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Anodic oxidation of 2,6-xylenol in acetonitrile on platinum
Elcctrochemica Acta, 1972. Vol. 17. pp. 1391 to 1400.
Pcrssunon Press. Printed In Northern Ireland
ANODIC OXIDATION OF 2,6-XYLENOL ACETONITRILE ON PLATINUM”
IN
C. IWAKURA, M. TSUNAGA and H. TAMKJRA Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka, Japan
was investigated Abstract-Anodic oxidation of 2,6-xylenol at the platinum electrodein acetonitrile by the various electrochemicaland chemicalmethods. Conductivitymeasurementsindicatedthat the phenolic hydroxyl of 2,6_xylenol is undissociated in acctonitrile solution containing lithium perchlorate. By cyclic voltammetry and chronopotentiometry,it was shown that anodic oxidation of 2,6-xylenol at the platinum electrodeproceedsin two steps. 2 electronsare involved in the first step, as determined by cyclic voltammetry using hydroquinone and DPPH as a reference compound. Polarographicstudy suggestedthat the first step is an electrophilicattack on the aromatic nucleus of the non-ionized phenol. The electrolysisproducts were analysed by thin-layer chromatography, ir and uv spectrophotometryand mass spectrometry. It is estimated that dimer, trimer and tetramer are produced by the anodic oxidation of 2,6-xylenol and that the dimer is 2,2’-dihydroxyl-3,3’dimethylstilbene. The formation reactions of trimer and tetramer probably proceed by the same mechanismas dimer formation. R&urn&Etude dam l’ac&onitriIe, au moyen de diversesm&hodes dlectrochimiqueset chimiques, de l’oxydation anodiquedu 2,6-xylenol a l’blectrodede platine. Les mesuresde conductiviteindiquent que l’hydroxylephenoliquedu 2,6-xylenol n’est pas dissociedans une solution d’ac&onitrilecontenant du perchloratede lithium. Par voltam&ie cycliqueet chronopotentiometrie,on montre que l’oxydation anodique du 2,6-xylenol a l’electrode de platine se deroule en deux &apes. 2 electrons sont impliqu& dans la premierecomme il r&.ultede la voltam&rie cyclique, en utilisant l’hydroquinone et le DPPH comme r&f&ences. L%tude polarographique sugg&reque la premiere &ape est une attaque electropbiledu noyau aromatique du phenol non ionise. Les produits de l’tlectrolyse sont analyses par chromatographieen couche mince, spectrophotometriesIR et UV et spectrom&rie de masse. On estime alnsi que le dim&e, le trim&e et le t&ran&e sont produits par oxydation anodiquedu 2,6_xylenolet que le dim&e est le 2,2’-dihydroxy-13-3’-dim&hylstylb&ne.Les reactionsde formation des trim&e et t&ramere pro&dent probablement par le meme meCanismeque pour la formation du dim&e. Zusammenfassu~-Die anodischeOxydation von 2,6-Dimethylphenolan einerPlatinelektrodewurde mit Hilfe verschiedenerelektrochemischerund chemischer Methoden untersucht, Leifftiigkeitsmessungen zeigten, dass die phenolische Hydroxylgruppe in 2,dDimethylphenol in einer Acetonitrilliisung,welche Lithlumperchloratenthalt, nicht dissoziiertist. Durch zyklischeVoltametrie und Chronopotentiometrie wurde gezeigt, dass die anodische Oxydation von 2,6-Dimethylphenol in zwei Schritten erfolgt. Durch zyklische Voltametrie (Hydrochinon und DPPH als Referenzsubstanzen) konnte gezeigt werden, dass am ersten Schritt zwei Elektronenbeteiligtsind. Polarographische Studienweisendarauf hin, dass der ersteSchritt ein elektrophilerAngrlff auf den aromatischen Kern des nicht ionisiertenPhenolsist. Die ElektrolyseproduktewurdendurchDilnnschichtchromatographic, IR-und UV-Spcktrophotometrie sowie Masscnspektrometrieuntersucht. Man nimmt an, dass bei der anodischen Oxydation van 2,6-Dimethylphenol such Dimere, Trimere und Tetramere gebidet werden,wobei das Dimere 2,2’-dihydroxy-3,3’-dimethylstilbenist. Die Bildung der Trimeren und Tetramerenverlluft wahrscheinlichnach demselbcn Reaktionsschemawie die Dimerisierung. 1. INTRODUCTION
POLYMERIZATION by oxidative coupling reaction is an essentia1 process in biosynthesis of the important
natural
products
such as lignins
and alkaloids.
