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A study of the polarographic reduction of some organic nitro compounds in anhydrous acetic acid Risk, James Berryman 1956

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A STUDY OF THE POLAROGRAPHIC REDUCTION OF SOME ORGANIC NITRO COMPOUNDS IN ANHYDROUS ACETIC ACID by JAMES BERRYMAN RISK 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 standard required from candidates for the degree of MASTER OF SCIENCE Members of the Department of Chemistry THE UNIVERSITY OF BRITISH COLUMBIA May, 1956 ABSTRACT Polarographic results for the reduction of a number of aromatic compounds i n anhydrous acetic acid were obtained. The compounds studied included: o-, m- and p-nitroanilines and nitro-phenolsj o-nitrobenzoic acidj 2 , . 4-dinitro substituted phenol, benzoic acid and toluenej 3 , 5 - d i n i t r o substituted phenol and benzoic acidj 2 , 4 . , 6 - t r i n i t r o substituted phenol, benzoic acid and toluene. It has been shown that activating substituents on the benzene nucleus hinder the reduction of nitro groups on the ring, the effect being most pronounced for the o- and p- positions. Hydrogen bonding has been shown to occur to a lesser extent i n acetic acid than i n aqueous solution. Deactivating groups have been shown to cause nitro substituents to be more easily reduced, the effect decreasing i n the order o<m<p. The polarographic reduction of a nitro group has been postulated to proceed with the same mechanism as i n aqueous s o l -vents. This reduction has been assumed to be a four or six electron irreversible reaction to the hydroxylamino or amino group repectively. Acetic acid, although limited i n the potential range available, has been determined to be a suitable solvent for polarographic studies of organic compounds. ACKN01LEDGEMENT The author wishes to express his thanks to Dr. H. M. Daggett, J r . for the assistance and guidance given throughout the present work and to the National Research Council of Canada for financial aid. TABLE OF CONTENTS INTRODUCTION 1 EXPERIMENTAL Materials 7 Apparatus 8 Procedure 11 RESULTS Capillary Characteristics 15 Electro capillary Curve . 15 Resistance Corrections . . . . . . . . . 18 Polarographic Results Mononitro Compounds 19 Dinitro compounds 29 Trinitro Compounds . . . 39 DISCUSSION General . . . . . . . . . . . . . AA Mononitro Compounds . 4-5 Dinitro Compounds - . 4-9 Trinitro Compounds . . . . 52 Mechanism of Reduction . . . . . . . . 54 CONCLUSIONS 58 BIBLIOGRAPHY 59 LIST OF TABLES Table I. Capillary characterization data . . • . • 15 I I . Electrocapillary data- for mercury i n acetic acid . . . . . . . . . . . . 16 I I I . Results obtained for o-nitroaniline . . . . 20 IV* Results obtained for m=citroaniline . . . . 21 V* Results obtained for p^nitroaniline . . . . 22 VI. Results obtained for o-nitrophenol . . . . 23 VII. Results obtained f o r m-nitrophenol . . . . 23 VIII. Results obtained for p-nitrophenol . . . . 24 IX. Results obtained for o-nitrobenzoic acid . . 24 X. Results obtained for 2,^. rdinitrophenol . . . 30 XI. Results obtained for 3 ,5-dinitrophenol . . . 31 XII. Results obtained for 2 ,4-dinitrobenzoic acid . 31 XIII. Results obtained for 3 ,5-dinitrobenzoie acid . 33 XIV. Results obtained f o r 2 , 4 - d i n i t r o t o l u e n e . . 34 XV. Results obtained f o r t r i n i t r o compounds (summary) 40 XVI. Results obtained for 2 , 4 , 6-trinitrophenol . . 40 XVII. Results obtained fbr 2 , 4 , 6-trinitrobenzoic acid 40 XVIII. Results obtained for 2 , 4 , 6-trinitrotoluene . 41 XIX. Summarized results for a l l compounds studied . 46 ILLUSTRATIONS Figure 1. Polarographic c e l l . . . . . 9 2. Residual current curve . . . 13 3. Half wave potential versus concentration: for o-nitroaniline . . . . . . . . . . .13 U* Electrocapillary curve of mercury i n acetic acid . 17 5. Potential versus logarithm i / ( i d - i ) f o r o-nitro-phenol 25 6. Potential versus logarithm i / ^ i ^ - i ) for o-nitro-benzoic acid .25 7. Polarogram for o-nitrobenzoic acid . . . . .26 8. Polarogram for o-nitrophenol . . 27 9. " Polarogram for o-nitroaniline . . 28 10. Polarogram for 2,4--dinitrophenol 35 11. Polarogram for 3,5-dinitrophenol 36 12. Potential versus logarithm i / ( i d - i ) from unresolved polarogram for 3»5-dinitrobenzoie acid . . . 37 13. Potential versus logarithm i/(i<i-i) from f i r s t resolved wave for 3,5-dinitrobenzoic acid . . 38 14-. Potential versus logarithm i / ^ i ^ - i ) from second resolved wave for 3,5-dinitrobenzoic acid . .38 15. Polarogram for 2,4,6-trinitrobenzoic acid . . . 42 16. Polarogram for 2,4,6-trinitrophenol 43 - 1 -INTRODUUTION Polarography i s the name applied to the study of the current-voltage relationship i n electrolysis taking place at a dropping mer-cury electrode (1, p. 1). This technique may he applied to either oxidations or reductions. In the present discussion only polarographic reductions are considered. As the voltage applied to a dropping mercury electrode i n a suitable supporting electrolyte containing a reducible compound i s increased no appreciable current i s observed u n t i l a definite potential, depending upon the reducible compound present, i s reached. Here the current increases at f i r s t slowly, then more rapidly, as the potential i s further increased. On increasing the potential s t i l l more the rate of increase of current i s found to pass through a maximum and f i n a l l y at sufficiently high potentials to once more become zero. The resulting nS" shaped current-voltage plot i s known as a polarographic wave, the potential at the inflection point on the curve being characteristic of the compound being reduced and the current on the plateau proportional to i t s concentration. The general theory of polarography has been reviewed very well by Jayadevappa (2, p.17) and a more detailed discussion of basic principals and derivations of equations i s presented clearly by Kolthoff and Lingane (3, Vol. 1). Only the most important points shall be considered here. It has been shown by Ilkovic that the magnitude of the diffusion current (that i s the average current observed at sufficiently high potentials that the rate of electrolysis i s entirely diffusion - 2 -controlled) i s given by tbe following equation (3, p.34)s i d = 607 n B 1 / ^ 2 / 3 ^ / 6 where i ^ = average current i n microamperes during l i f e of drop at potentials where the current i s entirely diffusion controlled. n = number of electrons taking part i n the reduction. D - diffusion coefficient of reducible compound i n cm^ sec--*-. C = concentration of reducible compound i n millimoles l i t r e - ^ " . m = rate of flow of mercury i n mg.sec"^. t = drop time i n seconds. From the above equation i t i s seen: ^/(cm^tVo) = 607 nD 1/ 2 s Constant for any given ion. This proportionality constant between the diffusion current and the concentration i s the basis for quantitative polaro-graphic analysis. Later workers (4, 5) have treated the problem more rigorously and applied corrections to the original Ilkovic equation but i n most instances the d i f f i c u l t y i n handling the more.cumbersome equations does not j u s t i f y the greater accuracy since the experimental errors may well be greater than the corrections. The relation between the current and the applied potential on the polarographic wave i t s e l f i s given by the equation; Ede = H/2 -°«0591/n log i / ( i d - i ) - 3 -where =potential applied to dropping mercury electrode. E I / 2 ~ n a 1 ^ w a v e potential of compound being reduced. n =• number of electrons involved in the reduction. i =average current at E ^ . i , =average diffusion current, a It is seen that a plot of versus logarithm i / ^ i ^ - i ) should give a straight line with slope equal to -0.059/n and an intercept of E - = E, / 0 when i = i - , / 2 . From this plot n can be determined for the ae 1/2 <x reduction being considered. The half wave potential E - ^ , * n a s definite thermodynamic significance and can be expressed in terms of universal constants, the standard redox potential of the compound and the activity coefficients of the oxidized and reduced forms of the compound 13, Ch. XI, XII). The above equations apply only to reversible reductions but in many cases reductions at the dropping mercury electrode are not reversible. This irreversibility is due to a slow reaction at the electrode surface, either chemical or electronic, such as to cause i d to no longer be entirely diffusion controlled. For irreversible waves plots of logarithm i / ( i d ~ i ) versus potential very often yield straight lines as are obtained with reversible reactions but the slopes of such plots cannot be used to determine the number of electrons taking place in the reaction. There are a number of criteria for