Cathodic macroelectrolysis of acetophenone: A careful
analysis of product distribution
Citation for published version (APA):
Janssen, L. J. J. (1988). Cathodic macroelectrolysis of acetophenone: A careful analysis of product distribution.
Electrochimica Acta, 33(7), 897-903. https://doi.org/10.1016/0013-4686%2888%2980086-0,
https://doi.org/10.1016/0013-4686(88)80086-0
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10.1016/0013-4686%2888%2980086-0
10.1016/0013-4686(88)80086-0
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Published: 01/01/1988
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CATHODIC
MACROELECTROLYSIS
OF ACETOPHENONE:
A
CAREFUL
ANALYSIS
OF PRODUCT
DISTRIBUTION
L. J. J. JANSSEN
Laboratory for Electrochemistry, Department of Chemical Technology, Eindhoven University of Technology, P. 0. Box 513, 5600 MB Eindhoven, The Netherlands
(Received 10 November 1987; in revised form 21 December 1987)
Abstract-The electrochemical reduction of acetophenone has been reinvestigated because of uncertainty, not only in the quantitative data given in the literature, but also in the knowledge of well known synthetic routes to both pure diastereometric pinacols and availability of advanced analysis techniques. The reduction of acetophenone has been carried out at a graphite electrode in a cell divided into two compartments by an anion exchange membrane. It has been found that the reaction products with a product yield higher than 1 o/0 at 25°C are I-phenylethanol, meso-pinacol and DL-pinacol. The diastereometric ratio, S&S,, between the selectivities of both forms of pinacol depends strongly on the pH of the solution, but for acidic media S,,/S, is practically independent of electrode potential, water content of water-organic solvent mixture, nature of alcohol used as organic solvent, and nature of added organic and inorganic compounds. The ratio S,JS, reaches a limiting value, viz about 1, at a pH of about 0.5. The selectivity for 1-phenylethanol formatIon is very low, viz about l-2 %.
1. INTRODUCTION
The electrochemical reduction of acetophenone has been studied under a wide range of conditions during the last 20 years. A large number of products of which only a part has been identified, can be formed. The best known products are methylphenylcarbinol or l- phenylethanol (carbinol) and 2,3-diphenyl-2,3- butanediol (pinacol). The overall reaction for the production of carbinol is given by Scheme 1 and that for pinacol by Scheme 2. The pinacol formed can consist of the optically inactive meso form (meso- pinacol) and the optically active D and L form (D pinacol and L-pinacol). The two last named forms are often combined and then indicated by DL-pinaCO1. The oldest and also most extensive studies of the macros- tale electrolytic reduction of acetophenon have been carried out by Swann and his co-workers[l-31. They have found that, for carbon and graphite electrodes in strongly acidic media at high temperatures, the major
OH COCH,
4
0 + 2e + 2H+ - Scheme 1.reduction products are pinacol with a maximum product yield of 8 % and bis-a-methylbenzylether (ether) with a maximum product yield of 54% and some resinous material of unknown composition (the ether is probably formed from the carbinol by dehyd- ration), and in alkaline media at high temperatures the major reduction product is pinacol with a maximum product yield of 47%. For a mercury cathode in alkaline solution, Elving and Leone[4] have also found that pinacol is the major reduction product of acetophenone.
Horner et al.[S-71 have established that besides pinacol, large quantities of carbinol can also be obtained by reduction of acetophenone at both mercury and carbon electrodes in alkaline solution. The ratio of carbinol to pinacol depends on many factors, eg nature of the electrode surface[7], electrode potential, pH, nature of the solvent and the conducting salt, acid or base and the temperature[6]. They have also studied the reduction of acetophenone in the presence of chiral salts. When (L)L or (D)L ephedrine hydrochloride is used, the carbinol formed is optically active and the pinacol formed simultaneously is optically inactive and consists of a mixture of the meso, D and L
forms[6,8]. Optically active pinacol has been pro- duced by using asymmetric conducting salts in an aprotic medium[9].
The ratio of Dbpinacol to meso-pinacol has been investigated extensively. From the thorough study by Stocker and Jenevein[lO] it follows that the dia- stereometric ratio DL/meso is 0.9 to 1.4 for acidic media
COCH,
2
0
0 + 2e + 2H+ -Scheme 2.
