and Grant (15). In order to convert the mass spectrometrically determined volume per cent sulfur compounds to weight per cent sulfur, weight-per cent analyses of the aromatics fraction were first calculated using estimated densities for the compound types found. Molecular weight values of the original oils before separation of the aromatics were used to complete the calculations. The comparisons are shown in Table XI. Gas chromatography values are generally higher than those found by mass spectrometry. Because the former procedure includes naphthenethiopheno as well as benzothiopheno types, higher values should be expected. The method has some obvious limitations. The highermolecular-weight fractions from petroleum may contain many compound types not considered here. Except for the three sulfur types and dibenzofurans, the heteroaromatic types are not specifically considered. While many of them will be included in the unidentified categories, at least a few types (15) R. L. Martin and J. A. Grant, ANAL.CHEM., 37,649 (1965).
must be calculated with the identified groups. For example, carbazoles contribute to both the acenaphthene and the fluorene values. Analyses of aromatics synthesized from relatively pure starting materials, as, for example, aromatic alkylates may not be satisfactory. Although the class analysis might be successful, irregularities in the (M-l)+ series could result in unreasonable values for the overlap types. Nevertheless the method gives reasonable analytical results for petroleum fractions. It provides more detail than earlier methods, and it can be used routinely when electronic computing of the results is available. ACKNOWLEDGMENT
The authors thank M. E. Fitzgerald, R. P. Page, and Arc0 Chemical Co., F. P. Hochgesang and Mobil Research and Development Corp., H. E. Howard and Union Oil Co., of Calif.; and J. B. Grutka and Universal Oil Products Co. for mass spectra of Wilmington gas-oil aromatics. RECEIVED for review May 20,1969. Accepted July 31, 1969.
Electrolysis of Organophosphorus Compounds Study of Mechanism of Reduction of Various Diethyl Aroylphosphonates at a Dropping Mercury Electrode K. Darrell Berlin,’ David S. Rulison,* and Paul Arthura Department of Chemistry, Oklahoma State Unioersity, Stillwater, Okla. 74074 Dialkyl aroylphosphonates in acetonitrile were electrolyzed at the dropping mercury electrode (DME) in the presence of benzoic acid to give dialkyl a-hydroxyarylmethylphosphonates as the exclusive product. Coulometric studies revealed an n-value of 2 electrons. In the absence of benzoic acid, benzoins and dialkyl hydrogenphosphonates were the only detectable products by GLC analysis. This situation requires C-P bond cleavage probably after an initial formation of a radical anion. The aroyl radical must couple rapidly at the electrode surface -0 0
0 0
I1 t
ArC-P(OR)2
+e
I t
--* A r $ - P ( O R ) *
4
to give a benzil which is further reduced to a benzoin. A wide variation in n-values was obtained in the electrolysis of dialkyl aroylphosphonates unless benzoic acid was added. A plot of E112 VS. Hammett U-values indicates the reduction is facilitated by electron-withdrawing groups as expected. Half-wave potentials for the dialkyl aroylphosphonates are more negative than for similarly substituted benzils. It is tentatively suggested that a conjugative effect on the carbonyl function by the phosphoryl group may be responsible in part for the decreased ease of reduction in the aroylp hosphonates.
THE POLAROGRAPHIC behavior of organophosphorus compounds at a dropping mercury electrode (DME) has been the subject of relatively few investigations. Reports have been To whom inquiries should be directed.