irreversible waves. A non linear logarithm plot is indicative.of irreversibility. From the Ilkovic equation i t is seen that i ^ is dependent on m2/3tV6 and hence on the height or pressure of mercury in the dropping mercury electrode. - 4 -In the case of irreversible waves the diffusion current is found -to be dependent on m^ /^ t^ /^  (3, p.274) and hence is independent of the pressure of mercury in the capillary. A discussion of irreversible waves is given by Meites (I, p.104). Since polarographic waves are diffusion controlled and not dependent on electrical migration 13, P«19), polarography is not limited to the study of ionic or charged species and may be applied to any compound whose reduction potential is not too high. Many organic compounds f a l l into this class but because of the limited solubility of many of these compounds in aqueous solutions, the polarographic method can be more readily applied in non-aqueous solvents. A discussion of the advantages and disadvantages of organic solvents as polarographic media and the suitability of various organic solvents together with an excellent survey of work in non-aqueous media has been given by Jayadevappa (2, pp.16, 32). x Comparatively l i t t l e work has been done using acetic acid as a polarographic medium. The f i r s t work reported was that of MacGillavry (6);who studied some inorganic acetates in this solvent, but with l i t t l e success. A more comprehensive study of inorganic ions was later made by Bachman and Astie 17). These workers used 0.25 M ammonium acetate as a supporting electrolyte and reported two types of waves. Normal polarograms were obtained for inorganic ions whose half wave potentials were more negative than -0.3 volts. Ions whose half wave potentials were more positive than -0.3 volts were a l l characterized by large discontinuous maxima which could not be suppressed. Bergman and James (8) have made a comprehensive study of - 5 -organic mononitro compounds i n anhydrous acetic acid using 1.0 M ammonium acetate as supporting electrolyte. These workers obtained well developed waves for the compounds studied and obtained linear logarithm i / ( i ^ - i ) versus potential plots, with slopes of about 0 . 0 6 , which they assumed indicated reversible one-electron reductions. The polarographic reduction of substituted and unsubstituted nitrobenzenes, i n buffered aqueous media, has been studied extensively by a number of workers. The f i r s t investigations were those reported by Shikata and coworkers (9, 10, 11), however these workers reported reduction potentials instead of half wave potentials. Reduction potentials are obtained from the zero current intercept of a tangent to the polarographic wave, hence these results cannot be correlated with half wave potentials reported i n later work. Astie and coworkers (12, 13, M> 15) have made a study of a number of substituted nitro-benzenes interpreting the results i n terms of the effects of hydrogen bonding. These workers determined the number of electrons taking part i n the reductions to be either four or six depending on the compound and the pH of the supporting electrolyte, indicating reduc-tion to the .hydroxylamine or amine respectively. It has also been shown that half wave potentials are dependent on the type of buffer used (15). Pearson (16, 17), who studied nitrobenzenes, nitrotoluenes, nitrophenols and nitroresorcinols, arrived at the same conclusions as to the number of electrons taking part- i n the reduction of the nitro group. In this work the reduction of the nitro group i s post-ulated to proceed through a potential determining step involving the - 6 -deposition on the electrode of atomic hydrogen which then reacts with the nitro group. On this assumption i t i s shown that a plot of potential versus logarithm i / ( i ^ - i ) should be linear with a slope, independent of the number of electrons taking part i n the reduction, equal to 0.059. Page, Smith and Waller (18) i n a study of the variation of the half wave potential of nitrophenols and nitrobenzoic acids postulate the potential determining step i s a reversible one electron reaction: W02 + e + H* -» BifO^M followed by one or more irreversible steps. These workers observed an increase of half wave potential with increasing concentration. The polarographic behaviour of disubstituted benzenes cor-relating the half wave potential with electron densities and activation energies for electrophilic substitution has been made by Gergely and Iredale (19). A study of the polarographic behaviour of nitroanilines i n absolute ethanol has been made by Runner (20). The object of the present work was to study the polarographic reduction of a series of substituted nitro compounds i n anhydrous acetic acid i n an attempt to determine the suitability of this solvent as a polarographic medium and to correlate the half wave reduction potentials to the type and position of the substituents on the benzene nucleus. - 7 -EXPERIMENTAL 1. MATERIALS (a) Purification of Mercury. The mercury used throughout this work was thoroughly oxidized by bubbling a i r through i t while i n contact with 1 N. n i t r i c acid for several hours. I t was then washed thoroughly by dispersing i n a counter current of water, f i l t e r e d through a wood f i l t e r stick, washed again, and f i n a l l y t r i p l y d i s t i l l e d under reduced pressure i n a stream of a i r . (b) Purification of Nitro Compounds. The nitro compounds studied i n this work were recrystallized from suitable solvents or synthesized as indicated i n the "Results'' section, page (15). (c) Purification of Acetic Acid. The acetic acid used as solvent i n this work was purified by the method of Eichelberger and La Mer (21) by refluxing with chromium trioxide and then drying by d i s t i l l i n g from boron triacetate prepared from acetic anhydride and boric acid. The melting point of pure acetic acid according to Eichelberger and La Mer (21) i s 16.59± .01° C., but according to Hess and Haber (22) i s 16.635± .002°C. The melting point of the acid used i n this work was determined to be 16.56t .02° C. The melting point of the unpurified Nichols Chemical Co. reagent grade acetic acid was 16.30+ .02° C. - 8 -2. APPARATUS A l l results reported were obtained with a manual polarograph. The apparatus used was essentially the same as that described by Jayadevappa (2, p.35). The galvanometer was calibrated by connecting i t i n series with an e.m.f. source and a standard resistance. A potentiometer was connected across this resistance thus enabling a calculation of the current. The galvanometer was found to have an accuracy of better than 1$ over the entire scale from 0 to 500 microamperes. Polythene tubing was used to connect the dropping mercury electrode to the mercury reservoir since i t was noticed that on standing i n Tygon tubing a scum formed on the surface of the mer-cury. An additional and more efficient vanadous sulfate scrubbing tower was introduced into the nitrogen purification t r a i n because of the extreme d i f f i c u l t y encountered i n removing the l a s t traces of oxygen from the acetic acid solutions. In this work, of course, the last tower i n the purification t r a i n contained acetic acid. The c e l l used i n the preliminary runs was identical to that described by Jayadevappa (2, p.35), but because of the d i f f i c u l t y of removing oxygen and introducing samples after a determination of the residual current had been made, this type of c e l l was found unsatis-factory. The c e l l used i n obtaining a l l results reported i s shown i n Figure 1. This c e l l consisted of two compartments joined by short tubes at two places. Through the neck of the main bulb of the c e l l was introduced a sintered glass cylinder for the introduction and dispersion of purified nitrogen used to scrub oxygen from the solution. To the upper portion of the neck was attached horizontally - 9 -Figure 1. Polarographic c e l l used throughout present work. - 10 -a second tube f i t t e d with a 3Tjoint. Into this tube were placed small platinum dishes containing weighed samples of the compound being studied. The platinum dishes could be pushed into the neck with a glass rod and hence into the solution whenever desired without exposing the solution to the atmosphere. The second compartment of the c e l l was used for introduction of the dropping mercury electrode, being simply a tube of 0.5 inches diameter connected to the bulb i t s e l f near the bottom and to the neck of the bulb higher up. With this arrangement, when the nitrogen was flowing, the solution circulated through the whole c e l l ensuring complete mixing. To the bottom of the dropping mercury electrode (henceforth called d.m.e.) compartment was fastened a tube for