J. Phys. Chem. 1985,89, 1243-1245
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Electrochemical Reduction of Benzene by Solvated Electrons in HMPA-Alcohol Solutions D. Pasquariello, J. Foise, R. Kershaw, G. Zoski: K. Dwight, and A. Wold* Department of Chemistry, Brown University. Providence, Rhode Island 02912 (Received: May 30, 1984)
Reduction of benzene to cyclohexadiene, cyclohexene, and cyclohexane is studied in ethanol-HMPA and 2-propanol-HMPA solutions. The effect of hydrogen bonding of the alcohol-HMPA on the repression of hydrogen production via alcohol is reported. In addition, the degree and nature of the reduction process are investigated as a function of the nature and concentration of the alcohol, the hydrogen overvoltage of the electrode, and the current applied to the cell. Under suitable conditions, over 60% of the benzene is reduced at a current of 240 mA with an overall current efficiency greater than 86%.
Introduction
Fraenkel et al.' have shown that hexamethylphosphoramide (HMPA) is capable of dissolving alkali metals, and Sternberg et a1.2first demonstrated that the electrolytic generation of solvated electrons in a solution containing HMPA-EtOH was possible. Sternberg also reported3 the reduction of benzene and olefins in an EtOH-HMPA solution by electrochemically generated solvated electrons. In a solution containing 66.6 mol % ethanol, reduction products were a mixture of 22.8% cyclohexadiene, 10.0% cyclohexene, and 67.2% cyclohexane, with an overall current efficiency of 95%. The reduction process was carried out in the presence of a large excess of benzene, and only 15% of the available benzene was reduced. Miyake et al.4 showed on the basis of cyclic voltammetry studies that solutions with an ethanol concentration less than 50 mol % showed only a reduction wave a t -3.4 V and no evolution of hydrogen gas. However, when the ethanol concentration was increased up to 50 mol %, a second peak was observed a t approximately -2.0 V. This peak became large at 67 mol % ethanol, and there was an observable evolution of hydrogen gas at the electrode surface. They proposed that this was caused by the electrolytic decomposition of the ethanol. In light of Miyake's studies, the role of ethanol concentration on the reduction of benzene as well as the competitive reduction of the alcohol to hydrogen needs to be reexamined. Sternberg also examined the distribution of the benzene reduction products produced when either a platinum or an aluminum cathode was used. He reported3 that the product distribution is not dependent on the cathode material. It would be of interest to see if any dependency existed when the cathode material possessed a larger overvoltage toward hydrogen production than either platinum or aluminum. The role of HMPA was shown to be the suppression of hydrogen evolution at the cathode as well as in solution. Muller et al.5 have indicatd that proton transfer via hydrogen bonding can take place between ethanol and HMPA. They observed a change in N M R chemical shift of several units when HMPA is diluted with an equal volume of solvent (75 mol % EtOH-25 mol % HMPA). However, the role of hydrogen bonding between HMPA and EtOH has not been discussed in terms of repressing hydrogen production in favor of the reduction of benzene. Experimental Section
Reagents. HMPA was first dried by refluxing over calcium hydride for 12 h, and then the middle fraction was vacuum distilled at 15 mmHg. Ethanol and 2-propanol were also refluxed over calcium hydride and distilled at ambient pressure. Lithium chloride was dried at 150 O C for 12 h prior to being used. Reaction Cell. The reduction cell and the electrode assemblies are shown in Figure 1. The anode assembly is similar to that of the cathode but does not contain the reference electrode insert. The electrodes are separated by a Whatman single-thickness paper extraction thimble (35 X 80 mm 0.d.). Hydrogen gas is bubbled 'Present address: GTE Laboratories, Waltham, MA 02254.