898 L. J. J. JANSS~N
of pH = 4.2 and 2.5 to 3.2 for basic media of pH > 11 and that the ratio is independent of the nature of cathode material (mercury, tin or copper). Several parameters, viz time of electrolysis, potential and current do not appear to have a significant effect on the diastereometric ratio.
Determination of the quantities of products formed by electrolysis have generally been carried out after an extensive procedure, consisting, for instance of distil- lation, extraction, neutralization and crystallization. Consequently, some results have only a relative value[5] or are even incorrect. Moreover, it may be possible that because of chemical instability of some species, conclusions on the primary yield distribution during the electrolysis are unreliable.
In the investigation described in the present paper, high-pressure liquid chromatography (HPLC) was applied to analyse the electrolysis samples in order to prevent an extensive procedure of product isolation.
The reduction of acetophenone has been reinvesti- gated because of uncertainty not only in the quantitat- ive data given in literature for the acetophenone system, but also in the knowledge of well known synthetic routes to both pure diastereometric pinacols and availability of advanced analysis technique. The results obtained will contribute to better understanding of the reduction of the carbonyl group, which will enhance progress in organic electrochemistry. In the present paper the study is limited to reduction of acetophenone on graphite electrodes in water-organic solvent mixtures.
2. EXPERIMENTAL 2.1. Electrolysis
The reduction of acetophenone has been carried out in a normal H-type electrolytic cell divided into two compartments by an anion-exchange membrane (Asahi Selemion ASV) 20 mm in diameter. The bottom of the working-electrode compartment consisted of a graphite disc. The graphite disc exposed to the solution was 12 cm’ in area and served as the working electrode. A 10 cm2 platinum plate was used as the counter electrode. The electrolysis was carried out either potentiostatically or galvanostatically. A saturated calomel electrode (see), was used as reference electrode, only for constant-potential electrolyses. The current or the potential was recorded and the quantity of charge passed through the cell measured by an ampere-hour meter. The working-electrode compartment of the cell was filled with 250cm” of solution containing ac- etophenone and the counter-electrode compartment with 250 cm3 1 M H,SO, or 1 M KOH. The solution in the working-electrode compartment was stirred magnetically unless a large quantity of acetophenone was being reduced because of the formation of a syrupy product. In the latter case the solution was stirred mechanically. To eliminate a loss of low boiling organic solvent from the working-electrode compart- ment, a heat-exchanger was usually put on the top of the working-electrode compartment.
2.2. Chemicals
The routine chemicals employed were either reagent grade or the best research grade obtainable and were
used as available. 2,3-dihydroxy-2,3-diphenyl-butane (pinacol) was prepared by electrochemical reduction of acetophenon, n-hexane was used to recrystallize and to separate pinacol into the meso- and the Dt.-form. In n- hexane the solubility of Dt_-pinacol is less than that of meso-pinacol. The melting point of the oL-pinacol obtained was 122°C (lit. 11: 122°C) and that of meso- pinacol obtained was 113°C (lit. 11: 117°C). The latter product was a mixture of meso- and DL-pinaCOl in a ratio of 86 to 14. The isolation of pure meso-pinacol is extremely difficult[ 121.
The proton NMR and ir spectroscopy were used for identificationc 13-J. 2,2-diphenyl-3-butanone was pre- pared according to the procedure of Sisido
et
a/.[141 and di[l-phenylmethyllether was supplied by the organic laboratory of the Eindhoven University of Technology. From an NMR spectrum of this product it has been concluded that it is a mixture of the meso- and DL-forms, probably in a I
:
1 ratio. The following special chemicals I-phenyl-ethanol, acetophenone, styrene-oxide and ethylbenzene are commercially available and were used without purification. 2.3. AnalysisSolution samples were analysed with an automatic high-performanceliquid chromatograph (HPLC)con- sisting of a high-pressure pump (SpectraaPhysics model 740B), sample processor (Waters Associates, Inc., WISP 710A), a column,a uv detector (Pye Unicam LC3) or a dual detector (uu and refraction index Knauer); recorder (Kipp’s Zonen BD8 multirange) and a computing integrator (LDC Shannon 30450).
