Indian Journal of ChemistryVol. 22A, May 1983, pp. 386-389

Electroreduction of Some Azoxine S Dyes: Mechanism &Structure- Reactivity Relationship

Y M ISSA* & B E EL-ANADOULI.Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt

Received 4 October 1982; revised and accepted 3 January 1983

The polarographic reduction of some azoxine S dyes (7-arylazo-8-hydroxyquinoline-5-sulphonic acids, sodium salts) hasbeen investigated in 40% ethanolic Britton-Robinson buffers covering the pH range from 3-12. The polarograms of thesecompounds exhibit over the entire pH range two reduction waves (A) and (B). A mechanism of the electrode process is proposedand the values of E'/2 of the wave (A) for all the compounds correlate well with 0+ rather than 0 or 0° constants.

Aromatic azo dyes are important class of compoundsand have been the subject of many polarographicinvestigations.". The introduction of substituents intothe aromatic ring completely alters the nature of theelectrode reaction". Heterocyclic azo compounds haverecently found wide use in spectrophotometric analysisand can now be regarded as important class ofchromogenic reagents". The polarographic behaviourof heterocyclic azo compounds:'. has received littleattention and no information is available on themechanism of reduction of these compounds at d.m .e.

The present investigation is devoted to study themechanism of reduction of some azoxine S dyes, viz (7-arylazo-8-hydroxyquinoline-S-sulphonic acids) atd.m.e. The substituent effect on the half-wavepotential is also investigated.

Materials and MethodsPolarograms were recorded at 2SoC using a pen

recording polarograph LP 60 (Laboratorni Pristroje,Prague). The UV spectra were recorded on aUNICAM SP 17S0 recording spectrophotometer. Thecapillary used had the following characteristics inoxygen: free 0.1 M KCI in open circuit; m = 1.8 mg/sand t =3.7 s at a mercury height of60 cm. The cell usedfor polarographic reduction was specially designed tohold IS ml of solution and was provided with a sinteredglass disc of porosity G4 sealed in its wall near thebottom. The cell was fitted into a calomel electrodewhose surface area was about 12 ern". The cell used forcoulometric studies was similar to that described byZuman". The pH values of the solutions weremeasured with the aid of a Prolabo pH-meter.

The azo dyes studied were prepared by a methodsimilar to that described by Uusitalo '. The stocksolutions (0.00 I M) of these compounds were preparedin absolute ethyl alcohol. The Britton-Robinsonbuffers were prepared as described before". All thechemicals were of AR grade.

386

Ethanol (4.S ml), the appropriate buffer solution (7.8ml), gelatin solution 0.S%(0.3 ml) and stock solution ofthe dye (I m!) were mixed in the polarographic cell,deoxygenated with purified nitrogen gas for IS min,the gas allowed to pass over the surface of the solutionin the cell and the polarogram recorded.

Results and DiscussionThe polarographic reduction of 7-arylazo-8-

hydroxyquinoline-S-sulphonic acids (sodium salts) (I)was studied at d.m.e. in 40% ethanolic Britton-Robinson buffers covering the pH range 3-12.

The polarograms of Ia as a representative exampleare shown in Fig. I. It is found that the compounds arereduced in two steps giving two reduction waves (A)and (B). At about pH 9 the limiting current of wave (A)starts to decrease as the pH increases and a definiteminimum appears at a potential slightly more negativethan that of wave (A). The minimum is not suppressedby the addition of excess amount of gelatin. Theminimum is not observed with compounds havingmore or less identical structures but devoid of S03Hgroup 7. Also with increasing the dipolarizerconcentration the minimum tends to be larger and itspotential range is widened. It is clear that the half-wavepotentials for waves (A) and (B) are pH dependent, theyshift to more negative potentials with increasing pH,thus indicating that the reduction process for waves (A)and (B) involve hydrogen ions in the rate-determiningstep.

