II

Indian Journal of ChemistryV~I: 20A, January 1981, pp. 49-52

I. "

Polarographic Reduction of 2-.Nitro- & 4-Nitro-diphenylamines.N ,N-Dimethylformamide & Acetonitrile

In

V. JOHN KOSHYt. C. S.VENKATACaALAM & C. KALIDAS*.Department of.Chemistry, Indian Institute of Technology. Madras 600 036

Received 28 January 1980; revised and accepted 2 July 1980. 1 ' . .,.

The polarographic behaviour of 2-nitro- and 4-nitro-d,iphenylamines in N, N-dimethylformamide (DMF) andacetonitrile (AN) in the presence of tetraethylammonium 'iodide (TEAl) and sodium nitrate supporting electrolytes;has been investigated. Both the depolarizers give two waves in DMF and AN in the 'presence of TEAl, the first \va~e .-corresponding to a single electron reversible step and the second wave to an irreversible five-electron process. However,,in the presence of sodium nitrate SUpporting electrolyte, while the first wave is shown to be an irreversible one-electron.process the second wave is found to be due to an irreversible three-electron reduction. The different types of reduc-ti~n behaviour of the depolarizersin the presence of two supporting electrolytes in the same solvent hav~ been explained.

INearlier work from this laboratory, the polaro-: . graphic reduction of ~-nitr6~.i;tnd4-ni~ro-di~henyl::"

ammes at the d.m.e. in methanol-water mixtures'was reported's", However, their reduction at d.m.e. indipolar aprotic solvents has not received any attentionin spite of the interesting possibility it provides forgenerating radical anions", This prompted us tostudy the. reduction of the title' diphenylamines at'd.m.e .. in the presence of tetraethylammonium iodide(TEAl)' and sodium nitrate supporting electrolytesin dimethylformamide (DMF) and acetonitrile (AN).

Materials and MethodsDMF and AN (Pfizer) were purified according to

the methods ..described in Iiterature+". TEAl (FJuka)specified for polarographic work was used as such.Sodium nitrate (AR, BDH) was dried in an air-ovenat 80°C and stored in a desiccator. The d.m.e,used in these" studies had the following capillarycharacteristics : m = 1.743 mg. see:": and t = 6.0see in O.IM KCl solution at a mercury pressure(uncorrected for back pressure) of 53.0 cm. AUmeasurements were made using a silver wire in thegiven solvent in the presence of the particular sup-porting electrolyte as reference, but these data wereconverted to SeE reference for comparison with£1/2 data reported for other compounds.

Results and DiscussionReduction in the presence of TEAl supporting

electrolyte - Both 2-nitro- and 4-nitro-diphenyl-amines were found to give two polarographic waves'in the presence of TEAl supporting electrolyte inDMF and AN with the ratio of wave heights (first

tPresent address : Chemical Physics Group. Tata Institute ofFundamental Research. Bombay 400 005' . , . .

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wave : second wave) being 1:5. Typical polaro-grams of the compounds in AN in the presence ofO.IM TEAl are presented in Fig. 1. The first waveof these compounds in the two solvents was reversibleand diffusion-controlled on the basis of the usualcriteria, and microcoulometric experiments revealedthe participation of a single electron in the electrodeprocess (Table 1). EPR spectra of the speciesgenerated: in situ by controlled potential electrolysisat the limiting region of the first wave confirmed thepresence of the anion radicals of the compounds,

The second wave in Fig 1 (and also in DMF) wasfound to involve a diffusion-controlled irreversiblemultielectron transfer step. Microcoulometric ex-periments performed at the limiting region of thesecond wave showed that six electrons participate inthe. overall, reduction and hence by difference fiveelectrons are involved in the second reduction stage.Table 1 summarises the polarographic and micro-coulometric data on the two compounds in DMF andAN in the presence of O.IM TEAl. '

..•3-

32.0

24.0

Z 16.0w

~u

8.0

~20· "I.Q I.~ 1.80

POTENTlAL.-E(VI vs see

Fig. 1- Polarograms, of 2-nitrodiphenyliunine (0-0) and4-nitrodiphenylamine (e-e) in AN containing O.IM TEA]. . supporting electrolyte

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INDIAN J. CHEM., VOL. 20A, JANUARY 1981

TABLE 1 - POLAROGRAPHICAND MICROCOULOMETRICDATA FOR REDUCTIONOF 2-NITRO AND 4-NITRO-DIPHENYLAMINESINDMF AND AN SOLUTIONSCONTAININGTEAl

[Depolarizer cone. = 1 x 10-3M; TEAl cone. = O.IM; temp.= 30°C]

Mierocoulometrie data-Compound Wave -E· 11' -(Ea14-EI/.) -£1>.

