Electrochemical and Spectrophotometric Study of Isomeric Stilbenediol Intermediates in the Reduction of Benzil in Acid Solution H. E. Stapelfeldt and S. P. Perone Department of Chemistry, Purdue University, Lufuyette, Ind. 47907
THEELECTROCHEMICAL reduction of benzil in aqueous-alcohol media has been the subject of several investigations. Polarographic studies indicate that the reduction is irreversible and requires two electrons over the pH range 2 to 10. It has been suggested that the reduction initially produces stilbenediol, which slowly rearranges to give benzoin, the latter being detected only at sufficiently long times after the electrolysis ( I ) . More recently, reduction of benzil electrochemically at the mercury electrode and by catalytic means was reported to give rise to two electrochemically oxidizable intermediates (2, 3). Cyclic voltammetric studies at the hanging mercury drop electrode ( H M D E ) indicated that the two oxidation processes differ by about 90 mV. In the case of constant potential reduction, the ratio of the two intermediate species formed depends on the electrolysis potential used for reduction, the solution pH, solvent composition, temperature, and the electrolyte composition. On the basis of these observations the following mechanism has been proposed (2): trans
4
OH
\
/
/
\
slow -\
c=c
0 0
(Benzil)
4
OH 0
I
4
HO
il
+CH-C+ (Benzoin)
OH cis
That is, the initial reduction of benzil produces a mixture of cis and trans stilbenediols which are unstable intermediates, finally rearranging to benzoin. These electrochemical results are of particular interest because of the apparent effect which the electrolysis potential, and other experimental parameters, have on the ratio of the isomeric intermediates formed. Therefore, a correct assignment of structure for the intermediates which are formed is desirable. Work in this laboratory involved the necessary combination of electrochemical, spectrophotometric, and kinetic data to make this assignment possible. EXPERIMENTAL
Instrumental. The instrumentation for the electrochemical experiments utilizing operational amplifiers has been described previously (4). Cyclic voltammetric data were ob(1) H. Pasternak, Helu. Chim. Acta, 31,753 (1948). (2) A. Vincenz-Chodkowska and 2. R. Grabowski, Electrochim. Acra, 9, 789 (1964). (3) E. J. Bauer, Ekcrroanal. Chem., 14,351 (1967). (4) S. P. Perone and J. R. Birk, ANAL.CHEM., 38, 1589 (1966).
tained using a Hewlett-Packard Model 202-A function generator modified for single-cycle operation. Voltammetric curves were recorded on an Esterline-Angus Speed Servo, 1 mV, '/,+econd recorder. Spectrophotometric results were obtained on a Cary 14 and Perkin-Elmer 202 recording spectrophotometers. Electrolysis Cell and Electrodes. The cell and electrode assemblies used were similar to those previously described (5), except that the cell bottom was drawn out to provide a mercury reservoir with attached Pt-wire contact. The working electrode for all preparative electrolysis was a large mercury pool (approximately 28 cm2 area). A hanging mercury drop electrode ( H M D E )was used for cyclic voltammetric experiments. Reproducible drops were obtained by dialing the appropriate drop size with a microburet type HMDE (Metrohm Ltd., Switzerland). The reference electrode was a large saturated calomel electrode. The reference electrode compartment was connected to the cell by a salt bridge composed of a fine porosity glass disk and a Luggin capillary probe containing 1M KCI solution. The counter x 3-inch graphite rod immersed in 1M electrode was a KCI solution, and separated from the sample solution by an ultra-fine frit. All electrochemical experiments were carried out at 25" i 0.5" C. Reagents. Sample solutions were prepared in 48z (by volume) ethanol and 50z (by volume) methanol solvent. All water was purified by distillation and passage over a mixed cation-anion exchange bed (Amberlite MB-3). Ionic strength of solutions was maintained constant when making quantitative studies as a function of pH, but was changed when necessary to optimize the ratio of reduction intermediates obtained. HCI, acetate and phosphate buffers were used and pH values given are measured values. Bulk alcohols (95% ethanol and 99% methanol) were checked for minor electroactive impurities using cyclic voltammetry and none were found. Spectrophotometrically, the alcoholwater solvents used were as transparent in the range of interest as identical solutions prepared from the