Goyal, R. N.

Malik, Warid U.

English

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INDIAN J. CHEM., VOL. HA, FEBRUARY 19768. KNAPMAN, F. G. W. & RANDALL, E. L., cu«. News,Land., 113 (1916), 265.9. HENDRIXSON,W. S., J. Am. chem. Soc., 45 (1923), 2013.10. TOMICEK, 0., Chern. Abstr., 19 (1925), 450.11. FEIGL, F. & SCHUMNER,0., Z. analyt, Chem., 64 (1924),249.12. FRESNO, C. D. & MAIRLOT,E.,Chem.Abstf'.,27 (1933), 211.13. TANDON,J. P. & CHAWLA,K. L., Bull. chem, Soc. Japan,40 (1967), 992.14. SUCHONELOVA,L., DOLEZAL,J. & ZYKA, J.,J. electroanal,Chem., 1 (1960), 403.15. MISRA,G. J. It TANDON, J. P., Chemist Analyst, 56 (1967),5.16. MISRA, G. J. & TANDON,J. P., Indian J. Chem., 5 (1967),343.17. BALE], J. & ROUSAR, 1., cu«. Listy, 51 (1957),695.18. MURAKI, 1., J. chem, Soc. Japan, 76 (1955), 193.19. MAHR, C., Chern. Abstr., 59 (1963), 8119d.20. SINGH, B. & SINGH,S., Analyt. chim. Acta, 13 (1955),222.21. SINGH, B. & SINGH,S., Analyt. chim, Acta, 14 (1956),213; 15 (1956), 277.22. SINGH, B. & SINGH,S., Z. analyt, cu«; 162 (1958), 325.23. FURMAN, N. H. & FENTON (Jr), A.., Analyt. Chem., 32. (1960), 745.24. VULTERIN, J. & ZYKA, J., Chem. Listy, 48 (1954),1762.25. SUSEELA, B. S., Z. analyt. Chem., 168 (1959), 112.26. BERKA, A., VULTERIN, J. & ZYKA, J., Chemist Analyst,52 (1963), 56.27. RAO, P. V. K. & RAO, G. B. B., Indian J. Chem., 10(1972), 746.28. VULTERIN, J., Chern. Abstr .• 74 (1971), 119807m.29. VULTERIN, J., Talanta, 10 (1963), 891.30. ERDEY, L., Chem, Abstr., 46 (1952), 849i.31. ERDEY, L. & SVEHLA, G., Z. analyt. Chem., 150 (1956),407.32. ERDEY, L., KASA, 1. & MEISEL, T., Talanta,'lO (1963),471.33. ERDEY, L. & BUZAS, L., Chem, Abstr., 49 (1955), 9431e.34. COQUET, C. D., Annis cu«. analyt., 22 (1917), 160;Chem, Abstr., 12 (1918), 794.35. MARTI, F. B., CONDE,F. L. & BOLLE-TACCHEO,S., Analyt.chim, Acta, 9 (1953), 299.36. YOSHIMURA,C., J. chem, Soc. Japan, 74 (1953),407.37. KOLTHOFF, 1. M., Z. analyt, Chem., 60 (1921),453.38. KOHN, M., Analyt. chim, Acta, 10 (1954), 405.39. MULLER, E., Z. anorg . a!lg. Chem., 135 (1924),265.40. DESH!dUKH, G. S. & SANT, .B. R., J. Indian chem. Soc.,29 (1952), 543.41. SINGH, B. & MALIK, I. I., J. Indian chem, Soc., 14 (1937),435.42. GOPALA RAO, G. & DIKSHlTULU, L. S. A., Talanta, 9(1962), 289.43. MCCAY, L. W., Ind. Engng Chem., 5 (1933), 1.44. PINTER, T., cu«. Abstr., 37 (1943), 2303.Polarographic Reduction ofSome Substituted PhenylthioureasWARID U. MALIK· & R. N. GOYALDepartment of Chemistry, University of Roorkee, RoorkeeReceived 23 December 1974; accepted 7 April 1975Polarographic reduction of substituted phenylthio-ureas in Britton-Robinson buffers and in O'IM NaOHgives one well-defined, diffusion-controlled, partiallyirreversible, two-electron transfer oxidation waves.Below pH 5·0 ill-defined waves are obtained. TheEl/2 shifts towards more negative value with the increasein pH and towards more positive potential with theincrease in [depolarizer]. Phenylthioureas do notundergo direct oxidation but form mercurous complexwith mercury at d.m.e. This is further supported bythe effect ofsubstltuents on El/2.