The column was packed with Polycosil R18 (5 pm) and the column packing was 10 cm in length and 0.46 cm in inner diameter. The mobile phase was a meth- anol/water mixture with a volumetric ratio of 40 to 60. The flow rate of the mobile phase was 0.0167 cm3/s at a pressure of about 100 bar. The sample processor injected a sample volume of 15~1 into the mobile phase. The adjusted retention time
tR
(the retention time minus the dead time), the specific peak area A (the quotient of the peak area and the quantity of solute), and the specific peak height (the quotient of the peak height and the quantity of solute) were de- termined for the available chemicals of interest in this investigation.3. RESULTS 3.1. Chromatographic data
The adjusted retention time t’a, the specific ultra- violet absorption peak area Ar,+” and the specific refractive indices peak area A,,, both at 254nm, for several chemical compounds are given in Table 1 and compared with those for Dt_-pinacol.
3.2. Products formed during the reduction of uceto- phenone
The reduction of acetophenone has been carried out at a high temperature, 75°C as well as at low tempera- tures, 1.5 and 25°C. From the chromatograms of catholyte samples it followed that a greater &mber of products is formed at 75°C than at 15 and 25°C.
Macroelectrolysis of acetophenone 899 Table 1. Relative adjusted retention time, relative specific ultraviolet
absorption peak area and relative specific refractive indices peak area, both at 254nm
Chemical compound tdta.oL A~.uvl&.uv.m_ &RII&. RI.DL
oL-Pinawl 1 1 1
Meso-pinacol 0.123 1.027 0.945 1-Phenylethanol 0.298 0.438 0.322 Acetophenone 0.377 38.8 0.360
Because of the great sensitivity of the uu detector, very small quantities of some compounds can give clearly distinguished peaks in the chromatogram. To establish whether a peak is due to a reasonable quantity of a substance, the chromatogram with the refractive index detector is more suitable.
From chromatograms for catholyte samples and for single compounds it followed that during the reduc- tion of acetophenone at both high and low temperature no detectable quantities of styrene, styrene oxide, ethylbenzene, di[l-phenylethyllether, dimethylstil- bene and dimethylstilbene oxide are formed. It is assumed that the retention times of the last two compounds are longer than for stilbene and stilbene oxide, respectively. It has been found that the reaction products with a product yield higher than 1 y0 at low temperature are I-phenylethanol, meso-pinacol, and DL-pinacol and those at high temperature are l- phenylethanol, meso-pinacol, DL-pinaCO1, 2,2- diphenyl-3-butanone and probably 1,2-diphenyl-2- methyl- l-propanone.
Swzmn
kt
ai.[l-31 have, however, found a high yield of dir I-~henvlethvllether. ie 22-51 Y for reduction of acetoph;none at &&odes bf differeni‘lypes of carbon at high temperature. Their results are not confirmed by the results given in this paper. To check their conclu- sion, electrolyses were carried out under practically the same conditions as described by Swann et aI.[l-33. The procedure of the product isolation was also similar to theirs. The resulting high-boiling liquid was frac- tionated by boiling at the same pressure as used by Swannet
aI.[3]. The largest fraction (about 40 % of the high boiling liquid) had a boiling point of 142°C at a pressure of 3 mm of mercury and the residue was about 50%. Analysis of the boiling fraction of 142°C by a combination of a gas chromatograph (HP 5790A) and a mass selective detector (HP 5970A) with an OV-1 column, shows four peaks whose retention times are 270,3 16,329 and 370 s with relative abundances of 20, 5, 100 and 7% respectively. From the experimental mass spectra it has been deduced that the 270-s peak indicates 2-methyl-1-phenyl-indene and the 329-s peak 2,2-diphenyl-3-butanone. The two small peaks could not be identified. It is well known that 2,2-diphenyl-3- butanone is formed with a yield of about 60% from pinacol by dehydration in heat aqueous sulnhuric acid[ 111. The reaction of pinacol and dimethylsul- foxide for 14 h at 190°C gives 2,3-diphenyl-butadiene (17 %) and a 65 % yield of ketones composed of 11% 1,2-diphenyl-2-methyl-1-oropanone and 89 % 2.2- diphenyl-3lbutanoni Isomer&ion of 2,3-diph&yldu- tadiene to 2-phenyl-3-methylindene is well known; the latter compound is an isomer of 2-methyl-l- phenylindene found in the fraction of 142°C.The final conclusion has to be di-[l-phenyl- ethyllether is not or practically not formed from acetophenone by electrolysis with carbon electrodes in aqueous acidic solutions at high temperature.
