The electrochemical reduction of acetonylacetone on a mercury
cathode in aqueous hydrochloric acid medium has been studied. It
has been shown that the electroactive species is the protonated
form of acetonylacetone. The experimental results indicate -that
the rate of electrochemical reduction is dependent upon the preceding
chemical protonation step and/or adsorption of the electroactive
species. The electrolysis at controlled potential (- 1.2 V
vs. S. C. E .) yielded the following products : acetaldehyde; acetone,
acetic acid, 2,5-hexanediol and 2,5-dimethyltetrahydrofuran. Possible
mechanistic routes leading to the formation of these products
are discussed

I. Tabaković

M. Laćan

Ž. Čeković

English

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CROATICA CHEMICA ACTA CCACAA 47 (4) 587-593 (1975) CCA-905 YU ISSN 0011-1643 541.138:547.284 Original Scientific Paper Electrochemical Reduction of Acetonylacetone in Hydrochloric Acid M. Lafon, I. Tabakovic, and Z. Cekovic Faculty of Technology, Zagreb, Technical Faculty, Banjaluka and Faculty of Sciences Be-0grad, Yugoslav ia Received August 15, 1975 The electrochemical reduction of acetonylacetone on a mercury cathode in aqueous hydrochloric acid medium has been studied. It has been shown that the electroactive species is the protonated form of acetonylacetone. The experimental results indicate -that the rate of electrochemical reduction is dependent upon the pre-ceding chemical protonation step and/or adsorption of the electro-active species. The electrolysis at controlled potential (- 1.2 V vs. S. C. E .) yielded the following products : acetaldehyde; acetone, acetic acid, 2,5-hexanediol and 2,5-dimethyltetrahydrofuran. Pos-sible mechanistic routes leading to the formation of these products are discussed. The electrochemical reduction of ketones has been extensively studied1• Depending on the reaction conditions used in electrochemical reduction, various compounds (pinacols, alcohols, metaloorganic compounds and alkanes) can be formed. By the presence of other carbonyl groups ·in a molecule the number of possible products is increased. Various diketones were a particulary interesting subject for electroche-mical study, not only because of the numerous 'posibilities for reduction pathways, but also because of potential synthetic use. Reduction of 1,2-diketones gave a-hydroxy ketones2• D. H. Evans et ar.3- 6 and E. Kariv et au-14 extensively studied the electrochemical behaviour of 1,3-diketones, which were shown to be affected by intramolecular interaction between two carbonyl groups. A. D. Thomsen and H. Lund12 reported very interesting work on the electrochemical reduction of acetylacetone in hydrochloric acid media. B. N. Gourley and J. Grimshaw13 have studied the reduction of some 1,5- and 1,6-diketones by pola-rography and product studies. E. Kariv and B. J. Cohen14 have extended their interest to the research on 'intramolecular interaction between carbonyl groups in 1,4-diketones, which was the first report in electrochemical literature on this subject. They investi-gated the reduction ·Of cyclohexanel ,4-dione and acetonylacetone in water and water-propane-2-01 mixture as solvent and tetraethylamonium p-toluenesulpho-nate as supporting electrolyte. Reduction of cyclohexane-1 ,4-dione yielded five products, while the preparative reduction of acetonylacetone at - 2.3 V vs. S.C.E. yielded no product and acetonylacetone was quantitatively recovered. This work prompted us to find suitable conditions for the reduction of acetonylacetone in order to study its electrochemical behaviour. We have 588 M. LACAN ET AL. found that acetonylacetone was reducable in the aqueous hydrochlodc acid medium in which its reduction was studied in this work. RESULTS AND DISCUSSI ON A reduction wave for acetonylacetone was obtained in aqueous hydro-chloric acid solution* (Fig. 1.). The limiting current of the polarographic wave presented in Fig. 1. ~s slightly decreasing