Du Pont Predoctoral Teaching Fellow, 1967-68; present address, E. I. du Pont de Nemours & Co., Waynesboro, Va. a Deceased. We dedicate this paper to the memory of Dr. P. Arthur. 2
1554
ANALYTICAL CHEMISTRY
published concerning polarographic studies of hydroxyalkylphosphines ( I ) , phosphonium salts (2), and other compounds such as triphenylphosphine (3), 0,O-dimethyl 2,2,2trichloro-1-hydroxyethylphosphonate (4, and various nitrophenyl phosphates (5). A series of papers has been released concerning the polarographic behavior of various metal ions and complex ions in the presence of certain organophosphorus compounds (6-9). A few electrochemical studies which describe the development of polarographic methods for analyses of such compounds as Malathion [S-(l,2-dicarbethoxyethyl) 0,O-dimethyl dithiophosphate] and Parathion (0,O-diethyl 0-p-nitrophenyl thiophosphate) have also been reported (10-13). Santhanam and coworkers (14) examined the behavior of tris-(p-nitrophenyl) phosphate at the DME and ~
~~~~
(1) K. Issleib, H. Matschiner, and M. Hoppe,Z. Anorg. Allg. Chem., 351, 251 (1967). (2) H. Matschiner and K. Issleib, ibid., 354, 60 (1967). (3) S . Wawzonek and J. H. Wagenknecht, “Polarography 1964,” Graham J. Hills, Ed., Macmillan, London, 1966, pp 1035-41. (4) Yu. M. Kargin and K. V. Nikonorov, Izv. Akad. Nauk SSSR, Ser. Khim.,1902 (1966); Chem. Absrr., 66, 4869 (1967). (5) W. M. Gulick, Jr., and D. H. Geske, J. Amer. Chem. SOC.,88, 2928 (1966). (6) H. Sohr, J. Elecrroanal. Chem., 11, 188 (1966). (7) H. Sohr and K. Lohs, ibid., 13, 107 (1967). (8) Ibid., p 114. (9) Ibid., 14, 227 (1967). (10) M. K. Saikina, Uch. Zap. Kazansk. Gos. Univ., Obshch. Sb., 116, 121 (1956); Chem. Abstr., 52, 296 (1958). (11) W. H. Jura, ANAL.CHEM., 27, 525 (1955). (12) C. V. Bowen and F. I. Edwards, Jr., ibid., 22,706 (1950). (13) D. E. Ott and F. A. Gunther, Analyst, 87, 70 (1962). (14) K. S. V. Santhanam, L. 0. Wheeler, and A. J. Bard, J. Amer. Chem. Soc., 89, 3386 (1967).
determined the mechanism for the electroreduction of this compound. A few diethyl acylphosphonates are recorded (15) to exhibit well-defined polarographic waves with halfwave potentials about -1.4 V us. the saturated calomel electrode. The authors conclude that the polarographic activity of the acylphosphonates is due to the carbonyl group in conjugation with the phosphoryl group. Neither product analysis nor the number of electrons transferred during reduction was determined. In a series of phosphorylated aldehydes in aqueous solution (16), an equilibrium was found between protonated hydrated and enol forms, the concentration of which depended upon the pH. In addition, a satisfactory correlation was found between the observed half-wave potentials and the Hammett U-values of the various ring substituents. The present study was undertaken to determine the mechanism of reduction of dialkyl aroylphosphonates at the DME. All experiments were performed in a nonaqueous medium to minimize the effects of hydrolytic decomposition characteristic of these compounds (27). In addition, attempts were made to correlate the observed half-wave potentials with the Hammett a-values of various ring substituents.
The useful cathodic potential range was from 0 to -2.50 V us. ACE. Numerous recrystallizations of the salt were required before electroactive impurities were found to be tolerable polarographically. This salt was dried in a heated vacuum desiccator at 90 "C and stored in the desiccator. Linde "Lamp Grade" nitrogen used for deoxygenating and drying solutions and purging the polarographic cell was purified using a modification of the apparatus developed in this laboratory (25). The additions to the original apparatus consisted of two scrubbers containing concentrated sulfuric acid, a large tube containing Linde Molecular Sieve 4A, and two large recirculating scrubbers containing acetonitrile which was maintained dry with Molecular Sieve. By means of these scrubbers, the purging gas was kept saturated with solvent. Dialkyl aroylphosphonates (I) were prepared according to the procedure in the literature (17). 0
II
qQJC' 1
P-
- OR'
OR'
EXPERIMENTAL
Instrumentation. Current-potential curves were recorded with a Sargent Model XXI polarograph using a Model A iR Compensator (18). All potentials were measured with reference to the acetone calomel electrodes (ACE) originally discussed by Arthur and Lyons (19). The characteristics of the cell have been previously described (20). The DME employed had open-circuit characteristics of t = 7.35 sec and m = 0.919 mg/sec. During electrolysis, a stable storage battery with a simple potential divider was used to supply a constant potential, the value of which was periodically measured with a potentiometer. An Aerograph 1520 gas chromatograph equipped with hydrogen flame and thermal conductivity detectors was used for product identification. The GLC column used was 10 feet X 1/4-in. diameter and was packed with 1 2 x SE 30 on acid-washed, 80/100 mesh, Chromosorb G. A flow rate of 75 ml/min of nitrogen was used, and temperature range examined was from 175-225 "C. Materials. All materials, unless otherwise specified, were reagent grade. Acetonitrile was the solvent used for polarographic reductions, and this was dried to a very low water content (0.0005 or less) using apparatus previously described (21). Determinations of water content were performed using a modified Karl Fischer titration (22). The supporting electrolyte was tetra-n-butylammonium perchlorate prepared by methods in the literature (23, 24).