Since 2,2-diphenyl-3-butanone and 1,2-diphenyl-2- methyl-1-propanone can be formed from the meso- form as well as the DL-forms of pinacol, it is likely that at high temperature the diastereometric DL/meSO ratio obtained from the analysis of catholyte samples can be affected by the rate of dehydration and of rearrange- ment of the different forms of pinacol. This means that the experiments carried out at high temperature are less suitable to study the effect of parameters as pH, nature of solvent and conducting salt and material of cathode upon the diastereometric DL/nESO ratio.
3.3. Course of electrolysis
The electrolyses were usually carried out at a constant potential difference between the working electrode and the reference electrode. Using the simple experimental set-up described in 2.1, it is impossible to perform a macroscale electrolysis for a long period, eg 200 h, at a constant cathode potential, since the composition of both the catholyte and the anolyte change during the electrolysis and the conductivity of both solutions as well. Serious attention was given to the pH-buffer capacity of the catholyte to keep an almost constant pH during the electrolysis. Preliminary experiments showed that a glutinous material was present on the cathode after a long electrolysis, and sometimes, for instance when using a water-dioxane mixture as catholyte solvent, demixing of the solution already occurred after a short time of electrolysis. In this case a glutinous and viscous substance of acetophenone and of products formed during the electrolysis was separated and practically no acetophenone and electrolysis products were present in the residual solution. To prevent deposition of glutin- ous material on the cathode and to keep a constant pH, the time of electrolysis was limited in order to obtain reliable results. From preliminary experiments it fol- lowed that pinacol is sufficiently stable at 25°C to carefully study the reactions taking place on and near the cathode surface. The temperature of electrolysis was kept at 25”C, unless otherwise mentioned.
A typical result for a potentiostatic electrolysis, ie an electrolysis at a constant potential difference between cathode and reference electrode, is given in Fig. 1. The catholyte used for the electrolysis of Fig. 1 was a water-ethanol mixture with a volumetric ratio of 20 to 80 and containing initially 0.2 M H2S04 and 0.035 h4 acetophenone. The pH of catholyte increased slowly during the electrolysis, viz from the initial value of
900 L. J. J. JANSSEN c/mM reaction products 02 c/mM acetophenone phenylethanol 0.20 i,A 1 0.1 5
Fig. 1. Concentration of acetophenone, oL-pinacol, mesopinacol and I-phenylethanol in catholyte and the current as a function of time ofelectrolysis. Cathode: graphite disc of 12 cm2: initial composition ofcatholyte:
water-ethanol mixture (20: 80), containing 0.2 M HZS04 to.07 M acetophenone.
0.71-0.74 at the end of electrolysis. The cathode potential, ie the adjusted potential minus the ohmic polarization, decreased from - 1.86 to -2.42 V. Figure 1 shows that, during the electrolysis, both the current and the acetophenone concentration decrease and the concentration of both forms of pinacol and l- phenylethanol increase at a decreasing rate.
An accurate determination of concentrations of l- phenylethanol smaller than about 0.001 M was rather difficult. Taking into account the inaccuracy in the results of analysis, it may be concluded thbt the overall selectivity for 1-phenylethanol, DL-pinacol and meso- pinacol is practically constant during the electrolysis. It has been found that the overall current efficiencies for the three reduction products mentioned decrease during the electrolysis due to the decline in ac- etophenon concentration. After a 24-hour electrolysis the overall current efficiencies are 0.007, 0.059 and 0.05
1
for, respectively, 1-phenylethanol, DL-pinacol and meso-pinacol.3.4 Cathode potential
In order to study the effect of the cathode potential on the selectivity of acetophenone reduction, potentios- tatic electrolyses were carried out at 25°C with a water-ethanol mixture (20 to SO), containing initially 0.070 M acetophenone and 0.2 M H,SO,. During these electrolyses the cathoode potential decreased with increasing time of electrolysis. The decrease in cathode potential is high, for instance, a decline in the current by about 50’!a causes a 600 V decrease in cathode potential. The relation between current and cathode potential was determined before and after addition of 0.0070 M acetophenone to a water+thanol mixture (20 to 80), containing 0.2 M HZSO,. It has been found that the addition of acetophenone has practically no effect.