when HCl concentration changed from 0.5 to 0.01 M ; half-wave potential is shifted more cathodically (from -1.13 V at 0.5 M to -1.27 V at 0.01 M HCl). N HCll 0,5 0,3 , 2 0,1 0,01 Fig. 1. Polarographic beha viour of acetonylace tone (3.2 X 10- 2 M) at different HCl concentrations starting at - 0.9 V vs. s. c. E. Polarographic behaviour of acetonylacetone 1 in a solution of changing HCl concentration shows that the rate of electrochemical reduction of aceto-nylacetone is dependent on the concentration of hydrogen ions17• The limiting currents of the reported polarographic waves are much smaller then those expected for a diffusion controlled process. These experi-mental results indicate that the electroactive species is not acetonylacetone 1, which probably exists in the solution in several forms and ·only one of them is electroactive (scheme 1). * Between pH 3 and 14 in buffered solution (Britton-Robinson buffers) no polarographic wave was observed. In some earlier papers S. I. Vityaeva and A . L. Markman15.1s reported two waves on the polarogram of acetonylacetone obtained with lithium chloride and tetraethylammonium iodide as supporting electrolyte in aqueous media. * The linear plot iL - h passes practically through the origin. 0 ~ 0 ELECTROREDUCTION OF ACETONYLACETONE O· ··H' Ny •2e . · 2H•- · -+ - · -H . .. O 2 OH ~ ·2e ·2H• 3 l (J-j OH 2CH3.C=cHz ! 589 OH )vy 4 OH 1 The results presented in Fig. 2 show the influence of the concentration of hydrochloric acid and of the height of the mercury reservoir on the limiting current*. Further increase of concentration of acetonylacetone (above 6 X 10-2 M) does not alter the rate of reaction significantly, indicating that the rate of electrochemical reduction is controlled by the reaction of protonation of acetonylacetone and/or by the adsorption of electroactive species 2. i1 [}JAJ 14 11 .10 ~~~~~-~~~~~-9~~-----~-~-2 4 6 8 10 12 14 cone. 20 30 40 50h IMJ · 10-2 [cm Hgl 5 6 7 VFi [cm Hgl Fig. 2. Influence of concentrat ion of a cetonylacetone and height of the m ercury r eservoir (concentr ation of aceton ylacetone : 3.2 X 10_, M) on the limit ing current. Suppor ting electroly te : 0.5 M HCl. Polarization curves obtained with a large mercury cathode (cca 19 cm2) show that the initial reduction potential, which could be determined by extra-polating the currentpotential curve to zero current (Fig. 3), becomes more nega-tive as the concentration of acetonylacetone in the solution increases. This effect could be explained by saturation of the electrode surface by the adsor-bed reactant and/or reaction products formed during reduction. A similar effect was noted by several workers18,19• Logarithmic analysis of the polarographic curves, taken from Fig. 1 indi-cates that the electrochemical reduction of acetonylacetone in 0.5 M HCl is a reversible process. Plotting of E (the applied potential) vs. log [(k - i)/i] gave straight line with a slope of 25 mV. The reduction of acetonylacetone in 0.5 M aqueous HCl solution was also studied by means of preparative el.ectrolysis and products distribution. Major 590 [mA1 500 400 300 200 100 0 M. LACAN ET AL. 0 -0,2 -0,4 -0,6 -0,8 -\0 -\2 -1/+ -1,6 Volts vs. S.C£. Fig. 3. Polarization curves at large mercury cathode for different concentrations of acetonyl-acetone. 1. 0.032 M, 2. 0.065 M, 3. 0.130 M, 4. 0.325 M , 5. Supporting electrolyte: 0.5 M HCl products were isolated by preparative gas chromatography and examined by i.r., n.m.r., and mass spectrometry. In the case of minor products (acetaldehyde and acetone) their indentification was performed by comparison of g. c. retent-ion times with those of authentic samples. Between several other unidentified compounds we have found organo-mercury compounds whose structure is under investigation at present. The yields of products, after electrolysis at -1.2 V vs. S. C. E ., are shown in the Table. TABLE Composition of the Reaction Mixture obtained by Electrolysis of 0.13 M Acetonylacetone