The oxidative
coupling reaction of phenolic compounds has particularly important significance.14 In this case, the reactions are often catalysed by metallic compounds in higher * Presentedat the 38th Aunual Meeting of the ElectrochemicalSociety of Japan, Osaka, May 1971; manuscriptreceived5 June 1971. 1391
1392
C. IWAKUK&, M. TSUNAGA
and H. TAMURA
oxidation numbers and in general two types of oxidation coupling reaction are well known; one-electron-transfer and two-electron-transfer reactions. PPO resin, which was first prepared by Hays and is now produced industrially by G. E. Company, is a polymer with excellent properties prepared by oxidative coupling polymerization of 2,6-xylenol. 6.7 Depending on the ratio of pyridine to cuprous chloride used as catalyst in this reaction, diphenoquinone or polyphenylene ether is predominantly produced, by C-C or C-O coupling respectively. In the present work, replacement of the catalytic oxidation reaction of 2,6xylenol with oxygen by anodic oxidation was tried. Preliminary experiments with tic* revealed that the products were not diphenoquinone and polyphenylene ether, -which might be formed by the oxidative coupling reaction. To clarify the differences between the two cases, the electrode reaction of 2,6-xyIeno1 was analysed by the various electrochemical and chemical methods. The electrolytic oxidation of phenolic compounds has been investigated by many workers.s-12 Vermillion et all2 have reported that two entirely different anodic reactions are possible with phenolic compounds. The first is an electrophilic attack on the aromatic nucleus of the non-ionized phenol with the irreversible removal of two electrons to give a mesomeric phenoxonium ion. The second is the reversible removal of one electron from the phenoxide anion to give a phenoxy free radical. 2.
EXPERIMENTAL
TECHNIQUE
2.1 Preparation and purification of chemical reagents Commercial grade 2,6-xylenol obtained from Wako Pure ChemicaI Ind. Ltd. and Nakarai Chemicals Ltd. was purified by recrystallization from n-hexane for up to three times.s No contaminants could be detected in purified 2,6-xylenol by ir analysis and gas chromatography. Commercial acetonitrile used as solvent was commercially available from the same sources. It was refluxed over phosphorus pentoxide for more than 8 h and then distilled at atmospheric pressure.13 Lithium perchlorate used as supporting electrolyte was dried at 150°C under reduced pressure for over 12 h. The test solutions prepared were agitated and de-aerated by bubbling argon for 10 min, just before each measurement. 2.2 Electrochemical measurements Polarization measurement by the stationary method, cyclic voltammetry, chronocontrolled-potential electrolysis, controlled-potential coulometry, potentiometry, polarography and conductivity measurement were employed. The cell assembly mainly used is shown in Fig. 1. The working electrode was platinum wire, O-10 cm dia., 0.18 cm length, plate, 1 cm x 1 cm or net, 100 mesh, 1 cm x 2 cm, and the counter-electrode was a smooth platinum plate of large surface area. Prior to each experiment, these platinum electrodes were polished with emery paper and then sufficiently washed with water and further treated with acetone for degreasing and drying. Ag/O. 1 M AgNO, was used as reference electrode.? Unless * Thin-layer chromatography. t The single electrode potential of this reference electrode is 0.337 V(~c.e).~~
hodic
oxidation of 2,6-xylenolin acetonitrileon platinum
1393
Prc. 1. Electrolytic cell. WE, working electrode (Pt); CE, counter electrode(I%); RE, reference electrode (A&O-l M AgNO,); G, glassfilter; L, Luggin capillary; P, potentiostat. otherwise stated, electrode potentials are referred to this electrode in the present paper. All instruments used were conventional. Temperature was kept constant at 30°C. 2.3
Chemical analysis of products
After the controlled-potential electrolysis, the electrolyte was chemically analysed, sometimes with pre-treatment for separation of the products. The separating procedure was as follows: after removal of lithium perchlorate from the electrolyte by salting-out techniques, the solution was condensed with an evaporator, and developed it with silica-gel column. The following solvents were used as developing agent in turn ; n-hexane, carbon tetrachloride, benzene, chloroform, ether and methanol. Each fraction was dried out and then analysed by tic, and by ir, uv and mass spectrometry. 3. RESULTS
AND
DISCUSSION
3.1 Analysis of electrode reaction The anodic polarization characteristic was measured by the stationary method in acetonitrile containing 0.5 M lithium perchlorate. The current values observed at 2 min after setting at each potential were recorded. Figure 2 shows the current/ potential results obtained for three different concentrations of 2,6-xylenol. It was observed that the sohttion in the immediate neighbourhood of the electrode was coloured reddish-brown and the coloured substance fell toward the bottom of the cell in the potential region where the current rose (Fig. 2). For each curve a Tafel line, slope 120 mV, was obtained in the potential range between O-6 and 0.8 V, from which value an, was caIculated to be O-50. The anodic currents had a minimum near 1-OV, and reached to the so-called limiting current when the potential was made more anodic. The limiting current was nearly proportional to 2,6-xylenol concentration.