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into the cell so that the gas impinges upon the surface of both electrodes. The gas diffuser (Pyrex frit 4-8 pm) and electrode assembly are inserted into tapered teflon plugs as shown in the figure. Current is applied by attaching alligator clip leads to the top portion of the stainless steel gas inlet tube. The gas exits the cell via the drip tube which is inserted into one of the 24/40 female joints. Attached to the drip tube are a water-cooled Graham condenser, a dry icelacetone cold finger, and a liquid-nitrogen trap. The exit gas is burned in a Bunsen flame. Procedure. The paper thimble is refluxed in a Soxhlet extractor with dry alcohol for 4 h prior to the electrochemical hydrogenation; the same alcohol is used in the extraction of the thimble as in the hydrogenation process. After extraction, 80 mL of the electrolyte solution is allowed to filter through the thimble. The platinum anode is a disk 0.8 cm in diameter; the cathode is a metal strip (Pt, Au, Pb) 2 X 8 cm. The reference electrode consists of a silver wire. The cell is assembled and flushed with dry nitrogen for 1 h. Both cathode and anode compartments are filled with the appropriate HMPA-alcohol solution which is 0.3 M with respect to lithium chloride. A total volume of 240 mL of solution is used in each reduction experiment. Three milliliters of benzene is added to the cathode compartment, and the inlet gas is changed from nitrogen to hydrogen. After 'I2h, constant current is applied from a power supply operating in a constant-current mode (Electronic Measurements Co., Inc., Model C 624A). At the end of the reduction process, the solution was analyzed by a modified procedure described by Stemberg? However, four extractions (with 15 mL of decane) were carried out on the entire solution rather than a single extraction of the catholyte as described by Sternberg. In addition, the thimble was also extracted with decane. The combined extracts were analyzed by gas chromatography. Nuclear Magnetic Resonance Studies. A series of ethanol/ HMPA solutions (0-82 mol % ethanol) were analyzed with a Bruker WM250 N M R spectrometer. The 31Presonance of HMPA solutions was compared to that of neat HMPA. From the results obtained it was possible to determine the effect of hydrogen bonding between the alcohol and HMPA. Results and Discussion
Sternberg2 has indicated that the electrochemical reduction of benzene in ethanol-HMPA solutions involves the stepwise addition of the solvated electron to the solvated benzene ring. The addition of electrons and protons leads to the formation of cyclohexadiene, cyclohexene, and cyclohexane. (1) Fraenkel, G.; Ellis, S . H.; Dix,D.T. J. Am. Chem. Soc. 1965,87,1406. (2) Sternberg, H.W.; Markly, R. E.; Wender, I. J. Am. Chem. Soc. 1967, 89, 186. ( 3 ) Sternberg, H. W.; Markly, R. E.; Wender, I.; Mohilner, D.M. J . Am. Chem. SOC.1969, 91, 4191. (4) Miyake, M.; Nakayama, Y.; Nomura, M.; Kikkawa, S . Bull. Chem. SOC.Jpn. 1979, 52, 559. ( 5 ) Muller, N.; Lauterbur, P. C.; Goldenson, J. J . Am. Chem. SOC.1956, 78, 3557.
0 1985 American Chemical Society
1244 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Pasquariello et al.
TABLE I: Shift of 31PNMR with Varying Concentrations of Ethanol-HMPA Solutions mol % ethanol 6, ppm vs. neat HMPA 0 11 21 36 43 67 82
0 0 0.1 0.4 0.7 1.5 2.0
TABLE 11: Reduction of Benzene as a Function of Alcohol Concentration"
21% ethanol 50% ethanol 67% ethanol 50% 2-
90.4 79.9 0 90.8
17.0 9.36 0 18.2
78.3 87.3 96.6 78.2
15.2 4.72 0 17.2
1.48 1.38 0 0.98
0.31 3.26 0 0.17
propanol "After 6'/2 h at a constant current of 60 mA, using platinum electrodes.
A competitive reaction involving the reduction of alcohol to hydrogen is also possible: e,-
+ ROH
-
0.5H2 + OR-
Suppression of hydrogen evolution is essential for maintenance of sufficient cathodic potential for the release of electrons into the solvent. Sternberg3 indicated that the role of HMPA was to coat the electrode and thereby suppress hydrogen evolution at the cathode. This HMPA coating of the electrode would also inhibit direct reduction of the alcohol at the electrode surface. Sternberg also indicated that HMPA must suppress hydrogen evolution in solution, although he gave no mechanism to account for this. The results of NMR data for solutions of HMPA-ethanol(0-82 mol % ethanol) are given in Table I. The observed downfield shift, 6, increased with an increase in the ethanol concentration. This downfield shift is consistent with a withdrawal of electron density from the 31Pnucleus by the protons involved in hydrogen bonding of the alcohol to HMPA. The effect of hydrogen bonding on the repression of ethanol decomposition can be deduced from Table 11. At the lowest ethanol concentration (21%), most of the available protons from the alcohol were bonded to the HMPA, and hence reduction of the benzene was favored over the reduction of the alcohol as shown by the high current efficiency. At 67% ethanol, no reduction of the benzene was observed and the hydrogen formed by reduction of the alcohol could be seen on the cathode when the external hydrogen flow was interrupted. The above results were obtained from experiments carried out for 6'/2
Figure 1. ElectrochemicalCell. Legand: A, teflon-coated stirring bar; B, 1-L reaction kettle; C, 400-mL beaker; D, Whatman extraction thimble; E, Pyrex cradle; F, fitting for drip tube and gas exit; G, gas inlets; H, ground-glass stopper; I, Ag reference electrode; J, gas diffuser; K, anode; L, cathode.
