Indian Journal of ChemistryVol. 19A, October 1980, pp. 956-960

Polarographic Investigations on (1, s-Llnsaturated Ketones1-(4'-Fluorophenyl)-3-phenyl-2-propenones

SARVAGYA S. KATlYAR* & M. LALITHAMBIKADepartment of Chemistry, Indian Institute of Technology. Kanpur 208016

Received 15 November 1979; accepted 30 January 1980

The reduction of various substituted 1-(4'-ftuorophenyl)-3-phenyl-l-propenones at the dropping mercury electrodehas been examined. In 50 % ethanol-water buffered media ftuorochalkones give two weD-defined waves in acidmedia and three waves in neutral and alkaline media. The one-electron electroreduction in the first two stepsresults in the formation of dihydrochalkone, which subsequently gets further reduced to the secondary alcohol. Alarge variety of substituents on the benzylidene moiety of t-( 4' -fluorophenyl)-3-phenyl-2-propenone, affect the half-wavepotentials according to the relation E111 = a[pH] + b. Substituents with a positive a value render the re-duction easier and shift the Ell. to more positive values, whereas groups with negative a values make the electro-reduction more difficult, resulting in more negative Ell. compared to the parent compound. A comparison of theE111 values of 0- and p-chloro substituted t-(4' -fluorophenyl)-3-phenyl-2-propenones shows that o-substituted derivativeundergoes reduction at a more positive potential indicating a positive ortho effect for this system.

THE polarographic investigation on (1, (3-un-saturated ketones (chalkones) has been asubject of much interest from the view pointof the mechanism of reduction at the d.m.e. Mostof the workerst+ have accepted the observation thatthe ethylenic group is reduced earlier than the car-bonyl. Studies on pH dependence of wave-heightand half-wave potential of the more negative waveof phenyl vinyl ketone established that saturatedketone resulted by the acceptance of the first twoelectrons. The results of our recent investigations'?on substituted 1-(2-thenyl)- 3-phenyl-2-propenonessupport Zuman'sll,12 observations that the firstelectron is accepted by the ethyIenic carbon ratherthan by carbonyl carbon as suggested by Bezuglyiand coworkerst+w. Moreover, (1,,a-unsaturatedketones are interesting as they provide a systemwhich is unsymmetrical with respect to the electro-active groups. Hence the study of the effect ofsubstituents in the aldehyde and ketone part ofthe molecule should give different values of p(Ham-mett). Existing literature shows lack of studies ofsubstituent effects on half-wave potentials of(I(,(3-unsaturated ketones especially when the substi-tuent is on the aldehyde moiety. In the presentinvestigation the polarographic reduction of anumber of fluorochalkones with substituents onaldehyde moiety has been investigated with a viewto studying the effect of substituents on the half-wave potentials.

Materials and MethodsVarious 1- (4' -fluorophenyl)-3-phenyl-2-propenones

used were synthesized by Claisen-Schmidt con den-sation" of substituted benzaldehydes with p-fluoro-acetophenone in the presence of KOH. The buffersolutions were prepared as described elsewhere'".All the polarographic measurements were made in

956

50 % (v/v) aqueous ethanol at a constant ionicstrength (0.20).

The polarograms were recorded at 25° ± 0.1°C)using a Sargent model XXI polarograph. pHmeasurements were made on an Elico pH meter.The capillary characteristics were: m= 1.8 mg sec-',t = 3.8 see at a mercury reservoir height of 65 cm(corrected for the back pressure).

Results and Discussion1.(4-' -Fluorophenyl)-3-phenyl-2-propenone has

been studied at the d.m.e. in various pH ranges.As our prime interest was to investigate influenceof various substituents on the half-wave poten-tials the behaviour of all the other fluorochalkoneswas investigated in the acidic pH range only.

