i l l be expressed i n v o l t s vs.  the saturated calomel electrode (SCE).  23.  RESULTS MD DISCUSSION  A. Nitrobenzene in aqueous solution Previous work in nitrobenzene has been carried out in aqueous buffers in presence of ethanol.  In ethanol-free solutions nitrobenzene  behaves similarly; the overall electrode reaction i s irreversible and proceeds in one wave to phenylhydroxylamine at a pH > 3.0. A small second wave, showing reduction to aniline, but ill-defined and coupled with at least one chemical reaction appears in solutions of pH lower than 3.0 (figure 5). There i s no agreement as to the nature of the chemical reaction (23, 37). Plots of Ej, vs the pH of the solution give straight lines up to a pH of about 9.0 with a slope approaching 0.059 volts per pH unit at 25"C (figure 6).  The steepness of the waves corresponds approxi-  mately to that of a monoelectronic reduction.  The value of the diffusion  current i s proportional to the concentration of nitrobenzene below 10 M. 3  The following information has not, to the author s knowledge, 1  been previously reported.  In ethanol-free solutions in presence of 0.01%  gelatin, the half-wave potentials of nitrobenzene are consistently more positive by 0;03 or 0.04 volts than those obtained in presence of 8% ethanol, as can be seen by comparing figure 6 with Pearson's data (19). In absence of maximum suppresors the current potential curves are well defined only at concentrations lower than 10  At higher concentrations  24.  - E Figure 5 Polarogram o f 3.92 x 10~  H  pH = 1.8  PhNO  A  pH - E i relationship. 2  26. maxima may occur.  I t was noticed that the tendency f o r maximum formation  i s greater at higher pH, but no quantitative r e l a t i o n s h i p i s apparent. Maxima may occur i n basic solution even at concentrations lower than 5 x 10" M (figure 7). J  The heights of the maxima decrease on standing.  The o r i g i n of maxima i n the current-potential curves of n i t r o benzene at the mercury/solution i n t e r f a c e .  The evidence f o r adsorption i s  shown i n the e l e c t r o - c a p i l l a r y curves (figure 8) of mercury i n a solution containing 8 x 10  M PhNO . z  In absence o f nitrobenzene or other adsorb-  able substances curve "a" i s obtained.  The lowering of the surface tension  of mercury i n presence of nitrobenzene (curve b) i s due to the adsorption of the l a t t e r at the mercury/solution interface.  Non-reducible  substances  having a higher surface a c t i v i t y than nitrobenzene suppress the maxima by being adsorbed at the interface instead of nitrobenzene (curve c ) . The point of coincidence of curves "a" and "c" corresponds to the desorption potential of Triton X-100.  B. Alternating current polarographic behaviour In absence of maximum suppresors, nitrobenzene gives small a. c. waves.  A r e l a t i v e l y high amplitude of the a. c. voltage had to be used  (50 mv) i n order to make the waves e a s i l y v i s i b l e i n acid solution.  The  same value of the a. c. voltage was used throughout t h i s work f o r the other solutions.  I t was noticed that the height of the a. c. waves increased  with increasing height of the mercury column (table I ) . i s d i f f e r e n t from that of a "well-behaved"  This behaviour  system, where i t i s found that  the a. c. does not vary with the mercury pressure.  The height of the a. c.  waves was found to increase with increasing pH of the solutions studied, although no quantitative r e l a t i o n s h i p i s apparent.  27.  