the introduction of the mercury pool after the oxygen had been removed from the solution. (According to Arthur and Lyons (23) the mercury may react with the dissolved oxygen, there-by changing i t s potential.) E l e c t r i c a l connection with the mercury pool was made by means of a platinum wire sealed through the bottom of the d.m.e. compartment. Two platinum discs, one inch i n diameter, were fastened into the bulb compartment of the c e l l to allow for pre-electrolysis of the solutions i n order to remove reducible impurities, however this .was found to be unsatisfactory. The c e l l was mounted i n a constant temperature water bath, maintained at 25.0+ .05° C , with a stationary clamp, and was always i n the same position each time i t was placed i n the bath. The d.m.e. was mounted on an assembly fastened to the wall to eliminate any vibration of the electrode by the sti r r i n g motor i n the water bath. The d.m.e. could be raised and lowered verti c a l l y , being held i n position with screw clamps. For each determination made the d.m.e. was always lowered - 11 -to the same position and since the c e l l had always the same position the separation of the d.m.e. t i p and the mercury pool was constant for a l l runs. Preliminary runs on a number of compounds studied were made with a Sargent Model XXI Recording Polarograph. The polaro-grams obtained with this instrument were used only to determine the position and general shape of the polarographic waves, duplicate runs being made i n a l l instances with the manual apparatus. 3. PROCEDURE (a) Characterization of Capillary. The capillary was characterized by the method of Muller (24., 25). The procedure followed was the same as that described by Jayadevappa (2, p.41). (b) Measurement of Polarographic Waves. The supporting electrolyte used for a l l determinations i n the present work was a 1.0 M. solution of anhydrous ammonium acetate i n anhydrous acetic acid. The solution was prepared by adding the required amount of ammonium acetate to a known volume of pure acid. The solution was then stored i n a specially designed solvent flask which enabled portions to be removed without exposure to atmospheric moisture. The preliminary runs, using the f i r s t c e l l , were carried out as described by Jayadevappa (2, p.43). However, because of the d i f f i c u l t y of removing oxygen from acetic acid, the solution had to be scrubbed with nitrogen for at least six hours before complete - 12 -removal of oxygen was obtained. Using the improved c e l l the procedure was modified slightly. In this case the cleaned and dried c e l l was weighed, a suitable volume of supporting electrolyte added, and the c e l l reweighed. The density of 1.0 M . ammonium acetate solution i n acetic acid was deter-mined to be 1.073 g./ml. From these values the volume of the solution was determined. The c e l l was mounted i n the constant temperature bath, and the substance to be determined weighed out i n the small platinum dishes, the amounts according to the concentration of the solution desired. These dishes were then placed i n the side arm of the polarographic c e l l and the joint, together with the glass rod assembly, put i n place. The solution was now scrubbed with a slow stream of nitrogen by means of the sintered glass cylinder i n the c e l l for a period of at least one hour. If-:the polarograms of the re-sidual current appeared to be abnormally large the nitrogen scrubbing was continued for an additional period. A typical residual current polarogram i s shown i n Figure 2. In instances when a maxima suppressor was used i t was added before the nitrogen scrubbing was begun and the residual current as measured then contained any contributions due to the maxima suppressor. After the residual current had been recorded the nitrogen flow was again started and the glass rod i n the side arm moved i n u n t i l the f i r s t platinum dish f e l l down the neck of the c e l l into the solvent. Because of the design of the c e l l the nitrogen flow caused circulation of the solvent. The nitrogen flow was continued for at least 20 minutes after the sample had dissolved to ensure that the solution was uniform. The nitrogen stream was now shut off and - 13 -r — i 1 0.00 0.30 0.60 0.90 1.20 1.50 Potential in volts against mercury pool. Figure 2. Residual current curve for 1,0 rao^ar ammonium acetate in anhydrous acetic acid containing 0.01% gelatin. 0.70 Figure 3. 0.72 0.74 0.76 0.78 Half wave potential in volts. Half wave potential versus concentration for o - n i t r o a n i l i n e 0 0.80 - 14 -the polarogram for the compound recorded. This process was repeated u n t i l a l l the samples i n the side arm had been introduced into the („ . . . . . . . c e l l . A series of polarograms for progressively increasing concen-trations of the compound being studied was thus obtained. This procedure was repeated for each compound studied, polarograms for four concentrations of each being recorded. The concentrations employed ranged-from about 0.4. to 4 .0 millimolar, the amount of compound required to make a 0.4 millimolar solution being the smallest amount that could be conveniently weighed out and concentrations above 4 .0 millimolar i n many instances giving rise to large maxima. - 15 -RESULTS 1. CAPlI.i.AKl OiiAKAOTERiSTiUS Several capillaries were fabricated and characterized before a satisfactory one was obtained, the f i r s t ones developing erratic drop times soon after having been made. The same capillary was used throughout this work, the characterization data being given i n Table I. According to Muller (24, 25 ) : 2.1567 x IO" 1 0 x length. where r = radius of capillary and X= applied pressure, P, divided by m. The length of the capillary used was 7.5 cm. and the average value of "ft from Table I i s 34-9 therefore r = 26.1 microns. Table I. Capillary characterization data. app t w m WV3 PBack cm. sec. mg. mg./sec. 30 71.9 58.3 0.81 1.79 1.6 35.1 40 52.9 58.4 1.11 1.79 1.6 34.7 50 41.7 57.5 1.38 1.80 1.6 35.1 60 34.3 57.9 1.68 1.80 1.6 34.8 2. ELECTftOCAPILLARI CURVES The data for the electrocapillary curve (Figure 4), obtained i n 1.0 ffl. ammonium acetate i n acetic acid, are given i n Table I I . From these results i t i s seen that the value of m 2/3tV6 i s constant over the entire potential range available i n acetic acid, variations of m and t compensating each other. For a l l compounds studied the drop - 16 -times were recorded as the polarographic data were obtained. Because of the period of time over which the present research was carried out, t, for a given potential, was found to vary slightly from time to time. In the calculation of the diffusion current constant the drop time actually observed at the potential the diffusion current was measured during the run under consideration was always employed. However, since the diffusion current constant i s dependent on the sixth root of t, the drop time had to change significantly before any correction needed to be applied. The value of m was checked periodically throughout the work but was found to remain constant. Table I I . Electrocapillary data for acetic acid, 1.0 M. i n ammonium acetate. E t m m2/^1/0 volts sees, mg./sec. 0.00 4.39 1.34 1.61 0.20 4.72 1.33 1.57 0.40 4-92 1.33 1.57 0.40 4-94 1.32 1.57 0.50 4.90 1.32 1.57 0.60 4.94 1.32 1.57 0.70 4.93 1.33 1.58 0.80 4.92 1.32 1.57 0.90 4.92 1.33 1.58 1.00 4.90 1.32 1.57 1.10 4-92 1.32 1.57 1.20 4.90 1.32 1.58 1.30 4.86 1.33 1.57 1.40 4.74 1.33 1.57 1.50 4.66 1.34 1.56 4.00 0.00 0.40 0.60 0.80 1.00 1.20 P o t e n t i a l i n v o l t s against mercury p o o l 0 1 . 4 0 F i g u r e 4. E l e c t r o c a p i l l a r y curve f o r mercury i n 1.0 molar ammonium acetate s o l u t i o n of anhydrous, oxygen f r e e , a c e t i c a c i d e - 18 -3 KfiSXsTANCE OOKKECI'IONS The resistance of the acetic acid was quite appreciable and hence iR corrections must be applied to the potentials recorded. A l l polarograms presented have been corrected for the residual current but not for iR drops. A l l results reported i n tables however have been corrected for iR drops, the corrections being applied i n the calculations. The resistance of the acetic acid solution was deter-mined with an a.c. bridge. I t has been shown by Ilkovic that the mean resistance, R a v, required for the iR drop correction i s given by: Rav = 4 / 3 Rmin. where Rmin. i s the minimum resistance observed durihg the l i f e of the drop (8). The mean resistance used for iR corrections through-out the work was determined to be 1 5 0 0 ohms. Using this value of R a v for iR drop corrections i t was observed that the half wave potentials varied with concentration for a l l compounds studied. In order to obtain comparable values for the half wave potentials a curve of E - ^ against concentration was plotted for each compound and the potential at zero concentration was obtained from this graph. This method of obtaining corrected half wave poten-t i a l values includes both iR drop corrections and variation of $>\j2 with concentration. A typical plot of Ei/2 versus concentration i s presented i n Figure 3 . This method of obtaining f u l l y corrected half wave potentials has been used throughout this work. - 19 -U. PuLAROGRAPHIC RESULTS (a) Mononitro Compounds. The mononitro compounds studied included: ortho-, meta-, and para- nitroaniline; ortho-, meta-, and para- nitrophenol; and ortho-nitro-benzoic acid. Most of the compounds gave well developed normal .waves (Figure 7). In some instances small maxima appeared which were easily suppressed. Plots of logarithm i / ( i ^ - i ) against applied potential were i n most cases reasonably linear but i n some cases were definitely curved. In these instances the best straight l i n e was drawn through points i n the v i c i n i t y of the half wave poten-t i a l i n order to get a value for the slope.