·Present address: Bundelkhand University, Jhansi.144pOLAROGRAPHIC investigations on thiols andsulphydryl compounds have been carried outby a number of workers-it. Two aspects have beengenerally studied: (i) practical applicability of thedouble waves of these compounds for diagnosticpurpose; and (ii) anodic depolarization. In thelatter case the general electrode process is repre-sented by the reactionR-SH+Hg~R-S-Hg+H't +eGenerally the electrode process is irreversible butin a few cases reversibility has also been claimed.Another class of compounds which could exhibitone of the above mentioned polarographic behaviourmay be the thioureas. Detailed studies on thepolarography of these compounds especially onphenylthioureas have not been undertaken so far.In this communication we report our results onthe polarographic behaviour of substituted phenyl-thioureas.Phenylthiourea and 2-methyl-, 3-methyl-,4-methyl-, 4-chloro-, 4-bromo-, 4-methoxy- and4-ethoxy-phenylthioureas were synthesized in thelaboratory by the action of ammonium thiocyanateon the hydrochloride of the corresponding amines-",0·01M solution of each of the above mentionedthioureas was prepared in dimethylformamide (AR).The polarograms were recorded on a Cambridge penrecording polarograph. The capillary characteristicwas 3·75 mg2/3 sec-1/2• The temperature coefficientwas calculated by using Nejedly's method" and thevalue of n, the number of electrons involved in theelectrode process was calculated by the method ofDeVries and Kroon? using a mercury pool cathodeand calomel electrode as a reference.The cell containing 10 ml of buffer was deoxy-genated by bubbling it with purified nitrogen for30 min prior to the recording of the polarogram.Similarly the polarograms of different thioureaswere recorded in Britton-Robinson buffer" and inNaOH. Each limiting current was corrected forthe residual current.All the substituted phenylthioureas gave a singlewell-defined oxidation wave at d.m.e. in buffersand in the alkaline solution. In the case of B.R.buffers, well-defined waves were obtained at higherpH.The nature of the oxidation waves was found tobe diffusion-controlled as revealed by the lineardependance of wave height to h1/2• The temperaturecoefficients of these thioureas were found to lie inthe range 1'20% deg-1 to 1·60% deg-1• The half-wave potential was found to shift towards morepositive potential with increase in [depolarizer],suggesting thereby complex formation" rather thanoxidation. Since the plots of Ed.e. vs [log U/id-i)Jwere not linear, the waves were analysed for ir-reversible nature, employing Koutecky's equationsfor irreversible cathodic waveEd.e. = Ei/2+0·0542/(1-oc)n [log i/(id-i) -0·546 log t]whereE~/2=-0·2412-0·05915/(1-oc)n log l'349K'b,h/DrPThe plots of Ed.e. VS [log (i/id-i)-0·546 log t) arelinear and the value of oc and Kg,h calculated fromTABLE 1 - VALUES OF VARIOUS KINETIC PARAMETERSFOR THE SUBSTITUTEDPHENYLTHIOUREAS(R~·~m-C-NHt)\ISNo. R E* 10-3X KO D6/2 X lll-'- ·1{2 at b,h(V) (em sec-1) (em sec1/2)1 H 0'400 0·775 1'620 3-902 2-CH3 0'380 0'775 1'545 3·273 3-CH3 0'390 0'835 1'608 3·704 4-CH3 0'395 0'750 2-525 3'245 4-CI 0·440 0·797 1'002 2-986 4-Br 0'420 0·780 2'415 3'657 4-0CH3 0·380 0·730 7'282 3·978 4·0C2Hs 0·390 0·730 2-812 3'20*At concentration 1x10-3}11.the slope and intercept of these plots is given inTable 1. The values of Kg,h. indicate the partialirreversible nature of the waves.The effect of pH on the E1/2 of various phenyl-thioureas was obtained from the polarograms runin different buffers. The El/2 was found to shifttowards more nagative value with the increasingPH (5·0-10·0). Such a behaviour was also observedby Fedoronko and coworkersw for urea, ~hiour~aand isothiourea. Plots of El/2 vs pH are lin ear IIIthe pH range 5·0 to 10·0. Below pH 5·0, ill-definedwaves were obtained.Since the value of n was found to be two, theanodic depolarization may involve the formation ofdisulphide or mercurous complex. In the formercase the Nernst equation should be given byEd.e.= El/2-RT/nF In [C%]I/C"where C,. and C" are concentrations of substitutedthiourea and its disulphide respectively.Putting n = 2, the above equation changes toEd.e.