h at a constant current of 60 mA using a platinum cathode and anode. The measured potential between the reference electrode and cathode was -4.2 V for the 21% and 50% ethanol solutions and -3.0 V for the 67% ethanol solution. These observations are consistent with hiiyake's studies4on the electrochemical decomposition of HMPA-ethanol solutions. Ethanol concentrations greater than 50 mol % resulted in appreciable formation of hydrogen. A comparison of the reduction products in 50 mol % 2-propanol and in 50 mol % ethanol, under identical experimental conditions,
TABLE IIk Reduction of Benzene in Ethanol-HMPA Solutions as a Function of the Cathode Overvoltage" mol 9% alcohol 21% EtOH 21% EtOH 21% EtOH 67% EtOH 67% EtOH
cathode Pt Au Pb Pt Pb
H2 overvoltage,b V 0.015 0.241 0.52 0.015 0.52
current efficiency, % 90.4 87.5 84.5 0 33.0
conversion, % 17.0 16.1 16.3 0 5.66
C6H6 78.3 78.3 79.6 96.6 88.6
mol 96 benzene recovered as C6H8 C6Hlo 1.48 15.2 1.80 13.7 14.6 1.42 0 0.49
0 4.56
C6H12 0.3 1 0.55 0.28 0 0.61
"After 6'/2 h at a constant current of 60 mA, using a platinum anode. b"National Academy of Sciences, International Critical Tables of Numerical Data"; McGraw-Hill: New York, 1929; Vol. 6, p 339.
TABLE I V Reduction of Benzene in a 21 mol % Ethanol-HMPA Solution as a Function of Time and Applied Currentn mol 9% benzene recovered as current current time, h 14 7 7
applied, mA 60 120 240
" Using platinum electrodes.
efficiency, 96 81.0 85.6 86.8
conversion, % 28.3 33.4 60.0
C6H6
59.4 60.5 30.4
I
C6H8 21.3 28.1 44.3
C6H10
4.82 4.23 10.8
C6H12
2.22 1.06 4.88
J. Phys. Chem. 1985,89, 1245-1249 is included in Table 11. The increase in the current efficiency of benzene reduction in the former solution can be related to the lower acidity6 of 2-propanol. The reduction in proton availability results in a repression of the competitive reduction of the alcohol and a decrease in the production of cyclohexane. Sternberg has indicated3 that the reduction of benzene is not dependent on cathode materials (Pt vs. Al) a t 5 mol % ethanol. The reduction products for Pt, Au, and Pb cathodes under identical experimental conditions are shown in Table 111. It can be seen that there is little dependence of the reduction process on the hydrogen overvoltage of the electrode at 21 mol % ethanol. Hence at relatively low ethanol concentrations, the reduction undoubtedly takes place in solution. At 67 mol % ethanol concentration, some benzene reduction was obtained with a lead cathode, whereas only the alcohol was reduced with a platinum cathode. It therefore appears that, at this concentration, some reduction occurs at the cathode, where the higher hydrogen overvoltage of lead represses the reduction of the alcohol. In all of the above reduction experiments, the current applied and time of the reduction process were arbitrarily chosen at 60 mA and 61/2h. The results of increasing the time or of doubling the applied current are given in Table IV. The platinum cathode dimensions were 2 X 8 cm, and the total number of coulombs applied was the same for the first two experiments. The higher percent conversion obtained for the large applied current is consistent with the larger current efficiency. It is not surprising that the total recovered products should be less for the longer experiment, 87.7% vs. 93.9%. The cell is being continuously flushed with a stream of hydrogen gas, and some loss of products would be expected despite the liquid-nitrogen trap. The maximum benzene reduction of 60% was achieved a t 240 mA in 7 h. The electrochemical reduction of benzene and related compounds in aqueous solutions has been reported by Coleman and Wagenknecht.' The authors claim that at optimum conditions the benzene may be converted to l,4-cyclohexadiene at 90% se(6) Hine, J.; Hine, M.J . Am. Chem. SOC.1952, 74, 5266. (7) Coleman, J. P.; Wagenknecht, J. H. J. Electrochem. SOC.1981, 128, 322.
