Voltammetry in Sulfolane: The Electrochemical Behavior of Benzaldehyde and Substituted Benzaldehydes Neal R. Armstrong and Rod K. Quinn Sandia Laboratories, Albuquerque, N.M. 871 15
N. E. Vanderborgh Department of Chemistry, University of New Mexico, Albuquerque, N.M. 8713 1
The electrochemical reduction of a series of para-substituted benzaldehydes in the aprotic solvent sulfolane has been explored by linear sweep and cyclic voltammetry. A single one-electron process is seen for the carbonyl reduction. The potential for the benzaldehyde reduction is shifted cathodically from previously reported potentials in other solvents and the chemical reaction following reduction is slow enough so that the reoxidation of the benzaldehyde intermediate is detectable at a sweep rate of 83.3 mV/sec. Benzaldehydes with an inductive substituent show an enhanced stability of the reduction intermediate. Rate coefficients for the disappearance of the intermediate are presented (e.g., k2 for the dimerization following reduction of benzaldehyde = 2.4 X l o 3 I./m*sec). The contribution of the dielectric constant, and low Bronsted acidity of sulfolane to this reduction behavior is discussed.
Several recent studies have described electrochemistry in the aprotic solvent sulfolane, tetrahydrothiophene-1,ldioxide. The reduction of several metals (1-4), and the reduction of oxygen ( 5 ) have been explored. These papers demonstrate the excellent solvent properties of sulfolane (3, 5, 6). The dielectric constant (43.3) ( 7 ) and wide electrolysis window (23.0 volts) (1) make this solvent unique for electrochemical investigations. No previous work has been reported to utilize these solvent properties for an investigation involving a more complicated reduction pathway than those mentioned above. Other dipolar, aprotic solvents such as acetonitrile, dimethyl sulfoxide, and dimethyl formamide have been used with some success to explore reaction pathways which include protonation steps and short-lived intermediates-ie., free radicals, and radical anions (8-12). Electrochemistry in sulfolane should be rewarding in elucidating these pathways. The electrochemistry of the carbonyl group has been studied extensively in aqueous media (8-10, 13-19). EquaJ F. Coetzee, J. M. Simon, and R. J. Bertozzi. Anal. Chem., 41, 767 (1969). J. F. Coetzee and J. M. Simon, Anal. Chem., 44, 1129 (1972). J. B. Headridge, D. Pletcher, and M. Callingham. J. Chem. Soc., 1967, 684. J. Desbarres, P. Pichet, and R. L. Benoit, Electrochim. Acta, 13, 1899 (1968). C. Louis and R. L. Benoit, Nectrochim. Acta, 18, 7 (1973). E. M. Arnett and C. F. Douty. J. Amer. Chem. SOC.,86, 409 (1964). R. Fernandez-Prini and J. E. Prue, Trans. Faraday SOC.,62, 1257 i1966). L. W. Nekrasov and A. D. Korsun, Elektrokhimiya, 4, 539, 996 (1968). A. D.Korsun and L. H. Nekrasov. Nektrokhimiya, 5, 212 (1969). L. H. Nekrasov, D. H. Soshchin, and V. H. Grarnenitskaya, Nektrokhimiya. 6, 15 (1970). V. J. Puglisi and A. J. Bard, J. Electrochem. Soc., 119, 829 (1972). A. J. Bard, Pure Appl. Chem., 25, 379 (1971). P. H.Given, J. Chem. SOC.,1958, 2074. P. H. Given and M. E. Peover, J. Chem. SOC.,1960, 385.
