Anodic Background Reaction in Moist Acetonitrile Louis C. Portis, J. Conrad Roberson, and Charles K . Mann Department of Chemistry, Florida State University, Tallahassee, Fla. 32306 The effect of water on the anodic behavior of acetonitrile at potentials more positive than +2.0 V VS. Ag/ Ag+ (0.1 M ) has been investigated, using NaBF4, tetran-propylammonium tetrafluoroborate and tetra-npropylammonium hexafluorophosphate as supporting electrolytes. With these electrolytes, potential regions exist in which the dry solvent electrolyte system is inert but in which an anodic reaction occurs if water is present. The reaction does not result in oxygen evolution in significant quantities, but instead a number of products, especially Con,acetamide, and methanol are formed as consequence of the involvement of the solvent. A reaction scheme is proposed.
THEPERFORMANCE of electrochemical reactions, especially for exploratory purposes, is facilitated if the solvent and supporting electrolyte used are unreactive in the potential region of interest. This has accounted for the widespread use of perchlorate salts in acetonitrile as medium for anodic reactions, since this system can easily be purified sufficiently t o produce very small currents up t o the potential at which perchlorate is oxidized. Recently there has been interest in using hexafluorophosphate or tetrafluoroborate as the supporting electrolyte anion t o carry out oxidations a t more anodic potentials than can be achieved with perchlorate (1,2). Potential limiting values of +2.48 V for C104-, +2.91 V for BF4-, and +3.02 V us. AgHO-ZM Ag+ for PF6- have been given for MeCN solution ( I ) . The criterion used was a potential which would cause the flow of 1 mA/cm2at a platinum anode. In this laboratory, limiting values of +2.1 V, $2.6 V, and $3.0 V for c104-, BF4-, and PF6-, respectively, have been observed. These limits are subjective because of the difficulty in defining current density when a cylindrical electrode is used in a H-cell and are values above which appreciable steady currents are observed in carefully purified systems. In using these supporting electrolytes at potentials greater than 2.4 V us. Ag/Ag+ (0.1M ) , the background currents observed are very dependent upon the water content of the system and quite small concentrations show appreciable reaction. This is in contrast t o the behavior of MeCN-C104for which no water reaction is observed unless relatively high concentrations, i.e., 6%, are present (3). We have undertaken t o examine the behavior of water in MeCN at very anodic potentials with the aims of determining under what conditions a reaction occurs and of identifying the products. Hopefully, this information can be used t o avoid interference from water and as a basis for decision about the advisability of taking precautions t o exclude water. This investigation has included NaBF4, tetra-n-propylammonium tetrafluoroborate (TPABF4), and tetra-n-propylammonium hexafluorophosphate (TPAPF6). EXPERIMENTAL
Experimental Apparatus. Electrolyses were performed with conventional electronic potentiostats, either Wenking Model 61RH or instruments of local construction which use (1) M. Fleischmann and D. Pletcher, Tetrahedroiz Lrrr., 1968, 6255. (2) T. Osa, A. Yildiz, andT. Kuwana, J . Amer. Ckem. Soc., 91, 3944
(1969). (3) J. P. Billon, J . EIocrroanaI. C'heni., 1, 487 (1960). 294
vacuum tube operational amplifier control and which allow application of a maximum of 0.2 A at 200 V. H-type electrolysis cells were used. These incorporated glass frit compartment dividers and were fitted with ground joints t o permit exclusion of atmospheric contamination. Platinum gauze cylindrical anodes were used which had approximately 5 cm2 external area. Mercury pool cathodes were used. The reference electrodes were silver wires immersed in 0.10M AgNOB in MeCN which made contact uia an asbestos fiber sealed in borosilicate glass, with a solution of supporting electrolyte in a guard tube which was similarly in contact with the solution in the anode compartment. Electrolysis Procedure. Supporting electrolyte and solvent were mixed in a reservoir fitted with joints to attach t o the electrolysis cell and with tubulation t o attach to a vacuum line. Solutions were degassed by, typically, three sequences of freezing and pumping with a mechanical pump. They were stored under an atmosphere of purified nitrogen or helium. Electrolysis cells were filled from the reservoir without contact with the atmosphere. Preelectrolyses were run at potentials about 0.1 V more anodic than were t o be used for subsequent reactions. Water was introduced by syringe after preelectrolysis and samples of solutions and head space were similarly taken for analysis. Reagents. TPAPF6 was prepared by reaction of TPAI with HPF6. To 125 grams of TPAI dissolved in 500 ml of EtOH, 50 ml of 65 % HPF6 was added. TPAPF6 was filtered off after cooling and was recrystallized twice from MeOH. The product had a melting point of 237-237.5 "C. Commercial NaBF, was recrystallized three times from MeOH. Using the criterion of amount of charge passed on preelectrolysis a t a potential approximately 0.1 V more anodic than that t o be used for a subsequent reaction, neither