The electrochemical reduction of benzalacetone (BA) has been
studied at mercury cathodes in N,N-dimethylformamide solutions
by the techniques of cyclic voltammetry and controlled potential
coulometry. The radical anion (BA\u27-) forms ion pairs with tetraethylammonium cations and then disappears in oligomerization
processes. Atentative structureof the dimer BA- BA"-...
(C2H5)4N+is suggested on the basis of quantum mechanical callations

Elžbieta Wagner

Marek K. Kalinowski

Slawomir Filipek

English

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CROATICA CHEMICA ACTA CCACAA 61 (4) 843-848 (1988)CCA-1831YU ISSN 0011-1643UDC 543.138.3Original Scientific PaperElectroreduction of Benzalacetone in N,N -dimethylformamideSlawomir Filipek, Elžbieta Wagner, and Marek K. KalinowskiDepartment of Chemistry, University of Warsaw, 1 Pasteur Str., 02-093 Warszawa(Poland)Received November 24, 1987The electrochemical reduction of benzalacetone (BA) has beenstudied at mercury cathodes in N,N-dimethylformamide solutionsby the techniques of cyclic voltammetry and controlled potentialcoulometry. The radical anion (BA'-) forms ion pairs with tetra-ethylammonium cations and then disappears in oligomerizationprocesses. Atentative structureof the dimer BA - BA"-...(C2H5)4N+is suggested on the basis of quantum mechanical cal-lations.INTRODUCTIONElectrochemical dimerization of activated olefins has been the object ofparticular1y active mechanistic studies. Most investigations in this field havebeen carried out in solvents of low proton activity such as acetonitrile andN,N-dimethylformamide (for review CP-4 and corresponding references the-rein). rt has been observed that under these conditions the discussed com-pounds undergo a radical - radical dimerlzation," visibly accelerated byaddition of water." Although such a mechanism seems to be a rather commonpathway of electrodimerization, some evidences exist that a radical - sub-strate coupling is also possible, particularly in aprotic media perfectJy purifiedfrom electrophilic contaminations.?The present report deals with the reduction of benzalacetone (BA) atmercury cathodes in N,N-dimethylformamide (DMF) solutions. The resultof these investigations is the evidence that und er experimental conditionsthe mechanism of the radical decay is profoundly affected by the concen-trati on of the reactant.EXPERIMENTALCyclic voltammetric measurements were made in a three electrode systemusing a Radelkis OH 105 polarograph. For higher sean rates, above 100 mV s-i,voltammograms were obtained using ameasuring system constructed from anEP-20A potentiostat (ELPAN, Poland) and a Hewlett-Packard 202A function ge·-nerator. Cyclic voltammetric curves were recorded on a Radelkis recorder or onDT 516A oscilloscope (KABID, Poland).The working electrode was a hanging mercury drop electrode" of 0.82 mmdiameter; the counter electrode was constructed of cylindrical platinum gauze.An aqueous SCE was employed throughout experiments and all potentials arereferred to it. Electrical contact between the SCE and the solutions investigated844 S. FILIPEK ET AL.was made through a two-part salt bridge with a Luggin capillary probe. Onebranch of this bridge was Iilled with saturated aqueous KCl, whereas thesecond one contained 0.1 M solution of tetraethylammonium perchlorate in DMF.Controlled potential electrolysis was performed at a mercury cathode with asurface area of 10 cm- using a Radelkis OH-404 apparatus.Benzalacetone was recrystallized several times from ethanol ; its melting pointwas in accordance with the literature data. Commercially available DMF wasdried first with anhydrous K2C03, later with CaH2 and then stored over molecularsieves of 4A type. The solvent was rectified at reduced pressure under argonatmosphere. Before each experiment, the solvent was passed through a columnof active alumina" (Woelrn neutral, lCN Pharmaceuticals). TetraethylammoniumperchJorate was recrystallized from triply distilled water and dried in vacuoat 60CC. All solutions were deaerated by argon which was passed through mole-cular sieves and presaturated by bubbling through the solvent. The measurementswere carried out at a temperature of 23 ± 0.1CC. Quantum chemical calculationswere performed with BASF/VM computer.RESULTS AND DISCUSSIONThe cyclic voltammogram of 0.5 mM BA in DMF solution containing0.1 M tetraethylammonium perchlorate exibits two cathodic peaks at