(Figures 5 and 6). In some cases a second poorly defined wave followed the main wave (Figure 8)-. These second waves invariably continued up to the decomposition potential of the supporting electrolyte and hence no analysis of any of these plots was attempted. It was noticed i f current readings were obtained by decreasing the applied potential from the diffusion current potential, the plot obtained by increasing the potential was not retraced (Figure 9). No explanation of this hysteresis effect i s attempted and results are reported only for data obtained by increasing the potential. (i) Ortho-nitroaniline. The o-nitroaniline used was recrystallized from water. Two series of measurements were made, one with no maxima suppressor present, the second with 0.0001% fuchsin present. The results obtained are tabulated i n Table III. With no maxima suppressor present small maxima developed with higher concentrations of the reducible compound but these .were completely suppressed by fuchsin. The polarograms obtained (Figure 9) showed only one normal wave and were reproducible. Table III Results obtained for o-nitroaniline. Gone. *d E l / 2 Encorr. E-wg iR Corr. Slope mM. c m ^ t 1 / 6 No maxima suppressor present 0.615 6.99 0.725 0.719 0.083 1.13 6.95 0.733 0.725 0.086 2.00 7.18 0.750 0.734- 0.087 4.29 7.12 0.784 0.746 0.088 0.0001% fuchsin 0.552 0.707 0.718 0.718 0.088 1.36 0.695 0.744 0.735 0.082 2.38 0.702 0.753 0.736 0.089 4.68 0.710 0.784 0.744 0.087 Average values: E-i./o f u l l y corrected 0.715 ,d t _.7.05 cm2/3tV6 Slope-_0.086 ( i i ) Meta-nitroaniline. The m-nitroaniline used was recrystallized twice from water. The polarograms obtained were normal "S" shaped curves with a second wave developing at more negative potentials and con-tinuing up to the decomposition potential of the acetic acid solution as was observed with mono nitrophenols.(Figure 8). Some d i f f i c u l t y was encountered i n obtaining satisfactory polarograms, the current not being reproducible and maxima being present. On allowing a solution which gave an abnormal wave to stand for twelve hours a - .21 -normal wave was obtained. Graphs of logarithm against po-tent i a l were found to give plots that were slightly curved. The results obtained are presented i n Table IV. Dio maxima suppressor was present i n the solutions. Table IV Results obtained for m-nitroaniline. Cone. *d E3./2 Uncorr. En/g iR Corr. Slope mM. em2/3tV6 0 . 6 3 7 4 . 2 9 0 .578 0.572 0 . 1 0 0 1 . 3 0 4 . 3 3 0 . 6 0 2 0 . 5 9 3 0 . 1 0 1 2 . 5 » — 0 . 6 2 5 0 . 6 1 0 0 . 0 9 4 • Concentration not known accurately. Average values: E-1/2 f u l l y corrected __0.565 1 d /..Tl ' cm2/3tV6 Slope- _ 0 . 0 9 8 ( i i i ) Para-nitroaniline. p-Nitroaniline, recrystallized twice from water, was used to make the solutions studied. Normal well developed waves were obtained for a l l concentrations, small maxima developing i n the more concentrated solutions. These were effectively suppressed with 0 . 0 0 0 1 $ fuchsin. Logarithm plots tended to be slightly curved as with m-nitroaniline. Only one wave was present i n polarograms obtained, the diffusion current for the f i r s t wave remaining constant up to the decomposition potential of the supporting electrolyte. Results obtained are given i n Table V # - 22 Table V Results obtained for p-nitroaniline. Cone. . &i/2 Uncorr. ®±/2 ^  Gorr. Slope rnM , C1U ' ^ t ' 0.477 6.43 0.718 0.717 0.098 1.17 6.25 0.731 0.721 0.092 2.511 6.38 0.743 0.725 0.088 4.83? 6.25 0.781. 0.746 0.100 4.779' 6.34 0.783 0.745 0.091 \ Maxima present '" 0.0001% fuchsin present Average values: E-i io fully corrected__0.713 ., 6.33 Slope. -0.098 (iv) Mononitrophenols. m-Witrophenol was used without further purification, the ortho and para compounds being1 recrystallized twice from water. The supporting electrolyte was 0.01% gelatin in the three series of measurements made on these compounds. Normal reproducible polaro-grams were obtained for a l l concentrations studied. The logarithm plots obtained from polarograms of o-nitrophenol were linear, (Figure 5j> but those obtained for the meta and para compounds had a definite curvature as illustrated for o-nitrobenzoic acid in Figure 6. A second wave followed the main wave closely in each case (Figure 8) and made an accurate determination of the diffusion current impossible, possibly accounting for the variation of the - 23 -current constant observed. The results f or o - , m—, and jpinitro-phenols are given i n Tables VI, VII, and VIII respectively. Table VI Results obtained for o-nitrophenol. Cone. g^z/^i/p E l/2 ^ n c o ^ ^ • %/2 ^ Corr. Slope 0.472 5.45 0.652 0.649 0.080 1.13 5.50 0.667 0.658 0.087 1.78 5.65 0.675 0.664 0.085 4.30 5.95 0.708 0.678 0.089 Average values &jy2 f u l l y corrected 0.650 ^73^ 1 / 6 ~ ~ Slope __0.084 Table VII Results obtained for m-nitrophenol. Cone, mil. cm ' t ' ^1/2 Uncorr. E l / 2 ^ Oorr* Slope 0.406 4.15 0.608 0.607 0.107 1.08 4.40 0.644 0.636 0.107 2.46 4.00 0.665 0.652 0.107 4.29 4.35 0.689 0.668 0.113 Average values f u l l y corrected. _ 0.62 ^2/3*1/6 Slope 0.109 - 2 4 -Table VIII Results obtained for p-nitrophenol. Cone. mM. > u cm2/3t0/6 E l / 2 Uncorr. El/2 ^ Corr. Slope 0.586 ,5.20 0.740 0.735 0.105 1.39 5.00 0.745 0.738 0.103 2.62 4.90 0.7766 0.749. 0.100 4.41 5.05 0.784 0.759 0.103 Average values Y,y2 f u l l y corrected 0.731 5.05 cm f ^v-f Slope 0.103 (v) Ortho-nitrobeneoic Acid. . .The o-nitrobenzoic acid was recrystallized twice from water. The supporting electrolyte was 0.01% i n gelatin. A well defined normal wave was obtained for a l l concentrations with indic-ations of a second wave developing (Figure 7)» The logarithm plots were slightly curved (Figure 6)• Results for this compound are tabulated i n Table IX. Table IX Results obtained for o-nitrobenzoic acid. Cone. , *T1 .. E]/2 Uncorr. E ] / 2 iR Corr. Slope mM. cm2/3tV6 0.430 3.63 0.607 0.606 0.116 0.978 3.64 §.620 0.617 0.113 1.91 3.60 0.628 0.620 0.125 3.73 3.A6 0.647 0.635 0.111 AAverage values f u l l y c o v e r e d - - °» 6 0 5 ffl2/3tl/S ~ ~ 3 # 6 2 Slope 0.115 - 25 -0.55 0.60 0.65 0.70 0.75 P o t e n t i a l i n v o l t s . Figure 5. P o t e n t i a l versus l o g a r i t h m i / U ^ - i ) from polarogram of 1.76* m i l l i m o l a r o-nitrophenol. Figure 6 P o t e n t i a l versus logarithm i / d ^ - i ) from polaro-gram of 1.91 m i l l i m o l a r 6-nitrobenzoic a c i d . 10.0 0.00 Figure 7 0.20 0.40 0.60 0.80 loOO Potential i n v o l t s against mercury pool, Polarogram for 1.91 millimolar o-nitrobenzoic a c i d . F i g u r e 6*. Polarogram f o r 1.78 m i l l i m o l a r o - n i t r o p h e n o l . - 28 -i 1 1 r ~ — i — — r 1 L _ I i - i i l 0.50 0.60 0.70 0.80 0.90 1.00 P o t e n t i a l i n v o l t s against mercury p o o l . Figure 9. Polarogram f o r 4.68 m i l l i m o l a r o - n i t r o a n i l i n e I n c r e a s i n g p o t e n t i a l . Decreasing p o t e n t i a l . - 29 -(b) Dinitro Compounds. The same general shape of wave was observed for a l l dinitro compounds studied. Normal "S M shaped polarograms were not obtained since &sr a definite inflection or change of slope was observed on the waves. The inflection point for the 2,4-dinitro compounds illustrated i n Figure 10 was more clearly defined than for the 3,5-dinitro com-pounds illustrated i n Figure 11. These waves were assumed to actually be two superposed waves. For the reasons, and by the method given la t e r , (see page 49 ) these waves have been resolved into two separ-ate waves for analysis. The logarithm plots of the whole waves were not linear (Figure 12) but cn separating into two waves a linear relationship was obtained for both waves as illustrated i n Figures 13 and U,. (l) 2,4-Dinitrophenol. 