of these salts was particularly satisfactory. A typical preelectrolysis of a solution that contained 1.5 mmoles of salt produced currents that dropped from approximately 0.5 mA/cm2 t o 0.06 mA/cm2 with passage of 0.1 meq of charge. The subsequent reaction in experiments reported here would typically involve nominal initial current densities of 1 mA/ cm2. In both cases, attempts a t further purification by additional recrystallization were ineffective. More satisfactory NaBF, was prepared by neutralizing HBF4 with NaOH in MeOH. Add 100 ml of 48% HBF4 t o 200 ml of MeOH that contains 20 grams of NaOH. Cool, filter, and recrystallize twice from MeOH. With this salt, preelectrolysis currents typically amounted t o 0.04 mA/cm2 maximum and dropped t o a small value with the passage of 0.01-0.02 meq of charge. High preelectrolysis currents were encountered with TPABF4 and a procedure for additional purification was worked out. To prepare TPABF4, dissolve 62 grams of TPAI and 30 ml of 48 HBF4 in 100 ml of water. Filter after cooling and wash with ice water. Recrystallize from 95% ethanol and wash with ether. The yield of TPABF, at this stage amounted t o 70 based upon TPAI used. For additional purification, electrolyze approximately 0.7M TPABF4 in MeCN at a platinum anode a t +1.6 V cs. Ag/Ag+ (0.1OM) t o remove I- by oxidizing it t o In. The use of more anodic potentials will vitiate the purification, owing to conversion of Iz t o I+. On completion of the electrolysis, remove the solvent together with most of the I2 by evaporation at about 50 "C under reduced pressure. Wash the residue with ether, recrystallize twice from 95% ethanol with an ether wash after each recrystallization, and dry at 80 "C under
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Table I. Oxidation of Water in NaBF,-MeCN' MeOH CH8CONHz coz Charge HsO consumed, produced, produced, produced, passed, mmoles mmoles mmoles mmoles meq 1 .2gb 1.36 0.27 ... ... 0.95b 1 .5gb 1.63 1.68
0.65 0.66
...
0.43
0.14 0.18 0.16 0.20
... ...
0.18 0.16
0.10 0.14 0.07 0.01
Table 11. Oxidation of Water in TPABF4-MeCNa Charge HzO con CHaCONHzb passed, consumed, produced, produced, meq mmoles mmoles mmoles 1.72 1.71 1.85
1.7 2.2 1.8
0.13 0.22 0.18
1.56 1.34 1.43
Reaction of 13 ml of degassed 0.1M TPABFa containing about 0.2M water at start.
Reaction of a solution containing about 0.15M water at start in 18 ml of 0.08M NaBF,.
b
CH3CONHzprimarily recovered from the cathode compartment.
* Solutions degassed and run under Nz atmosphere.
vacuum for 24 hours. The yield a t this stage was 45 %. Preelectrolysis of solutions of this salt produced currents that dropped from 0.2 t o 0.02 mA/cm2 with passage of about 0.01 meq of charge, a reduction of about a n order of magnitude as compared with salt not subjected t o the additional purification. Acetonitrile was purified as previously described ( 4 ) . Product Analyses. Products were examined by gas chromatography, in each instance comparing the retention and peak height of the unknown with that of a valid sample taken under identical conditions. Compound sought; column used; temperature as follows: C o t ; 6-ft Porapak Q ; 30 "C. H 2 0 ; 6-ft Porapak N ; 200 "C. CH3CONH2,HCOOH, CH3COOH, NCCH2CH2CN; 6-ft Porapak Q ; 200 "C. CH,; 6-ft Porapak Q; 30 "C. N2 and 0 2 ;6-ft 13A Molecular Sieve; 25 "C. MeOH and H C N ; 6-ft Porapak N ; 100 "C. N H 3 ; 3-ft Chromosorb 103,75 "C. To support the G L C identifications, peaks thought to correspond to MeOH and t o AcNH2 were trapped from the chromatograph effluent. The infrared and mass spectra of these samples did correspond t o those of valid samples. In addition, the melting point of AcNH2 recovered from electrolyses mixtures was identical with that of a valid sample. From reaction mixtures produced with TPABF4electrolyte, a nonvolatile residue was recovered which was insoluble in polar solvents. The mass spectrum of this material showed the repetitive pattern extending t o very large mass numbers that is characteristic of polymers. The formation of this material is suggested by Equation 4A. RESULTS
Sodium Tetrafluoroborate Supporting Electrolyte. When NaBF, is used, the products include COZ, MeOH, and MeCONH2which are produced in sufficient concentrations t o be readily identified and determined. Data obtained by analyzing reaction mixtures are presented in Table I. Oxygen was found t o be produced at the start of these reactions, but decreased after a period of time. Presumably it reacts with intermediate products after the reaction has proceeded for a time. In addition, small amounts of CO, HCN, and CH4 were detected by GLC. The overall process is evidently quite complex; variations in product yields were greater than could be accounted for by analytical error. Measurements were made on solutions that contained about 0.15M water at the start. The same reaction apparently occurs at lower water concentrations, but at a lower rate. Tetra-n-Propylammonium Tetrafluoroborate Supporting Electrolyte. With TPABF,, significant reaction was not observed, even at +2.4 V 1;s.Ag/lO-l Ag+, unless the water concentration was approximately equal t o that of the elec(4) J. F. O'Donnell. J. T. Ayres, and C. K. Mann, ANAL.CHEM., 37, 1161 (1965).