po-tentials -1.66 V and -2.45 V, respectively, for sweep rate 0.1 V S-I. Noanodic currents complementary to these peaks were observed during reversalscan. The first peak, which is the object of our interest, is proportional tothe reactant concentration within the range from 0.08 to 0.7 mM, and to thesquare root of the sweep rate (v), at least within the range for 0.05 to 5 VS-I. The peak potential EpcI moves toward more positive values as v dimi-nishes and the concentrations of benzalacetone (CBA) and electrolyte (cdincrease. Corresponding characteristics are collected in the Table.TABLEEssential Characteristics of the First Reduction Step of BenzalacetoneParameter11Epc]/11log v11Ep/ /11log CBA11Epc'/l1logCEExperimental value-18 ± 2 mY/log unita20 ± 3 mY/log unit''55 ± 6 mY/log unit"a Scan rate range: 0.05- 2.9 V S-I; CBA = 0.5 mM.I> CBA range: 0.08- 0.54 mM; sweep rate 0.1 V S-I.o CE range: 0.02- 0.15 M; CBA = 0.5 mM; scan rate 3.2 V S-I.These values, together with the number of electrons transferred in electro-reduction (see text below), are fully consistent with the radical - radicaldimerizationw-P influenced by the ion pairs formation between the benza-lacetone radical anions and the cations of the supporting electrolyte. Analyzingthe dependence of EpcI versus v (Figure 1), we have found the apparentrate constant of dimerization; the value of kdim = (8 ± 2) X 104 M-l S-I.Figure 1 was constructed for the solution containing 0.5 mM BA and0.1 M (C2Hs)4NCI04. Under these conditions, at a higher potential sweeprate (i. e. 2.9 V S-I and more) D..EpcI/D..logv = O and the first reduction peakof benzalacetone becomes fully reversible. The corresponding pair of cathodicELECTROREDUCTION OF BENZALACETONE 845- E~CmV1670•1690f• v = 2.9Vs-'•1650-1 o log VFigure 1. Variations of the first peak potential with the logarithm of the sweeprate for a 0.5 mM solution of benzalacetone in N,N-dimethylformamide containing0.1 M tetraethylammonium perchlorate.and anodic peaks is now observed with (Ep} - EpcI)= 60 ± 2 mV and ipaI/ip/ =1.0. Thus, it is possible to estimate the association constant value, Kas> properfor the equilibrium.(1)in which BA- and BA- ... (C2Hs)4N+denote the benzalacetone radical anionand the corresponding ion pair, respectively.It is known that the Kas value can be calculated on the basis of therearranged Nernst equation'"(2)where Eoip and Eo stand for the formal potentials of the electro de reactiondetermined in the presence and in the absence of ion association, respecti-vely, whereas aM+ marks the activity of a cation interacting with the radicalanion. Large singly-charged cations, such as tetraethylammonium ions, donot generally show ion pairing with the anionic products of electro de react-ions. The situation is somewhat different in the case investigated by us;therefore, we were not able to determine the value of E; in direct experiments.Consequently, the ion pair formation constant of the BA- ... (C2Hs)4N+systemwas estimated by the method proposed by Chauhan et aI.13 for solutionsof varying ionic strength.Transforming eq. (2) into exponential form(3)l846 S. FILIPEK ET AL.where f = FIRT, we have analyzed the plot of exp (f Eoip) against a(C,H5),N+. Thevalu es of Eoip were meas ured from cyclic voltammetric curves registered foreBA= 0.5 mM at sean rate 3.2 V S-I. The strategy of calculations of a(C2H.),N+wasthe same as in paper-š, i. e. any variation in the liquid junction potentialbetween the working and reference electrodes has been estimated by theHenderson equation for different concentrations of tetraethylammonium per-chlorate in electrolytic cell. According to Prue and Sherington-t we haveassumed that (C2Hs)4NCI04is not associated in DMF solutions. The resultingvalue of Kas is 10 ± 4 M-l.Of particular significance to the present investigations is the fact thatcoulometric studies of BA on the diffusion plateau of the first polarographiewave produces concentration dependent n-values. As it can be seen fromFigure 2, the number of electrons transferred in electroreduction (n) isequal to 1.00 ± 0.05 but only with in the eBArange up to 0.5 mM; at higherconcentrations, the n values diminish progressively. We assume that the lasteffect is eaused by oligomerization of benzalacetone, as it was suggestedearlier by Simonet-" and Evans et aU6 At present, we are not able to separatetheoligomeric products; our results indie ate only that an attack of the ionpair BA- ... (C2Hs)4N+on the unreduced BA molecule is an initial step ofoligomerization. Obviously, such a process is tantamount to a radical -substrate dimerization.n1~~0.8 IO 20 1.0 60 c.1O-SMosOO 1000 2000 JOaO c'10-5 MFigure 2. Plot of the number of electrons transferred during the first electro-reduction step of benzalacetone against the concentration of the reactant (0.1 Mtetraethylammonium perchlorate in N,N -dimethylformamide).What is, however, the tentative structure of the dimer BA-BA- ...