Figure 7 Influence of pH on tendency to maximum formation  a) 4.43"1c 10" M PhNOj, pH = 1.8 S  b) 4.48* x 10'  s  M PhNO^, pH z 9.0  28,  -E (va SCE) Figure 8 - E l e c t r o c s p i l l a r y curves of Hg i n a) Buffer, pH 7 b) Same, plus 8 x 10" M PhN0 c) Same as a), plus 0.05$ Triton x 100 3  2  Table I I Irregular dependence of the alternating current on the height o f the mercury column and the concentration of nitrobenzene i n presence and absence of Triton Z-100  pH 11.9  Concentration  Pressure, cm. Mercury  Triton  Current _______  4.65  lO^M  33.4  —  2.4  4.65  IO" M  84.0  —  6.17 ••  4  9.3  IO  M  33.4  0.002$  5.46  9.3  10 -« M  84.0  0.002$  5.62  - 4  1.39  10 -*M  33.4  0.002$  10.56  1.86  IO  33.4  0.002$  14.00  - 3  M  30. In presence of 0.01$ g e l a t i n or 0.001$ Triton X-100 the height of the mercury column ceased to a f f e c t the a. c. wave heights, although the other a. c. c h a r a c t e r i s t i c s mentioned above persisted. the e f f e c t of pH i s i l l u s t r a t e d .  In f i g u r e 9  The higher wave i s s t i l l much smaller  than that corresponding to a reversible one-electron transfer.  I t can  also be noticed that the "summit p o t e n t i a l " o f the a. c. wave (referred to below as E ) does not correspond exactly to E s  v  (figure 9).  The con-  centration-dependence of the a, c. wave heights was not l i n e a r (table I I ) .  C. E f f e c t s of surface-active  agents  The e f f e c t of increasing concentrations of Triton X-100 on the polarographic reduction of nitrobenzene i n basic solution (pH 11,9) i s i l l u s t r a t e d i n f i g u r e s 10 and 11.  I t can be seen that, as a l i m i t i n g  property, two d i s t i n c t waves are formed, the sum of t h e i r heights being equal to the height of wave "a"', while t h e i r r a t i o becomes 1*3. the o v e r a l l process involves 4 electrons,  Since  t h i s r a t i o i s evidence f o r a  monoelectronic reduction f o r the f i r s t wave.  At lower concentrations of  Triton the e f f e c t i s more pronounced the longer the drop time of the d. m. e. (curves b and c ) .  In neutral solution the same s p l i t t i n g i s found, and the E ^ of the f i r s t and second waves appear at about the same potentials as i n basic solution.  However, the l i m i t i n g r a t i o 1*3 i s not reached, even  a f t e r increasing the concentration of Triton to more than 1$ (figure 12). In acid solution no s p l i t t i n g occurs, but the E^_ i s s h i f t e d 2.  to considerably more negative potentials.  31.  a) pH = 1.8,  3.92 x 1 0 ^ M PhN0  b) pH = 9.6, 3.92 x IO'* .  c  2  M PhNO,.  . polaro^ram o f "a"  0.65  0.75  0.85  0.95  1.05  1.15  -E Figure 10 Effect of Triton X-100 on the d. c. polarograms of 5.6 x 10~* M PhN0 , pH 11.9 4  5.6 x IO* M PhNO i n a buffer of pH 11.9 t  a) 0.0006$ Triton, 't at E,^ : b) 0.0017$ Triton, t at  4.4 seconds  : 4.4 seconds IT,  c) 0.0017$ Triton, t at E^ t 6.4 seconds d) 0;01$ Triton, t at E,. : 4.2 seconds  .  Oi 65  0.75  0.85  0.95  1.05  -E  Figure 11 a. c. polarograms of the same •solutions as in figure 10  1.15  33.  34  1.1  0.9 .1  0.5  0.3  °-  1.3  7  -E Figure 1^  a  b  ) pH 1.8, »° ) pH 1.8, c )  T r t t 0 n  0.05*  pH 7.4, »°  e  ™  ™  ) pK 9.6, «  f )  T r i t  pH 9.6, 0.05*  °  n  36. In neutral and basic solutions of PhNO^ the a. c. polarograms show waves whose height increases with increasing concentration of Triton, u n t i l a l i m i t i n g value i s reached at the same concentration of the surfaceactive agent at which a l i m i t i n g r a t i o is: attained f o r the d. c. waves (figure 13). .No substantial increase i s found i n acid solutions.  The  height of the a. c. wave i s maximum i n basic solution, when the r a t i o i s 3 f o r the two d. c. waves has been obtained. Camphor and basic fucshin were found to cause the same e f f e c t as T r i t o n .  Gelatin was t r i e d , too, but i t caused only p a r t i a l s p l i t t i n g  at a concentration of 0.1$ (figure 14).  '•  The s p l i t t i n g of multi-electronic waves by non-reducible active agents was f i r s t reported by Kolthoff and Barnum (39). Wiesner (40)  surface-  Later  found the same e f f e c t i n the electroreduction of quinones  caused by eosine dyes. longer drop-times.  