trolyte. This is in contrast t o the behavior with NaBF4. Similarly, n o oxygen o r C O was detected at any time during the reaction. The major products were COSand CH3CONH2, as was true with NaBF,; analytical results are summarized in Table 11. In addition, small amounts of MeOH, Ns, and N H , were detected by GLC. Methane was found as a major component in the cathode compartment and as a minor component in the anode compartment. During the reaction, a pronounced greenish yellow color can be observed in the cathode chamber. A sample of this solution was withdrawn by syringe and subjected t o ESR examination. A weak spectrum was obtained which consisted of a 1 :1 :1 triplet with about 5 gauss splitting, presumably owing t o discharge of the TPA+ ion, t o the radical which reacted t o produce CHI and Pr3N. This reaction was not investigated further. With TPABF4, the yield of acetamide is much larger than when NaBF, is used. Most of this is formed in the cathode compartment and is thought t o result from hydrolysis of the solvent, catalyzed by basic products formed on reduction of TPA+. It was ascertained that acetamide can be formed by reaction of MeCN and tetraethylammonium hydroxide with approximately the same concentrations of water that were present in the electrolyses. The hydrolysis did not occur when NaBF, was used, presumably because the basic cathodic product, N a amalgam, was not in intimate contact with the solution. It was also determined that if a solution was electrolyzed for 1 hour and allowed t o stand overnight, CH3CONH2, but no COSwas produced during the period when n o current was passing. Tetra-n-Propylammonium Hexafluorophosphate Supporting Electrolyte. In MeCN solutions which have water concentrations below about 10 mM, TPAPF6 is a useful electrolyte at anodic potentials up t o approximately $3.0 V 1;s.Ag/O.lM Ag+. It is sufficiently soluble t o form 0.5M solutions. By contrast, alkali hexafluorophosphates are insufficiently soluble in MeCN t o produce solutions that are adequately conductive. With water present at the 85mM concentration level, however, the behavior of PF6- salts is very complex, the reaction causes the formation of solid material at the anode side of the cell divider. An examination of this material by TLC indicated that it was a complex mixture. No detailed investigation was made. Anodic Behavior of Proposed Intermediates. The anodic behavior of several compounds in MeCN, for which information could not be found in the literature, was briefly examined. Methanol, formaldehyde, and formic acid were shown to be degraded t o C 0 2and CO, as might be expected. Acetic acid, even in the presence of considerable water, was unreactive a t +2.4 V.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
295
Acetamide, the behavior of which had previously been examined in NaC104-MeCN solutions (3, was oxidized in both NaBF4-MeCN and in TPABF4-MeCN at +2.4 V VS. Ag/O.lM Ag+. Major products in NaBF4-MeCN were COZ, MeOH, and HOAc. There was some indication, by GLC, of HCOOH. In TPABF4 with about 200mM water present, the major anodic products were COZ(42 mole of reacting AcNH2) and CHI (24 mole Z). In addition, HOAc and MeOH were present.