2,4-Dinitrophenol, recrystallized from water, gave well devel-oped waves for a l l concentrations studied except the lowest with no maxima appearing. The lowest concentration gave a poorly developed wave which ?;as possible due to experimental errors. The supporting electrolyte was 0.005% gelatin. Only one main wave was observed, a well defined diffusion current being obtained. A definite inflection point was observed on the wave, (Figure 10), and on separating into two waves linear logarithm plots were obtained. The results are re-corded i n Table X. - 30 -Table X Results obtained for 2,4-Dinitrophenol. Cone. M 1^/2 Uncorr. Ei/2 ^  Corr. Slope mM cm2/ 3tV 6 First Second First Second First Second wave wave wave wave wave wave 0.342 6.85 0.749 5.85 1.65 5.50 2.60 5.03 0.480 0.473 0.489 0.498 0.730 0.725 0.758 0.770 0.477 0.467 0.477 0.482 0.722 0.710 0.726 0.720 0.097 0.097 0.087 0.090 0.083 0.094 Average Values: Slope - - - — First Wave Second Wave 0.094 0.088 E]_/2 fully corrected" _ First ¥ave_ _ 0.473 Second Wave __ 0.715 (ii) 3,5-Dinitrophenol. As with 2-4 dinitrophenol well defined waves were obtained for a l l concentrations* however, unlike the 2-4 dinitro compound, no def-inite inflection point was present on the wave. (Figure 11) 0.01% gelatin was present as maxima suppressor and although no maxima were observed abnormal galvanometer oscillations were noticed over short potential ranges with 2.17 and 4.60 millimolar solutions. The re-sults obtained are recorded in Table X I . ( i i i ) 2,4-Dinitrobenzoic Acid. 2,4-Dinitrobenzoic acid was prepared according to the method given by Vogel (26) from phenylacetic acid and fuming nitric acid and purified by three recrystallizations from 20% ethanol. The polaro-grams obtained had the same form as those obtained with 2,4-dinitrophenol, illustrated in Figure 10, but the inflection point was not as clearly - 31 -defined. A second poorly defined wave followed the main wave con-tinuing up to the decomposition potential of the supporting electro-lyte. The supporting electrolyte contained 0.01% gelatin and no maxima were observed in any of the polarograms. Results for 2,4-di-nitrobenzoic acid are tabulated in Table XII. Table XI Results obtained for 3,5-Dinitrophenol. Cone. mM. c m2 / 3?l / 6 El /2 Uncorr. E1/2 iR Corr. Slope First Second First Second First Second wave wave wave wave wave wave 0.479 3.88 0.433 0.590 0.431 0.583 0.076 0.097 1.00 3.73 0.444 0.593 0.442 0.583 0.095 0.092 2.17 3.75 O.468 0.620 O.46O 0.593 0.098 0.088 4.60 3.78 0.502 0.672 0.482 0.613 0.097 0.098 Average Values: xd , Slope... First Wave__ 0.091 cm2/3tI/6 Second Wave_ _ 0.094 3 ..79 E-1/2 fully corrected_ First lave 0.426 ' Second Wave_ .. 0.577 Table XII Cone. raM. Results obtained for 2,4-Dinitrobenzoic Acid. E]_/2 Uncorr. E1/2 iR Corr. 0.340 0.742 1.39 2,82 cm2/3?l/b 3.52 3.50 3.47 3.56 First Second wave wave 0.472 0.656 0.493 0.670 0.501 0.683 0.522 0.716 First Second wave wave 01472 0.490 0.495 0.511 0.653 0.660 0.667 0.681 Slope First Second wave wave 0.098 0.037 0.092 0.057 0.100 0.091 0.087 0.092 Average Values: uu .,,^d, Slope._ First Wave__0.094 cm2/3tV°_3.51 Second Wave-.0.089 E]y2 fully corrected ._ _ First Wave..0.474 Second ¥ave_.0.651 - 32 -(iv) 3,5-Dinitrobenzoic Acid. 3,5-Dinitrobenzoic acid was recrystallized from water. With no maxima suppressor present large maxima developed even with low concentrations (0.701 millimolar) of the reducible sub-stance. Addition of 0.001% fuchsin suppressed the maxima for concentrations upetoj.1.2 millimolar but with higher concentrations the maxima reappeared.- The polarograms have a shape similar to those obtained with 3,5-dinitrophenol, illustrated in Figure 11, having a very poorly defined inflection point. The logarithm plots for the whole wave had the characteristic "S" shape. (Fig-ure 12) On separating the wave into two separate waves more nearly linear plots were obtained but were s t i l l definitely curved. (Fig-ures 13, 14) The slope, taken by drawing the best straight line through points in the vicinity of the half wave potential varied : over a considerable range. This could have been due to maxima interference but i t appears that the difference in half wave poten-tials for the two waves is so small that an arbitrary separation of the waves into tv.-o waves leads to large errors. Results for 3,5-dinitrobenzoic acid are given in Table XIII. (v) 2,4-Dinitrotoluene. 2,4-dinitrotoluene recrystallized from ethanol gave well developed waves for a l l concentrations studied. The polarograms obtained had the same general shape as those obtained for 2,4-di-nitrophenol, (Figure 10) and 2,4.-dinitrobenzoic acid having a well developed inflection point. A second poorly defined 7/ave devel-oped at higher negative potentials and continued up to the decom-- 33 -position potential of the acetic acid. 0.0005% methyl red was effective as a maxima suppressor for concentrations below 1.6 millimolar. Using 0.005% gelatin no maxima were observed up to concentrations of 2.56 millimolar. The results are presented in Table XIV. Table XIII. Results obtained for 3,5-Dinitrobenzoic Acid. Cone. mM. El/2 First wave Uncorr. Second, wave E]_/2 iR Corr. First Second wave wave Slope First Second wave wave 0.701'" •»' 3.04 0.395 111 0.393 111 0.041 111 0.693 3.07 0.392 0.528 0.390 0.520 0.046 0.073 1.19 2.98 0.400 0.530 0.395 0.517 0.070 0.077 2.81'' 2.25 0.422 1'1 0.413 111 0.061 111 5.211' 3.24 111 111 111 t • 1 1 > 1 111 ' No maxima suppressor present '' Maxima present • • » M a s k e d by m a x i m a Average Values: i d _ _ . _ - 2.92 Slope_. First Wave. _ No r e l i -cm 2/3t 1/6 a b l e v a l u e Second Wave 0.075 E]_/2 fully corrected First Wave _ ._ 0.384 Second Wave 0.525 - 34 -Table XIV. Results obtained for 2,4-Dinitrotoluene. i L _ Ei/p Uncorr. Ei/p Cone. - ^  ^  E l / 2 . Uncorr. ^ 2 IR Corr. Slope mM. cm^  F i r s t Second F i r s t Second F i r s t Second wave wave wave 7/ave wave wave 0.0005$ Methyl Red present. 0.395 4.39 0.516 0.683 0.512 0.683 0.073 0.082 0.762 5.00 0.517 0.700 0.5U 0.685 0.101 0.083 1.63 4.93 0.526 0.722 0.518 0.695 0.076 0.090 2.44 4.98 0.540 0.750 0.523 0.703 0.078 0.084 0.005% Gelatin present 0.398 5.20 0.511 0.700 0.510 0.694 0.082 0.084 0.790 5-30 0.520 0.736 0.532 0.707 0.086 0.086 2.56 5.10 0.554 0.763 0.540 0.716 0.087 0.092 Average Values:- Using data from determination with gelatin present. Ad . 5.10 Slope,_ F i r s t Wave 0.083 cm' Second Wave 0.088 E1/2 f u l l y corrected .__ F i r s t Wave<_ _ 0.504 Second Vfave 0.693 0.00 0.20 0.40 0.60 0.80 . 1.00 1.20 1.40 P o t e n t i a l i n v o l t s a g a i n s t mercury p o o l . F i g u r e 10. Polarogram f o r 1.6? m i l l i m o l a r 2 , 4 - d i n i t r o p h e n o l i l l u s t r a t i n g a r b i t r a r y r e s o l u t i o n of wave. Recorded curve. Resolved curves. 30.0 24.0 ft i o o •H -P' 18.0 12.0 0.00 0.20 0.40 0.60 0.80 1.00 P o t e n t i a l i n v o l t s against mercury p o o l . 1.20 1.40 Figu r e 11. Polarogram f o r 2.17 m i l l i m o l a r 3 , 5 - d i n i t r o p h e n o l i l l u s t r a t i n g a r b i t r a r y r e s o l u t i o n of wave. Recorded curve. — ftesolved curve. 0.35 0.40. 0.45 0.50 0.55 0.60 P o t e n t i a l i n v o l t s . Figure 12. P o t e n t i a l versus l o g a r i t h m i / d ^ - i ) from complete polarogram of 1.19 m i l l i m o l a r 3 , 5 - d i n i t r o b e n z o i c a c i d . - 38 -0.30 0.35 0.40 0.45 0.50 P o t e n t i a l i n v o l t s . Figure 13. P o t e n t i a l versus logarithm i / ( i d - i ) f o r f . i r s t r esolved wave from polarogram of 1.78 m i l l i m o l a r 3 , 5-dinitrobenzoic a c i d . 0.45 0.50 0.55 0.60 0.65 P o t e n t i a l i n v o l t s . Figure 14. P o t e n t i a l versus logarithm i / ( i d - i ) f o r second resolved v/ave from polarogram of 1.78 m i l l i m o l a r 3,5-dinitrobenzoic a c i d . - 39 -(c) Trinitro Compounds. Three .symmetrical trinitro compounds were studied; 2,4.,6-trin-itrobenzoic acid, recrystallized from water, 2,4.,6-trinitrophenol, recrystallized from ethanol, and 2,4,6-trinitrotoluene, recrystallized from ethanol. Well developed waves were obtained for each compound, the trinitrobenzoic acid (Figure 15) and: trinitrophenol (Figure 16) giving only one wave but the polarogram for trinitrotoluene having a second irregular and poorly defined wave following the main wave closely and continuing up to the decomposition potential of the supporting electrolyte. These trinitro compounds, like the dinitro compounds, did not give normal "S" shaped curves. In these cases at least one, and sometimes two inflectionspoints or changes of slope could be observed; however, these were not well developed. It was assumed that these waves consisted of three super-posed single waves and, as with the dinitro compounds, logarithm plots for the whole waves were not linear. Because of the arbitrary method of obtaining three separate waves logarithm plots were not drawn for these compounds and the half wave potentials were taken directly from the polarograms. The supporting electrolyte con-tained 0.01% gelatin. This was found effective in suppressing maxima except at the higher concentrations.of the nitro compounds. The results for these compounds are presented in Tables XVI, XVII and XVIII. These results are summarized in Table XV. - AO -Table XV. Results obtained for t r i n i t r o compounds studied. Compounds Avg. i d c m 2 / 3 t l / 6 2,A*6-trinitrophenol 2,At6-trinitrobenzoic acid 2,4,6-trinitrotoluene 4.54 5.64 4.55 E i / 2 f u l l y corrected F i r s t Second Third wave wave wave 0.323 0 . 