Indian Journal of ChemistryVol. 21A, December )982, pp. 1087-1089

Polarographic Behaviour of Some artha-Substituted Nitrobenzenes:Evaluation of Polarographic arrha-Shift

K GANAPATHY· & M RAMANUJAM

Department or Chemistry. Annamalai University, Annamalainagar 608002

Received 29 May 1982; recised 25 August 1982; accepted 6 September 1982

The polarographic characteristics of thirteen ortho-substituted nitrobenzenes have been studied at 35· ±O.I'C in a well-buffered medium of 50~~ (v/v) aqueous ethanol. having pH 5.8. Reduction process for all the compounds is found to beirreversible. The heterogeneous rate constant k?h, transfer-coefficient, :1II., free energy of activation, !'iG, diffusion currentconstant, I and the radius, r, of the solvated depolarizer have been determined. The deviation of ortho-derivatives from thelinear plot of El/2 versus Hammett's sigma constant has been accounted for on the basis of steric hindrance and the formationof hydrogen bonding. An evaluation of polarographic orf/Zo-shift, !'io for all the groups has been attempted.

Although a number of investigators! -7 have studiedthe polarographic reduction of substituted nitroben-zenes under different conditions, a great deal ofconfusion appears to exist regarding the effect ofortho-substituents on the mode of reduction. By andlarge anomalous behaviour of ortho-substitutednitrobenzenes towards the polarographic reductionhas been attributed to steric factor. It, therefore, seemspertinent to study the polarographic reduction of someortho-substituted nitro benzenes in detail. In thepresent paper we report the polarographic characteris-tics of thirteen ortho-substituted nitrobenzenes and aquantitative parameter, the so-called 'polarographicorlho-shift (~i)' has been evaluated.

Materials and Methodso-Nitrotoluene, o-nitrophenol, o-nitrobenzoic acid,

o-nitroaniline and o-chloronitrobenzene, all of BDHquality, were purified by standard methods. 0-

Nitr o an iso le , o-ni trobenzo n it r ile. o-bromo-nitrobenzene, o-nitroacetophenone, o-methylmer-captonitrobenzene, o-nitrophenyl methyl sulphoxideand o-nitrophenyl methyl sulphone were preparedby the literature methods" - J 4 Solids wererecrystallised to constant melting points and theliquids distilled under reduced pressure until theboiling points matched with the literature values.

Polarographic measurement-The polarograms ofthe fully deaerated solutions (2.0 mol dm -3 indepolariser) were recorded at 35: ±O.\"C on anautomatic pen-recording polarograph (ELICO CL 25)wi th LR 10IP recorder. The capillary had thefollowing characteristics: m = 1.1377 mg s -l, t = 4.8 sat -1.0 V vs SCE and m2/3 (1/6 = 1.4154 mgll3 s -1/2 ata mercury height of 65 em. For all the polarograms, thecurrent at the end of the drop-life was taken instead ofthe average current because the determination of

kinetic parameters is based on the Koutecky'scalculations which are more accurately reproduced bymeasuring the maximum current. The pH of themedium was maintained at 5.8 with sodium acetate(0.1 mol dm - 3) and acetic acid (0.1 mol dm - 3). In eachcase 0.03~/:) gelatin was used as the maximumsuppressor.

The viscosities and densities of solution weremeasured using Ostwald viscometer and specificgravity bottle (10 ml capacity) respectively. Theviscosity of the medium was found to be 1.6376 c pat35'-'.

Results and DiscussionAll the ortho-substituted nitrobenzenes in the

present investigation give well-defined single wave.The slope values of the plot of - Ed,e versus log (i/id-I)

are higher than those obtained theoretically, indicatingthe reduction process to be irreversible. Theirreversibility is further ascertained from the k?'h D -1/2

values which are less than 0.05 s - t /2, a condition fortotal irreversibility. The polarographic parameters arelisted in Tables 1 and 2. These findings are inaccordance with our previous report 15 thatnitrobenzene is reduced in 50% (v/v) aqueous ethanolat pH 5.8 to N-phenylhydroxylamine (Eq. I).C6HsN02+4H++4e-,-,C6HsNHOH+H20 ... (I)

The kinetic parameters (heterogeneous rateconstant, k?,h, transfer coefficient, ~n.) have beendetermined by Koutecky's" method as extended byMeites and Israel' 7. The free energy of activation (~G)for the reduction process has been calculated asdescribed earlier I s. The radius, r, of the solvateddepolarizer has been calculated by combining theviscosity data of the solution with Ilkovic and Stokes-Einstein 18 equations.