He, too, found the e f f e c t more pronounced f o r  Wiesner i n t e r p r e t s these e f f e c t s as caused by f i l m  formation on the electrode surface, the rate of which depends on the concentration of the dye. At low concentrations substance the formation  of the surface-active  of a f i l m i s not immediate, and t h i s would explain  the more pronounced e f f e c t noticed with slowly dropping mercury. interpretation seems to apply to the present case.  This  Current-time curves  obtained f o r single drops during the electroreduction of nitrobenzene (figure 15.) show the. process to be more hindered at the end of the droplife.  But then Wiesner i n t e r p r e t s the s p l i t t i n g of the wave as due to a  "sieve action" which prevents part of the electroreducible substance from :  reaching the cathode u n t i l more negative potentials are applied to the d. m. e.  I f t h i s were true f o r the present work, the l i m i t i n g r a t i o 1S3  f o r the two waves, and the increase i n the a. c. current with increasing  36.  6  i  2  0.A  0;2  Oi.6  0*8  Figure 13 a. c. polarograms of the same solutions as i n f i g u r e 12 a  —  ...»d  b  e  c  » f  >  37.  8  _  7  _  6 _  5  _  4  _  3  -  2  _  1  _  Figure 14 Polarograms of5 a) 4. 5 x 10" M PhN0 , 0.1$ g e l a t i n 41  L  b) 5.0 x 10" M PhN0 , 0.006$ fuchsin 4  t  c) 3.3 x IO" M PhN0 , 0.1$ camphor 4  t  d) -,Tenspntmetrie wave of camphor  39.  Triton concentration could not be explained.  Probably the quinones studied  by Wiesner were reduced i n two one-electron stages, the f i r s t one corresponding to the r e v e r s i b l e formation of a semi-quinone.  A re-investigation  of Wiesner* s work seems to be desirable. The author's r e s u l t s obtained with d. c. polarograph are similar to those of Holleck and Exner (24 - 5) f o r other aromatic nitrocompounds already mentioned.  Their interpretation i s here re-examined i n the l i g h t  of the experimental evidence obtained with a, c. polarography. In basic solution the primary reaction seems to be the one proposed by Holleck and Exner, that i s PhNOa. The f a c t that the E ^  f  e  •» PhNOl  (Al)  of PhNO i s independent of pH i n basic solution x  i s i n agreement with t h i s step.  The increase i n the alternating current  due to the addition of Triton i n d i c a t e s that the process i s rendered more r e v e r s i b l e by hindering the following i r r e v e r s i b l e reactions.  This hin-  drance also shows that the second step i s not a pure electron transfer (for a discussion of hindrance of electrode processes, see references 41,-43, 27). A probable second step would be PhNO" +  HjO ^ e  • PhNO +-20H"  (A.2)  Since the reduction potential of PhNO i s much more p o s i t i v e than the value at which the above reactions occur, the reduction to phenylhydroxylamine proceeds at once. The Triton would tie-up the PhNOJ and hinder i t s reaction with H 0 u n t i l an overpotential i s applied to the d. m. e. high enough to a  46. • overcome the b a r r i e r introduced by the Triton.  Holleck and Exner (24  - 5) used tylose, camphor, g e l a t i n and  agar-agar as surfa.ce-active agents i n an extensive study of the polarographic reduction of p - n i t r o a n i l i n e .  They found the desorption p o t e n t i a l  of camphor to nearly coincide with the second wave of the reducible substance i n basic solution.  They concluded that, i n general, the second wave  i n the reduction of nitrocompounds occurs only a f t e r the surface-active agent i s desorbed. desorbed at -1.80  The author found t h i s conclusion, not true!  vs SCE at a concentration of 0.05$ as can be seen from  the e l e c t r o c a p i l l a r y curves of mercury i n solutions with and Triton (figure 8). of pH 9.6  Triton i s  without  However, the second wave of nitrobenzene i n a solution  occurs at about -1.15  v. vs SCE.  I t seems then that the r e s u l t s  of Holleck and Exner were j u s t a coincidence.  This was conclusively  proven by obtaining a polarogram of p - n i t r o a n i l i n e i n presence of 0.05$ Triton.  