, ' Indian Journalof Chemistry Vol. 23A, September 1984, pp. 732-735 I, Voltammetric Studies of 3-Bromo-4-hydroxYbenzophtmone A SIVAKUMAR & S JAYARAMA REDDY· Department of Chemistry, Sri Venkateswara University, Tirupati 517502 ~-. " ' " ", . . -: ~. Received 7 Febru't!rYi984; reviS!!d;anti izc&;ted 27 --JH:iI;19S4. ,) ;. Electrochemical kgucti~n Of'3'\J~omoA-~y~~b~be',lz?Phen~~ 1~s ~ri car;'i~d'oU~i~ jq~e~~ ~rt:d'50~ a~.ethanolic • media in the pi'esenceof different suppottlng 'electrolytes --using' cyclic' voltammetry, chronoamJx*dmetty and' chronopotentiometry at hanging mercury drop electrode. The reduction process is found to be irreversible leading to the formation of diphenylcarbinol in all the supporting electrolytes except in 0.1 M HCI in 50~~aq ethanol where pinacol formation is observed. Diffusion coefficients, (In. values and heterogeneous rate constants evaluated using different techniques are compared. The electrochemical reduction of halo substituted benzophenones is complicated because of the reductive cleavage of the carbon-halogen bond. For example Nadjo and Saveant 1 and M'Halla 2 identified three waves in the reduction of chloro- and bromo- benzophenones using cyclic voltammetry. It was concluded that the first wave which was observed at low sweep rate and preceded the carbonyl reduction waves was due to the cleavage of carbon-halogen bond. These facts and our interest in the electroreduction of aromatic carbonyl com- pounds ' -7, led us to carry out the title investigation using cyclic voltammetry, chronoamperometry and chronopotentiometry at hanging mercury 'prop electrode (HMDE), The data have been utilised to evaluate «n, values, diffusion coefficients and heterogeneous rate constants. l Materials and Methods . . 3-Bromo-4-hfdn?xybenlO~~IJOne:~ ~*s. ~JJr~$~ed according to the'literatureprocedure". Ttie.:s()luotion,of t the e1ectroactive species was prepared by dissolving it in minimum amount of ethanol and then making up . with the supporting electrolyte to the desired concentration, The supporting electrolytes were prepared using AR reagents in doubly distilled' water. Phosphate buffers of pH 6.85 and 1\.7, and 'bOrate. buffer of pH 9.2 were used as supporting electrolytes for studies in aqueous media while 0.1 M He!, 0.1 M KCI and 0.1 M NaOH prepared in 50":, ethanol were the supporting electrolytes employed' for' studies' in non-aqueous media. The electrode assembly and the experimental procedure employed were the same as given in our earlier cornrnunications'. 0. All the '. experiments were carried out at 24 ± 0.1 .C. Results and Discussion Studies in aqueous media The cyclic voltammograms of 3-bromo-4-hydroxy- benzophenone indicate that the compound undergoes 732 - irreversible reduction in all the supporting electrolytes employed as can be seen from the absence of peak in the reverse scan and variation of peak potentials. Ep. with voltage scan rate, 11 [For example Ep varied from - 1.46V to -1.54V (vs SeE) for a change in 1 from 0.01 Vs -1 to 0.10 Vs -1]. Typical cyclic voltam- mogram of the compound in phosphate buffer of pH 11.7 is shown in Fig. I. In all the supporting electrolytes employed, only one peak is observed unlike in polarography where two waves are observed corresponding to the reductive cleavage of C -Br bond followed by the reduction of carbonyl group 7. Thus. the single peak in cyclic voltammetry includes both the processes. Merger of the two peaks may be due to higher sweep rates employed in cyclic voltammetry '(also see refs. I and 2). , The plots of ip (peak current) versus V 1 / 2 in all the ' supporting electrolytes are linear and pass through the' O·IY , ~O.5IlA . '( ;~ z t- Z ••• ~ o VOLTAGE (vs.SCE) -I·OOV Fig. I-Typical cyclic voltammogram of 3-bromo-4-hydroxy- benzophenone in phosphate buffer of pH 11.7 at [depolarizer] =0.741 mM and sweep rate =0.04 vs :'
Transcript
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Indian Journalof ChemistryVol. 23A, September 1984, pp. 732-735
I,
Voltammetric Studies of 3-Bromo-4-hydroxYbenzophtmoneA SIVAKUMAR & S JAYARAMA REDDY·
Department of Chemistry, Sri Venkateswara University, Tirupati 517502~-. " ' " ", .
Electrochemical kgucti~n Of'3'\J~omoA-~y~~b~be',lz?Phen~~ 1~s ~ri car;'i~d'oU~i~ jq~e~~ ~rt:d'50~ a~.ethanolic •media in the pi'esenceof different suppottlng 'electrolytes --using' cyclic' voltammetry, chronoamJx*dmetty and'chronopotentiometry at hanging mercury drop electrode. The reduction process is found to be irreversible leading to theformation of diphenylcarbinol in all the supporting electrolytes except in 0.1 M HCI in 50~~aq ethanol where pinacolformation is observed. Diffusion coefficients, (In. values and heterogeneous rate constants evaluated using different techniquesare compared.
