-
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 -@[email protected] 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.