highest quality alcohols available (Baker spectro-grade methanol and Commercial Solvents Corp. "Gold Seal" ethanol). All other chemicals were reagent grade and were used without further purification. Oxygen was removed from sample solutions by high purity nitrogen. To remove traces of oxygen from the nitrogen, the gas was bubbled through two gas-washing bottles containing chromous chloride and amalgamated zinc (6). The nitrogen train passed through a third bottle containing solvent and finally was dispersed in the cell by a coarse porosity frit. Procedure. Twenty-five-ml portions of sample solution were added to the electrolysis cell and deaerated for at least 10 minutes prior to reduction of the benzil. Preparative electrolysis was carried out at constant potential while the solution was stirred and deaerated. (Vincenz-Chodkowska et al. ( 2 ) and Bauer ( 3 ) have shown that appropriate variation of the electrolysis potential and solution pH permit the selective preparation of either isomeric stilbenediol as the major (5) S. P. Perone, and T. R. Mueller, ANAL.CHEM., 37,2 (1965). (6) L. Meites, Anal. Chim. Acta, 18, 364 (1958). VOL. 40, NO. 4, APRIL 1968
81 5
Table I. Kinetic Data for the Rearrangement of trans Stilbenediol, 50 Methanol Solvent, Ionic Strength = 0.1,” a t Various pH’s PH
k t , (sec-l) X IO3 f std dev
8.0 7.5 7.2 6.5 5.5 3.3 3.3 1.0 1.0
13.7b=t4 . 1 7.9b A 0 . 6 7 . 3 b=k 1 . 4 5 . 0 b =k 0 . 5 3.0* f 0 . 2 1 . 6 bf 0.1 1.7c f 0 . 1 1 . 1 b A 0.1 1.2< iz 0.1
p = 0.13 for pH 7.5 solution. Obtained from electrochemical i ( H M D E ) us. time plots, in stirred solution, at 25 O C. c Obtained from spectrophotometric absorbance 6s. time plots, not thermostated.
a
W 0
z a m a
0
m m
a
b
product of the benzil reduction.) At the end of a measured electrolysis period, the sample-intermediate mixture was examined both electrochemically and spectrophotometrically with respect to time. Electrochemical measurements involved either constant-potential current measurement with stirring or fast sweep voltammetry. In either case, the HMDE was used. Spectrophotometric measurements were made by removing an aliquot of sample-intermediate mixture with a syringe and placing it in an unthermostated cell in the spectrophotometer.
210
230
250
270
290
310
330
350
WAVELENGTH (mi) Figure 1. Spectrophotometric results after 10-minute electrolysis of benzil solution. 48z ethanol, pH = 1.0, 0.81 X M benzil initially Scan from 350 mp to 210 mp at 1mp per second (a) start at 2.5-minutes after the 10-minute electrolysis (b) start at 10.0-minutes after the 10-minuteelectrolysis (c) start at 30.0-minutes after the 10-minute electrolysis (d) start at 75.0-minutes after the 10-minute electrolysis
RESULTS AND DISCUSSION
Ultraviolet absorption studies of several a- and a,a’substituted stilbenes (7, 8) have provided certain generalizations regarding the spectra of these compounds. (1) The conjugation or K-band is shifted to shorter wavelengths and reduced in intensity with increasing number and size of substitutents on the ethylenic carbons. (2) The trans isomer absorption of stilbene and a-methylstilbene occurs at longer wavelength than the corresponding cis isomer. On the other hand, the reverse is true for the a,a’-dialkylstilbenes. (3) The trans isomer absorption (K-band) is more intense than that of the cis isomer. Correlations have been made(8) for A,, in the actual spectra and the most probable spatial configuration of the alkylated stilbenes based on a simple LCAO calculation. However, the necessary assumptions break down for correlations involving similar compounds having substituents with nonbonding electrons. On this basis, no sound predictions can be made as to the position of A, in the UV spectra of the isomeric stilbenediols. For those substances which exhibit geometric isomerism such as the stilbenes, one isomer more closely approaches a coplanar configuration. This is usually associated with the position of “maximal elongation” of the isomeric structure. As a result, the absorption properties of the isomers will differ, the more elongated isomer (trans) absorbing more intensely (9). On this basis a structural assignment can be made. With regard to the two electro-oxidizable intermediates formed when benzil is reduced, this assignment requires a combination of electrochemical and spectrophotometric observations. In (7) H. Suzuki, Bull. Chem. SOC.Jap., 25, 145 (1952). (8) Zbid., 33, 396 (1960). (9) R . S. Mulliken, J . Chem. Phys., 7, 364 (1939).