(C2Hs)4N+?From the formal point of view, various coupling reactions involvingthe formation of new C-C bonds must be taken into account. Hence, tosolve the problem, we have computed the total energies of supersystemsBA-BAR using the MMP2 method.F-" For simplicity, the radical in proto-nated form, BAR, was considered instead of the ion pair BA- ... (C2Hs)4N+.According to the adopted tactics, we have taken into account the supersurfaceELECTROREDUCTION OF BENZALACETONE 847of interaction of BA and BAH' molecules, approaching the last one fromvarious directions, turning it around benzalacetone and rotating it about thelocal axis so as to find the best orientation. The essential result of ourcalculations is presented in Figure 3. We suppose that it reflects the bestorientation of the interacting molecules, at least from the energetical pointof view. Other possibilities, e. g. an addition of BAH' to the C= O groupof the BA molecule, seem to be less probable.Figure 3. Structure of the dimer benzalacetone-benzalacetone radical anion com-puted by the MMP2 method: a = 1130, j3 = 1150; the dihedral angle between theplanes of the phenyl rings is y = 48°.Obviously, the structure proposed in Figure 3 is purely speculative andit will be verified in spectroscopic experiments. At present, it seems reaso-nable to suggest that the BA-BA- ... (C2Hs)4N+radical is then capable ofthe BA mole, lenghthening progressively an oligomeric chain. As it followsfrom Figure 2, its length is dependent on the concentration of benzalacetonein solutions.AcknowLedgement. - This work was sponsored by the Scientific ProgramCPBP 01.15.REFERENCES1. M. M. B a i z e r, in Organic ELectrochemistry, M. M. B ai z e r, ed., p. 679,Marcel Dekker, New York 1973.2. D. H. Eva n s, in EncycLopedia of ELectrochemistry of the Elements, A. J.Bar d and H. L u n deds., vol. 12, chapter 1,2.5, Marcel Dekker, New York1978.3. V. D. Par k e r, in Advances in PhysicaL Organic Chemistry, V. Gol d andD. Bet h e Ll eds., vol, 19, chapter 2, Academic Press, London 1983.4. J. M. Save a n t, Acta Chem. Scand. B37 (1983) 365.5. E. La m s, L. Nad j o, and T. M. Save a n t, J. ELectroanaL Chem. 42 (1973)183.6. E. La m y, L. Nad j o, and J. M. Save a n t, J. ELectroanaL Chem. 50(1974) 141.7. V. D. Par k er, Acta Chem. Scand. H35 (1981) 219.8. W. K e m ula and J. Za w a dow s ka, Fresenius Z. anaL Chem. 300 (1980)39.9. O. Ham m eri c h and V. D. Par k e r, ELectrochim. Acta 18 (1973) 537.10. C. P. And rie u x, L. Nad j o, and J. M. Save a n t, J. ELectroanaL Chem.26 (1970)147.11. C. P. And rie u x, L. Nad j o, and J. M. Save a n t, J. ELectroanaL Chem.42 (1973)223.12. M. E. P e ove r and J. D. Da vie s, J. ELectroanaL Chem. 6 (1963) 46.13. B. G. C h a uha n, W. R. F a w cet t, and A. Las i a, J. Phys. Chem. 81(1977) 1476.14. J. E. P rue and P. J. S h eri n g t o n, Trans. Faraday Soc. 57 (1961) 1795.15. J. S i m o net, C. R. Acad. Sci. Paris C263 (1966) 1546.848 s. FILIPEK ET AL.16. J. F. Zi m m e r, J. A. Ric h ard s, J. C. Tur ner, and D. H. Eva n s,AnaL. Chem 43 (1971) 1000.17. N. L. Ali n g e r, in Advances in Physical Organic Chemistry, V. Gol d andD. Bet h e l l eds., vol. 13, chapter 1, Academic Press, London 1976.18. N. L. Ali ng e r and Y. H. Yu h, MM2jMMP2,QCPE Program No 395, 1980.SAŽETAKElektrodna redukcija benzalaeetona u N,N -dimetilformamiduS. Filipek, E. Wagner i M. K. KalinowskiMetodama cikličke voltammetrije i kulometrije uz kontrolirani potencijal istra-žena je redukcija benzalacetona na živinoj kato di u otopini u N,N-dimetilformamidu.Produkt elektrodne redukcije, radikal-anion benzalacetona, stvara ionski par s tetra-etilamonijevim ionom te potom ulazi u proces oligomerizacije.Predložena struktura dimera, BA-BA- ... (C2H;)4N', proizlazi iz provedenihkvantno-mehaničkih računa.

Electroreduction of Benzalacetone in N,N -dimethylformamide

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