The plots of log id versus log h (h = mercury height)are linear with slopes amounting to O.S for waves (A)and (B) indicating that the electrode process for suchsteps is mainly diffusion controlled. Logarithmicanalyses of the two waves indicate that the electrodeprocesses are irreversible. The plots of limiting currentversus the concentration of the depolarizer for all dyesare linear passing through the origin indicating thevalidity of Ilkovic equation.

iSSA & EL-ANADOULI: ELECTRO REDUCTION OF AZOXiNE S DYES

Fig. I-Dependence of polarographic waves of ia (2 x 10-4 M) onpH in 40% ethanolic Britton-Robinson buffers, [Curves starting at

O.OV]

Mechanism of reduction oj' dye molecules-Forunderstanding the course of the electrode process,controlled potential coulometry (cpc) was carried outwith Ia as a representative compound at pH lOon twosteps, the potential of each step being constant at theplateau of the waves (A) and (B). The absorptionspectrum of Ia before reduction exhibits twoabsorption bands at 480 and 320 nm, the first one isassigned to n-it" electronic transition within the azogroup influenced by intramolecular charge transferand the second to n-tt" transition within the quinolinemoiety. In the absorption spectrum of the productafter cpc at -0.8V, which is the potential correspond-ing to wave (A) at pH 10, the 480 nm band vanishescompletely indicating the reduction of the azo group.The 320' nm band is shifted to 295 nm due to lack ofconjugation in the molecule. When the cpc is carriedout at -1.7 V which is the potential corresponding tothe wave (B), the 295 nm shifts to lower wavelength dueto reduction of the quinoline ring at this step":".Furthermore, the results of coulometric measurementson wave (A) show that the number of electronstransferred in the electrode process is approximatelyfour. Finally aniline was detected among the productsof cpc at potential corresponding to wave (A) by spottest 10.

The data presented above permit some generalconclusions to be drawn regarding the course of theelectrode processes in the reduction of azo compoundsstudied. The observed linear dependence of half-wavepotential of wave (A) with pH suggests theparticipation of hydrogen ions in the electrode reactionand the azo linkage is reduced in the protonated formto two molecules of amines in acidic and alkaline

media in accordance with the following overallequation II:

Finally, the reduction of the quinoline moiety proceedsvia wave (B) by the uptake of two electrons to give thedihydro compound as previously reported 12 -17. Inlight of the given information, Scheme I can besuggested for the reduction process.

If it is assumed that the potentials for the steps (2)and (5) are more or less the same, i.e. EA ~ Ec, then onlyone four-electron wave should be observable. Theappearance of the minimum which is not suppressedby the addition of excess amount of gelatin may beattributed to the adsorption of the sulphonic group onthe surface of the mercury dropI8.19.

Re-duction of -N:N - groupH Ar

W/(

< "'" NI

S03Na( a)

(b) -+ 2~-

/Ar

OH 'NH

W""'N~ IQ

(b) S03Na

Ar

OH NH'

f"NA~-vy (2)

S03Na(c) Ar

o&t ......("(d) S03Na

(1)

\ c) + H'fast--

( d)

387

INDIAN J. CHEM .. VOL. 22A. MAY 1983

0-

(¢j",,-NH •.~

"" EC

S03Na

(g)

•••••• (5)«JNH2

503 Na

( h)

Rf'duction of thf' quinotine moif'ty:

( h)~~A-NH2~ ••••• (6)

S03Na

(i)

H OHI .cqNH

2 ••••• (7)

( j) S03Na

Schl'ml' ,

Table l=-Values of (J Constants and Half-wave Potentials ofAzoxine S Dyes at Different pH Values

Subs' /1+ /1 /10 -E'2atpH

7 8 9

P-OCH3 -0.78 -0.27 -0.12 0.43 0.51 0.59P-CH3 -0.31 -0.17 -0.15 0.39 0.47 0.55m-CH3 -0.07 -0.07 -0.07 0.38 0.43 0.52H 0.00 0.00 0.00 0.37 0.44 0.51p-I 0.13 0.18 0.27 0.37 0.44 0.51m-CI 0.40 0.37 0.37 0.35 0.41 0.50

O/H'NO

~N~~ X

~SOJNa

( I)

WhereX=H (a).p-OCH3 (b).P-C~ (c).m-C~(d).P-I(l')and m-CI(f)

Structure-Reactirity relationship-It is found thatthe values of ana for all compounds studied are more orless constant with deviations of ± 0.1 indicating thatall the compounds are more or less reduced at d.m.e. byan identical mechanism.