x
(15) B. Ackerman, T. A. Jordan, C. R. Eddy, and D. Swern, J . Amer. Chem. Soc., 78, 4444 (1956). (16) A. I. Razumov, G. A. Savicheva, and G. K. Budnikov, Zh. Obshch. Khim., 35, 1454 (1965). (17) K. D. Berlin and H. A. Taylor, J. Amer. Chem. Soc., 86, 3862 (1964). (18) P. Arthur and R. H. VanderKam, ANAL. CHEM.,33, 765 ( 1961). (19) P.Arthur and H. Lyons, ibid., 24,1422 (1952). (20) K. D. Berlin, D. S. Rulison, and P. Arthur, ibid., 40, 1389 (1968). (21) D. S. Rulison, P. Arthur, and K. D. Berlin, ibid.,40, 1015 (1968). (22) W. M. Haynes, Ph.D. Thesis, Oklahoma State University, May 1966. (23) I. M. Kolthoff and J. F. Coetzee, J . Amer. Chem. SOC.,79, 870 (1957). (24) E. J. Cokal and E. N. Wise, J. Electroanal. Chem., 11, 406 (1966).
Diethyl a-hydroxybenzylphosphonate (IIa) and diethyl a-hydroxy-p-chlorobenzylphosphonate(IIb) were prepared by known routes (26). Apparently unknown, IIb was obtained in a yield of 5 0 x , m.p. 71-2 "C. A n d . Calcd. for C,HlsO4C1P: C, 47.40; H, 5.79; P, 11.11. Found: C, 47.02; H, 5.51; P, 10.98. Infrared and NMR spectra from both IIa [mp 83-84 "C; lit. (26) mp 83-4 "c]and IIb were in accord with the proposed structures. Diethyl hydrogenphosphonate was obtained from Virginia-Carolina Chemicals and was used with purification in the preparations of IIa and IIb. 4-Chlorobenzaldehyde needed in the synthesis of IIb was washed with aqueous sodium bicarbonate and recrystallized from benzene before use. 4,4'-Dichlorobenzoin was prepared according to the method of Lutz and Murphy (27), mp 86.5-7.5 "C [lit. (27) mp 87-8 "C]. Air oxidation of 4,4'-dichlorobenzoin gave the corresponding benzil, mp 196-7 "C [lit. (28) mp 195-6 "c]. Experimental Procedure. The details of setting up a polarographic run have been described elsewhere (20). Anhydrous techniques using hypodermic syringes and smalldiameter polyethylene tubing were utilized to minimize hydrolytic decomposition of the dialkyl aroylphosphonates. Techniques for polarographic runs were identical with and without a proton source present. All solutions were prepared immediately before use. After deoxygenation and placement of the solution in the cell, initial polarographic data were obtained. Samples were then allowed to electrolyze for periods up to 10 days, during which time points on the
(25) P. Arthur, ANAL.CHEM.,36, 701 (1964). (26) 0. Gawron, C. Grelecki, W. Reilly, and J. Sands, J. Amer. Chem. Soc., 75, 3591 (1953). (27) R. E . Lutz and R. S. Murphy, ibid., 71, 478 (1949). (28) G. T. Barry and R. Boyer, Can. J . Research, 26B,518 (1948). VOL. 41,NO. 12, OCTOBER 1969
1555
Table I.