x
DISCUSSION
These results indicate that a complex sequence of reactions occurs when water is electrolyzed in MeCN. A reaction scheme is presented in Equations 1-11 t o account for the results. In a system as complex as this, it is likely that reaction steps occur which have not been detected and that this scheme is accordingly incomplete. HzO -”; HzO+. 1 -4e-
HzO -3 0 2 OH 1
+ CHBCN
I
-H*
CHBC=N. 2
OH
0
I
11
+ H . * CHIC=NH CH3CNH2 R . + Pr4N+--+ R H + [XI‘ -+ Polymer R . + CH3CN * R H + CHzCN H+ .CHzCN + 1 +[HzO-CHzCN]+ -+ HCHO + HCN 2
HCHO
other system components. Radical attack on the solvent is shown in Equation 2; however, it is recognized that other components of the system may also participate. At several points in the reaction scheme, addition of a hydrogen atom is indicated. This is intended to imply hydrogen atom abstraction from some donor, of which there may be several in the system. Two possibilities are suggested in Equations 4A and 4B. It was observed that with TPABF4MeCN, but not with NaBF4-MeCN, a high molecular weight product was formed. It would be anticipated that abstraction from MeCN, as in Equation 4B would lead to succinonitrile. This, in fact, does not occur to a significant extent. However, HCN was identified as a product. Equation 4C is plausible since hydroxyacetonitrile is known to decompose as shown (6). Formaldehyde was not detected in the products; however, it was ascertained that it does react under these conditions to form COZ and CO, which is indicated in Steps 5 and 6. Some acetamide was recovered from the anode compartment, but very little from the cathode compartment, after oxidation of water in NaBF4-MeCN. With TPABF4MeCN, small amounts were recovered from the anode compartment and a large amount was found in the cathode compartment. Step 3 is thought t o account for the electrochemical formation of AcNHz. AcNHz was shown to be oxidized to CO?, MeOH, CHI, HOAc, and HCO?H, as described above. Actually only small amounts of AcNH2 are recovered when NaBF4 is used, even though it should be a major product according to the proposed reaction scheme. A scheme for the degradation of AcNHz is presented, starting with Equation 7.
6
I + Hi0 CHr-C-NHz
+ 4
* -e-
0
I AcNHz --+ e - CH3C-NHz -Hf
AcOH
OHz+
+ .NHz
+H. -----)
“3
0
HO=C 4
3
-
+ ‘CH3 + HOC-”* /I
-e-
3 y z CO 3
+1
-H+
HCOOH
0
I‘ HO-C-NHz
HCOOH
-2e-
.CH3
COz
~-
--___
(5) J. F. C)’Donnell and C. K. Mann, J . Electronnu/. Cliem., 13, 157 (1967).
296
-H+ __+
NH3
CH30H
.CH3 + H . CHd
’Ihe solvent-supporting electrolyte systems used are un-
reactive in the potential range of interest. On addition of water, a reaction starts; accordingly Step 1 is suggested. In electrolysis of aqueous solutions, this leads to evolution of oxygen from the anode in a reaction shown in Equation 1A. This reaction is an obvious one to consider here and care was taken in determining whether oxygen is a product. No appreciable oxygen evolution occurs with TPABF4. With NaBF4, a small amount of oxygen is produced at the outset, but the oxygen content of the anodic head space drops thereafter to a low value, presumably because oxygen reacts with secondary products. Recovery of products that have C-0 bonds implies reaction of the initial electrochemical product 1 with
+1
*COz f
CH30H
-2e-
HCHO
Several pieces of supporting evidence can be cited in support of this scheme. Equation 8A shows a cleavage of the radical intermediate 4 at the C-N bond. Some AcOH is detected in the mixture produced by oxidation of AcNH2, as well as in the water reaction products. Small amounts of N H 3 were found in the water reaction prcducts as well as small amounts of nitrogen. It was previously shown that N H 3is oxidized to N2in MeCN (7). (6) J. D. Roberts and M. C. Caserio, “Basic Principles of Organic Chemistry,” Benjamin, New York, N. Y . , 1965, p 438. (7) K. K. Barnes and C. K. Mann, J . Org. Chem., 32, 1474 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Cleavage of the C-C bond of intermediate 4 t o form methyl radicals and carbamic acid is shown in Equation 8B. Carbamic acid would decompose as indicated in Step 9 t o form COz and NHa, which are products of the reaction. The methyl radical could serve as the source of methanol (Equation 10A) and of methane (Equation 10B). Methanol is degraded to COzand CO, presumably after oxidation to formaldehyde, as shown in Step 11. These results show that, contrary t o expectations (8), anodic oxidation of MeCN-water mixtures with water concentrations as high as 0.2M, does not simply cause discharge of water t o oxygen and hydrogen ion. Furthermore, the course of the reaction is strongly affected by the nature of the supporting ___-
.~
(8) R. N. Adams, “Electroanalytical Chemistry At Solid Electrodes,’’ Marcel Dekker, New York, N. Y . , 1969, p 30.
electrolyte cation. In solutions containing sodium ion, the products of the interaction of solvent with the initial electrode reaction are degraded t o COZ and CO through acetamide, methanol, and formaldehyde. With tetrapropylammonium ion, rather than sodium, the process is to a considerable extent terminated with formation of protonated acetamide. Smaller amounts of COZand methanol are formed. We cannot offer an explanation for this difference, other than the obvious observation that sodium would be expected to be hydrated t o a greater extent than tetrapropylammonium. Accordingly, the environment in which the follow-up chemical reaction occurs would be affected by the nature of the cation. RECEIVED for review June 14, 1971. Accepted September 22, 1971. The support of the National Institutes of Health through Grant No. G M 10064 is acknowledged.