tion 1 shows a summary of the proposed pathways for reduction under several conditions of acidity (19). Of particular interest here is the high pH pathway (A); the one-electron transfer to form the anion radical, in the case of benzaldehyde, C 6 H 5 C H O ~which , then undergoes protonation followed by dimerization. H
I
C,H,&~H
y(H , d i m e r i z a t i o n )
C,,H,CHO
CGH,-c-oH
I
H
Large cathodic potential shifts have been observed in the half-wave or formal potentials with increasing p H and with decreasing aqueous mole fraction (8-10, 13-15). This behavior is commensurate with a reduction mechanism initiated by a polarization of the functional group immediately prior to electron transfer and followed by protonation immediately after electron transfer, as assumed by Elving and Leone ( 1 7 ) (phenyl ketone reductions) and Rudd and Conway (16) (acetophenone reduction). Using a ring-disk electrode in a 0.1M NaOH, 40% ethanol water basic media, Nekrasov and coworkers ( & I O ) , have shown a single, one-electron process a t E112 = -1.5 V us. SCE for the reduction of benzaldehyde. This is a potential shift of some -0.7 V from the potential observed in 1M H2S04. The reoxidation of the reduction intermediate could be simultaneously detected a t the ring electrode, but the oxidation current was only a small fraction of the reduction current indicating a rapid following chemical reaction. This reduction intermediate was not detected in more acidic media. Stable electrolysis intermediates have also been observed by ESR spectroscopy in dimethyl formamide solutions and acetonitrile solutions (20,21). There are indications that the dimerization following reaction is a heterogeneous, surface-dependent process. (15) S. Wawzonek and A. Gunderson, J. Electrochem. Soc., 107, 537 (1960). (16) E. J. Ruddand B. E. Conway, Trans. FaradaySoc., 1971, 440. (17) P. J. Elving and J. T. Leone, J. Amer. Chem. Soc., 80, 1021 (1958). (18) P. Zuman et a/., Collect Czech. Chem. Commun., 33, 2548, 3090, 3205, 3213 (1968); and "Topics in Organic Polarography," P. Zuman, Ed., Plenum Press, New York, N.Y., 1970, pp 37-48. 49-56, 57-65, 66-79. (19) B. Kastening and L. Hollack, J. Electroanal. Chem., 27, 355 (1970). (20) P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 37, 281 1 (1962). (21) H. Steinberger and G. K. Fraenkel, J. Chem. Phys., 40, 723 (1964).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 12, OCTOBER 1974
1759
T a b l e I.
Purification M e t h o d s Compound
Origina
T e t r a b u t y l a m m o n i u m perchlorate (TBAP) Benzaldehyde (C6H;CHO) p-Nitrobenzaldehyde ( P N B ) p-Cyanobenzaldehyde (PCNB) p-Chlorobenzaldehyde ( P C B ) p-Bromobenzaldehyde (PBB) p-Phenylbenzaldehyde (PCsHjB)
(A) (K) (A) (A) (K)
1,2-Diphenyl-l,2-ethanediol
(K)
E
(E) (E)
Purification method*
(1) (2) (1) (1) (1) (1) (1) (1)
Eastman White Label, Eastman Kodak Co.; K = K and K Laboratories Inc.; A = Aldrich Chemical Co. (1) = recrystallization from ethanol and vacuum dried a t 60 'C. (2) = distilled a t reduced presaure from molecular sieves. =
Udupa e t al. ( 2 2 ) ,have shown the products of the aqueous benzaldehyde reduction to be dependent on the current density. At high current densities, high surface coverage, the pinacol predominates, while at low current density, low surface coverage, the alcohol is preferentially formed. Stereochemical analysis of the pinacol of this reduction also indicates a surface dependent formation (23). We report here, the systematic study of the electrochemistry of a series of para-substituted benzaldehydes in sulfolane to demonstrate its novelty as an electrochemical solvent and its effect on these carbonyl reductions.
EXPERIMENTAL Solvent Purification. Several techniques have been reported for purification of sulfolane (1, 3 , 4 ) .The solvent used in this study was prepared using a purification scheme which was suited to our electrochemical studies. Purity levels compare favorably to those previously reported ( I , 3 ) . Sulfolane, Eastman Chemicals, was heated to approximately 100 O C and maintained a t this temperature as it was eluted through a 125 X 4-cm diameter column of separate beds of activated alumina (8-14 mesh), followed by molecular (Linde 4A) sieves. A continuous stream of high purity argon was passed through the heated eluant. The eluant, approximately 3 liters in volume, was subsequently distilled from molecular sieves, using a Norelco stainlesssteel, spinning-band still a t 80 "C and lo-* atm; the 80% centercut was retained. This fraction was again distilled under identical conditions; volumes of this distillate were received directly into specially equipped electrochemical cells which served as the final receptacles in the purification scheme (details are given under the equipment section). The purified solvent had a maximum conductivity of 1 X lo-* ohm-' cm-' and a water content of