1394
C.
IWAKURA,
M.
TSUNAGA
and H. T-
Potential,V(Ag/O-I M AgNO*) FIG. 2. Current/potentialcurves of 2,&xylenol at various concentrations. a, 0 M; b, O-01 M;
c, 0.05 M;
d, 0.10 M.
A typical voltammogram of 2,6-xylenol obtained with the multi-sweep method is given in Fig. 3. The anodic reaction evidently occurs in two steps, giving two anodic peak currents at 1-Xand 1.5 V. These are designated peak 1 and 2, respectively. The chronopotentiograms also revealed two well-defined waves at the corresponding potentials. The small anodic peak at ca 0.65 V in Fig. 3 might be caused by the further oxidation of the oxidized product of 2,6-xylenol because it has disappeared
Potential, V(Ag/O*I M AgNOJ FIG. 3. Typical cyclic vdtammogram of 2,6-xyknol
(multi-sweepmethod).
1395
Anodic oxidation of 2,6-xylenolin acetonitrileon platinum
with the single sweep, as indicated in Fig. 4. Since it can be presumed that the oxidation of 2,6-xylenol occurs at peak 1 and that the product, probably dimer, is further oxidized at peak 2, some detailed discussion follows.
Potential, V(Ag/O*I M AgNOJ FIG. 4. Typical
cyclic voltammograms (single-sweep method). hydroqtinone; c, DPPH.
According to the theory of cyclic voltammetry, the peak EP and the half-peak I&, is given by % -
E,/2
=
a, 2,6-xylenol;
b,
the potential increment between
0*0485~cW~v,
(1)
for a totally irreversible system at 30X!,” where ocis the transfer coefficient and n, is the number of electrons involved in the rate-determining step. Substituting avz, = 0.50 in (I), we obtain ED - Ep lz= O-097V. The experimental value is in good agreement; for instance, the potential increment was 0.10 V at u = 0.024 V/s for concentration 0.01 M. Therefore, the reaction is considered to be a totally irreversible reaction. For such a system, the current can be expressed as i=
nFAc"diix(/?t>,
(3
where i is the current (A), n the number of electrons, A the surface area of the electrode (cm2), co the bulk concentration of the species (mole/cm3), D,, the diffusion coefficient (cma/s) and B = cu@~/RTwith u the potential sweep rate w/s). The other notations have their usual significance. Here the current exhibits a peak at x(Bt) = O-282. Cyclic voltammetry was carried out in the sweep-rate range between O-024 and l-47 V/s with varying the 2,6-xylenol concentration from 0*01-O-05 M. Straight lines with good linearity were obtained by plotting ip against co or 2/;; on the basis of the voltammograms. The slope of the ip vs co curve (v = 0.11 V/s) was calculated to be 33 and therefore ?l(az2,)““D;~” = 4-93 x 10-S. (3) The following value was also obtained by calculating in the same way from the slope of ip vs 2/;; curve (c” = 1 x 1O-6 mole/cm3), IZ(cW~)1’2D;‘a = 5.91 x 10-s. Eggins and Chambers I6 have reported that hydroquinone and then reduction with an irreversible two electron-transfer
(4) undergoes oxidation at l-1 and O-3 V(sce).