in Aqueous 0.5 N HCl Solution at - 1.2 V vs. S. C. E.0 •0 Products Acetaldehyde Acetone Acetic acid 2,5-dimethyltetrahydrofuran Acetonylacetone (unreacted) 2,5-hexanediol Not defined • Amount of electricity passed: 2000 C " Amount of acetonylacetone reduced: 2 ml Yield (Of&) 2 4 5 10 35 26 18 ELECTROREDUCTION OF ACETONYLACETONE 591 The first step in the reduction is addition of two electrons to protonated acetonylacetone 2, which is the electroactive species forming the diradical 3 (scheme 1). The conversion of the diradical 3 into the main product 2,5-hexa-nediol 4 may occur by further addition of two electrons through formation of a dianion following its protonation. A mixture of diastereoisomedc 2,5-hexa-nediols was obtained in the electrochemical reduction of acetonylacetone. It is known20 that the eliminative cyolisation of disecondary 1,4-dfols, leading to the forimation of tetrahydrofurans 5, proceeds stereoselectively by SN2-type mechanisms with inversion of configuration at one chiral centre. The cyclisation of meso 2,5-hexanediole leads to the formation of trans-2,5-dimethyltetrahy-drofuran, while the respective diasteroisome:ric substrates are converted exclu-sively into cis-2,5-dimethyltetrahyd:rofuran. By g . 1. c~analyses we have found that the ratio meso/dl 2,5-hexanediols, after electroly,sis at -1.2 V vs. S. C. E ., was about one. We feel that formation of acetaldehyde and acetic acid, which are rather unusual products of electrochemical reduction, might be explained by homo-litic degradation of starting compounds by means of present radicals, but this hypothesis needs to be tested. The formation of acetone 6 through cleavage of diradical 3 is due to 1,4-interaction, since the direct cleavage of such a bond is not known to occur in the potential range studied here. A similar cleavage was postulated in ele-ctrochemical reduction14, Clemmenson reduction21 and reduction with lithium in liquid ammonia22• The stereochemistry of the electrode reaction during the reduction of acetonylacetone, as well as some other 1,4-diketones is under investigation in our laboratory. EXPERIMENTAL Gas chromatography A varian Aerograph instrument series 1200 (flame ionisation detector) or a Perkin-Elmer F-11 instrument was used for analytical work, and a Varian Aerograph instrument model A-700 (thermistor detector) for preparative purposes, with columns Carbowax 20M, Poropak Q or 1,2,3-tris-(2-cyanoetoxy)-propane adsorbed on Chro-mosorb P. Other conditions for gas chromatography analyses were similar to those described in our previous work23• Separations of cis-2,5-dimethyltetrahydrofuran (shorter retention time) and trans-2,5-dimethyltetrahydrofuran (longer retention time) were performed using a 1,2,3-tris-(2-cianoetoxy)-propane column. Spectroscopic methods N. m. r. spectra were recorded on a Varian A-60A spectrometer (CC14 was used as solvent and tetramethylsilane as internal standard), i. r . spectra on a Perkin-Elmer Infracord instrument model 337 and mass spectra on a Varian mass spectrometer CH5. Polarography Polarograms were recorded on a Polariter P04-polarograph (Radiometer Co-penhagen). The cell used was a modified H-type of 25 ml capacity with dropping mercury electrode and saturated calomel electrode connected via a salt bridge of 30/o agar in saturated potassium chloride solution. Temperature was kept constant at 25.0 ± 0.1 ()C by means of a thermostat. Preparative electrolyses Electrolysis at controlled potential were carried out by means of a potentiostat (Amel - 555 - SU). An equal amount of electricity (- 2000 C), measured by electronic 592 M . LACAN ET AL. integrator, was transferred during each experiment. Representative results are shown in the Table. Preparative electrolyses were made in a jacketed water-cooled cell divided by a sintered-glass diaphragm into anode and cathode compartments. The cathode was a mercury pool (19 cm2) and the anode was a Pt-wire. A saturated calomel reference