t
Coulometric Data for Reductions of Various Diethyl
0 0
I! t
Benzoylphosphonates, R-C6H4C-P(OC2H&~ Concn ratio of benzoic Range of acid/ A concns studied phosphonate n H 1.3-4.3 X 10-ZM 0 0 . 3 -0.7 22 1.95-2.03 p-t-C4H9 0.9-2.6 X 10-ZM 0 0.4 22 2.02 0-c1 1.3-3.6 X 10-2M 0 0.3 0.8 0.75 12 2.07 u-Cl 1.0-3.8 X 10-2M 0 0.7 1.5 1.8 >2 1.95-2.00 o-CH~O 1.2 2.8 X lO-’M 0 0.4 1.4 0.4 2.8 0.66 p-CHa 1 . 0 2.7 X IO-ZM 0 0.85 22 1.97 +
0
Iz w
aa
3 0
-E v s ACE Figure 1. Polarogram for reduction of Ia, 2.038
x
+ Solution contained 0.2M (n-C*H&NCIO 4 in acetonitrile;
10-2M current
sensitivity was 0.3 pA/mm
All solutions were 0.2M in (n-C4H&N c104-.
current-time curve were periodically measured for later coulometric calculations. After sufficient electrolysis had occurred, the solutions of reduced materials were transferred to the special containers already described (20). Acetonitrile was then removed under reduced pressure and 0.5 ml of a 50/50 mixture of benzene and 2,2,4-trimethylpentane was injected to extract the organic materials. No effort was made to dry the extracting solvent. The supporting electrolyte was insoluble in the solvent mixture. Samples were removed by syringe for GLC analysis from these containers, care being taken not to remove small particles of the insoluble supporting electrolyte. The retention times of known materials were compared to those of the peaks in the reaction mixtures. The method of using mixed injections of standards with the reaction mixtures over a wide temperature range gave excellent confirmatory data on the identities of individual components in the mixtures. Infrared spectra of all compounds were identical to those of authentic samples. RESULTS
The polarograms of all the dialkyl aroylphosphonates analyzed are similar (Figure 1). All polarograms exhibit prewaves, the heights of which do not vary with the height of the mercury column and are evidently not adsorption-controlled (29). An examination of the variation of the heights of the second (main) waves with the height of the mercury column indicates that the waves are not diffusion-controlled. Polarograms of Ia-If also exhibit a small wave well after the main wave and very near the rise corresponding to the discharge of the supporting electrolyte. No correlation between wave heights and concentration could be found for any of the esters. For identical concentrations of the same ester, the main wave height depended approximately upon the time of deoxygenation. In addition, studies showed that with no electrolysis occurring, the polarographic wave heights did not remain stationary with time but
rose abruptly over the first few hours and then slowly decreased over the next few days. The half-wave potentials moved toward more positive values during this same period. Since it was suspected that these effects were connected with the hydrolytic decomposition of the esters, a comparison was made of a polarogram of a mixture of benzoic acid and diethyl hydrogenphosphonate with one of a sample of Ia to which water had been added. The polarograms were essentially identical. Consequently, it is suggested that the lowering of the polarographic wave heights is due, at least in part, to hydrolytic decomposition of the ester.
II t ArC-P(OCZH5)z
1556
ANALYTICAL CHEMISTRY
t H 2 0 --* A r C 0 2 H
+ H tP ( O C 2 H 5 ) z
The initial rise in current of the main wave is probably due to the formation of an aroylphosphonate hydrate, which evidently has a greater diffusion coefficient than the unhydrated form. When sufficient water is present, the esters hydrolyze, thus lowering the polarographic waves. The prewaves are due to very small amounts of the hydrated form present at the start of the experiments. Coulometric data were calculated from timed reductions of the aroylphosphonates in acetonitrile; the time was from one to ten days in length, and conversions up to 50% of the aroylphosphonates to products were realized. Noninteger values less than one for the number of electrons transferred were obtained as shown in Table I. This is evidently due to the fact that during the electrolysis, the polarographic waves were being decreased by a process other than reduction, namely hydrolysis. The half-wave potentials appear to be independent of drop time, indicating a reversible reaction (30). It was possible by GLC analysis of the mixture from the reduction of Ia to identify all components. The main products were benzoin and diethyl hydrogenphosphonate. The only other component, which was minor, was IIb. Benzil and 4,4’-dichlorobenzil reduce, under electrolysis conditions identical to those used with the aroylphosphonates,
(29) General principles are described in a recent text. See L.