The E'  y  'a.  be about -1.4  of the second wave i n a solution of pH 7.0 was found to  v o l t s vs SCE (figure 16)..  For a c i d i c solutions the German authors postulate the following mechanism : RNO^  -t-  e  +  H\  *• ..RN0H x  (A3)  which they consider to be e s s e n t i a l l y d i f f e r e n t from that i n basic s o l ution.  They also claim that the two processes overlap i n neutral solution.  Recent work by V. Stackelberg and Weber (44) electroreduction of nitrobenzene  has shown, however, that the  and several other organic compounds pro-  ceeds through electron transfer as the i n i t i a l step, even at low  over-  voltage cathodes, and that the dependence of the reduction p o t e n t i a l on  41  42. the pH in buffered solution is due to a fast reaction of the intermediate semi-quinone with hydrogen ions. It appears, then, tha.t the polarographic reduction of nitrobenzene obeys the following general mechanism: PhNCj  +  e  (A3)  » PhNO ~  followed by PhNO"  +•  » PhNC^H  (A4)  in acid or neutral solutions, or by PhNO^  +  H,0  +  e  > PhNO +  20H~  (A5)  (or other reaction involving H 0) in basic solution. The experimental results obtained in this work can thus be explained as follows:  as the pH increases, the rate of reaction (A4)  decreases, therefore on obtaining an a. c. polarogram there will be a higher concentration of unreacted PhNO^ just before the positive half of the alternating voltage cycle at the E  $  of the wave.  Thus, increasing  a. c. wave heights should be expected with increasing pH.  This is actually  the case. In the alkaline pH region the semi-quinone reacts with water. Therefore the half-wave potential should be independent of pH, again in agreement with experiment. Surface-active agents hinder reactions involving water at a mercury/solution interface more effectively than reactions involving  43. hydrogen ions.  This explains why  the subsequent reaction of the semi-  quinone with PhNO., i s only p a r t i a l l y hindered i n neutral solution, where the H  f  concentration i s several orders of magnitude higher than i n solu-  tions where the reaction i s hindered completely. nitrosobenzene  i s s t a b i l i z e d by hindrance  The p o s s i b i l i t y that  (due to the presence of T r i t o n  X-100) of i t s f u r t h e r reduction to phenyl-hydroxylamine was ruled out by obtaining a. polarogram of nitrosobenzene under the same conditions (figure 17). Strassner and Delahay (45), too, studied the influence of surfaceactive agents on the polarographic reduction of nitro-compounds.  They  investigated the behaviour of p - n i t r o a n i l i n e i n the presence of g e l a t i n , and observed s p l i t t i n g of the wave i n two not very d i s t i n c t parts.  They  made use of the theory of i r r e v e r s i b l e waves f o r t h e i r i n t e r p r e t a t i o n of the phenomena.  Unfortunately they applied the treatment to the whole  d i s t o r t e d six-electron wave without f i r s t obtaining more information as to the nature of the process.  Their conclusions can not, therefore, be  accepted.  In the present case, however, enough information has been obtained to make use of the theory.  already  The f i r s t double step of the  electroreduction i n acid or neutral solution, PhN0  x  +  H  +  +•  e  >  PhN0 H z  i s rapid, otherwise no l i n e a r relationship; would be obtained between E,.  '2. and pH.  S i m i l a r l y , the f i r s t step i n basic solution PhNO^  +  e  >-PhNO ~ r  i s f a s t , since i t follows r e a d i l y the changes i n the p o t e n t i a l of the  0.0  0.2  0.4  0.6  0.8  - E Figure  17  Polarogram of nitrosobenzene (sbout 1.5 x 10"* M) pH 3  0.05$ Triton  7.4,  45>.  d. m.  e. caused by a superimposed alternating voltage of 6 0 cycles per  second.  Nitrosobenzene,  which i s known to be formed as an intermediate  i n the electroreduction of nitrobenzene, i s reduced r e v e r s i b l y at potent i a l s much more p o s i t i v e than the reduction p o t e n t i a l of nitrobenzene, other things being equal ( 4 3 ) .  