Diphenylamine (V vs SCEr V (Vvs SCE) Potential n(V vs SCE)

SOLVENT: DMF

2-Nitro- 1 1.059 0.055 1.068 1.350 1. 012 1.571 0.070 1.635 2.000 5-99

4-Nitro- 1 1.245 0.055 1.253 1.400 0.942 1.629 0.065 1.643 2.000 5.96

SOLVENT: AN

2-Nitro- 1 1.057 0.055 1.059 1.200 1.082 1.425 0.090 1.525 1.800 5.96

4-Nitro- 1 1.177 0.080 1.179 1.320 0.992 1.485 0.065 1.505 1.800 5.99

(a) Accurate to ±2mV; (b) a. e. summit potentia! accurate to ± 4mV; and (c) accuracy ± 0.05

16·0

:i 1203.•...zUJa: 8.0a:OJu

4·0

1.10 1.30 1.50 I 70

POTENTlAL,-E(V} vs seE

Fig. 2 - Polarograms of' 2-nitrodiphenylamine (0-0) and4-nitrodiphenylamine (e-e) inDMFeontaining O.IMNaNOa

supporting electrolyte

Based on the above results, the mechanism of re-duction of these compounds in DMF and AN maybe represented as

5e-,6H+R~N02+e-~[R-NO:J ~~-_ R~NH2 .. (1)

(where R=C6H5~NH-C6H4-) with the corres-ponding amine as the final product.

Reduction in the presence of sodium nitrate GUS-

porting electrolyte - Both 2-nitro- and 4-nitro-diphenylamines gave two well-defined polarographicwaves in the ratio first wave: .second wave = 1:3,in DMF in the presence of NaN03 supporting elec-trolyte, with the first wave corresponding to a diffu-sion-controlled irreversible single electron transferby the application of the usual criteria. This wasfurther confirmed by the EPR spectra obtained on

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TABLE2 - POLAROGRAPHICAND MICROCOULOMBTRICDATAFOR REDUCTIONOF 2-NITRo- AND4-NITRo-DIPHENYLAMlNESIN

DMF SOLUTION CONTAINING NaNO.

[Depolarizer cone. = 1.075 x 10-3M; NaN03 cone. = O.IM;temp. = 30°C]

Wave -Ei/2 -(E.,,- -E~(V vs SCE) Ell,)" (V) (V vs SCE)

Microcoulometricdata

Potential(V vs SCE)

n

2-N ITRODIPHENYLAMINB

12

1.031.558

0.105 1.0400.105 1.800

1.250 1.00

4-NrnODIPHENYLAMlNB

12

1.0441.556

0.0950.190

1.0561.840

1.250

(a) Accuracy same as mentioned in Table 1.

the species generated in situ by controlled potentialelectrolysis at the limiting region of the first wave ofthe compounds, which showed the presence of anionradicals. Typical polarograms of the two com-pounds in DMF in the presence of O.lM NaNOaare shown in Fig. 2.

The second wave was found to involve diffusion-controlled irreversible multielectron transfer step.Since the limiting region of the second wave in thiscase was not well-defined, it was not possible to deter-mine the total number of electrons (n) experimentally.However this was estimated from the ratio of theheights of the first wave to the second wave. Table 2summarizes the polarographic and microcoulometric

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KOSHY et al. : POLAROGRAPIDC REDUCfION OF 2-NITRO & 4-NITRO-DIPHENYLAMINES

data on the two compounds in this solvent in the ----------------------presence of O.IM NaNOs.

The following mechanistic scheme may be suggestedto account for the above observations for the reduc-tion of the compounds in DMF in the presence ofNaNOs supporting electrolyte :

4H+,3e-R-NOz+e-?[R-N02l'- ------+ R-NHOH

.. (2)

The protons required in the second stage of thereduction in the processes (1) and (2) presumablyresult from the presence of trace amounts of waterassociated with the solvents. This view is justified inthe case of DMF as it is known to be a highly hygro-scopic solvent and it is very difficult to remove the lasttraces of water during the purification of this solvent".These arguments apply to AN solvent also? andfurther AN can supply protons" according to thefollowing equilibrium :

.. (3)

since the conjugate base of AN is a resonance stabi-lized species.

It is thus seen that the reduction behaviour of thesedepolarisers in DMF in the presence of NaNOasupporting electrolyte is different compared to thatin the presence of TEAl supporting electrolyte in thesame solvent. Such an observation involving differentreduction behaviour of a compound with a change ofsupporting electrolyte does not seem to have beenreported hitherto in literature and can be rationalisedon the basis of the greater electron accepting tendencyof NEtt cation.