816
ANALYTICAL CHEMISTRY
the work reported here, it was found that the more easily oxidized species was the trans stilbenediol, while the cis isomer was more difficult to oxidize. The results on which these conclusions are based follow. Observations at Low pH. Electrolysis of benzil a t -0.950 V cs. SCE in 48x ethanol or 50x methanol, pH = 1.0, produced a single electro-oxidizable intermediate (trans) as predicted by previous work (2, 3). Typical spectrophotometric results are shown in Figure 1. Under conditions where the majority of the benzil (>90z) was reduced during the 10-minute electrolysis, an initial absorption maximum appeared at ca. 3 1 0 mp. At longer times after the electrolysis this absorption decreased and a new absorption maximum appeared at ca. 260 mp (unreduced benzil plus benzoin). The absorption at ca. 310 mp was assigned to the electrooxidizable intermediate observed on the basis of rate studies at pH 1.0, included in Table I. That is, the rate of decay of this absorption was found to be the same as that of the trans species as determined electrochemically. At low pH the overall reduction process is
for which the second step is a first-order pH-dependent process as indicated by the data in Table I. Because the electrolysis is actually a coulometric reduction in a stirred solution, we may write (10) -d[Benzil]/dt = kC*eVkt
(2)
(IO) P. Delahay, “New Instrumental Methods in Electrochemistry,” Interscience, New York, 1954, pp 283-4.
I
0
W 0
z a m a
0
0 v,
I
z=
m
a
41
-
0
0
2
4
6
8
IO
210
230
PH
(3)
Solving this differential equation for the concentration of trans at the end of the electrolysis gives (4)
where, T is the total time of the coulometric electrolysis, kr7 is the rate constant for the trans rearrangement, and [trans], is the concentration of trans isomer at the end of the electrolysis. For a given sample run, it is possible to evaluate k (from the coulometric current-time curve), k t r (from the change in absorbance of trans or the plot of ilr (HMDE) us. t after the electrolysis), and trans]^ from the above equation. Combining the calculated concentration at the end of the electrolysis with the total absorbance change at 310 mp (extrapolating the plot of log absorbance us. time to 7) permits calculation of an extinction coefficient for the trans stilbenediol at a pH of 1.0. 8310
(trans) = 1.9
x
310
330
350
M benzil initially. Scan from 48% ethanol, pH = 7.5, 1.06 X 350 mp to 210 m l at 1 mp per second (a) start at 2.0-minutes after the 10-minute electrolysis (b) start at 10.0-minutes after the 10-minute electrolysis (c) start at 30.0-minutes after the 10-minute electrolysis
where, C* is the initial benzil concentration k is the "rate constant" for the coulometric reduction (a function of stirring rate, cell geometry, etc., all of which were kept constant during the experiment). The overall rate of change of the intermediate (trans) during the electrolysis, is given by kt7[trans].