The current-voltage curves of 1-(4'-fluorophenyI)-3-phenyl-2-propenone in buffers of pH 3.6, 9.9 and10.7 in 50 % ethanol-water are given in Fig. 1.The two waves obtained in acid medium are quitewell-defined and well-separated. The first andsecond wave almost coalesce at pH 9.9 and complete-ly merge beyond this pH value but the third reduc-tion wave makes its appearance in neutral and alka-line media. The polarographic characteristics ofthese waves in buffers are reported in Table 1. Itmay be noted that the half-wave potentials shift tomore and more negative values as the pH of themedium increases (Fig. 2). The shape of the curvesfor waves 1 and 2 is found to be almost similar to1,3-diphenylpropenonell and other heteroanaloguesof chalkone'>. In this case the d(ElI2)I/dpH andd(ElIJJdpH in the pH ranges 0-6.5 and 0-3.0are 83 and 70 mY/pH respectively. The secondwave half-wave potential is found to be constantabove pH 3. Because of the pH independent natureof the second wave above pH 3 and pH dependentnature of the first wave up to pH 8, there appears a

----.-,--. -- -- --- .-_~_-

KATIYAR & LALITHAMBIKA : POLAROGRAPHIC BEHAVIOUR OF (X,~-UNSATURATED KETONES

TABLE 1 - POLAROGRAPHICCHARACTERISTICSOF 1-(4'-FLUOROPHENYL)-3-PHENYL-2-PROPENONE

[Concentration = 3.21 X 10-'M; temp. = 25°C]

pH -(Elf2) , -(E,/2). -(E" 2)3 Id, Id. Ida n, no n.V V V

0.81 0.510 0.900 0.76 0.72 1.0 0.881.42 0.575 0.945 0.80 0.82 0.41 0.902.63 0.645 1.050 0.84 0.84 1.20 0.903.60 0.735 1.155 0.80 0.80 0.67 0.614.01 0.795 1.200 0.82 0.82 0.64 1.0

4.91 0.84 1.200 0.78 0.80 0.90 1.265.62 0.785 1.21 0.84 0.82 1.07 0.888.0 1.042 1.215 0.84 0.68 0.83 0.829.9 1.15 1.68 1.60 0.84 0.9 0.83 0.81

10.7 1.20 1.71 1.60 0.84 1.4 1.1 0.9

u,..~D-

Eogi

I0·4 Microamptre~

-0.6

Fig. 1 - Polarograms of 1-(4'-fiu0rophenyl)-3-phenyl-2-pro-penone in buffers of pH 3.6, 9.9 and 10.7 containing 50~oethanol [(1) pH 3.6; (2) pH 9.9, and (3) pH 10.7. Polaro-

gram at the bottom is for the buffer blank].

1.7.------------.1.5

N;:,\l.l10.9

1WavfZ

/2 4 6 8 10 12

pH

0.7

0.5

0.3o

Fig. 2 - Variation of half-wave potentials of 1-{4'-fluoro-phenyll-Sephenyl-z-propenone with pH.

definite tendency of these two waves to coalesce.Above pH 8 they are two close to be separated andat pH 10.7 first wave is a composite wave of thetwo waves that appeared in acid medium. This issubstantiated by the diffusion currents of the firsttwo waves in acid and alkali media. It is evidentfrom Table 1 that the ratio of i1/i2 is almost thesame in acid media, whereas at higher pH thecomposite wave has the height equal to the sumof the heights of the first two waves.

The plot of log (i/id-i) versus E is linear for thefirst two waves. The values of n, the number ofelectrons involved in the electrode process, corres-ponds to 1 for both of these steps, as shown inTable 1. The effect of temperature and concentra-tion on the polarographic waves indicates thediffusion-controlled nature of the waves in acidmedium. Polarograms of 1-(4' -fluorophenylj-Lphe-nyl-2-propenone have also been recorded at varioustemperatures between 15° and 35°C. Temperaturecoefficient of waves 1 and 2 corresponds to 1.5-2 %,which is in agreement with the diffusion-controlledprocess, whereas for the third wave it averages toabout 5 % and indicates kinetic control. The effectof concentration of 1-(4'-fluorophenyl)-3-phenyl-2-propenone has been investigated in the concentra-tion range 0.5-2.0 X 10-' M. Both the waves arelinearly dependent upon the concentration, suggestingthat waves are either diffusion- or kinetic-controlled,but rules out the possibility of catalytic or adsorptionwaves. The variation of diffusion current with theheight of mercury head is shown in Fig. 3. It canbe seen that the variation of i« with .[fi is linear;but the linear plot does not pass through the origin.This finding may be interpreted to indicate thatthere is some probable kinetic contribution to thewaves and they are not totally diffusion-controlled.