Table 2 shows the effect of the cathode potential E on the overall selectivities So,, S, and S, and on the overall selectivity ratio SDJS,. Moreover, the starting cathode potential E,, the end cathode potential E, the starting current density i, the end current density i, and the fractional conversion of acetophenone X, are given in Table 2. Though the cathode potential is not constant during an electrolysis, from the results of Table 2 it may be concluded that the cathode potential in the range from - 1.6 to - 3.1 V has no effect upon the selectivity of acetophenone reduction.
3.5 Water content in catholyte
Experiments were carried out at 25°C with various water contents in the ethanol-water mixtures; the water content varied between 5 and 40 “4. The poten- tial difference between cathode and reference electrode was adjusted so that the starting current was about 0.2 A. The fractional conversion for acetophenone varied between 0.68 and 0.99. The mean values for the overall current efficiencies for DL-pinacol, meso-pinacol and
1-phenylethanol were, respectively, 0.047, 0.042 and 0.004. No significant effect of the water content has been found. Table 3 shows the overall selectivities So, S, and S, and the overall selectivity ratio So@,,, for the acetophenone reduction in catholyte with various water contents. From Table 3 it follows that the water content does not affect the selectivity of the ac- etophenon reduction.
3.6. Nature of supporting electrolyte
TO investigate the effect of the supporting elec- trolyte, galvanostatic electrolyses with a current of 1 A were carried out at 25°C for an acidic water-ethanol mixture (volumetric ratio 38 to 62), containing 0.2
Macroelectrolysis of acetophenone
Table 2. The effect of the cathode potential on the reduction of acetophenone on a graphite cathode of 12cma at 25°C and in a water-ethanol mixture (20 to 80)
containing initially 0.2 M H,SO, and 0.070 M acetophenone Experiment number
1 2 3 4
Starting potential E,,V End potential E,,V Starting current i,,A End current &,A
Fractional conversion X oc Overall selectivity SD, Overall selectivity 5, Overall selectivity 5, Overall selectivity ratio S&S, Overall current efficiency Qor Overall current efficiency Q, Overall current efficiency Qe
- 1.57 - 1.86 -2.13 -2.40 - 1.95 - 2.32 - 2.74 -3.10 0.145 0.200 0.250 0.290 0.064 0.101 0.120 0.140 0.999 0.980 0.978 0.838 0.320 0.320 0.349 0.342 0.219 0.282 0.308 0.308 0.018 0.014 0.013 0.017 t.147 1.134 1.134 1.11 0.03 1 0.046 0.046 0.033 0.027 0.041 0.041 0.030 0.002 0.002 0.002 0.002
Table 3. Effect of the volumetric water content in ethanol-water mixtures containing initially 0.2 M H2S04 and 0.070 M acetophenone on the selectivities for the reduction ofacetophenone
on a graphite cathode at 25°C Parameters
Starting potential E,, End potential E, v Starting current isA End current i,,
Fractional conversion X, Overall selectivity So, Overall selectivity S, Overall selectivity S, Overall selectivity ratio S&S, Overall current efficiency Q,, Overall current efficiency Q, Overall current efficiency Q,
Volumetric water content
40% 30% 20 ‘% 10% 5 % - 1.81 - 1.80 - 1.86 - 2.25 -2.15 - 2.01 -2.10 - 2.32 -2.97 - 2.43 0.200 0.200 0.200 0.175 0.095 0.105 0.100 0.101 0.103 0.08 1 0.98 1 0.974 0.980 0.999 0.68 0.324 0.333 0.320 0.310 0.324 0.285 0.295 0.282 0.274 0.290 0.013 0.016 0.014 0.012 0.01 s 1.14 1.13 1.13 1.13 1.12 0.045 0.047 0.046 0.049 0.052 0.040 0.042 0.041 0.043 0.046 0.002 0.002 0.002 0.002 0.002 901
M HCl or 0.2 M H,SO, or 0.2 M H,SO, and 0.2 M benzenesulphonic acid or acetic acid or 0.02 M anthra- nilic acid, tefraethylammonium tetrafluoroborate,
D( + ) gluconic acid-o-lacton, ephedrine or tetraethyl-
ammonium perchlorate. The initial concentration of acetophenon was 0.16 M. The time of electrolysis varied between 25 and 50 hours and the fractional conversion between 70 and lOO’y& TabIe 4 shows the effect of the nature of organic compounds added to the sulphuric acid-water-ethanol mixture upon the over- all selectivity ratio S&S,,,. From Table 4 it follows that for all organic compounds the ratio S,,/S, is of