290
Figure 3. Spectrophotometric results after 10-minuteelectroly sis of benzil solution
5Oz methanol, all buffers of ionic strength 0.1. 0 from electrochemical i(HMDE) us. t plots, 25" C. 0 from spectrophotometric absorbance (310 m d tis. t plots
-
270
WAVELENGTH (my)
Figure 2. pH dependence of the rate of rearrangement of trans stilbenediol
d[rrans]/dr = kC*e-kr
250
l o 4 l/m-cm i= 5 % re1 std dev
Observations at pH's between 4 and 8. Anodic voltammetry at the H M D E after the large scale electrolysis of benzil in the pH range 4 to 8 showed the presence of two electrooxidizable intermediates. For example, at a pH of 7.5, voltammetric peaks were observed at ca. - 0.450 V and
-0.360 V us. the SCE. The more easily oxidized intermediate species was observed to be the same intermediate (trans) produced at low pH. The more anodic peak, ascribed to the cis isomer, appeared gradually with increasing pH. Both peak potentials shifted cathodic with increasing pH. The trend in kinetic data for the trans species obtained over the pH range of 1 to 8, is demonstrated in Figure 2. The rate constant for the decay of the cis isomer appeared to be about ' 1 3 of that of the trans isomer in pH range 4 to 8. Variation of solvent and the electrolysis potential in the limiting region at a given pH resulted in a change in the ratio of the intermediates [cis]/[trans]obtained. In 50 methanol at a pH of 7.5, voltammetric experiments following electrolysis at -1.200 V us. SCE at the HMDE showed a ratio of [cis]/ [trans] < 0.1. Using the approach outlined in the previous section, except that an optimum electrolysis time of two minutes was used, the maximum extinction coefficient of the trans species at a pH of 7.5 was obtained experimentally. e310
(trans) = 1.6 X lo4 l/m-cm
+ 9% re1 std dev
In contrast, at a pH of 7.5 in 48 % ethanol, a maximum value of the [cis]/[trans]ratio was observed for electrolysis potentials between -0.950 V and -1.000 V us. the SCE. On a time scale short as compared to the rearrangement process, voltammetric experiments show the instantaneous value of the [cis]/[trans] ratio to be greater than 10. At the end of the electrolysis procedure under these experimental conditions, less than 5 % of the total intermediate is the trans isomer and therefore the reduction scheme could be written ream
+2e-
Benzil
+ ( H g pool) cis
7Benzoin.
(5)
The value of the rate constant k for the benzil reduction is obtained directly from the coulometric current-time curve. VOL. 40, NO. 4, APRIL 1968
817
[Benzil] Table 11. Kinetic Data for Rearrangement of cis Stilbenediol in 48x Ethanol Solvent, Ionic Strength of 0.13
PH
k , (sec-l) X loa =k std dev
7.5
2.44f 0 . 3
k, (sec-l) X IO* -2.w
0 Obtained from time dependence of i p fqr cis stilbenediol, 25OC. b Obtained from time dependence of ip for benzoin, 25' C.
The rate constant k , for the rearrangement of the cis isomer can be evaluated by obtaining the time dependence of the voltammetric peak current i, for either the cis isomer or for the benzoin (which is reduced ca. 0.6 V cathodic of the benzil reduction). See Table 11. The relative amounts of benzil, benzoin, and cis stilbenediol under these experimental conditions can be determined by using the respective rate constants and Equation 4, or by measuring the peak current i, for the cis species and relating it to the bulk concentration of the cis isomer. In either case, it is assumed that C* = [Benzil] [Benzoin] [cis]. In the latter case, the proportionality constant relating i, to concentration is assumed to be the same for benzil and the cis stilbenediol. Using either approach, at the end of a 10minute electrolysis, the composition of the sample solution (48 ethanol, pH = 7.5)was found to be
+
+
[Benzoin] [cis]
15% 50%
3575
Spectrophotometric results for the same sample-intermediate mixture after the electrolysis are shown in Figure 3. These results show only a slight increase in absorbance at ca. 255 mp over the period of time (based on electrochemical monitoring of both cis and benzoin) required for the rearrangement to be completed. Because at short times (when the concentration of cis is still appreciable) the absorbance at 255 mp is too great to be attributed to benzil and benzoin alone, it appears that the cis intermediate also has an absorption maximum at that wavelengthLe., A,
(cis) 'v 255 mp
+ 5.