1396
C. IWAKURA, M. TSUNAQA and H. TAMURA
respectively, under similar conditions as the present experiment. This was again confirmed in our laboratory. Good agreement was obtained for all sweep rates studied by a comparison of the peak height of 2,6-xylenol with that of hydroquinone. A typical result is given in Fig. 4. A similar curve for 1,l-diphenyl-2-picrylhydrazyl (DPPH) is also shown: DPPH is known to undergo a pseudo-reversible reaction with one-electron transfer .lG20 The peak height of 2,6xyIenol is nearly double the peak height of DPPH. Consequently, we infer that 2,6-xylenol is oxidized at peak 1 with a two-electron transfer.* This is supported by controlled-potential coulometry. The coulometry was made at l-1 V for an acetonitrile solution containing 5 ml of 0.01 M 2,6-xylenol and O-5 M lithium perchlorate. The electrolysis was stopped when the current reduced to about 5 per cent of the initial value. The observed charge passed was lo-17 C and the calculated 9.65 C assuming n = 2. Using n = 2 and ouz, = 0.50, D, can be calculated with (3) and (4) as 1.22 x 1O-s and l-75 x 1O-5 cm2/s respectively, which are reasonable values. Do’s of quinone and hydroquinone, which have similar structure to 2,6-xylenol, have been reported to be 2.9 x 10N5and (3.16 -& O-15) x 1O-5 cm2/s respectively.16 Theelectrical conductivity of 2,6-xylenol in acetonitrilesolutioncontaining 0.001 M lithium perchlorate was measured by an ac method. A 1 kc/s sine wave was put into the impedance bridge and the equilibrium point was detected with an oscilloscope. O-1 M potassium chloride aqueous sohrtion was used as the standard solution for determining the cell constant. 21 The results are summarized in Table 1. The specific TABLE 1. E~rzr~o~rrrc. CONDUCTANCE OF ~,~-XYLENOL M ACETONITRILE SoLuTIoN CONTAINING 0.~1 M LITHILW PERCHLORATE Concentration of 2,6-xylenol M
Resistance c-2
specifk resistance R-cm
specific conductivity mho/cm x lo-’
:X)0156 ow313 000625 0.0125 00250 0~0500 0-1000
2316 2310 2310 2310 2310 2315 2330 2334
6401 6385 6385 6385 6385 6399 6440 6451
1562 1.566 1.566 l-566 1.566 1,563 1.553 1.550
resistance and the specific conductance remain in constant over a wide range of 2,6-xylenol concentration and furthermore are approximately the same as those of the blank solution without added 2,6-xylenol. This indicates that 2,6_xylenol is scarcely dissociated in the solution. The half-wave potentials for a totally irreversible electrode reaction at 25°C may be correlated by the Hammett12 equation -0*059 1 E l/2 - El”l2 = UP, (5) BnB
where Ella, ET/s are the half-wave potentials of substituted and non-substituted l Under the same experimental conditions, ip varies depending on n, aur, and D, with the different compounds. Here ovl, can be determined also from (1) and D, can be estimated by the similarity of the structure of the compounds.
Anodic oxidation of 2,6-xylenol in acetonitrife on platinum
1397
compounds, respectively. p is the transfer coefficient for irreversible oxidation and ng is the number of electrons transferred in the potential-determining step of the electrode process. 0 is Hammett’s substituting constant derived by Gould22 and p is Hammett’s reaction constant. According to Vermillion ef aZ,12 (5) is in agreement for the hindered phenols of formula
(R = CH,, t-butyl, CHO, COOH) R and plots of El12 ag ainst d for the R group gave a straight line plot. In this case, the oxidation reaction is an electrophilic attack on the aromatic nucleus of a non-ionized phenol. On the other hand, for a reversible electrode reaction at 25”C, E l/2 --
E& = -o-0591ap,
where the reaction is a reversible removal of one electron from the phenoxide anion. The half-wave potential of 2,6-xylenol was determined by polarography at a rotating platinum electrode as l-1 to 1-3 V. The b+-value of 2,6-xylenol was calculated as -0.138 with the assumptions proposed by Vermillion et uLX2 The necessary values for this calculation are in the literature.= The Elf2 [14-l-6 V(sce)] and o+ (-0.138) values are plotted in Fig. 5, from which it can be seen that the point for 2,6-xylenol
3 8 >
I
1
-0.8
I
-0-4
I
I
0
0.4
u+
Hammett b+ plot. a, 2,6-dimethoxyphenol; b, 2,4,6-tri-tert-butylphenol; c, 2,6-di-tert-butyl-p-01; d, 2,6-di-tert-butylphenol; e, 3,5-di-tert-butyI4hydroxybenzaldehyde; f, 2,6-xylenoI. 0, data of Vermilion et al;” 0, present study. FIG. 5.