The electrochemical reduction of halo substitutedbenzophenones is complicated because of the reductivecleavage of the carbon-halogen bond. For exampleNadjo and Saveant 1 and M'Halla 2 identified threewaves in the reduction of chloro- and bromo-benzophenones using cyclic voltammetry. It wasconcluded that the first wave which was observed atlow sweep rate and preceded the carbonyl reductionwaves was due to the cleavage of carbon-halogenbond. These facts and our interest in theelectroreduction of aromatic carbonyl com-pounds ' -7, led us to carry out the title investigationusing cyclic voltammetry, chronoamperometry andchronopotentiometry at hanging mercury 'propelectrode (HMDE), The data have been utilised toevaluate «n, values, diffusion coefficients andheterogeneous rate constants.
l
Materials and Methods . .3-Bromo-4-hfdn?xybenlO~~IJOne:~ ~*s.~JJr~$~ed
according to the'literatureprocedure". Ttie.:s()luotion,of t
the e1ectroactive species was prepared by dissolving itin minimum amount of ethanol and then making up
. with the supporting electrolyte to the desiredconcentration, The supporting electrolytes wereprepared using AR reagents in doubly distilled' water.Phosphate buffers of pH 6.85 and 1\.7, and 'bOrate.buffer of pH 9.2 were used as supporting electrolytesfor studies in aqueous media while 0.1 M He!, 0.1 MKCI and 0.1 M NaOH prepared in 50":, ethanol werethe supporting electrolytes employed' for' studies' innon-aqueous media. The electrode assembly and theexperimental procedure employed were the same asgiven in our earlier cornrnunications'. 0. All the '.experiments were carried out at 24 ± 0.1 .C.
Results and DiscussionStudies in aqueous media
The cyclic voltammograms of 3-bromo-4-hydroxy-benzophenone indicate that the compound undergoes
732
- f
;~ ;
irreversible reduction in all the supporting electrolytesemployed as can be seen from the absence of peak inthe reverse scan and variation of peak potentials. Ep.with voltage scan rate, 11 [For example Ep varied from- 1.46V to -1.54V (vs SeE) for a change in 1 from0.01 Vs -1 to 0.10 Vs -1]. Typical cyclic voltam-mogram of the compound in phosphate buffer of pH11.7 is shown in Fig. I. In all the supportingelectrolytes employed, only one peak is observedunlike in polarography where two waves are observedcorresponding to the reductive cleavage of C -Br bondfollowed by the reduction of carbonyl group 7. Thus.the single peak in cyclic voltammetry includes both theprocesses. Merger of the two peaks may be due tohigher sweep rates employed in cyclic voltammetry'(also see refs. I and 2). ,
The plots of ip (peak current) versus V1/2 in all the 'supporting electrolytes are linear and pass through the'
O·IY
, ~O.5IlA
. '( ;~ z
t-Z•••~o
VOLTAGE(vs.SCE)-I·OOV
Fig. I-Typical cyclic voltammogram of 3-bromo-4-hydroxy-benzophenone in phosphate buffer of pH 11.7 at [depolarizer]
=0.741 mM and sweep rate =0.04 vs :'
SIVAKUMAR & REDDY: VOLTAMMETRIC STUDIES OF 3-BROMO-4-HYDROXYBENZOPHENONE
Table I-Cyclic Voltammetric Data of 3-Bromo-4-hydroxybenzophenone[depolarizer] =(1) 0.385 mM; (2) and (3) 0.741 mM; sweep rate = 0.04 VS -I
SI Supporting -Ep ip eln. D x 10' (em' s -I) kr. b
No. electrolyte (V vs SCE) (J.lA) (em s -I)C.V. C.A.
Phosphate buffer of 1.20 2.80 0.80 0.49 1.03 6.63 x 10 -20
pH 6.85
2 Borate buffer of 1.51 6.20 0.80 0.41 0.67 1.34 x 10 -23
pH 9.2
3 Phosphate buffer of 1.58 5.70 0.80 0.41 0.53 5.97 x 10 -2'
pH 11.7
Table 2-:Chronopotentiometric Data of 3-Bromo-4-hydroxybenzophenone
[depolarizer] =(1) 0.385 mM; (2) and (3) 0.741 mM
T iT'12Dx 10' kr. bSI Supporting Applied
(s)an.
