Indian Journal of ChemistryVol. 18A, December 1979, pp, 495-498

Polarographic Behaviour of Diacetyl Monoxime BenzohydrazoneM. A. MORSIt, A. M. A. HELMYt, & H. M. FAHMY*

Department of Chemistry, Faculty of Science, CAIRO University, Giza, Egypt, A. R. E.

Received 8 February 1979; revised 2 May 1979; accepted 18 May 1979

The polarographic behaviour of diacetyl monoxime benzohydrazone (I) has been investigated in 10 % ethanolicaqueous buffered media. A comparison with the polarographic behaviour of diacetyl monoxime shows that oxime groupin (I) is reduced first in preference to the hydrazone group.

THE polarographic behaviour of diacetyl mono-xime benzohydrazone (I) has been investigatedin different hydrogen ion concentrations in

CHaNOH. I II

ph-CO-NH-N =C~C-CHa

(I)

order to study the possible electro-reduction modesof such a molecule contai ning more than one reduciblefunctional groups.

Materials and MethodsDiacetylmonooxime benzohydrazone (I) was pre-

pared by refiuxing together 5.0 g diacetyl monoximein 80 ml of absolute dry ethanol and 5.0 g benzo-hydrazide for 4 hr. After refiuxing the mixture wasallowed to cool when faint yellow lustrous .needlesseparated out. These were filtered off, recrystallizedfrom ethanol and finally dried, m.p. 195°. Puritywas checked by TLC and elemental analysis [Found:C; 60.30; H, 6.0; N, 19.20. Calc.: C, 60.26; H,5.97; N, 19.17].

Polarographic procedure - Polarographic curveswere recorded with an LP60 polarograph (Lab ora-torni Pristroje, Prague). The capillary possessed thefollowing characteristics (in H20, open circuit). :t = 3.75sec drop-I; m=2.l5 mg sec " for h=56.5em. All the experiments were carried out at 25± 1°.

pH

The half-wave potentials were measured graphicallyand expressed (versus saturated calomel electrode)with an accuracy of ± 0.005V. 10-3 M stock solu-tions were prepared in absolute ethanol. Britton-Robinson! modified universal buffers (prepared fromAR grade chemicals) were used as supporting electro-lytes. Ethanol and the appropriate buffer solutionwere introduced into the polarographic cell. Thesolution was then deaerated by bubbling H2 gas for5 min. The calculated amount of stock solution wasthen introduced into the cell so that the final con-centration of compounds under study pn 10 ml ;of10% (by volume) ethanolic buffers was 10-4 M.

Results and DiscussionThe polarograms of 1O-4M diacetyl[monoxime

benzohydrazone (I) in 10% ethanolic Britton-Robin-son buffers are illustrated in Fig. 1. The compoundreveals two consecutive waves A and B in acid mediaofpH < 5. It is noteworthy that the limiting currentof the second wave (wave B) has a value practicallyhalf that of the wave A. Above pH 5 another ill-defined wave C is displayed; however no deep analysis

.was carried out for it. At pH .-- 8 only wave A isobserved in the polarograms which, however, de-creases in height with increase in the pH of the solu-tion in the form of a dissociation curve- as shown inFig. 2. At this stage a fourth wave (wave D) withpK' ,.....9.3appears which, in turn, increases in heighton the expense of the less negative wave A.E 1/2-pH

Fig. 1 _ Typical polarograms of 10-4 M diacetyl monoxime benzohydrozone in 10% ethanolic B. R. buffers.

-0.1-'1 vs scr

tDepartment of Chemistry, Faculty of Science, El-MansouraUniversity, Egypt, A.R.E.

I

(

495

INDIAN J. CHEM .• VOL. 18A. DECEMBER 1979

E:/2 = -0.25 - 0.062pH (1) ,H HO 4.20

CHJ

E:/2 = -0.46 - 0.070pH (2)~=oC=NOH.. CHJ~

E~/3 = -0.76 - 0.060pHci

(3)

7

Wove Ao

5

o

3

2

•I~Wove A

2 4 6 pH a 10

Fig. 2 - if, pH-dependence of polarographic waves of 10-'Mdiacetyl monoxime benzohydrazone in 10% ethanolic B. R.

buffers

plots for the waves are shown in Fig. 3. It is obviousthat all the waves are pH-dependent in El/z exceptthe wave D which is pH-independent. The shift inEll I with the increase in pH can be described for eachwave by the following equations:

Routine analyses of the waves including test ofreversibility", effect of mercury-height' and effect ofconcentration" 'showed that wave A' is an adsorptionpre-wave; the other waves are diffusion-controlledirreversible waves. The mean value of dBl/2/dpHforthe waves is -0.065, an indication that H+ ions areinvolved in the electrode process and most probablya mono-protonated form is the reducible species.G

The compound under investigation has two mainfunctional groups, oxime and hydrazone, both ofwhich are polarographically active. In order toassign the waves more precisely, and hence the actualelectro-reduction mechanistic pathways, the resultshave been supplemented by investigating anotherknown model compound diacetyl monoxime (whichcontains the oxime functional group.

