J. H. Carney and 0. J. Mullia University of Alabama University, Alabama 35486
Preparative Reduction of Benzil Use of a polarographic analyzer and simple current booster
Electrochemical synthesis and electroorganic chemistry continue to receive attention as indicated in recent reviews by Fry ( I ) and Chang (2). Iversen (3) has developed an experiment which illustrates the preparation of dichloroacetic acid by reduction of trichloroacetic acid. However, this reduction requires a fairly long electrolysis and workup procedure. We wish to report a simple lahoratory experiment which demonstrates some of the preparative and mechanistic electrochemistry of ketones in acid solution. Also, we will describe a simple and inexpensive apparatus for preparative scale electrochemistry. There are generally two problems encountered in designing an experiment to illustrate the electrochemical technique of synthesis. The first problem is equipment. For selective reductions or oxidations a potentiostat capable of 0.5-2.0 A is necessary. A potentiostat is a device for keeping the potential difference between the reference electrode and a working electrode, at which the electrochemical reaction is carried out, a t some preset value. This function can be accomplished by the circuit shown schematically in Figure 1. The potential between the working electrode and the reference electrode is detected by the potential comparison device and compared to the desired potential. If the two potentials are not equal, an error signal is generated and transmitted via the feedback link, shown here as a dotted line, to cause the voltage applied between the counter and working electrodes to change until the working electrode-reference electrode potential difference reaches the desired value. In manual potentiostats, the potential comparison and error signal feedback are accomplished by a potentiometer and the human eye, brain, and hand. Further information on potentiostats and controlled potential electrolysis is contained in the review by Fry (4). There are three basic choices of instruments which will accomplish the potential control scheme shown in Figure 1: a relatively expensive automatic potentiostat or programmable power supply, a manual instrument that requires close attention by the operator, or a current boater that can be added to existing polarographic equipment. We chose the third possibility because of cost considerations. Though it is t m e that the cost of a polarographic analyzer is comparable to that of commercial high current potentiostats, polarographic equipment is very often already available in the laboratory. Also, it is often necessary to determine the polarographic behavior of a system prior to and during electrochemical synthesis. If one limits himself to either reductions or to oxidations, the cost of the hwster with power supply is less than $30. If a 50-V, 1-A supply is already available, the cost is less than $15. The second problem in designing an experiment is finding a compound that can he electrolyzed in reasonable amounts during one lab period. Also the product should be fairly easily separated from the electrolyzed solution. We have found that henzil (I) meets these requirements, and furthermore provides a rather interesting and straightforward example of some of the complexities that can arise in reduction of organic compounds. Polarography' of henzil in acid solutions shows only one, two-electron
'..
'
potential cornpanson devlce
Figure 1. Schematic diagram of potentiastat: C.E. Counter electrode: R.E. Reference electrode; W E . Working electrode.
wave (5). I t is reduced to the enediol (II), which is not reducible and only 0
0
H
H
0
0 0
I
I
slowly tautomerized to henzoin (III), which can he further reduced in 0.1 M HCI a t -0.9 V versus Ag/AgCI (5). Thus a polarogram of henzil shows only one wave; hut a polarogram of partially reduced henzil shows an anodic wave for oxidation of stilhenediol which decreases in time as i t forms henzoin. After reduction is complete, polarography reveals only a cathodic wave for reduction of henzoin. The electrochemical generation of benzoin from henzil is a good example of a preparation which requires potential control. Reduction of benzil by passing current between two electrodes a t uncontrolled voltage will produce at best a mixture of henzoin and its reduction product. However, by controlling the working electrode potential a t -0.6 V with respect to a Ag/AgCI reference electrode, henzoin will he generated in good yield and will not be further reduced. One can amroach the svstem from the s t a n d ~ o i n tof preparative ei&ochemist&, demonstrating that'a selective electrochemical reduction of benzil can he carried out to produce a compound that is easily isolated and identified. Or one can also pursue details of the mechanism; first by determining the number of electrons in the reduction by coulometry and then by determining the rate of d i s a ~ ~ e a r a nof c ethe intermediate diol. T& necessary current integration for coulometry can he accomplished by a chemical or electronic integrator, but the simplest and least expensive method makesuse of the exponential relation between current and time for a convection-controlled mass transport process (6)
where io is the current a t the start of the electrolysis and Volume 51. Number 5, May 1974
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kc the mass transport constant. Integration of the current for complete electrolysis gives the total charge passed Q = iok, By plotting In fi) versus time, kc, i0, and thus the charge Q can be obtained. Finally, application of Faraday's law allows determination of the number of equivalents per mole. Further mechanistic details can be demonstrated by interrupting the electrolysis and following the decay of stilbenediol and buildup of benzoin by polarography, cyclic sweep voltammetry, or even uv spectrophotometry (7). The polamgrams of Figure 2 illustrate details of the reduction and subsequent tautomerization a t p H 4.55. This p H was chosen to simplify detection of the enediol which, in 0.1 M HC1, appears near the anodic limit of mercury. The polarograms were obtained a t the times indicated after a 10 min electrolysis of henzil. By plotting the polamgraphic diffusion current for oxidation of stilbenediol, one can obtain a first-order rate constant for the tautomerization. A check on the rate constant can be obtained from the increase in current due to reduction of the final product, benzoin. Values obtained agree favorably with that determined by Perone (7). Further information concerning the effects of p H on the reduction and the surprisine fact that the ratio of cis to trans stilbenediol can be cokrolled by the potential a t which the reduction is carried out has been ~ u b l i s h e dhv Vincenz-Chodkowska and Grabowski (8). Experimenlal
supply capable of at least 30 V dc and 0.5 A. Voltage stabilization in the power supply is not necessary. A maximum of 70 V is set to protect the two transistors. The amplifier and current meter were mounted in a small chassis with Qz on a heat sink. All connections were made with patch cords so that polarographic measurements could be m with the same polarographic analyzer used to control the booster. The potentiostat was found to be capable of at least 1.0 A through a dummy cell. The potentiostat configuration described here has been used daily in our laboratory for 6 mo with no problems. Wwdward, Ridgway, and Reilly have described a more versatile current booster which can perform both oxidations and reductions, but it is necessarily more complicated and is limited to about 100 mA (9). Cell
The cell employed and the Ag/AgCl reference electmde have been described by Iversen (3). However, it was found that only a single porous glass frit divider between the two cell compartments was necessary. Preparative Reduction of Benzil To each compartment of the cell 150 ml 95% ethanol was added. Then 1.0 g of benzil was added to the cathode compartment. After the henzil had dissolved, 150 ml of 0.2 M HCI was added to each compartment. A polamgram was run to determine the potential at which the electrolysis should he carried out. The potentiostat was then set st -0.6 V and electrolysis carried out at the mercury pcol until the current indicated that the reduction 1 Oxidations may be carried out by substituting a pnp Darlington power transistor such as an HEPS9141 for the two transistor npn amplifier. The power supply leads are reversed but all other connections remain the same.