electrode was connected to the cathode compartment through a Luggin capillary. The 0.13 M solution of acetonylacetone 1 obtained by dissolving 1 in 100 ml of catholyte (0.5 M aqueous HCl) was electrolysed at controlled potential. Temperature during electrolyses was between 18 and 20 °c. After stopping the electrolysis 20 g NaCl was added to the reaction mixture, and the mixture extracted five times with ether or ethylacetate. The combined extracts were dried (MgS04) and the solvent removed by distillation to a volume of 50 ml. The remaining mixture was subjected to g. 1. c. analyses. Acetaldehyde, acetone and 2,5-hexanediole w ere identified by comparison of their respective retention times with authentic samples on at least two different chromatographic columns. Acetic acid, 2,5-dimethyltetrahydrofuran and unreacted acetonylacetone were isolated by preparative gas chromatography and identified by comparison with authentic samples (g. 1. c., n. m . r., i. r ., and m ass spectra) . At least three additional products whose structure has not been identified remain in the reaction mixture. REFERENCES 1. L. G. Fe o kt is to v and H. Lund in : M. M. Ba ii z er (Ed.), Organic Electro-chemistry, Marcel Dek!ker, Inc., New York 1973. 2. P. H . Given, M. E. Pe over, and J. J . Schoen, J. Chem. Soc. (1958) 2674. 3. D. H. Ev ans and E. C. Woo db u r g, J . Org. Chem. 32 (1967) 2158. 4. B . C. Bu ch t a and D. H. Ev ans, Anal. Chem. 40 (1968) 2181. 5. B. C. Buch t a and D. H. Ev ans, J. Electrochem. Soc. 117 (1 970) 1494. 6. B . C. Buchta and D. H . Ev ans, J. Org. Chem. 35 (1970) 2844. 7. E. Kari v , B. J . Cohen, and E . G i 1 ea di, Tetrahedron 27 (1971) 805. 8. E. Kari v, J. Her mo 1 in, and E. G i 1 ea di, J . Electrochem. Soc. 117 (1970) 342. 9. E . Kari v, J . Her mo 1 in, and E. G i 1 e a di, Electrochim. Acta 16 (1971) 1453. 10. E . Kari v, J . Her mo 1 in, I. Rubinstein, and E . G i 1 ea di, Tetrahedron 27 (1971) 1303. 11. E . Kari v and J. Her mo 1 in, Electrochim. Acta 18 (1973) 417. 12. A. D. Thomsen and H . Lund, Acta Chem. Scand. 25 (1971) 1576. 13. B. N. Gour 1 e y and J . Grims h aw, J. Chem. Soc. C (1968) 2388. 14. E. Kari v and B. J . Cohen, J . Chem. Soc. Perkin Trans. II (1972) 509. 15. S. I. Vi t ya e v a and L. Markman, Tr. Sredneaziat. Politekh. Inst. 2 (1957) 17; C. A. 54 (1960) 11765. 16. S. I. Vi t ya e v a and L. Markman, Dokl. Akad. Nauk U zbek. 8 (1957) 33; C. A. 53 (1959) 2886. 17. P. Zuma n , »The Elucidation of Organic Electrode Processes« Academic Press, New York 1969. 18. A. B. Er sh 1 er, G. A. Te o do r a dz e, M. Fa k h mi, and K . P . But in, Elektrokhimiya 2 (1966) 319. 19. B . K a st en in g and L . Ho 11 e ck, Talanta 12 (1965) 1263. 20. M. Lj. Mihailovic, S. Gojkovic, and Z. Cekovic, J. Chem. Soc. Perkin Trans. I (1972) 2460. 21. E. Wenker t and J . E. Loder, J. Org. Chem. 35 (1970) 2986. 22. J. Grims h aw and R. J . Has 1 et t , J . Chem. Soc. Chem. Comm. (1974) 174. 23. M. Lac an and I. Tabak o vi c, Bull. Soc. Chim. Beograd 38 (1973) 297. ELECTROREDUCTION OF ACETONYLACETONE SAZETAK Elektrokemijska redukcija acetonilacetona u solnoj kiselini M. Lafon, I . Tabakovic ii Z . Cekovic 593 Ispitana je elektrokemijska redukcija acetonilacetona na zivinoj katodi u razri-jedenoj solnoj kiselini. Pokazano je da je protonirani oblik acetonilacetona 2 elek-troaktivan. Eksperimentalni rezultati pokazuju da brzina elektrokemijske redukcije ovisi o prethodnom stupnju protoniranja i (ili) adsorpciji elektroaktivne vrste. Elek-troliza kod kontroliranog potencijala (-1,2 V vs. S. C. E.) daje slijedece produkte: acet-aldehid, aceton, octenu kiselinu, 2,5-heksandiol i 2,5-dimetiltetrahidrofuran. Disku-tiran je mehanizam nastajanja ovih produkata. TEHNOLOSKI FAKULTET 41000 ZAGREB TEHNICKI FAKULTET BANJA LUKA 1 PRIRODNO-MATEMATICKI FAKULTET 11000 BEOGRAD Prispjelo 15. kolovoza 1975. 

Electrochemical Reduction of Acetonylacetone in Hydrochloric Acid

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