Therefore, i t s electroreduction, at the  reduction potential of nitrobenzene, i s fast-. electrons, while the f i r s t consumes one.  This step consumes two  Since the o v e r a l l reduction  involves four electrons, the step corresponding to the formation of n i t r o sobenzene from PhNOjH or PhNO/ , which i s probably the rate-determining step, consumes one electron.  Theref ore,o<  should be equal to o< . Eo\r-  ;  ever, i t i s seen from table I I I that values of a( z_, higher than u n i t y can be obtained when Delahay s theory i s applied. 1  Since the t r a n s f e r coef-  f i c i e n t can not be higher than u n i t y (according to Delahay, reference 7,', i t f a l l s between 0 . 3  and 0 . 7 )  t h i s seems to contradict the reasoning i n  t h i s paragraph according to which z^-< 1 .  However, the following consider-  ations must be taken into account before reaching a f i n a l conclusion.  It  i s not certain that polarographic reductions can occur through the simultaneous transfer of two or more electrons ( 2 7 ) .  I f only one electron can  be transferred at a time, then, f o r any single step ex z,_ = f a c t that oi z; f o r several processes ( 7 ,  .  Then, the  4 $ ) has been found to be higher  than u n i t y could mean that, either a rate-determining step' includes 11  1,  a c t u a l l y more than one single step, or that Heyrovsky's theory of the multi-stage reduction i s not true. be desirable.  Further work on t h i s subject seems to  Another factor that should be taken i n t o consideration i s  the probable dimerization of two PhNO'"' r a d i c a l s before f u r t h e r reduction takes place, i n which case no f i n a l conclusion as to the meaning of the value of <J( z«. can be arrived at.  Table III Values of the parameter <x  f o r the reduction  of nitrobenzene at three d i f f e r e n t pH values Concentration . 5*24 * Triton:  pH  10~ M 4  0.0008$  oi  4.2  1.18  7.8  0.82  9.6  1.39  47. D. Nitro-compounds i n a c e t o n i t r i l e Polarograms of nitrobenzene and o-nitrophenol i n 0.1 M C^H^NI i n anhydrous a c e t o n i t r i l e were obtained at d i f f e r e n t concentrations and d i f f e r e n t heights of the mercury column. diffusion-controlled.  The waves were found to be .  The r e s u l t s are summarized i n tables IV and V.  Previous r e s u l t s obtained on o-nitrophenel by Jayadevappa (3) do not seem to be acceptable.  The present author could reproduce h i s  r e s u l t s only after i n t e n t i o n a l l y contaminating the solutions by bubbling moist a i r through them f o r several hours i n presence of mercury. The shape of the polarograms of both compounds i s shown i n f i g u r e 18.  Analogy with the reduction of nitrobenzene i n basic solutions  i n the presence of surface-active agents i s apparent.  Since i n t h i s  l a t t e r case the hindered reaction i s the combination of the intermediate semi-quinone with water (according to the mechanism proposed above), i t i s to be expected that the hindrance would be more pronounced i n absence of water.  That t h i s a c t u a l l y happens i n a c e t o n i t r i l e seems to support  the above mechanism.  Another i n t e r e s t i n g behaviour i s the s h i f t of the  second wave of the nitro-compounds i n a c e t o n i t r i l e to more p o s i t i v e values by the addition of water, while the E y of the f i r s t wave i s not appreciably shifted.  This i s to be expected, since the f i n a l reaction must  involve hydrogenation, easier i n presence of water or acids, while the f i r s t wave involves only electron transfer.  In the case of basic aqueous  solution, surface-active agents produce only a small s h i f t of the step corresponding to the formation of a semi-quinone, while the step corresponding to hydrogenation i s shifted to much more negative potentials.  Table IV Polarographic reduction of nitrobenzene.  