Booster The circuit shown in Figure 3 is a Darlington amplifier with the emitter of Q1 connected to the base of QZ to increase the input impedance of the amplifier.' The current booster was driven by a model EUA 19-2 Heath polamgraphic module. We used the Heath 1P20 as a power supply; however, it can be any low ripple
BOOSTER
B
A 4 CE. Q W.E.
POWER SUPPLY
Figure 3. Booster and power supply schematic: 012N3643 O2 TR61 TTTransformer
Figure 2. Polaragrams r& after lo-min electrolysis of benril solution. 48% ethanol, pH = 4.55, 1.25 mM benzil initially. Polarogram started at a ) loo s, b) 600 5 , and c) 2100 s after the 10-min electrolysis. Region I: reduction of benzil; Region 11: oxidation of stilbenediol; and Region Ill: reduction of benzoin. 344
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Journal of Chemical Education
D
(Stancor p8197) Four IN3256 diodes
C,,C,250MF108V R39ohms. 10 W
Connectto counter electrode jack of polarographic analyzer B. Connect to reference electrode jack of polarographic analyzer
A.
C. E. Counter electrode R. E. Referenceelectrode
W. E. Working electrode
hr). Stirring was accomplished with a magnetic stirrer and a 4 cm spin bar. The potential was kept positive to -0.80 V to prevent reduction of benzoin as it formed from the enediol intermediate. Solution color changed from pale yellow to colorless during the reduction. A palmgram at the end of the electrolysis indicated that the wave due to b e n d had disappeared and a new wave with an Ex,, of -0.91 V appeared. The solution in the cathode compartment was evaporated to one-third its original volume and extracted five times with 75-ml portions of diethyl ether. The ether solutions were combined and evaporated, leaving a crude precipitate. Recrystallization from 15 ml of ethanol yielded a white precipitate (7W800 mg). This product was identified as benzoin: rnp 131-132'C (lit. 1309C, Ref. (7)); uv;., (EtOH) 246 nm; ir(KBr) 34W em-' (O-H), 1660 em-' ( G O ) . Additional confirmation was obtained from elemental analysis and mass spectroscopy. was complete (about 3 4
acetate of 0.2 M. The concentration of b e n d was 1.0 mM. Electrolysis was carried out as before but a t -0.7 V, then stopped after 10 min and polamgrams run a t 5-min intervals until the anodic current for the enediol oxidation had substantially decreased. A plot of the logarithm of the stilhenediol diffusion current as a function of time yielded the first-order rate constant for tautomerization. One critical consideration is that the rate of stilhenediol generation must exceed the tautomerization rate. To this end the ratio of mercury surface area to solution volume and the stirring rate were made as large as pcasible. Acknowledgment T h e authors would like t o express their appreciation to
H. A. Moore of t h e Electronics Shop, Department of Chemistry, for his help in ronstmction of t h e booster.
Coolometry While in theory, current integration as described above could be accomplished on the same solution employed for preparative reduction, in practice, solution concentration in the range of 1.0 mM gave better results for our particular cell and stirring arrangement. Electrolysis was carried out as before at -0.65 V, and the current was read from the meter on the boaster every 100 s over a 20-min period. Then in i was plotted versus time to obtain is, kc and thus Q. The number of electrons obtained by this method was 2.0 0.1.
*
Tautomerization Rate
Literature Cited (1) Fty, A. J., "Synthetic Organic Electmcbemistry," Harper and Row, In.
.. New York.
111%~
(2) Chang, J.. Large, R. F., and Popp, G., "Physical Methoda of Chemistry," Pan n B.. (Editors: weisrbeqer. A., and b i t e r , B. W..) Wilcy-Inu.sclcnce. New York, I97I.Vol. I, pp. 1-89, (3) Ivcrsen, P. E..d. CHEM.EDUC.. 48; 136 (1971). (41 hy,A. J.. "Synthotle Oqmlc Elebmchcmistw," H m r and Raw,h e . , New Ywk. 1972. pp. 9 M 1 2 . (51 Pastemsk,R.,Hclu. Chim Acto, 31.753(194%l. (61 Delahay, P., "New Instrumental Methoda in Eloetrahemistw," Wllry-Interseience. New Ymk. 195C no. 283-284.
The buffer was prepared by mixing 95% ethanol in a 1:1 ratio with pH 4.55 acetate buffer, which had a concentration of d i u m
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