Meites, “Polarographic Techniques,” Interscience, New York, 1965, Chap. 3.
0
0 0
(30) Ibid., p 229.
I
0
1
I
I
I
I
I
1
2
3
4
5
6
I 1
7
benzoic a c i d / p h o s p h o n a t e c o n c . r a t i o
c
&yr
I t ’
I
I
0.55
0.00
I
I
-E
VS.
I
I
I .65
1.10
2.20
ACE
Figure 2. Polarogram for reduction of Ia, 1.306 X 10-2M
Figure 3. Plot of id/C os. ratio of concentrations of benzoic acid/aroylphosphonate. Concentrations of aroylphosphonate are the same as in Table I
+
Solution contained 0.2M (n-C4H&N C10 2 in acetonitrile with 0.04M benzoic acid. Current sensitivity was 0.6 pA/mm
in two one-electron steps (average values 0.900 and 1.125 electrons, respectively). In each case, the potential used to electrolyze the aroylphosphonate was at a point on the plateau of the polarographic current. This potential corresponded to a point on the plateau of the second polarographic wave of the corresponding benzil. Coulometric studies on the first wave of benzil in the presence of excess benzoic acid yielded an n-value of 2 electrons. Philp and coworkers report a 2-electron mechanism in the reduction of benzil to the dianion of stilbenediol in dimethylformamide (31). The small amounts of the diethyl a-hydroxyarylmethylphosphonates are evidently an indirect result of slow hydrolysis of the ester (Ia or Id). A small quantity of the corresponding acid is formed and acts as a proton source.
Ia (or I d )
H20
,
ArC02H
0 0
It t ArC-P(OC2H5)2
0
+ HfP ( O C 2 H 5 ) 2 OH
2ArC02H 2e
0
’ ‘ J ArCH-
(OC2H5)2
In addition to product analysis, further evidence for the formation of the benzoins was found upon examination of the DME during electrolysis. At potentials more negative than the initial current rise, a fleeting crimson or red-violet color was observed opon the surface of each mercury drop. In a very recent study of a-oxo phosphine oxides of the type 00 11 1 /R’ , RCP in dimethylformamide and acetonitrile, Saircheva \R” and coworkers (32) found unstable colored radical anions of aromatic carbonyl compounds. The first step in the reduction was suggested to be a reversible one-electron transfer when R
(31) R. H. Philp, Jr., R. L. Flurry, and R. A. Day, Jr., J. Electrochem. Soc., 111, 328 (1964). (32) G. A. Saircheva, M. B. Gazizov, A. V. Il‘yasov, and A. U. Razumov, Zh. Obshch. Khim., 37, 2785 (1967).