Sup-  porting e l e c t r o l y t e ! O J M tetra-n-butylammonium iodide icentration 10' M  E 0% z  3  Wave No.  m t (mg *sec*) 2  -Ei,volts vs^Hg pool  0.363  0.00  1  1.53  0.57  2.00  0.363  0.00  2  1.42  1.55  4.46  0.882  0.00  1  1.53  0.58  4.71  0;882  0;00  2  1.42  1.57  9.50  1.200  0;92  1  1.53  0.58  6.22  1.200  0.92  2  1.42  1.45  1.190  1.83  1  1.53  0.58  1.190  1.83  2  1.42  1.35 *  1.68  0.00  1  1.53  0.60  8.6  1.68  0. 00  1  2.02  0;60  10.9  1.68  0.00  2  1.42  1.6  18.3  1.68  0.00  2 •  1.87  (?)  13.9 6.20  4  13.6  22.8  Average value of d i f f u s i o n current constant 1^ = 6.68 ft Value only approximate! wave distorted by maximum (?) Wave too distorted to allow measurement of E  4  49. Table V Polarographic reduction of o-nitrophenol i n a c e t o n i t r i l e . Supporting e l e c t r o l y t e  icentration IO" M  0;1 M t e t r a n-butylammonium iodide. - E i , volts vs Hg pool  H 0$  Wave No.  0.386  0.00  1  1. 53  0.33  2.46  0.386  0.00  2  1.42  i.d.  i.d.  0.920  0.00  1  1.53  0.35  5.20  0,920  0.00  2  1.42  1.44  16.40  2.005  0,00  1  1.53  0.34  10.52  2.005  0.00  2  1.42  1.43  35.30  2.005  0.00  1  1.85  0.35  12.24  2.005  0.00  2  1.72  1.44  41.90  3.330  0.00  1  1.53  0.35  16.60  3.330  0.00  2  1.42  1.44 *  58.10  3.240  3.24  1  1.53  0.33  16.40  3.240  3.24  2  1.42  1.08 *  62.20  3  t  (mg*'* gee •.) Z  Average value of d i f f u s i o n current constants 8.58 & Approximate value onlys wave d i s t o r t e d by maximum  50.  20  _  15  -  10 _  5  -  Figure 1 8 d.  c.  poiajrographic behaviour of o-jtf-trophenal and nitrobenzene i n a solution of 0.1 M C^iH^NI i n a c e t o n i t r i l e  a) 3.6.3 x K T * M nitrobenzene  b) 9.2 x 1 0 ^  M  o-nitrophenol  51,  CONCLUSIONS  From the r e s u l t s obtained i n t h i s work the following  conclusions  can be arrived at. In aqueous solutions the reduction of nitrobenzene proceeds v i a a reversible formation of a semi-quinone PhNO a,  +•  e  •—»  intermediate! PhNO^~~  The steps i n the o v e r a l l electrode process involving reaction of the semi-quinone are i r r e v e r s i b l e , and as a r e s u l t an i r r e v e r s i b l e d. c. wave i s produced.  By means of a. c. polarography i t was  shown that i t i s pos-  s i b l e to hinder the i r r e v e r s i b l e steps i n the electrode process i n neutral and basic solution, by addition of non-reducible surface-active agents. The i r r e v e r s i b l e steps occur then, only after a high overpotential i s applied to the d. m.  e., but desorption of the surface-active substance  does not necessarily precede them.  In acid solution the whole electrode  process i s hindered, and the wave i s s h i f t e d to more negative potentials. I t was  shown that a mechanism of reduction that postulates two d i f f e r e n t  reactions depending on whether the solution i s acid or basic  (Holleck s 1  theory) i s not necessary to explain the experimental f a c t s .  In anhydrous a c e t o n i t r i l e the reduction of nitrobenzene i s stepwise.  The semi-quinone formed i n the f i r s t step i s electrochemically  stable over a wide range of potentials. addition of water.  I t s s t a b i l i t y decreases upon  52. I f the discussion of the l a s t paragraph i s correct, a prediction can be made!  a. c. waves of nitro-compounds i n a c e t o n i t r i l e corresponding  to the f i r s t step should be considerably higher than those corresponding to the second d. c. wave.  Upon addition of s u f f i c i e n t amounts of water  or acids the height of the f i r s t a. c. wave should decrease, because the electron transfer would be accompanied by hydrogenation, which would make the step l e s s reversible.  These tests have not yet been made.  I f per-  formed, t h e i r r e s u l t s could e i t h e r confirm the mechanism proposed above for nitrobenzene i n aqueous solutions, or, i f giving r e s u l t s i n disagreement with the prediction, demand a re-examination of the problem.  