The influence of the substituents on wave (A) for] isin accordance with expectation, i.e. p-methoxyderivative (I b) exhibits the most negative value of E1/2

whereas the m-C1 derivative (If) exhibits the mostpositive one. Figure 2 illustrates the correlation of thehalf-wave potentials for I at different pH values withthe three sigma constants a, a+ and aO as taken fromthe tabulation by Ritchie and Sager+". These values arecollected with E I 2 at pH values 7, 8 and 9 in Table I.The data in Table I have been treated statisticallyusing the least-squares method according to Jaffe 2 I ,

and the calculated parameters are given in Table 2. Thepositive value of the reaction constant (p) shows thatthe substrate with an electron withdrawing substituentis reduced at less negative potential than is the parentcompound. It is obvious from Table 2 that the half-

Table 2-Statistical Treatment of the Results of Azoxine S Dyes

Parameter /1 at pH /1+ at pH /10 at pH

7 8 9 7 8 9 7 8 9

-E;'2 0.382 0.451 0.631 0.374 0.441 0.521 0.383 0.455 0.535{J 0.102 0.126 0.122 0.066 0.084 0.082 0.091 0.111 0.103r 0.884 0.841 0.840 0.991 0.966 0.970 0.722 0.687 0.702±s 0.014 0.021 0.021 0.0004 0.010 0.009 0.021 0.029 0.028

N».wI

o pH0.6~ .

• 0~9 0

--. --------0.50 .-.~.

~-o- -0- 70.4 o~

o

o

~o-'~ .-

-O~o

- 0.,2-0..1 0. 0..1 0..2 0..3

o0.8 -0.6 -0..4-0..2 0. 0..2

()+0.2 0.1 0 0..1 0.2 0..3

c:PFig. 2-Dependence of E, 2 on different /1 constants

388

ISSA & EL-ANADOULI: ELECTROREDUCTION OF AZOXINE S DYES

wa ve potential with (J or (Jo gives poor correlationindicating that the conjugation of the substituents withthe azo group produces considerable effect, that is whythe best correlation was found with (J+.

ReferencesI Florence T M, Aust J Chern, 18 (1965) 609, 619 and references

cited therein.2 Marcznko M, Spectrophotometric determination 0/ elements

(John Wiley, New York) 1976,45.3 Florence T M, Johnson D M & Batley G E, J electroanal Chern.

50 (1974) 113.4 Monousek 0 & Zuman P, Colin Czech chem Commun, 29 (1964)

17.5 Uusitalo E, Ann Acad Sci Fennicate Ser A, II (1957) 87.6 Britton H T S, Hydrogen ions, Vol I (Chapman & Hall, London)

1955, 364.7 Issa Y M & EI-anadouli B E, Indian J Chern, 21 A (1982) 698.8 Khalifa H, Issa Y M, Badawy S S & Kenawi I M, Egypt J Chern,

22 (1979) 355.

9 Issa I M, Issa R M, Temerk Y M & Mahmoud M R. ElectrochemActa, 18 (1973) 265.

10 Fiegl F, Spot test, Vol 2 (Elsevier. London) 1954,377.

II Patai S, The chemistry ofthe hydra:o, a:o and a=oxy groups (JohnWiley, New York) 1975,476.

12 Stock J T & Williams T R, J chem Soc. (1960) 3221.

13 Stock J T. J chem Soc. (1949) 686.14 Stock J T, 1st international polarographic congress. Prague,

(1951) 371.15 Turner W R & Stock J T. J chem Soc. (1956) 1506.

16 Kolthoff 1 M & Lingane J J, Polarography (Interscience. NewYork) 1952.819.

17 Zuman P. The elucidation oforganic electrode process (AcademicPress, New York) 1969.

18 Gupta S L. Proc Indian Acad Sci. A47 (1958) 254.

19 Parny J M & Parsons R, Trans Faraday Soc, 59 (1963) 241.20 Ritchie C D & Sager W F. Progress in physical organic chemistry,

Vol 2 (Intcrscience, New York) 1964.334-336.21 Jaffe H H. Chern Rei. 53 (1953) 1916.

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