Thus, the formation of quinonediimine inter-mediate- 2 is facilitated by this cation from thehydroxylamino diphenylamine formed earlier. Themechanism of reduction is shown in Scheme 1.

SCHEME 1

H Et

O~fj ~~-O-H + Et-lEt- I~···· e-,H P Et

!

Et

0""0 EJ/, N ~-H + N- Et + HI- - I'

(diimine intermediate) g Et

lI2H-.2.-

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TABLE 3 - DIFFUSION CuRRENTS FOR FIRST AND SECOND WAVESOF 2-Nrmo- AND 4-NITRO- DIPHENYLA,MrNESIN DMF AND AN

[id is given in microamps and "7) in cp]

Solvent id1 idl"7)l/1 idz ids"7)l/Z

2-NffRODIPHENYLA.MJNE

DMF 3.27 2.92 20.6 18.38AN 5.28 3.12 31.9 18.87

4- N ffRODIPHENYLAMINE

DMF 3.06 2.73 21.4 19.1AN 4.97 2.94 31.9 18.87

Once the diimine species is formed, its further re-duction to the amine occurs readily by the uptakeof two protons and two electrons. The above men-tioned type of interaction of the species R-NHOHwith sodium cation of NaN03 is not possible due toits inert gas configuration and hence the reductiondoes not proceed beyond the hydroxylamine stage inthe presence of NaN03 supporting electrolyte inDMF. It may be stated that a similar suggestioninvolving the participation of the inert electrolytein the electrode reaction was made by Wawzoneket a/.' in their study on the electroreduction of stil-bene in the presence of tetra n-butylammoniumiodide but no mechanistic details were provided bythem.

Comparison of diffusion coefficients of 2-nitro- and4-nitro-diphenylamines in DMF and AN - A com-parison of the currents for the reduction of 2-nitro-and 4-nitro-diphenylamines in DMF and AN withTEAl as supporting electrolyte shows that the currents(both at the end of the first and second waves) are ingeneral larger in AN than in DMF. The diffusioncurrents for the first and second waves of the twodepolarisers in both the solvents in the presenceof TEAl are presented in Table 3. Assuming thevalidity of Stokes-Einstein relationship'? for thediffusion coefficients of these depolarisers, it wouldbe expected that id"l)l/2will be a constant. An exa-mination of Table 3 shows that this product is fairlyconstant for each of the waves of the two depolarisersin both the solvents. It may thus be concludedthat the observed difference in diffusion currents iscaused by changes in diffusion coefficients due to thedifferences in viscosities of the two solvents and thatno specific solvation effectsll,lZ between the depo-larisers and the solvents are associated with thesechanges.

References1. VARKEY, V., VENKATACHALAM, C. S. & KALIDAS, C., Trans.

SEST, 9 (1974), 162.2. VARKEY, V., VENKATACHALA,M, C. S. & KALIDAS, C., J.

elctroanalyt. Chem., 89 (1978), 191.3. KAe"IENING, B. in Progress in polarography, Vol. ITI. edited

by P. Zuman, L. Meites & I. M. Kolthoff (Wiley-Interscience, New York), 1972, Chapter 4, 265.

4. JULLlARD, J., Pure & appl. Chem., 49 (1977), 885.5. JAYADEVAPPA, E. S., Indian J. Chem., 7 (1969), 1146.

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INDIAN J. CHEM .• VOL. 20A. JANUARY 1981

6. COVINGTON.A. K. & DICKINSON.T.. Physical chemistryof organic solvent systems (Plenum Press. New York),1973. Chapter 1.

7. LUDWIG.P .. LAYLOFF.T. P. & ADAMS.R. N .• J. Am. chem.Soc .• 86 (1964). 4568.

8. MANN. C. K. in Electroanalytical chemistry. a series ofadvances. Vol. 3. edited by A. J. Bard (Marcel Dekker.New York). 1969.

, ..,

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9. WAWZONEK.S.• BLAHA. E. W.• BERKEY,R. & RUNNER.M. E., J. electrochem, Soc., 102 (1955). 235.

10. MEIlES. L., Polarographic techniques (Interscience, NewYork), 1965. Chapter 3. 141.

H. WASA,T. & ELVING,P. J.• J. electroanalyt. Chem .• 91 (1978).249.

12. HEYROVSKY,J. & KU1'A, J., Principles of polarography(Academic Press. New York). 1966. Chapters 6 and 20.