is on the same line as found by Vermillion et al. On the other hand, the calculated point was largely deviant from the line based on (6). Consequently the anodic oxidation of 2,6-xylenol in acetonitrile is presumed to be an electrophilic attack on the aromatic nucleus of the non-ionized phenol. 3.2 Analysis of electrode reaction by chemical approaches A spectral change by electrolysis of 2,6-xylenoi is shown in Fig. 6. It is clear from the similarity between (b) and (c) that 2,6-xylenol does not react with lithium
C. IWAKURA, M. TSUNAGA and H. TAMURA
1398
Wave
length,
tam
FIO. 6. Spectral change by electrolysis of 2,6-xyleno1 (qualitativeanalysis). a, blank solution (acetonitrile-O~S M LiClOJ; b, blank solution, 2,6-xylenol added after ektrolysis; c, before electrolysis with 2,6_xyienoI; d, after ekctrolysis with 2,6-xykllo1.
perchlorate and acetonitrile. And it can be found from the difference between (c) and (d) that new absorption peak appear at 253 and 260 nm and the peak of 2,6-xylenol at 270-280 nm is reduced by electrolysis. The products of electrolysis at 1-l V for 24 h were separated by column chromatography. Their fractions were analysed by mass spectroscppy and the following relations between the developing agents and the fractionated substances were obtained. (mw 240) n-hexane, carbon tetrachloride: dimer : trimer (mw 358) benzene : tetramer (mw 476) chloroform, ether : mixture (mean mw 450). methanol Here, the mean molecular weight of the methanol fraction was measured by the VP0 method, using acetonitrile as a solvent. Absorption bands observed in ir and uv spectra of the dimer were assigned to Ir spectrum stretching vibration of phenol. 3380 cm-l: O-H stretching vibration 1650 cm-l : &C C-O stretching vibration of phenol. 1190 cm-l: Uv spectrum 210 nm (strong Et absorption band): aromatic w 4 m* transition. 260 nm (strong K absorption band): presence of chlomophore conjugated with a ring. 280 nm (weak K absorption band): characteristic absorption of ctistilbene. Furthermore,
the ir spectrum of the product was simiIar to that of cis-stilbene. From
Anodic oxidation of 2,6-xylenol in acetonitrileon platinum
1399
the above facts, the dimer was estimated as 2,2’-dihydroxyl-3,3’-dimethylstylbene, OH
OH CH=CH,/!,/
Cl%
.
It is presumed that trimer and tetramer are produced by further polymerizatiqn in the same way, since the peak fragment of dimer (mw = 240) could be observed in their mass spectra. 3.3 Reaction mechanism On the basis of the above discussion, we propose the following mechanism for the anodic oxidation of 2,6-xylenol at the platinum electrode in acetonitrile. Since the phenol& hydroxyl is undissociated, the non-ionized 2,6-xylenol is presumed to form the resonance OH
r
@OH
@OH
1
H,C
H’ (III)
(11)
_I
Two electrons are withdrawn from (II) or (III) by electrochemical oxidation with the simuhaneous removal of hydrogen ion, and it forms a mesomeric phenoxonium ion according to ,CH.