4 0 2 0 . 6 0 1 0.324 0.432 0.571 0.382 0 . 5 4 1 0.692 Table XVI. Results obtained for 2 ,4,6-trinitrophenol. Cone. mM. i d c m 2 / 3 t l / 6 E l / 2 Uncorr. E l / 2 iR Corr. F i r s t 1 Second Third F i r s t Second Third wave wave wave wave wave wave 0 . 2 9 4 4.50 0 . 3 3 6 0 . 4 2 2 0 . 6 3 0 0 . 3 3 4 0 . 4 1 7 0 . 6 2 2 0.778 4 . 5 7 0 . 3 9 2 0.470 0.708 0 . 3 8 8 0 . 4 5 7 0.687 1 . 1 6 4 . 6 0 0 . 4 0 0 0.515 0 . 7 4 0 0 . 3 4 4 0 . 4 9 6 0 . 7 0 9 2 . 1 4 1 4-50 — Maximum present. TableXXVII. Results obtained for 2 ,4,6-trinitrobenzoic acid. Cone id cm*/3tVb f E l / 2 Dncorr. h./2 iR Corr. F i r s t Second Third F i r s t Second Third -'X - - wave wave wave wave wave wave 0 . 1 3 5 0.385 0.754 1 . 4 0 5.60 5.67 5.67 5.63 0 . 3 2 5 0.340 0.360 0.368 0.438 0.455 0.475 0.500 0.575 0.595 0.610 0.640 0 . 3 2 4 0.337 0.355 01359 0.435 0.447 O .46O 0.472 0.571 0.582 0.585 0.594 - 41 -Table XVIII. Results obtained for 2,4,6-trinitrotoluene. Cone. mM. id cm2/3lV6 h/2 Uncorr. El/2 iR Corr. First Second Third First Second Third wave wave wave wave wave wave 0.450 0. 842 1. -73 2.6# 4.60 4.60 4.50 4.53 0.385 0.410 0.425 0.440 0.550 0.580 0.602 0.633 0.705 0.735 0.774 0.810 0.383 0.405 0.416 0.426 0.543 0.566 0.575 0.591 0.693 0.712 0.728 0.755 - U2 -10.0 a> U 0) a s aJ o u o •H s c •H •P c u 3 O 0.10 0.30 '0.50 0.70 0.90 Potential in volts against mercury pool: Figure 15. Polarogram for 0.385 millimolar 2,4,6-trinitro-benzoic acid i l l u s t r a t i n g arbitrary resolution of wave. • Recorded wave. Resolved waves. *• - A3 -0.10 0.30 0.50 0.70 0.90 1.10 P o t e n t i a l i n v o l t s against mercury p o o l . Figure 16. Polarogram f o r 0.778 m i l l i m o l a r 2 , 4 , 6 - t r i n i t r o -phenol i l l u s t r a t i n g a r b i t r a r y r e s o l u t i o n of waves. — — — — — Recorded curve, — — Resolved curves. - uu -Discussion As is shown by the representative polarograms presented a l l nitro compounds studied are seen to give the same general type of curve. This indicates the reduction mechanism to be similar in every case. From previous work reported on the polarographic reduction of nitro compounds in aqueous (12 to 19) and in absolute ethanol solutions (41) this i s also observed. Mono-nitro compounds are found to give a polarogram with one well defined wave present within certain instances, a second poorly defined wave developing at more negative potentials. Dinitro compounds are observed to also give one well defined wave but this wave is assumed to be a composite wave resulting from the superposition of two waves whose half wave potentials differ by a small amount. Polarograms for trinitro compounds are similar to those for the dinitro compounds, the composite wave being composed of three separate waves instead of two. As for mononitro compounds a second poorly defined wave in certain cases developed at more negative potentials for both the di and trinitro compounds. Maxima were found to. develop with higher concentrations of a l l compounds studied. The occurence of maxima did not appear to depend on the compound being reduced nor directly on its con-centration -but ^ rather on the diffusion current observed for the particular polarogram being recorded. The diffusion currents for di and trinitro ..compounds were observed to be approximately two and three times..respectively those observed for mononitro compounds and maxima tended to occur at much lower concentrations of the former. These maxima could be suppressed with either methyl red, fuchsin or gelatin. Gelatin was found to be the most satisfactory, suppressing maxima for higher concentration of the reducible sub-stance than either of the other two as well as not having the dis-advantage of itself being reduced at the dropping mercury electrode, (a) Mononitro Compounds. marized and presented in Table X I X . The order of ease of reduction of nitroaniline and nitrophenols i s seen to be m«o<p. In the case of nitroaniline however, half wave potentials for the ortho and para isomers can be considered to be equal within experimental error. These results for nitroaniline agree qualitatively with those obtained in aqueous solvents (19), absolute ethanol (20), acetonitrile (2), and acetic acid (8) by other workers. In the case of nitrophenol the results reported agree with those observed in acetonitrile by Jayadevappa (2) however in acetic acid Bergman and James (8), and in buffered aqueous solutions Page, Smith and Walker (18) and Pearson (16, 17) report the order of ease of re-duction to be m«o<p. No explanation of the discrepancy between the values obtained in the present work and those reported by Bergman and James obtained under identical conditions can be given. the accepted effects of resonance and hydrogen bonding. o-Nitro-phenol is known to resonate between structures as illustrated: The results obtained for a l l compounds studied are sum-The above results can be explained from consideration of / \ i O ! H - 46 -Table XIX Polarographic results obtained for compounds studied. Compound \/2 - Slope volts cm2/3tl/6 o-Nitroaniline 0.715 0.085 7.05 m-Nitroaniline B 0.565 0.098 6.33 p-Nitroaniline 0.713 0.095 6.33 o-Nitrophenol n 0.650 0.094 5.50 m-Nitrophenol" 0.62 0.109 4.25 p-Nitrophenol1? 0.713 0.103 5.50 o-Nitrobenzoic acid 0.605. 0.116 3.62 2,4-Dinitrophenol n a] 0.473 a) 0.089 b] 0.715 b) 0.088 3,5-Dinitrophenol" a] 0.426 a) 0.091 3.79 h] 0.577 b) 0.095 2,4-Dinitrobenzoic acid" a-] 0.474 a) 0.095 3.51 b; 0.651 b) 0.089 3,5-Dinitrobenzoic acid" a] 0.384 a) - 2.92 M3.§25 b)0.075 2,4-Dinitrotoluene" a] 0.507 a)0.083 5.10 b) 0.693 b) 0.088 2 , 4 , 6-Trinitrophenol n 0.323 — 4.54 b] 0.402 — c] 0.601 -2,4,6-Trinitrobenzoie acid a] 0.324 5.64 b] 0.432- — c] 0.571 -2,4,6-Trinitrotoluene" a] 0.382 4.55 b] 0.541 — c] 0.693 -Second wave followed main wave at more negative potentials. - Al -This quinoid type of resonance is also known to be present in the para isomer and when the hydroxyl group is replaced by an amino group. It is seen that when such resonance is present the nitro group has a resultant negative charge. This nitro group will be more resistant to reduction than the normal group as is present in meta substituted nitroaniline or nitrophenol where such reson-ance is not possible. The above consideration indicate*the meta isomer would be reduced at a lower potential than either the ortho or para isomers. Both the hydroxyl group and the amino group have negative inductive effects, that is they tend to draw electrons from the ring. This effect tends to cause the nitro group to be more easily reduced, however the effect would be slightly greater for the ortho substituted isomer thus the ortho compound should be reduced before the para. From consideration of the above factors one would expect the order of reduction to be m<o<p. The ortho isomers of both nitrophenol and nitroaniline are capable of intramolecular hydrogen bonding. Such bonding, i f present, would tend to hinder the normal resonance present within the nitro group itself and thus reduce its reduction potential to such an extent that the ortho isomer may be reduced before the meta. If such stable hydrogen bonds formed the order of reduction would be o < m < p. In both the present work and as previously noted for other solvents, the order of reduction of nitroanilines is m<o<p. This supports the above argument and indicates that hydrogen bonding - 48 -between the nitro and amino group does not occur to any extent with o-nitroaniline. The order of reduction of the nitrophenols ' i s also observed to be m<o<p , i n disagreement with the order os:m<p, obtained i n aqueous solvents (16, 17, 18). I t i s known that a nitrophenol forms hydrogen bonds i n aqueous solutions, hence the order of reduction should be o»m<p as reported but with acetic acid as a solvent, o-nitrophenol, l i k e o-nitroaniline, must form weak hydrogen bonds. Since the supporting electrolyte used i n the present work was 1,0 M. ammonium acetate i n acetic acid i t i s ^possible that the nitrophenols were present as pheno-late ions