Mechanism of the electrode process - Like styryla.-thenyl ketones'>, fluorochalkones give two polaro-graphic reduction waves involving two electronseach, which may be further broken in three or fourwaves in some pH ranges. All the fluorochalkonesbehave in a similar fashion. Up to pH 6 the gene-ration of the two one-electron waves corresponds tothe reduction of ethylenic bond as shown in Scheme 1.

957

INDIAN J. CHEM., VOL. 19A, OCTOBER 1980

1.7

1.1

1.0

~ 0.9

u50.8-.;:J

~ 0.7

0.6

0.5

6 & 9(f\

10 II

Fig. 3 - Variation of diffusion current with the Vb for thewaves of 1-(4'-fiuorophenyl)-3-phenyl-2-propenone.

The addition of the first electron to ethylenic linkagein acid medium has also been suggested by Zumanand coworkers". This is proved by the reductionof unsubstituted and substituted chalkones. Polaro-graphic reduction of 1-(4' -fluorophenyl)-3-phenyl-2-propenone and 1,3-diphenyl-2-propenone in neutraland alkaline medium afford identical half-wavepotentials. This shows the occurrence of reductionat the electro active center far from fluoro substi-tuent so that it does not feel the influence of thesubstituent. However, when acetophenone and p-fluroacetophenone are polarographed under identicalconditions it is observed that the half-wave potentialof p-fluoroacetophenone becomes more positive,indicating that the electron addition to the carbonylgroup is affected by the fluoro substituent.

Electrolysis experiments on 1,3-diphenyl-2-pro-pen one (chalkone) carried out by Zuman et al.11demonstrated that the first electron was accepted bythe ethylenic bond. On the other hand Lavrushinand coworkers'> maintained that the first electronaddition takes place on the carbonyl group resultingin a free radical which electromerized to a new freeradical of the enolic form of the ketone which wasfurther reduced to a saturated ketone or got dimeriz-ed. If this is true the existence of dimers should beindicated in the polarographed solution. This hasbeen proved as follows : The controlled potentialelectrolysis of 1-(4-fluorophenyl)-3-phenyl-2-pro-penone is carried out over 40 hr on the d.m.e. at thepotential of limiting current of wave-I, in the hy-

F -@-C-CH =CH~ liJg X~ F -@-C-CHz-CH-@....B x

drochloric acid-acetate buffer of pH 5.0. When theelectro lysed solution is transferred to the buffers ofhigher pH (8-13) and examined at the d.m.e., no-wave.o.f the reduction product is detected. Only therernammg chalkone waves are found. This suggests~he absence of the following dimers(I) and (II), sinceIf these

oIIc.rr, C CHI CHR

ICoHo C CHI CHR

IIo

J II

were to form in solution, a reduction wave similarto that of benzoyl group would have been observed.

A portion of the electrolysed solution is added to'a periodate solu~ion of kn~wn concentration at pH 5,and the change In wave height of periodate with timeis recorde

KATIYAR & LALlTHAMBlKA : POLAROGRAPHIC BEHAVIOUR OF «,~-UNSATURATED KETONES

(ivl

Ell2 versus pH plots. The data in Table 3 show thatthe values of a are comparatively constant, showingthe similarity in the mechanism of electroreductionof these substituted compounds. This observationmakes it possible to correlate the half-wavepotentialswith the substituent constant a. As expected,it is seen from Table 2 that those groups with apositive a value render the reduction easier andshift the half-wave potentials to more positive values,whereas the groups with negative a values makethe reduction more difficult compared to the reduc-tion of the corresponding parent compounds re-sulting in more negative values for E1I2•

The correlations between the first half-wave poten-tials of fluorochalkones with Hammett a parameters

· are shown in Fig. 4. For substituted chalkones ofthis series E1I2 versus a plots have been obtained attwo pH values (0.84 and 2.65). It is observed from

· these plots that 4-CI, 4-0CHa, 4-0CHa-3-0H, and4-0H-3-0CHa substituents obey Hammett equationexcellently and other groups deviate from the reg-ression line.

A comparison of the half-wave potentials of 0-and p-chloro substituted 1-(4'-fluorophenyl)-3-phenyl-2-propenone shows thatortho substituted derivativeundergoes reduction at a more positive potential

'pointing out a positive ortho shift for this system.Quantitatively El/2(0.01)- El/2(1'-01) = .60 = + 0.015,0.012 and O.oI5 atpH 0.84,2.65 and 3.32 respectively.