Table 4. Effect of the nature ofan organic compound added to a water-ethanol mixture (20 to 80) contain- ing initially 0.2 M H,SO, and 0.070 M acetophenone on the selectivity ratio for the reduction of ac-
etophenone at a graphite cathode Extra organic compound
None Bcnmnesulphonic acid Acetic acid Anthranilic acid Tetraethylammonium tetrafluoroborate D( + ) Gluconic acid-Llacton Ephedrine Tetraethylammonium perchlorate SDLf5ln 1.16 1.06 1.15 1.14 1.13 1.16 1.12 1.12
the same order of magnitude. It has been found that the S.&S,,, ratio for the solution containing hydro- chloric acid is practically equal to that for the solution containing sulphuric acid.
3.7. Nature of organic solvent
Galvanostatic electrolyses with a current between 0.15 and 0.24 A were carried out at 25°C for acidic mixtures of water and methanol, ethanol, propanol-1 and propanol-2. The volumetric ratio between water and the organic compound is 20 to 80. The mixtures contained initially 0.035 M acetophenone and 0.2
M HsSO,. Table 5 shows the effect of the nature of the organic compound in the solvent mixture upon the overall selectivity ratio SuJS,. Moreover, the frac- tional conversion for acetophenone is given in Table 5. From Table 5 it follows that the S,JS,ratio increases in the sequence of methanol, ethanol, propanol-1 and propanol-2.
3.8. pH of catholyte
Experiments were carried out with a water-ethanol mixture (80: 20) containing 86 mM acetophenone, 0.2 M sodium toluene sulphonate and 0.2 M, 1.2 M KOH, 0.2 M HISO* or a pH buffer. The starting current varied from 40 to 150 mA. the temperature was kept at 25°C.
902 L. J. J. JANSS~N
The pH of the catholyte was measured at the start of the electrolysis and after some periods
of
electrolysis.
Figure 2 shows the overall selectivity ratio S,,/S,as a function of pH.the overall current efficiency for pinacol was about 0.30 during the first 4 hours of electrolysis.
Unfortunately, the pH of the catholyte changed during some electrolyses. In these cases, the pH range is indicated by the length of the horizontal straight. The increase of pH was caused by too low a capacity of the buffer system due to the low solubility of its com- pounds. To limit the pH change during the electrolysis, in some experiments the time of electrolysis was reduced from about 22 to about 4 hours. From Figure 2 it follows that the S,,/S,ratio increases from 1.1 to about 3.3 for a pH increase from 1 to 14.
For experiments with alkaline solutions with a pH of about 14 (0.2 M or 1.2 M KOH as extra addition), the total selectivity for pinacol was about 0.89 and practically no 1-phenylethanol was formed. Moreover,
The number of identified compounds formed by reduction of acetophenone on a graphite cathode depends on conditions of electrolysis, especially tem- perature and pHC3.13. In acidic media at high tempera- ture, pinacol is unstable and rearranges to ke- tonesC3.11. To obtain the diastereometric S,,/S,ratio for pinacol formed at the electrode surface and to diminish the number of reaction products, the elec- trolyses were generally carried out at 25°C. The distribution of reduction products in acidic media differs strongly from that in alkaline media. Hence, both media are discussed separately. It has been found that in acidic media at 25°C the identified major reduction compounds are D, L-, and meso-pinacol (the
sum of the three forms of the pinacol is indicated by pinacol). Moreover, I-phenylethanol is formed in small quantities. The total selectivity for pinacol and I- phenylethanol lies between 0.61 and 0.66CTables 2-41. Consequently, about one third part of acetophenone is converted into unidentified compounds. Preliminary experiments showed that some glutinous material is formed by reduction of acetophenone. Some resinous material has also been found by Swann et aI.[l-31. From the results of Horner et a/.[71 and of Stocker and JeneveinClO] it follows that during their electrolyses a large part of acetophenone was also converted to unknown by-products.