= aryl. The second step was concluded to be an irreversible one-electron transfer. The effects of adding a proton source during electrolysis of our phosphonates were also examined. Benzoic acid was selected as the proton source since it was soluble in the medium and was a weak acid [strong acids lead to slow hydrolysis of the esters (In].The waves were raised to much higher currents than previously observed when benzoic acid was not added. The effect on the main wave by added benzoic acid was similar to that found when water was purposely added to the solutions. In addition, no prewaves were visible on polarograms of any of the aroylphosphonates, and the half-wave potential was shifted to a more positive value in each case. No small waves were visible after the main waves. Maxima usually appeared with aroylphosphonate concentrations above 0.015M. A typical polarogram is US. log t have slopes of shown in Figure 2. Plots of approximately 70 mV indicating that the electrode reaction is irreversible (30). Without electrolytic reduction, the wave heights remain stationary for many days, indicating that no decomposition is occurring. During electrolysis, no red color is observed on the surface of the mercury drops. Studies show that the polarographic wave height is directly proportional to concentration of the aroylphosphonates when the benzoic acid concentration is at least twice that of the aroylphosphonate. Figure 3 contains a plot of id/C (current per unit concentration) US. the concentration ratio of benzoic acid/aroylphosphonate, and shows that id/C becomes constant when the concentration ratio 2 2 . This indicates that two protons are required for the reduction of each molecule. Figure 4 shows a plot of the variation in half-wave potentials with the same ratio. As illustrated in Table I, coulometric determinations, when the above mentioned ratio is 2 2 , yield values very near 2 electrons for the reduction processes for all compounds studied with the exception of Ie. Analysis by GLC of the mixture from reduction of Ia in the presence of benzoic acid showed only one product, which was identified as being IIa. No benzoin was detected in the mixture. A similar study using Id revealed that the single product was IIb, and again the corresponding benzoin was absent. Returning to Table I, notice that regardless of the benzoic acid/aroylphosphonate ratio, a value of 2 electrons is not obtained for the reduction of Ie. The polarographic wave height decreases slowly when no reduction occurs. Wavelowering may be a function of the slow decomposition of Ie,
VOL. 41,
NO. 12, OCTOBER 1969
0
1557
i\‘\\
I
0.8
0
I
I
1
2
3
i
0r d
& I
4
I
I
I
5
6
7
0.9 -.4
l
l
l
i
l
i
l
-.3
-2
-.I
0
+I
f 2
f.3
benzoic ocid / phosphonote conc. ratio
l
+.4
t5
U
Figure 4. Plot of observed half-wave potentials us. ratio of concentrations of benzoic acid/aro ylphosphonate
Figure 5. Plot of observed half-wave potentials US. Hammett U-values
Concentrations of aroylphosphonate are the same as in Table I
M benzoic acid, and 0.2M (n-CdHg)4N ClOa in acetonitrile
possibly by increased ease of hydrolysis by an acid-catalyzed process. Polarograms reveal that the reduction process is not voltage-independent as is the case with the other esters. An attempt was made to correlate observed half-wave potentials for the aroylphosphonates with Hammett U-values for the various ring substituents. Figure 5 shows the plot obtained using the half-wave potentials from solutions of 0.01M aroylphosphonates and 0.04M benzoic acid. The results from Ie were not included in the plot, as they were greatly different. This deviation of the o-CH30 group has been previously reported (33). The point for ICwas calculated by using a method by Zuman (33). Using a method from the literature (34), calculations were performed using the slope of the plot of Figure 5 . An approximate p of +4.7 was obtained, indicating that electron-withdrawing substituents facilitate reduction. Behavior of this nature has been attributed to the nucleophilic character of the electrode (35, 36). Zuman (36) has made an extensive study of the application of the Hammett equation to polarographic half-wave potentials. Numerous types of compounds were analyzed, including substituted benzaldehydes, benzophenones, and acetophenones. In 38 of the 40 reaction series of the benzenoid type, the sign of the reaction constant was found to be positive (electron-withdrawing substituents facilitate reduction). The reaction constant for the aroylphosphonates is $0.67 V as calculated using the method of Zuman. This is larger than the majority of reaction constants in that report, indicating
the susceptibility of the aroylphosphonate reduction to substituent effects. Calculation of id/C for IC (Figure 3) indicates that the 0-C1 group does not interfere with the reduction process. However, the low id/Cvalue and shape of the polarographic wave of Ie indicate that the bulky o-CH30 group retards electron transfer at the site of reduction (the carbonyl group).
Each solution contained 0.01M aroylphosphonate,0.04+
DISCUSSION
From available results, the reduction of the dialkyl aroylphosphonates in the presence of benzoic acid appears to proceed uia a two electron-two proton mechanism to yield the appropriate dialkyl a-hydroxyarylmethylphosphonate. In the absence of acid, an ECE mechanism (37) apparently operates to yield the corresponding benzoin. The postulated reaction sequence is summarized as follows:
a
R
0-
‘ P c4-oR: LR, +
i
o
e
R G ‘ ‘ i - - O R ‘
09‘
Ill
With acid:
(33) P. Zuman, “Substituent Effects in Organic Polarography,” Plenum Press, New York, N. Y.,1967, pp 43, 47.