=E1/2-O·059/2 In (ill-i)'I'/iwhere i represents the amount corresponding tothe concentration of the oxidized substance and idto the total limiting current. If the formation ofdisulphide is involved the plots of Ed.e. VSIn (id-i)2/ishould be linear with slope of 0·059/2. However,it was found that the slopes of these plots are muchhigher (in the range 0·06-0·07) than the theoreticalvalue. On the basis of this observation the possibi-lity of direct oxidation of the substituted phenyl-thioureas was ruled out.The second possibility of complex formation canbe accounted for by considering the zwitterionformation'- of thioureas. These species would reactwith mercurous ions to give a complex. As theconcentration of Hg~+ will be controlled by thesolubility product S of the mercurous compound,the Nernst equation in this case can be representedasE = EOHgt++RT/2F In S-RTfF In [C.•]After considering the diffusion of substitutedphenylthiourea ions towards the electrode surfaceat i=0 the above equation changes toEl/2 = EO-RTjF In C,,/2where EO = EOHg:++RT/2F In S.NOTES4-0C H 4·CH 202 5 0o4--0CH3 2-CH 3N0·38 ::::UJI0·360·0(JFief. 1 - Plots of £1/2 versus (J for substituted phen ylt hio-o ureas to test the applicability of Hammett equationFrom this equation it is clear that the El 2 shouldbe independent of flow rate, dr?p. time and theheight of the mercury column. ThIS.ISwhat ha~ b~nobserved in the case of all these thioureas. SImilartype of complex formation have also. been reportedby Edsberg'! in the case of uns~bshtuted thiourea.An examination of the data in Table 1 revealsthat electron donating substituents, viz. methoxy,ethoxy and methyl groups, shift the E1/2 towardsmore positive potential, whereas electr~n withdraw-ing groups like chloro and bromo shifts the E1/2towards more negative potential. This behaviourcan be explained on the basis of zwitterion forma-tion. An electron withdrawing substituents attractselectron towards itself due to which tendency ofzwitterion formation would be decreased therebymaking the reduction more difficult. An electrondonating group would impart a negative charge,thereby increase the zwitterion formation tendencywith the result E1/2 slightly shift towards morepositive potential.The relation between half-wave potential ofsubstituted phenylthioureas and the Hammettsubstituent constant (0') which include both polarinduction effect and polar conjugation effect isshown in Fig. 1. It is observed from this plot thatsubstituents like H, 3-CH3, 4-CHa, 4-0CH., 4-OCzH5,4-Br obey Hammett equation while other groupsdeviate from the regression line.References1. KOLTHOFF I. M. & BARNUM, C., J. Am. chem . Soc., 62(1940), 3061.2. SAXENA, R. S. & GUPTA, K. C., Electrochim, Acta, 13(1968), 1749. .3. KRALL, H. & GUPTA, R. D., J. Indian chem . Soc., 12(1935), 629.4. CLERMONT, DE., J. chem, soc., 30 (1870), 70.S. BRITTON, H. T. S., Hydrogen ions, Vol. 1 (Van Nostrand,New York), 1956, 360.6. NEJEDLY, V., Co1l11 Czech. Chen>. Commun., 1 (1922),319.7. DEVRIES, T. & KROON, J. L., J. Am. chem . Soc., 75(1953), 2484.8. KOLTHOFF, I. M.. Polarography, Vol. 1 (Interscience,New York), 1952, 202.9. MEITES, L., Polarographic tech-niques (Interscience, NewYork), 1969, 34.10. FEDORONKO. 1\1., MANOUSEK, O. & ZUMAN, P., Chem.Listy, 49 (1955), 1494.11. EDSBERG, R. L., J. analyt . Cbem., 26 (1954), 724.145

Polarographic Reduction of Some Substituted Phenylthioureas

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Polarographic Reduction of Some Substituted Phenylthioureas

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