The polarograms of 10-4 M solution of diacety-monoxime (Fig. 4) show that it is reduced along awell defined 4-electron irreversible diffusion controlledwave A in acid media of pH < 5. In the pH range5-9.7 two additional consecutive 2-electron wavesBand C appeared at more negative potentials whichwere assigned to the reduction of the carbonyl moietyto ~CHOH~ followed by loss of HP molecule

(

Wove 0 ---........13

rz

1.1

1.0

Wove B 0w 0.9uVl

<II>> 0.8

s:w1 0.7

Wove A

0.6

A0.5

Pre-Wove A'

2 I. 6 pH

Fig. 3 - Ells-pH dependence of the polarographic waves of10-' M diacetyl monoxime benzohydrazone in 10%ethanolic

B.R.buffers

urv •• seeFig. 4 - Typical polarograms of diacetyl monoxime in 10%

ethanolic B. R. buffers

and saturation of the -C=C~ group as illustratedby the following representative mechanistic pathways(Scheme 1).

The above mechanism is evidenced by the factthat wave-height measurements indicate that the limit-ing current of the first wave (wave A) correspondsto a 4-electron process (i1=1 fI. amp for 1O-Il Mconcentration) as compared with wave-heights ofwaves Band C which corresponded to two-electron-reduction processes. Furthermore, Ella values ofwave A are in good agreement with previouslyreported El/2 values of the oxime'. Also, the pH-independence of El/3 of waves Band C as illustratedin Fig. 5 is a further indication that the carbonylgroup is the reduced moiety at wave B together withthe position of the wave at highly negative potentials. 8

Careful inspection of the polarographic data ofthe compound indicate that the wave-height of waveA (Fig. 2) is a two-electron wave. Comparison of the

MORSI et af. : POLAROGRAPHY OF n1ACETYLMONOX1ME' BENZOHYDRAZONE

[

~H3 'J . . -'IC=O' . EaI H++4; + 3H+~

C=NOH -H2

0I '

CH .3 .We.ve A

. CHI 3C=OICHNH2ICH3

(4.)

Wave C

Schemel

(6)

EI, CH (8)2e"'+2H+ , 3

-H 0 • ph-CO-NH-N=~2 C=~H+

I 2CH3

E2- 2 + CH3+2e + H • ph-CO-NH-N=C: (9)

I +7HNH)CH3

( 10)

CH3 E~/2 CH3I + + I +C=NH 2e +2H CH.NH

3I 2 •• I + (12)CH,NH3 CH.NH

3~H3 lH

3(ill-deiined third wave C)

Scheme 2

polarograms with those of the model compound indi-cates that the most probable reduced functionalgroup is the oxime. The very small difference inE1/2 and il values of the model as compared to those

(

1.6

I.S

,..Wove 8 •• • •11. . ..

••• -s ,

\3,WUen~ 1.2

:>:_\Nw, II

1.0

09

Wove A13.8

2 4 6 8 10pH

Fig. 5 - ~1/2·pH dependence of the polarographic waves of10-< M diacetyl monoxime III 10% ethanolic B. R. buffers

of the compo'l;lndunder mvestigation is most probablydue to the differences m the molecular dimensionsdiffusion coefficients and the orientation of thes~molecules at the surface of the electrode».

Based on the foregoing, results the most probablemechanism of electro-reduction of the compoundunder investigation could be summarized as follows :(Scheme 2).

In the aforesaid scheme it is assumed that theenergy of step 8 (E1) is nearly equal to that of step9 (E2), thus no separation could be observed in thepolarogram. Consequently only one 4-electron waveA (E'~2) is-displayed (Fig. 1) . Obviously, in step 10protonation will occur on the hydrazonic moiety andhence the attack will be directed on this latter func-tional group. Reduction will then proceed normally asfor typical hydrazones via a four electron reduction

I

splitting ofthe -C= N-NH- moiety. That the limit-ing current of wave C is lower than that of a normaltwo-electron process wave is proably due to the easeof hydrolysis of compound,

+NH2 NH3

II ICHa-C-CH-CHa

in both acid and alkaline media as proved from thedecrease of t, with time, a behaviour which is verysimilar to that reported for amidines of aliphatictsand aromatic acids=. In alkaline medium this groupis not reducible since its reduction is only possiblein the protonated form while the unprotonated formis polarograpbically inactive'. In such media, wave A

497

INDIAN J. CHEM., VOL 18A, DECEMBER 1979

starts to decrease in height with the appearance of anew wave D which is an indication that the unpro-tonated form is the reducible form at such pH-values.

6. ZUMAN, P., Colln Czech. Chem. Commun., 13 (1968),2548.

7. ZUMAN, P., Topics in organic polarography (Plenum Press,London), 1970, 392.

8. ZUMAN, P. & PERRIN, C. I., Organic polarography (Inter-References science Publishers), 1969, 249. .

9. LUND, H., Acta chem. scand., 13 (1959), 249.1. BRITTON, H. T. S., Hydrogen ions, vol. 1 (Chapman andHall, London), 1955, 365. 10. ZUMAN, P. & EXNER, 0., Colln. Czech. Chem. Commun .•

30 (1965), 1832.2. ZUMAN, P., The elucidation of organic electrode processes(Academic Press, New York), 1969, 20. ..)1. ZUMAN, P., Substituent effects in organic polarography

3. KOLTHOFF,I. M. & LINGANE,J. J., Polarography (Publishing (Plenum Press, New York), 1967, 211.House of Czechoslovak, Academy of Sciences), 1965, 68. 12. KANE, P.O., Z. anal. chem., 173 (1960), 50.

4. HEYROVSKY,J. & ILKOVIC, D., Colin Czech. Chem. 13. LUND, H., Electrochemistry of the carbon-nitrogen doubleCommun., 7 (1935), 198. bond, in The chemistry of the carbon-nitrogen double

5. ZUMAN, P., The elucidation of organic electrode processes bond, edited by S. Patai (Interscience Publishers,(Academic Press, New York), 1967, 7. New York), 1970, 538.

•

498

,(