thus decreasing the amount of hydrogen bonding. From table i i i i t i s seen that the nitrophenols are generally reduced at more positive potentials than the nitro-anilines. This may not be expected since the hydroxyl group has a stronger mesomeric effect leading to the addition of relative-l y more electrons to the ring, however since the 'oxygen of the hydroxyl group i s more electronegative than the nitrogen of the amino group i t exerts a more positive inductive effect, i h i s results i n a greater lowering of the reduction potential for a nitro group on a phenol molecule relative to one on an aniline molecule. This effect would be more pronounced for the ortho than for the para ^position but i s opposed i n these two positions by the resonance effects. For the meta position only the inductive effect w i l l be present. From the above considerations the nitrophenol i s expected to be reduced at the lower potential with the d i f f e r -ence i n reduction ^potential to decrease i n the order m< o< p. From - 4V -Table XIX this is seen to be the order observed. o-Hitrobenzoic aced was found to have a lower half wave potential than either o-nitrophenol or o-nitroaniline. Since the carboxyl group is not capable of contributing to a resonance structure and also has a positive inductive effect, a lower re-duction potential is to be expected. vb) Dinitro Compounds AS has been previously noted, well defined single waves were obtained for a l l mononitro compounds studied, whereas com-posite waves were obtained with the dinitro compounds. ihe arbitrary resolution of these composite waves into individual waves has been carried out in such a manner that at any given potential the observed current is the sum of the current con-tribution, at that potential, of each individual wave present. ihe resolution of these composite waves into separate waves is justifiable for two reasons, r'irstly, since there is no reason to suspect the mechanism for the reduction of poiy-nitro compounds to be any different than that of the mono "compounds, a linear logarithm plot should be expected with the polynitro compounds, 'i'his is only obtained on resolution of the waves (Figures 13, M). secondly the limiting currents observed with the" di- and trinitro compounds are respectively approximately two and three times those observed with equiva-lent concentrations of mononitro compounds. Currents of this magnitude for the polynitro compounds could be obtained with a single wave only i f two or three groups were reduced at the - 50 -same potential . This could possibly,, occur with the 3,5-dinitro compounds since these positions are equivalent, i n the case of 2,4-dinitro compounds the positions are not equivalent and hence the nitro groups would not be reduced at the same potential , bince the same general type of wave was obtained for a l lpolynitro com-pounds i t has been assumed that simultaneous reduction of two equivalent nitro compounds does not take place i n any instance and a l l polarograms for these compounds have been resolved into separate waves corresponding to a stepwise reduction of the nitro substituents (Figures 11, 12). rrom T a b l e i i i i t i s seen that the half wave potentials for the f i r s t wave for a l l dinitro compounds i s consistently lower than that obtained for any of the mononitro compounds. This i s to be expected because of the inductive effect of the second nitro group on the benzene r i n g . From the known inductive and mesooeric effects of the hydroxyl, amino, and methyl groups and consideration of possible hydrogen bonding, the relative half wave potentials of the 2,4-disubstituted nitro compounds studied can be explained, i t i s here assumed that reduction of the nitro group ortho to the v a r i -able substituent i s responsible for the f i r s t wave. This i s ex-pected for the toluene and w i H occur for the phenol and benzoic acid i f hydrogen bonding i s present. From the directing i n f l u -ence of hydroxyl and carboxyl groups, as noted i n the discussion of mononitro compounds, one would expect" the dinitrophenol ~fem bs -51 -to be reduced at a relatively higher potential. Since the f i r s t half wave potentials for the phenol and benzoic acid are equal, the hydrogen bonding must be stronger for the hydroxyl group. The f i r s t half wave potential of the 2,4-dinitrotoluene i s ob-served to be larger than that of the benzoic acid, indicating that the negative inductive effect of the carboxyl group i s s t i l l predominant and therefore the hydrogen bonding present must be •relatively weak. The polarographic reduction of the nitro groups i s assumed to proceed to the hydroxylamine or completely to the amine. ( see p. 54) "ihe second wave of the 2,4-dinitro com-pounds i s therefore due to the reduction of the nitro group of a 4-nitro-2-hydroxylamino or a 4-nitro-2amino compound. Since the previously reduced group i s meta to the nitro group undergoing reduction, any.mesomeric effect w i l l be small, and only the inductive effect w i l l affect the half-wave potential, of a nitro group meta to i t . bince the mesomeric and inductive effects are both small and oppositely directed, they w i l l tend to cancel out. i n the case of 2,4-dinitro substituted compounds the influence of the original substituent i s s t i l l present. From these considerations the order of reduction of the second nitro group of these compounds studied should be GOGH<CH^< OH. This i s the order observed It i s .seen from Table XiX that the half wave potential for the second wave of 2,4-dinitrophenol i s the same as that for p-nitrophenol. This also indicetes that the hydroxylamino or - 5 2 -amino group resulting from the reduction of the f i r s t nitro group has l i t t l e or no effect on the half wave potential of the second nitro group. The half wave potentials for both wavesof the 3,5-dinitro compounds studied are found to be lower than the corresponding potentials for the 2,4-substituted compounds. All substituents on these compounds are meta to each other hence half wave pot-entials w i l l be affected primarily by the inductive effects of the substituents. As has been shown for the meta substituted mononitro componds, this should result i n lower half wave pot-entials for the 3,5-dinitro compounds than for the 2,4,-dinitro compounds. This i s found to be the case for the dinitrophenols and dinitrobenzoic acids studied. 3,5-Dinitrobenzoic acid was found to be reduced more readily than 3|5-dinitrophenol. This i s i n accordance.-with the stronger inductive effect of the car-boxyl group. (c) Trinitro Compounds Trinitro compounds gave polarographic waves.similar i n form to those obtained with the dinitro compounds. These waves were assumed to be composed of three single superimposed waves, xhree separate half wave potentials were obtained for. each compound, corresponding, as for dinitro compounds, to the successive reduction of the threenitro groups present. The half wave potentials recorded were obtained directly from the ar b i t r a r i l y resolved polarographic curves (i'lgures 15, 16) and may be appreciably i n error. The introduction of a third nitro group into the benzene ring, as i s expected, lowers the half way reduction potential of that compound. The half wave potentials for the three t r i n i t r o compounds studied are appreciably lower than those of the cor-responding dinitro compounds. As with the 2,4-dinitro compounds, the order of reduction of the f i r s t nitro group of the 2,4,6-tri-nitro compounds was observed to be OHsCOOH < Ch^, tne effect of the third nitro group being only to reduce the half wave potenti-als of each compound by an equal amount. This not found to be true i n the case of the second wave. From considerations previously presented, the second nitro group to undergo reduction w i l l be the one i n the 6 position. This position i s equivalent to the position of the nitro group that has already undergone reduction. Since the reduction product has l i t t l e effect on the half wave potential of the remaining nitro group, the order of reduction should be the same as that ob-served for the f i r s t group reduced. The half wave -potentials of the. f i r s t wave i s , as previously noted, equal for the 2,4,6-trinitrophenol and 2,4,6-trinitrobenzoic acid, however the second half wave potential for the phenol os lower than that of the acid. This discrepancy may be due to other unexplained effects, or possibly to experimental, error. The third wave on the polarograms of the 2,4,6-trinitro compounds i s due to the reduction of the nitro group i n the 4 posi-tion.. The half wave potentials for the reduction of this group should be the same as those observed for the second wave of the dinitro compounds. The third waves of the trinitrophenol