, ;-OCH3.3-0H pH: ].65

4-0CH34-0H,3-0CH3 °3•4-d;OCH3

.·CH30.9Ho

w 0.8'-' J ,L-diOCH3

o 4'OCH3 3-0H2 ,/t- OICCH3.5 Br

o 0o l_':.~" 0

213-djC"'~"j . H0.54-CI

2-CP

C. t. L.--'-------="::---1.0 -0.5 o 0.5

Fig. 4 - Correlation between the first half-wave potentials offluorochalkones with Hammett a.

It is clear from the .60 values of chloro substituentthat these are pH dependent. The half-wave poten-tials of 4-0CHa, 2,3-diOCHaand 3,4-di-OCHasubsti-tuted derivatives of the series are -0.575, -0.515and ""-o.545Vrespectively. If the system experiencesthe cumulative effects of the substituents the half-wave potential should have increased in the orderof increasing a values. The a values of 4-0CHa,3,4-di-OCHa and 2,3-diOCHa groups are -0.26,-0.16 and -0.28 respectively. But it is observed thatthe half-wave potentials are in the order 4-0CHa >3,4-diOCHa > 2,3-diOCHa which is inconsistentwith the values obtained by additivity principle.Also for 4-0H-3-0CHa and 4-0CHa-3-0H substi-tuted compounds of series 2 it is seen that the addi-

TABLE 2 - HALF-WAVE POTENTIALS OF SUBSTITUTED 1-(4'-FLUOROPHENYL)-3-PHENYL-2-PROPENONES

-El/s(vs S. C. E.) at pH Corr. E1I3t at pHSubstituent" 0.84 2.65 3.32 0.84 2.65 3.32

H 0.515 0.632 0.682 0.520 . 0.628 0,682· 4-CI 0.485 0.616 0.675 0.473 0.6182-CI. 0.470 0.602 0.660 0.479 0.593 0.654

· 4-CH3 0.522 0.660 0.720 0.555 0.659 0.7274-0CH3 0,575 0.700 0.750 0.575 0.694 0.7454-N(CH,). 0.537 0.715 0.7654-NHCOCH3 0.560 0.685 0.7352-0H 0.582 0.700 0.7574-0H-3-0CH3 0.567 0.722 0.742 0.560 0.721 0.7454-0CH3-3-OH 0.575 0.715 0.757 0.5752,3-diOCH3 0.515 0.745 0.7053,4-diOCH, 0.545 0.655 0.727 0.553 0.657 0.7252,4-0CH3-5-Br 0.530 0.655 0.705

*Substituents are on the benzylidine part of the molecule.tE1/s values are corrected by least squares fit.

959

INDIAN J. CHEM., VOL. 19A, OCTOBER 1980

TABLE3 - VALUESOF a AND b OFEQUATION-ElI.=a[pH}-+-bIN THEpH RANGE0.8-5.0

SI Substituent" a bNo.

1. H 0.066 0.4652. 4-Cl 0.071 0.4303. 2-CI 0.073 0.4004. 4-CH. 0.071 0.4705. 4-0CR. 0.074 0.4306. 4-N(CH3). 0.091 0.4607. 4-NHCOCH3 0.069 0.4908. 2-OH 0.078 0.5109. 4-0H-3-0CH3 0.076 0.500

10. 3.4-diOCH3 0.069 0.48011. 2,3-diOCH3 0.073 0.53012. 3-0H-4-0CH. 0.066 0.52513. 2,4-0CH3-6-Br 0.067 0.540

*Substituents are on the benzylidene part of 1-(4'-fluoro-phenyl)-3-phenyl-2-propenone.

tivity rule is flouted. The values of t1 of the abovegroups are -0.26 and -0.15 respectively, but the4-0H-3-0CH3 substituted compound is found toget reduced at a more positive potential (-0.567 V).The positive ortho shifts and non-additivity of sub-stituents could be due to the steric effects caused bythe substituents. This phenomenon is observed invarious other systems also18,19.

The values of p, characteristic of the reactionseries, obtained from the Ell 2. versus t1 plots at differentpH are given below :

p values (V) at pHSeries

2.23

0.175

3.32

0.2002

0.00

0.250

0.77 0.81

0.225

The p values obtained in this series in acid mediumare quite comparable to the value of p for stilbenein which there is no carbonyl group (+ 0.22 V).It could be seen that the values change with pH ofthe medium. The change in the value with the pHof the medium is a common observation for othersystems also2o,21.