Table 5. Effect ofthe nature of thealcohol in the water-alcohol mixture (20 to 80) containing initially 0.2 M H,SO, and 0.070 M acetophenone on the selectivity ratio for the reduction ofacetophenone at a graphite
cathode Organic solvent X, Methanol 0.60 Ethanol 0.97 Propanol-1 0.83 Propanol-2 0.98 - sDL,sm 1.02 1.11 1.23 1.24 3.0 SOL
i
i
t?
i
lb
ri
li
--,PHFig. 2. The diastereometric S,,/S, ratio as a function of the pH of catholyte. Cathode: graphite disc of 12 cm’; initial composition of catholyte: water-than01 mixture (20: 80), containing 0.087 M acetophenone, 0.2 M sodium toluenesul- phonate and different electrolytes, viz 0.2 M H,SOL (1). (0.2 M KOH (2), 1.2 M KOH (3), acetic/sodium acetate (4). NaHCO,/Na,CO, (5), H,P0,/NaH,P04 (6), NaH,PO,/Na,HPO, (7), Na,HPO,/Na,PO, (8),
ethylenediamine/toluenesulphonic acid (9).
4. DISCUSSION
The total overall current efficiency for the formation of identified compounds is only in the order of 10 ‘52 (Tables 2-4). Neglecting the charge used for the production of glutinous material, the rate of hydrogen evolution corresponds to about 90’5; of the total current, this explaining the slight effect of the addition of acetophenone to an acidic ethanol-water mixture on the potential-current relation[3,4]. To elucidate the question whether the mass transfer of acetop- henone to the graphite cathode is the rate-determining step for the acetophenone reduction, an estimate of the limiting diffusion current density was made. It has been found that, during the first 4 hours of electrolysis, the current density used for acetophenone reduction is about a factor of 10 smaller than its diffusion limiting current density. This means that the rate-determining step of the acetophenone reduction is an electrode reaction; probably the reduction of acetophenone to the carbinol-free radical.
It can be shown that during almost the whole time of electrolysis the concentration of acetophenone at the cathode surface is almost equal to that in the bulk of the catholyte. Probably this explains the independence of the selectivity ratio S,/(S,,+ S,,,) (Table 3). Brown and Horner[6] have found that, for the reduction of acetophenone at a mercury cathode the selectively ratio S,/(S,,+ Sd increases strongly with decreasing pot- ential, ie from 0.08 at E = - 1.34 V to 10 at E = - 1.80 V. The performance of their electrolyses differed much from the one applied in the present study. They continuously added a 0.05 M acetophenone solution to the catholyte, to electrolyse at a very low, but un- known, acetophenone concentration. The stereochem-
Macroelectrolysis of acetophenone 903 istry of pinacol formation in acidic media is the main
object of this study. It has been found that the selectivity ratio S,JS, is practically independent of the time of electrolysis (Fig. 1), the cathode potential and the water content of ethanol-water mixture for a water content varying between S and 40 %. The ratio depends on the nature of the organic compound of the solvent and increases in sequence of methanol, ethanol, propanol-1 and propanol-2 (Table S), corresponding to the sequence in decreasing dielectricconstant for the pure solvents. Moreover, the nature of organic salts or acids has also a moderate effect on the selectivity ratio S, JS, (Table 4). Stocker and Jenevein have con-
cluded that the So JS,,, ratio is independent of the
nature of cathnde material, the time of electrolysis and current, but the spread in their results is rather large. In alkaline media, the formation of I-phenylethanol is undetectable and the identified major reduction compounds are the three forms of pinacol, ie D-, L- and meso-pinacol. The total overall selectivity for pinacol is about 89 %; this is significantly higher than the one in acidic media. Thus, the formation of unknown by- products is also much less in alkaline than in acidic media. Moreover, the current efficiency for pinacol formation in alkaline solutions of pH = 14 is a factor of about five higher than that in acidic solutions of pH = 0.7. The So JS, ratio at pH = 14 agrees well with
the result found by Stocker and Jenevein[lO].
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