In addition, because the maximum absorbance value increases only slightly over a period of time sufficient to allow a substantial conversion of cis to benzoin, it must be concluded that, at a pH of 7.5, ezsa(cis)
'v
m(Benzoin) 'u 1.2
x l o 4 f 5%
This observation substantiates the assignment of the cis isomeric structure to the less easily oxidized intermediate species, because at the same pH eels is smaller than RECEIVED for review December 4, 1967. Accepted January 18, 1968. Work supported by Public Health Service, Grant No. CA-07773 from the National Cancer Institute.
Solvent Extraction-X-Ray Spectrometric Measurement of Microgram Quantities of Tantalum E. A. Hakkila, R. G. Hurley, and G. R. Waterbury Los Alamos Scientific Laboratory, University of California, LQS Alamos, N . M . 87544 A RAPID ANALYTICAL METHOD was required for determining tantalum when present in concentrations between 0.5 and 10 ppm in pure silver. Several spectrophotometric methods (1-6) for measuring tantalum exist, but these methods suffer from nonspecificity or poor sensitivity. Trace element analysis using x-ray spectrographic techniques has become competitive in recent years with spectrophotometric and emission spectrometric methods. Luke (7) used a curved crystal focussing spectrometer for measuring submicrogram quantities of various elements that were concentrated on ion exchange resin discs '/*-inch in diameter. Limits of detection as low as 0.01 pg were claimed for elements havipg Ka: x-rays in the wavelength region between 1.4 and 1.9 A (zinc and iron). Campbell, Spano, and Green (8) used a flat ( 1 ) C. L. Luke, ANAL.CHEM., 31, 904 (1959). (2) Y . Kakita and H. Goto, fbid.,34,618 (1962). (3) J. 0. Hibbits, H. Oberthin, R. Liu, and S. Kallmann, Talunta, 8, 209 (1961). (4) J. C. Guyon, Anal. Chim. Acta, 30, 395 (1964). (5) S. V. Elinson and A. T. Rezova, Zh. Anal. Khim., 19, 078 (1964). (6) G . R. Waterbury and C. E. Bricker, ANAL. CHEM.,29, 474 (1957). (7) C. L. Luke, fbid.,36,318 (1964). (8) W. J. Campbell, E. F. Spano, and T. E. Green, Ibid. 38, 987 (1966).
818
ANALYTICAL CHEMISTRY
crystal spectrometer to determine trace elements adsorbed on an ion exchange filter paper. A limit of detectioa of 0.55 pg was claimed for lead using the La1 x-ray at 1.175A. The separation of tantalum from silver using ion exchange papers or discs is not readily accomplished. Extraction of microgram quantities of tantalum from various elements has been reported, however, from either hydrochloric, nitric, or sulfuric acid solutions containing hydrofluoric acid into 4-methyl-2-pentanone (hexone) (6, 9, IO). An investigation of the extraction of tantalum from either a nitric-hydrofluoric or a hydrofluoric-hydrochloric acid aqueous phase into hexone led to the development of a method for the separation and x-ray fluorescence measurement of microgram quantities of tantalum in silver. EXPERIMENTAL
Apparatus and Reagents. A Philips Electronics, inverted, three-position spectrograph, a molybdenum-target x-ray tube, and a scintillation detector were used. Samples were evaporated on optical cover glasses that were 18 mm in
(9) P. C. Stevenson and H. G . Hicks, fbid.,25, 1517 (1953). (IO) G. R. Waterbury, L. E. Thorn, and R. E. Kelly, U. S . At. Energy Comrn. Rept. LA-3465, 1966.