(34) The equation used for this calculation is described in the literature, see reference 29, page 286. (35) A. Streitweiser, Jr., and C. Perrin, J. Amer. Chem. SOC.,86, 4938 (1964). (36) P. Zuman, Collection Czech Chem. Commun., 25, 3225 (1960). 1558
a
ANALYTICAL CHEMISTRY
(37) A. C. Testa and W. H. Reinmuth, ANAL.CHEM., 38, 1320 (1966).
Without acid :
0
2R*c'
,
II
R*l-!*
0
Table 11. Comparison of Observed Half-Wave Potentials of Diethyl Aroylphosphonates and Benzilsa - E l / % US. ACE (V) Second wave Compound First wave Ia 0.967 0.772 1.57 Benzil Id 0.929 4,4 '-Dichlorobenzil 0.558 1.44 a All solutions were 0.01M in the material of interest, 0.04M
0
R
+
in C6H6C02H, and 0.2M in (n-C4H&Nc104-.
P'P'
P PH
The radical anion I11 agrees with the results of Saircheva and coworkers (32) in the related system and could account for the color observed at the electrode surface. In the absence of a proton source, this initially formed intermediate anion undergoes C-P bond cleavage to yield benzoyl radicals which subsequently dimerize. However, the reaction must occur very rapidly so that the benzil is subsequently reduced at the electrode surface and does not diffuse into the bulk of the solution (benzil or 4,4'-dichlorobenzil could not be detected by GLC analysis in mixtures from Ia or Id, respectively). Because two polarographic waves could not be resolved under the experimental conditions examined, it was not possible to use methods such as that of Nicholson and coworkers (38) to determine the magnitude of the reaction rate constant for the intermediate chemical step. Interestingly, it was previously suggested that orbital overlap may occur between the nonbonding electrons on the oxygen atom of the P 0 group and the p orbital on the adjacent carbonyl carbon atom (17, 39). It might be assumed that electron transfer to the carbonyl group in electrolysis would be less easy with the aroylphosphonates than with a-dicarbony1 compounds. This difference is observed as shown in Table 11, which compares the observed half-wave potentials of Ia and Id with the corresponding benzils. A search of the literature did not reveal a comprehensive and similar study of substituted benzils.
-
(38) R. S. Nicholson, J. M. Wilson, and M. L. Olmstead, ANAL. CHEM., 38, 542 (1966). (39) K. D. Berlin and D. H. Burpo, J. Org. Chern., 31, 1304 (1966).
It is apparent that the substituents in the ortho and para positions greatly influence the ease of reduction of the aroylphosphonates. The classical valence bond forms which can be written for If, for example, suggest that the electronic deficit
on the carbon atom of the carbonyl group (created by polarization) may be partially alleviated by resonance interaction involving the unshared pairs of electrons on the methoxyl group. Consequently, the greater difficulty in reduction of this ester is predictable in this system as with aldehydes and ketones (38). Further work is in progress on the chemistry of 0 0
11
I
the highly reactive acyl-phosphoryl (C-P) linkage. The relatively long times used for the reduction attest to the stability and versatility of the cell described previously (20) for the electrolysis of organic systems. Thus, it is possible to obtain sufficient conversions of organic compounds to obtain very ample quantities of products which can be analyzed with precision by gas chromatography, especially with the sensitive hydrogen flame detector. In addition, reproducible coulometric data are provided which together with a thorough product analysis lay a foundation of evidence upon which to deduce the mechanism of electrolysis. Another advantage is that the whole process is only a relatively smallscale operation and should prove useful to organic chemists.
RECEIVED for review April 24, 1969. Accepted July 17, 1969. Presented in part, 156th National Meeting, ACS, September 1968, Atlantic City, N. J. Work supported by Public Health Service, Grant No. G M 10367-06.
VOL. 41, NO. 12, OCTOBER 1969
1559