Rotating Gold Ring-Disk Study of Tin(ll) in 4.0 Molar Hydrochloric Acid V. A. Vicente and Stanley Bruckenstein Department of Chemistry, State Unicersity of New York at Buffalo, Buffalo, N . Y . 14214 Cyclic voltammetry at a rotating gold disk electrode showed that Sn(ll) in 4.0M HCI is oxidized to Sn(lV), dr2 = +0.17 V (SCE) at lO-4M Sn(ll). Ring-disk experiments showed that Sn(ll) is adsorbed on gold at E 5 +0.2 V, and underpotential deposition of Sn(0) occurs at -0.6 V < E < 0. Bulk deposition of Sn(0) begins at -0.6 V. About 230 &/cm2 of Sn(ll) are adsorbed and/or deposited underpotentially as Sn(0) at -0.15 V. About 128 WC/cm2 of the Sn(0) deposit i s oxidized at -0.15 V to soluble Sn(ll) during the anodic scan. Strongly adsorbed Sn(ll) (102 pC/cm2) i s not removed until the disk potential reaches $0.15 V, where the adsorbed Sn(ll) is oxidized to soluble Sn(lV).
-
THEADSORPTION of a monolayer of tin Dn a platinum electrode from an acid solution of Sn(I1) at a potential more anodic than that required to produce a multilayer deposit has been reported by Bowles et a/. ( I , 2 ) . By the use of the Mossbauer effect, Bowles and Cranshaw distinguished between monolayer formation by adsorption in the electrical double layer and actual plating of a monolayer on to the platinum substrate. Mossbauer resonance spectra showed that the monolayer of tin was not held in the double layer but that it was in fact remarkably similar to metallic tin (i,e., underpotential deposition of tin metal). In this investigation a gold ring-disk electrode is employed in studying the electrochemistry of tin in 4.OM HCI. Sn(I1) in 4.OM HCl deposits on gold at underpotential as tin metal. (1) B. J. Bowles and T. E. Cranshaw, Phys. Lett., 17, 258 (1965) (2) B. J. Bowles, Nature, 212, 1456 (1966).
Electrode
a
No.
RI (cm)
R P(cm)
92 93
0.3788 0,3920
0,3985 0.3996
However, Sn(I1) also adsorbs strongly on gold in the double layer region and this adsorbed species does not desorb until the gold reaches potentials more anodic than +0.15 V (SCE). EXPERIMENTAL
The instrument used for independent potentiostatic control of the ring and disk has been described elsewhere, as has the cell and high speed rotator (3). Unless stated otherwise, a rotation speed of 2500 rpm was used. Stock 10-2M Sn(I1) solutions were prepared from reagent grade SnClz.2Hz0 crystals and oxygen-free Baker analyzed hydrochloric acid, which were used without further purification. A new stock solution was prepared if significant oxidation of Sn(I1) was detected. All solutions were prepared using triply distilled water. Nitrogen, which had passed over hot copper on infusorial earth ( 4 ) and subsequently presaturated with water, was used to remove oxygen from the solutions. A nitrogen atmosphere was also maintained above the solution while electrochemical measurements were made. The gold ring-disk electrodes were made by the Pine Instrument Co. of Grove City, Pa., and were polished using standard metallographic techniques. Final polishing to a mirror finish was done using 0.05-11 alumina on Buehler microcloth and was repeated before each experiment. The dimensions of the electrodes employed are given in Table I.
______-__ (3) D. C. Johnson. Ph.D. Thesis, University of Minnesota, 1967. (4) F. Meyer and G. Ronge, Atlgew. Client., 52, 637 (1939). ( 5 ) W. J. Albery and S. Bruckenstein, Trans. Faraday Soc., 62, 1925 (1966).
Table I. Electrode Parametersa Disk area R 3 (cm) (cm2) 0.4180 0.4218
0.4508 0.4584
N
B2/3
S
0.1612 0.1793
0.3161 0.3438
0.4900 0.4785
See reference 5 for definitions of these symbols.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
297