1087

INDIAN 1. CHEM., VOL. 21A, DECEMBER 1982

~~~~~.-~~- ~-.- -- --

of Some Ortho Substituted NitrobenzenesTable \- Polarographic Characteristics

Ortho - £1/2 ~n.substi- (V)tuent

CH3

OHOCH3

SCH3

SOCH3

S02CH3

COCH3

CNCHOCOOHNH2CIBr

0.6340.4840.5220.6800.5160.4580.5360.5180.5300.5440.5910.5650.589

0.72350.29720.75270.29430.65960.89910.64440.76820.77990.29870.29870.25200.4105

Table 2 - Diffusion Current Constant (/), Radius (r) andFree Energy of Activation (~G) of Polarographic Reduction

of Some Ortho Substituted NitrobenzenesOrtho I r x 10" AGsubsti- UtA mol -I m ' (em) (kl mol "")tuent mg -2/351/2)

CH3 7.948 1.39 75.42OH 7.153 1.71 54.96OCH3 8.213 1.30 68.31SCH3 8.478 1.22 60.03SOCH3 6.623 2.00 65.92S02CH3 5.299 3.12 67.93COCH3 5.299 3.12 67.30CN 7.418 1.59 68.73CHO 5.043 3.46 70.94COOH 5.829 2.58 57.22NH2 7.683 1.48 57.89CI 7.153 1.71 55.84Br 8.213 1.30 61.49

--------- -_.--"----

It is observed that the ortho-derivatives deviate fromthe linear plot of E I, 2 versus Hammett (J constants andthis can be attributed to steric hindrance andformation of hydrogen bonding. In the case of 0-

nitrophenol, o-nitrobenzoic acid and o-nitroaniline,the reduction occurs at more positive potentials thanfor the analogous meta derivatives and at substantiallymore positive potentials than for the para derivatives.This difference is due to the formation of hydrogenbonds between the hydrogens of these substituents(-OH, -COOH and - NH2) and the oxygen of thenitro group, leaving the nitrogen of the nitro groupmore positively charged. Further, resonance within thegroup is also decreased. These factors contribute tofacile reduction of nitro group in these compounds.Hydroxyl and amino groups, in contrast to the nitrogroup, favour o,p-substitution by electrophilicreagents, and must therefore be regarded as essentiallyelectron-donating ( + T effect). This will stabilize the

1088

D2 X 106

(em" s .1)kP.hD 1,2

(s . I 2)

2.055 x 10-8

6.006 X 10'3.279 x 10.7

8.438 x 10 6

8.421 xlO '3.786 x 10-'4.871 x 10-7

2.811 X 107

1.183xlO'2.491 x 10 -s1.934 x \0 '4.318 x 10 5

4.747 x 10 6

10.7168.6801.444

8.6807.4424.7634.7639.3354.2985.763

10.0138.680

11.442

6.277 X 10-6

2.039 x 10.2

9.695 x IO -,2.039 x 10 2

3.087 x IO - 41.735 x 10 42.232x1049.202 x 10 '5.707x10'1.038x10-2

6.113 x 10 3

1.466 x 10'1.403 x 10 .l

bonding of oxygen atoms in the nitro group and makethem more difficult to be removed. In line with thisconcept, it is found that the lowering of half-wavepotentials as compared with nitrobenzene (-0.614V)IS is less with the corresponding meta and parasubstituted nitrobenzenes 19 This lowering of half-wave potential also stabilises the hydroxylamineformed, so that reduction beyond this stage is notpossible.

For the remaining ortho-substituents both the stericand polar effects operate. However. a quantitativeseparation of these factors could not be made.Nevertheless, for o-Cl, o-Br. o-CHJ and o-OCHJ

derivatives a correction in E 1 2 has been made for sterichindrance using Eq. (2)20

E12(corr)=EI2-[()n.R(EJ,,_,] ... (2)

where steric reaction constant, 15 n,R, is given by Eq. (3)

b~,R =(I1EI2 - fI rr.R(J" J/(EJo x ... (3)

The values of steric substituent constant, (EJoX'are taken from Iiterature/". The value of thereaction constant, Pn.R is taken as 0.123 V from adetailed study 1 'I of meta- and para-substitutednitro benzenes under the present experimentalconditions.

The value of ()n.R is found to be 0.14 V. The values ofcorrected EI/2 of these ortho derivatives are plotted(Fig. 1) against Hammett (Jo~x' and the reactionconstant, P".R is found to be 0.116 V (corr. coefficient= 1); this compares favourably with that (0.123 V)obtained using meta and para derivatives.

It cannot be easily concluded whether theapplication of (Jo-x and (EJo--x' which express polarand steric effects in ester hydrolysis, is justified for thetreatment of structural effects in electroreduction sincethe number of ortho substituted compounds employedin the present investigation is very few. It is preferable