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lectivity with nearly quantitative current efficiency. However, the total yield of reduction products obtained in a batch reaction was no better than those reported in this study when a current of 240 mA was applied for 7 h in a cell containing a 21 mol % ethanol-HMPA solution. In the continuous electrolysis process which they describe, a considerable loss in the reduction products occurred, presumably because of the evaporative losses which occurred at the operating temperature of 60 OC.
Conclusions At low ethanol concentrations, the reduction of benzene by solvated electrons appears to take place primarily in solution since it is independent of the hydrogen overvoltage of the electrode used. NMR measurements are consistent with the existence of hydrogen bonding between HMPA and alcohol, and this bonding can be expected to affect the relative rates of benzene and alcohol reduction. At high ethanol concentrations ( 2 6 7 mol % ethanol), no benzene is reduced except when the cathode has a sufficiently high hydrogen overvoltage, e.g., Pb. Only the alcohol is reduced when a platinum cathode is used, and hydrogen is evolved. This dependence upon hydrogen overvoltage indicates that under these conditions some reduction must occur at the cathode surface. The substitution of 2-propanol for ethanol decreases proton availability for the same alcohol concentration. This represses reduction of the alcohol, and consequently greater benzene reduction is observed. Acknowledgment. Acknowledgment is made to the National Science Foundation (Grant DMR 79-23605) for the support of D.P. In addition, we thank the Office of Naval Research (N00014-77-C-0387) for the support of J.F. and K.D. A.W. thanks the GTE Laboratories (Waltham, MA) of the GTE Corporation for partial support during this work. Acknowledgment is also made to Brown University's Materials Research Laboratory program, which is funded through the National Science Foundation. Registry NO. Pt, 7440-06-4; Au, 7440-57-5; Pb, 7439-92-1; HMPA, 680-3 1-9; benzene, 7 1-43-2; cyclohexadiene, 29797-09-9; cyclohexene, 110-83-8; cyclohexane, 110-82-7; ethanol, 64-17-5; 2-propanol, 67-63-0.
Vibrational Excitation due to Atom-Molecule Collision in an Intense Laser Field C. A. S. Lima Instituto de Fhica, Universidade Estadual de Campinas, 13100- Campinas, SP, Brazil
and L. C. M. Miranda* Instituto de Estudos Avancados, Centro TZcnico Aeroespacial, 12200-S.J. Campos, SP, Brazil (Received: July 26, 1984)
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A theory for the nonresonance vibrational excitation of diatomic molecules under impact excitation conditions in an intense laser field is developed. Specific consideration is given to 0 n transitions in He-LiH collisions. It is found for this system that at relatively large collision times direct many-photon processes are dominant. However, with decreasing collision time the compensation for resonance mismatch requires fewer photons to assist the vibrational excitation. The behavior of the different excitation processes for the 0 n transition on the laser field strength is also presented.
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Introduction The development of powerful laser sources in the infrared range has stimulated a great deal of work in the areas of selective excitation of and many-quantum dissociation.F10 In (1) Karlov, N. V.;Petrov, Yu.N.; Prokhorov, A. M.; Stelmarkh, 0. M. JETP Lett. 1970, ZI, 135. (2) Meyer, S.W.; Kwok, M. A.; Gross, R. V. E.; Spencer, D. J. Appl. Phys. Lett. 1970, 17, 516.
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the later process, also known as collisionless dissociation of molecules, the original idea was to use a laser frequency w resonant (3) Basov, N. G.; Markin, E. P.; Oraevskii, A. N.; Pankratov, A. V. Sou. Phys.-Dokl. (Engl. Transl.) 1971, 16, 445. (4) Ambartsumyan, R. V.; Letokov, V. S . Appl. Opt. 1972, 11, 354. (5) Jortner, J.; Mukamel, S . In "The World of Quantum Chemistry"; Daudel, R., Pullmann, B., Eds.; Riedel: Dordrecht, The Netherlands, 1974; p 145. (6) Quack, M. J . Chem. Phys. 1978, 69, 1282.
0 1985 American Chemical Society