53  REFERENCES  1.  Heyrovsky, J . , Analyst 81, 189 (1956).  2.  Kolthoff, I. M. and Lingane, J . J . , Polarography, Interscience Publishers, Inc., New York, (1952).  3.  Muller, 0. H., The Polarographic Method of Analysis, Chemical Education Publishing Co.  4.  Jayadevappa, E. S., MSc. Thesis, 1955.  5.  Eyring, H., Marker, L. and Kwoh, T. C., J . Phys. Chem. _>_i, 187 (1949).  6.  Delahay, P., J . Am. Chem. Soc. 2 2 , 4944 (1951 )i  7.  Delahay, P., New Instrumental Methods i n Electrochemistry, Interscience, New York, pp. 72-86 (1954).  8.  Delahay, P., J . Am. Chem. Soc. 7j_, 1430 (1953).  9.  Breyer, B., Gutman, F. and Hacobian, S., Aust. J . S c i . Res., A-3, 558 (1950).  10.  Breyer, B. and Hacobian, S., Aust. J . S c i . Res., A5, 500 (1952).  11.  Breyer, B. and Hacobian, S., Aust. J . of Chemistry 2, 225 (1954).  12.  Randies, J . E. B., Discussions Faraday Soc. 1, 46, 1947.  13.  Delahay, P. and Adams, T. J . , J. Am. Chem. Soc. 7j>, 5740 (1952).  14.  Delahay, P., Proceedings of the Sixth Meeting of the International % . Committee of Electrochemical Thermodynamics and Kinetics, Butterworths S c i e n t i f i c Publications, p. 522-4 (1955).  15.  Grahame, D. C., J . Electrochem. Soc. 9_9_, c370 (1952).  16.  Shikata, M. J . , Agr. Chem. Soc. Japan 1, 533 (1925). comments, about t h i s paper and papers 17 and 18 are taken from the a r t i c l e by Page, J.E., Smith, J . W. and Waller, J . G., (21), Trans. Farad. Soc. 2_, 42 (1925).  17.  Shikata, M. and Hozaki, N., Mem. C o l l . Agr., Kyoto University, 17, 1, 21 (1931). J  18.  Shikata, M. and Watanabe, M., J . Agr. Chem. Soc. Japs*,  19.  Pearson, J . , Trans. Faraday Soc., 4_4, 683, 692 (19#).  20.  Pearson, J . , Trans. Faraday Soc. 4_5> 199 (1949). /"  21.  Page, J . E., Smith, J . W. and Waller, J . G., Ji^Phys. and C o l l o i d Chem., i l , 545 (1949).  924 (1928).  j  54. 22.  Korshunov, I. A. 8nd K i r i l o v a , A. S., J . Gen. Chem. (U.S.S.R.) 18, 785 (1948), Chem. Abs. 4585 (1949).  23.  Stocesova, D., C o l l e c t i o n Czechoslov. Chem. Communs> 1^, 615 (1949).  24.  Holleck, L. and Exner, H. J . , Proceedings of the I International Polarographic Congress, Prague, 97 (1951).  25.  Holleck, L. and Exner, H. J . , Z e i t Elektrochem. j>6, 46 (1952).  26.  Holleck, L. end Exner, H. J . , Z e i t Elektrochem. j>6, 677 (1952).  27.  Heyrovsky, J . and Matyas, M., C o l l e c t i o n Czechoslov. Chem. Communs 16, 455 (1951).  28.  Bergmann, I. and James, J. C., Trans. Faraday Soc. ^8, 956 (1952).  29.  Risk, J . B . , MSc. Thesis, University of B r i t i s h Columbia, 1956.  30.  Wawzonek, S., Blaha, E. W., Berkey, R. and Thomson, D., Technical Report, AECU - 2927, U. S. Atomic Energy Commission, U, S. Government P r i n t i n g Office, Washington, D. C., 41, (1954).  31.  Bruss, D. B. and DeVries, T., J . Electrochem. Soc. 100. 445 (1953).  32.  Ref. 2, P. 626.  33.  Vogel, A. I., A Textbook of P r a c t i c a l Organic Chemistry, Longmans, Green and Co., London, P. 603 (1951).  34.  Wawzonek, S. 8nd Runner, M. E., J . Electrochem. Soc. 22, 457 (1952).  35.  Laitinen, H. A. and Wawzonek, S., J . Am. Chem. Soc. 64j 1765, (1942).  36.  M i l l e r , D. M., Can. J . Chem. 2A> 942 (1956).  37.  Bergmann, I. and James, J . C , Trans. Faraday Soc. j>0, 829 (1954).  38.  Smith, J . W. and Waller, J. G., Trans. Faraday Soc. 46, 290 (1950).  39.  Kolthoff, J . M. and Barnum,  40.  Wiesner, K., C o l l e c t i o n Czechoslov. Chem. Communs 12. 594 (1947).  41.  Heyrovsky, J . , Dis. Faraday Soc. 1, 212 (1947).  42.  Frumkin, A. N., Doklady Akad. Nauk U. S. S. R. 8J>, 373, Chem. Abs. 4jS, 10956 (1952).  43.  Heyrovsky, J . , C o l l . Czechoslov. Chem. Communs 19„, Suppl. 2, 58 (1954).  J . Am. Chem. Soc. 63_, 3061 (1941).  44. vStackelberg, M. and Weber, P., Zeit. Elektrochem. j>6, 807 (1952). 45.  Strassner, J . E. and Delahay, P., J . Am. Chem. Soc. 2At 6237 (1952).  

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