GANAPATHY & RAMANUJAM:POLAROGRAPHY OF SUBSTITUTED NITROBENZENES

--------_.0'660

0'640

o> 0·620cNci°.600

I 4 5

0·580 ."n-C:-,,-'--;;-':;;;-'-' -~';:-;;-'--:-'-:-:-.l-LI --'-----'-----'---'---1-0·1,0 - 0·30 -0·20 -0·10 0 0'10 0.20

(J

Fig. I-Plot of corrected Eli1 versus Hammett's a-constant [I,Methoxy; 2, methyl; 3, hydrogen; 4, chloro; and 5-bromo

substituents]

to use special O"o-x constants instead of O"p_x (used forortho substituents as a first approximation) so as tofollow the derivatives.

As the polar effect of a particular substituentoperates with approximately equal strength from orthoand para positions, the values of O"o-x generally are notvery different from those of ap=:«: In the polarographicreduction process where a mesomeric interactionbetween the polarographically active group Randsubstituent X takes place, the magnitude of ortho effectcan be obtained from the difference in half-wavepotentials of ortho and para substituted compounds.The non-polar ortho effect, described as 'polaro-graphic ortho shift, 110, is defined as

(E1/2)" -x --(EI12)p -x = 110 ... (4)

which means that

11" = Prr,Joo-x -- a p __) + ()n,R(E.)o- x +(M rr.,,-x - M rr.p-)

'" (5)For substituents which have no appreciablemesomeric interaction with the reaction centre theterms Prr,R(ao-x-ap_) and (M1[o-x-Mrr,p-) in Eq.(5) can be neglected. Thus 110::::::()rr R(EJ -x since a0

-x:::::: ap_x' the value of 11" is a measure of the non-polarcontribution to the ortho effect. The magnitude of 11" inthe series investigated has been determined for all thegroups. The values of corrected E1/2 and ortho shift arerecorded in Table 3.

Except for compounds bearing o-Br and a-SCH3

substituents, the values of 110 are positive for others,indicating that these ortho derivatives are reduced atmore positive potentials than their para isomers. Suchortho shifts are termed as 'positive ortho shifts'. Thelargest positive ortho shifts are observed for 0-

nitrophenol and o-nitroaniline. The negative orthoshift of o--methylmercaptonitrobenzene is due to thegreater mesomeric interaction of - SCH 3 group with

------------- -----------

Table 3-Corrected Half-Wave Potentials andPolarographic Or tho Shifts of Some Nitrobenzenes

Ortho - E 1/2 (V) ~osubsti-tuent Obs, Corr.

CH3 0.634 0.634 0.002OCH3 0_522 0.660 0.125CI 0.565 0.590 0.032Br 0.589 0.589 -0.005SCH3 0.680 -0.058SOCH3 0.516 0.027S02CH3 0.458 0_070COCH3 0.536 0.014CN 0.518 0.015CHO 0.530 0_071COOH 0.544 0.038OH 0.484 0_168NH2 0.591 0.113

the benzene ring, while that of o-bromonitrobenzene isdue to the bulkiness of bromine. It is difficult torationalise the high positive ortho shift in the case of 0-

methoxynitrobenzene.

AcknowledgementWe are deeply grateful to the CSIR, New Delhi for

the award of a senior research fellowship to one of us(M.R.)_

ReferencesI Pearson J, Trans Faraday Sac, 44 (1948) 553, 683.2 Page J E, Smith J W & Waller J G, J Phys Colloid Chern, 53(1949)

545_3 Suzuki M, J electrochem Sac (Japan). 22 (1954) 112.4 Bergmann I & James J C, Trans Faraday Sac, 48 (1952) 956_5 Kastening B, Electrochim Acta, 9 (1964) 241.6 Koopmann R & Greischer H, Ber Bunsengs, 70 (1966) 127.7 Astle M J & Cropper W P, J Am chern sac, 65 (1943) 2395_8 Reynolds G A & Hasner C R, Organic Syntheses, 30 (1950) 71.9 Beech W F, J chern Sac, (1954) 1300_

10 Oxley P, Patridge M W, Robson T D & Short W F, J chem Sac,(1946) 763_

II Bogert M T & Stull A, Organic Syntheses, Coli Vol 1(1958) 220.12 Foster D G & Reid E E, J Am chem Sac 46 (1924) 1936_13 Barbieri G, Cinquini M, Colonna S & Montanari F, J chem Soc.

(C) (1968) 659_14 Waldron W R & Reid E E, J Am chem Sac, 45 (1923) 2400_15 Ganapathy K & Ramanujam M. Z Phys Chemie, Leipzig

(Communicated).16 Koutecky J, Colin czec' chem Commun, 18 (1953) 597_17 Meites L & Israel Y, J Am chem Sac, 83 (1961) 4903.18 Einstein A, Ann Physik, 17 (1905) 549; 19 (1906) 371;

Elektrochem, 14 (1908) 235_19 Ramanujam M, Ph_D_ Thesis, Annamalai University, 1981.20 Zuman P, Substituent effects in organic polarography (Plenum

Press, New York) 1967.

1089