Bezuglyi's experiments on the effects of substi-tuents on acetophenone and benzylidine part of themolecule showed that the p values were differentfor reduction of carbonyl and ethylenic bond, thevalues being 0.50 and 0.20 V, respectively at pH 7.Yet in another Ct, ~-unsaturated ketone (I-methyl-3-phenyl-2-propenone, CH3 COCH = CHCeH5)the pvalues for the first reduction wave in buffered mediaof pH 8.5 and 10.5 corresponded to + 0.20 and+ 0.28 respectively. In the case of substituted

960

1,2-diphenylethylene (C6HuCH = CHC6Hu) in 75 %dioxan-water medium containing 0.1 MN (CaH5)41the El/2 versus t1 plot gave a p-value + 0.25, com-patible with the corresponding p-value of stilbene.The latter value corresponding to the reduction of-CH = CH- is numerically the same as that ob-tained in the present investigation. The close simi-larity of the p values for the first reduction wavewith those of stilbene and 1,2-diphenylethylene anda comparison of the p values of benzophenone(+ 0.37) and the 1-(4'-fluorophenyl)-3-phenyl-2-propenones establishes that the reduction of ethylenicbond precedes the reduction of the carbonyl.

AcknowledgementThe authors gratefully acknowledge the help

rendered by Dr D. N. Dhar of our Department, inthe synthesis of chalkones and for a generous giftof some fluorochalkones.

References1. PASTERNAK,R., Helv. chim. Acta, 31 (1948), 753; Chem.

Abstr., 42, 5354d.2. GEISSMAN,T. A. & FRIESS, S. L., J. Am. chem. Soc., 71

(1949), 3893.3. PREVEST,C. H., SONCHARY,P. & CHANVELIER,J., Bull.

Soc. chim. Fr., 18 (1951), 715.4. COULSON,D. M. & CROWELL,W. R., J. Am. chem, Soc.,

74 (1952), 1294.5. CASSIDY,J. E. & WHITCHER, W. J., J. phys, Chem., 63

(1959), 1824.6. JOHNSON,C. W., OVERBERGER,C. G. & SEAGERS,W. J.,

J. Am. chem. Soc., 75(1953), 1495.7. TIROUFLET,J. & CoRVAISIER,A., Bull. Soc. chim. Fr.,

535 (1962).8. SIMONET,J. & ALBISSON,A., Bull. Soc. chim. Fr., 1125

(1971),9. ZUMAN,P. & MICHL, J., Nature, 192 (1961), 655.

10. KATIYAR, S. S., LALITHAMBIKA,M. & JOSHI, G. C., J.electroanal. Chem., 53 (1974),439.

11. RYVOLOVA-KEJHAROVA,A. & ZUMAN, P., J. electroanal.chem., 211969),197.

12. BARNES,D. & ZUMAN,P., J. electroanal. Chem., 16 (1968),575.

13. LAVRUSHIN,V. F., BEZUGLYI, V. D. & BELOUS,G. G.,Zh. Obshch. Khim., 33 (1963), 1711.

14. LAVRUSHIN,V. F., BEZUGLYI, V. D. & BELOUS, G. G.,Zh. Obshch. Khim., 34 (1964), 13.

15. BELOUS,G. G., LAVRUSHIN,V. F. & BEZUGLYI,V. D., Zh.Obshch. Khim., 37 (1967), 2169.

16. GILMAN, H. & BLATT, A. H., Organic syntheses, CoIl.Vol. I (John Wiley and Sons, New York), 1961, 78.

17. LALITHAMBIKA,M., DEVAPRABHAKARA,D. & KATIYAR,S. S., J. electroanal. Chem., 31 (1971), 219.

18. FIELDS, M., VALLE, C. & KANE, M., J. Am. chem. Soc.,71 (1949), 421.

19. ZUMAN,P. & VOADEN,D. J .• Tetrahedron, 16 (1961), 130.20. MARUYAMA,M. & FURUYA, T., Bull. chem. Soc. Japan,

30 (1957), 657.21. KOZENY,M. & VELICH,V., Colin Czech. Chem. Commun.,

25 (1960), 1031.