No. electrolyte current (em' s -I) (em s -I)i{J.IA)
Phosphate buffer of 2.0 5.0 4.470.84 0.35 1.61xlO-19
pH 6.85 3.0 3.0 5.19
2 Borate buffer of 2.0 9.5 6.16 0.78 0.65 1.81 x 10 -22pH 9.2 2.5 8.0 7.07
3 Phosphate buffer of 4.0 9.0 12.000.92 0.66 5.57 x 10 -27
pH 11.7 5.0 7.5 13.69
origm indicating the process to be diffusion-controlled, except in phosphate buffer of pH 6.85where a deviation in these plots may be due toadsorption of the reduction product at the electrodesurface. The lower values of peak current in thismedium compared to the other media suggest weakadsorption of the reduction product. The peak currentfalling off in the second and subsequent cycles alsoprovides evidence for this adsorption phenomenon.
The number of electrons (n) involved in thereduction process has been obtained from thepolarographic data 7, which indicate two-electronaddition to the carbonyl group and one-electron forthe reductive cleavage of C-Br bond. Also, productanalysis indicates that the reduction of carbonyl groupfollows the same path as that followed by otherbenzophenones studied.' -6. Typical cyclic voltam-metric data of the compound together with an; values,diffusion coefficients and heterogeneous rateconstants (kt J are presented in Table I.
Chronoamperornetric studies of the compound,carried out at fixed step potentials chosen from thecyclic voltammograms, indicate the reduction processto be adsorption-free, except in phosphate buffer of pH6.85 where a negative' slope of the it''? versus t plotindicates weak adsorption of the product. Diffusion
coefficients evaluated from chronoamperometric dataare given in Table 1.
Chronopotentiometric i-£ curves of the compound,have been recorded at two current densities in eachsupporting electrolyte. The product of current andsquare root of transition time (rt/2) is found to be fairlyconstant in all the media indicating the process to bediffusion-controlled. Ill-defined chronopotentio-grams, obtained in phosphate buffer of pH 6.85 areprobably due to weak adsorption of the product.Chronopotentiometric data of the compound alongwith a.n., diffusion coefficient and kr~ h valuesevaluated using equations given in the literature" arepresented in Table 2.
Studies in 50% aq ethanolThe polarograms of 3-bromo-4-hydroxybenzo-
phenone in 50% aq ethanol in 0.1 M KCl and O.i MNaOH are characterised by the appearance of twowaves, the first one corresponding to the cleavage of C-Br bond and the second to the reduction of carbonylgroup. Typical polarogram of the compound in 0.1 MKCl is given in Fig. 2a. However in 0.\ M HCI in 50/0aq ethanol, a single wave appears, due to the merger ofthe two waves, similar to that observed in phosphatebuffer of pH 6.85 under polarographic conditions 7•
733
INDIAN J. CHEM., VOL. 23A, SEPTEMBER. 1984
0-
ZWexex:::;)u
VOLTAGE (vs. SeE)- I·OOV
0·1 V
® IO'S,MA
-1·2 V VOLTAGE (v s. SeE)
0-
ZWQ:Q::::;)U
Fig. 2-{a) Typical polarogram and (b) cyclic voltammogram of 3-bromo-4-hydroxybenzophenone in 50% ethanol-water (v/v) in 0.1 M KCIat [depolari.er] = 1.0 mM; (a) drop time = 3 see; and (b) sweep rate =0.04 Vs-I
The plots of log i/id - i versus Edme and Tomes'criterion indicate that the reduction of the carbonylgroup in 0.1 M KCl and 0.1 M NaOH is reversibleinvolving two electrons while the cleavage of C-Br isirreversible. In 0.1 M HCl, the process appears to beirreversible. Comparison of diffusion currents (iJobtained in 0.1 M HCl with those in the othersupporting electrolytes indicates one-electron additionto the carbonyl group in this medium. The plots of idversus h~~~r(height of mercury column) are linear andpass through the origin indicating the process to bediffusion-controlled. Typical polarographic data anddiffusion coefficients evaluated using Ilkovic equationare presented in Table 3.
As in aqueous supporting electrolytes, the cyclicvoltammograms in 50% aq ethanol (Fig. 2b) indicateonly one reduction peak which is found to beirreversible and diffusion-controlled. Comparison ofip/vl/2 C values obtained here with those obtained inaqueous buffers indicates n to be two in 0.1 M HCI andthree in 0.1 M KCl and 0.1 M NaOH. Reversiblepolarographic behaviour also confirms the value of n.
Chronoamperometric curves obtained at steppotentials, corresponding to the diffusion-limitedregions of the cyclic voltammograms, show a smooth
current-time decay. The plots of it1/2 versus t also donot indicate any adsorption of the reduction product.
Cyclic voltammetric data along with an., D and kf~ h
values are presented in Table 4.Based on the results obtained herein, the reduction
of 3-bromo-4-hydroxybenzophenone is expected totake place similar to the other ketones studied.' -6,
after the expulsion of the bromine atom from the anionradical formed by the first electron addition. The two-electron addition to the carbonyl group leads to theformation of carbinol and one-electron reduction topinacol which was the product isolated.
Table 3-Polarographic Data of 3-Bromo-4-hydroxybenzo-phenone in 50% Aq Ethanol