and - 5A -trinitrobenzoic acid are, hewever, found to occur at lower poten-t i a l s than" the second waves of the dinitro compounds. This dis-crepancy-is too large to be accounted for by experimental errors, but no explanation i s attempted. —(d) Mechanism of Reduction. ~ In aqueous media (12-17) and i n absolute ethanol (20) the polarographic reduction of a nitro group procedes i n a single step via a four or six electron reduction to the hydroxylamino or amino group respectively. As the diffusion .coefficients of the compounds studied i n acetic acid are not known, a calculation of n, the number of electrons taking part i n the reduction, from the Ilkovic equation i s not-possible. Also i t i s impossible to get a value of n from logarithm plots since these gave a non-integral value less than one. No independent determination of n was carried out i n this work. However, since the polarograms obtained were very similar to those obtained i n different solvents by other workers, the reduction mechanism i s assumed to be the same. This assumption i s further j u s t i f i e d by consideration of the observed current constants. Although these vary considerably, they are of the same magnitude as those reported i n other media, indicating approximately the same number of electrons taking part i n the reaction. As indicated i n Table XIX, many of the polarograms obtained had a second poorly defined wave appearing at higher potentials. If the reduction of the nitro group had proceeded completely to the amine, no further reduction would have been possible. The - 55 -appearance of a second wave indicates that the f i r s t wave i s due to a part i a l reduction of the nitro group. This i s assumed to be a four electron reduction proceeding-to the hydroxylamine. For o- and p-nitroaniline, o-nitrobenzoic acid and 2,4-,6-trinitro-benzoic acid no second waves are apparent within the potential range available i n acetic acid. This could indicate complete reduction to the amino group, but this i s not necessarily the case. A second wave may develop at a higher potential than i s available i n acetic acid. It i s seen from Table XIX that the diffusion current con-stants for o- and p-nitroaniline are significantly larger than those for m-nitroaniline or o-, m- and p-nitrophenol. Since second waves developed for these l a t t e r compounds, indicating the f i r s t wave to be due to a four electron reduction, i t i s reasonable to assume that the o- and p-nitroanilines are completely reduced to the diamino compound i n one step. In the case of o-nitrobenzoic acid no definite statement may be made, but from the small value of the diffusion current constant, a four electron wave i s indicated. With 2,4.,6-trinitro-benzoic acid, however, the relatively large diffusion current constant indicates a six electron reduction. Additional data are required before~a more definite statement may be made regarding these compounds. From Table XIX i t - i s seen that the values of the slopes of the majority of waves obtained for the compounds studied l i e within the range 0.085 to 0.095. These values are indicative of - 56 ~ irreversible reductions. Bergman and James (8) obtained values of about 0.06 for a series of nitro compounds studied i n acetic acid. These workers interpreted this as indicating reversible one electron reductions. In view of the above discussion, the writer feels this interpretation i s erroneous, since the diffusion current constants obtained by these workers were of the same order of magnitude as those obtained i n the present work. In a study of the variation of the half wave potential with pH for a number of substituted nitrobenzenes i n buffered aqueous media, Pearson (16) has presented a theory which predicts the observed slope values. I t i s assumed i n this theory that the potential governing step i s the reduction of hydrogen ions to atomic hydrogen on the*surface of the mercury drop. The current observed i s dependent on the rate at .which hydrogen atoms are removed from the surface of the drop or to the rate of reaction of the reducible compound with the atomic hydrogen. That i s : i - KGg.Cg where i i s the current observed, K i s a proportioality constant and and-Cg..are the concentrations at the electrode surface of the reducible species and of atomic hydrogen respectively. I t i s assumed that K i s independent of potential, but i f such i s tha case this equation breaks down, since from polarographic theory (3,p.8 ) when i = i , = 0. The slopes obtained i n the present work correspond to those obtained by Pearson (16,17) indicating that the reduction mechanisms are similar. A number of workers (27 to 33) have presented theoretical treatments for irreversible reductions, but equations are very involved and cumbersome. It is the writers opinion that no simple treatment will satisfactorily explain the mechanism of the reduction of organic nitro compounds at a dropping mercury "electrode. - 58 -CONCLUSIONS The half wave potentials for the polarographic reduction of substituted aromatic nitro compounds i n acetic acid indicate that the reduction potentials are i n accordance with the known mesomeric and inductive effects of the substituents considered, providing hydrogen bonding i s assumed to occur i n certain instances. Compounds containing more than one nitro group are found to give polarograms consisting of two or more superposed waves, one corresponding to each nitro group present. From the general shape of the polarograms, the values of the current constants and the slopes of logarithm ±/(i^-l) plots, i t i s apparent that the polarographic reductions of nitro compounds procede by the same mechanism i n acetic acid as i n other solvents. This i s a four or six electron irreversible reduction to the hydroxylamine and amine respectively. Acetic acid has been found to be a satisfactory solvent for polarographic studies of organic compounds, with, however, some drawbacks. The resistance of an anhydrous acetic acid solution was found to be appreciable and necessitated iR drop corrections. Oxygen had to be completely removed from the solution before satisfactory polarograms were obtained. It was found that complete removal of oxygen"from acetic acid was d i f f i c u l t . Maxima, which tended to develop i n acetic acid, were not suppressed for higher con-centrations of the reducible compound with concentrations of gel-atin as great as 0.01%. The development of maxima i s not, however, unique with acetic acid solutions. BIBLIOGRAPHY 1. Meites, L. 'Polarographic Techniques' Interscience Publishers Inc., New York, N. Y. 1955. p. 104. 2. Jayadevappa, E. S. M. Sc.'Thesis University of British Columbia. Oct. 1955. 3. Kolthoff, I. M., and Lingane, J . J . 'Polarography' Inter-science Publishers Inc., New York, N. Y. 1952. 4. Lingane, J . J., and Loveridge, B. A. J . Am. Chem. Soc. 72: 438. 1950. 5. Streklow, H., and von Stackelberg, M. ,Z. Electrochem. - 54: 51. 1950. 6. MacGillavry, D. Trans. Faraday Soc. 32: 1447. 1936. 7. Bachman, G. B., and Astle, M. J . J . Am. Chem. Soc. 64: 1303. 1942. 8. Bergman, J . and James, J . C. Trans. Faraday Soc. 48: 956. 1952. 9. Shikata, M. Trans. Faraday Soc. 21: 42 1925. 10. Shikata, M., and Hozaki, N. Mem. C o l l . Agr. Kyoto Imp. Univ. 17: .1. 1931. 11. Shikata, M., and Watanabe, W. Mem. CoH. Agr. Kyoto Imp. Univ. 4: 1934. 1928. 12. Astle, M. J., and McConnel, W. V. J . Am. Chem. Soc. 65: 35. 1943 13. Astle, M. J., and Cropper, W. P. J . Am. Chem Soc. 65: 2395. 1943. 14. Astle, M. J., and Stevenson, S. P. J . Am. Chem. Soc. 65: 2399. 1943. 15. Dennis, S. F., Powel, A. S., and Astle, M. J. J. Am. Chem. Soc. 71: I484. 1949. 16. Pearson, J . Trans. Faraday Soc. 44: 683. 1948. 17. Pearson, J . Trans. Faraday Soc. 45: 199. 1949. 18. Page, J . £», Smith, J . W . , and Waller, J-. G. J . Fhys. Chem. 53: 545. 1949. 19. Gergely, E., and Iredale, T. J . Chem. Soc. 3226. 1953. 20. Runner, M. E. J . Am. Chem. Soc. 74: 3567. 1952. 21. Eichelberger, W. C , and La Mer, V. K. "J. Am. Chem. Soc. 55: 3633. 1935. 22. Hess, K., and Haber, H. Ber. 70B: 2205. 1937. 23. Arthur, P., and Lyons, H. Anal. Chem. 24: 1422. 1952. 24. Muller, 0. H. J . Chem. Ed. 18: 172. 1941-25. Muller, 0. H. J . Am. Chem. Soc. 66: 1019. 1944. 26. Vogel, A.I. 'Practical Organic Chemistry' Longmans, Green, and Co. New York N. Y. 1948. p. 719. 27. Delahay, P. J . Am. Chem. Soc. 73: 1944. 1951. 28. Delahay,IP. ;J. Am. Chem. Soc. 74: 3506. 1952. 29. Delahay, P. J . Am. Chem. Soc. 75: 1430. 1953. 30. Delahay, P.,.and Strassner, J . E. J . Am. Chem. Soc. 73: 5219. 1951. 31. Kivalo, P. J. Am. Chem. Soc. 75: 3286. 1953. 32. Brdichka, R. J. Am. Chem. Soc. 76: 907. 1954. 33. Kern, D. M. H. J . Am. Chem. Soc. 76: 4234. 1954. 


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