,CH*
-20
(II),
cm
z
* (7)
+-+
H 09
A similar kind of phenoxonium ation,ls
+ CH,OH
+
\ H,C-
KC
0-1
ion as (V) and (VI) has been isolated by methoxyl-
0
/R
/@
0
‘OCH,
(V) undergoes the further removal of hydrogen ion from a methyl group to produce the intermediate 0
cv)
_H+*Ha% II FHa
0\I WI)
,
(9)
C. IWAKURA, M. TSUNAGA and H. TAMURA
1400
followed by its dimerization
to form (VIII),
2
H2C\
OH
OH
0
II
0\’ (VII)
(VIII)
involves an electrophilic attack on the aromatic nucleus at a position to the phenolic hydroxyl, the intermediate may be (IX), which produces (X) by dimerization,
If the reaction
para
..
CHa 0
ox>
It was confirmed by tic and ir spectrophotometry that diphenoquinone (X) was not produced. Though the same process is likely for trimer and tetramer formation, detailed discussion cannot be made without further investigations on the second step of the It should, however, be noted that the proportion of dimer in the electrode reaction. products is usually larger with decreased concentration of 2,6-xylenol and with shorter This may be related to the observed facts that the current duration of electrolysis. smoothly decreased with the electrolysis time at low concentration of 2,6-xylenol, whereas a minimum in the current/time curves was observed at high concentration. Acknowledgement-The authors wish to express their deep gratitude to Assistant Professor Toshiyuki Shono, Osaka University for his helpful suggestions in the course of this work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
W. I. TAYLOIZ and A. R. BA~ERSBY, Oxidatiue Coupling of Phenols. Dekker, New York (1968). A. I. Scorn,, Quart. Rev. 19, 1 (1965). K. FREUDE~ERC+, Science 148,595 (1965). B. FRANR, 0. BLA~CHKE and G. SCHLING~FP, Angew. Chem. 75,957 (1963). A. S. HAY, J. Am. them. Sot. 81, 6335 (1959). T. SHONO. K. YAMANOI, T. MATSUSK-XITA and K. SHINRA, Kogyo Kagaku Zasshi 70,2062 (1967). (1969). T. SHONO, M. MORI and K. SHINRA, Kogvo Kagaku Zasshi 72,178l J. F. HEDENB~R~and H. FREISER,Ana&. Chem. 25,1355 (1963). V. I. GINZBERC),Zh. fiz, Khim. 33, 1504 (1959). V. F. GAYLOR, A. L. CONRAD and J. H. LEUDERL,Analyt. Chem. 25,107s (1953). V. F. GAYLOR, A. L. CONRAD and T. H. L~UDERL, Analyt. Chem. 29,224 (1957). F. J. VERMILLION,JR. and I. A. PEARL, J. ebctrochem. Sot. 111, 1392 (1964). A. J. BARD, Electroanalytical Chemistry, Vol. 3, p. 57. Dekker, New York (1969). P. DEL-Y, New Instrumental Methods in Electrochemistry, p_ 115. Interscience, New York (1954). B. R. E-INS and J. Q. CHAMBERS,J, electrochem. Sot. 117, 186 (1970). E. SOLON and A. J. BARD, J. Am. them. Sot. 86,1926 (1964). E. SOLON and A. J. BARD, J.phys. Chem. 68,1144 (1964). D. A. HALL and P. J. ELVING, Electrochim. Acta 12, 1363 (1967). B. L. FUNT and D. G. GRAY, Can. J. Chem. 46,1337 (1968). C. IWAKURa and H. T AMURA, unpublished work (1970). H. C. PARKERand E. W. PARKER,J. Am. them. Sot. 46,3 12 (1924). N. S. GOULD, Mechanism and Structure in Organic Chemistry, p. 790. Holt, Rinehart & Winston, New York (1959). H. H. J&, Chem. Rev. 53, 191 (1953).
Pcrssunon Press. Printed In Northern Ireland
ANODIC OXIDATION OF 2,6-XYLENOL ACETONITRILE ON PLATINUM”
IN
C. IWAKURA, M. TSUNAGA and H. TAMKJRA Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka, Japan
was investigated Abstract-Anodic oxidation of 2,6-xylenol at the platinum electrodein acetonitrile by the various electrochemicaland chemicalmethods. Conductivitymeasurementsindicatedthat the phenolic hydroxyl of 2,6_xylenol is undissociated in acctonitrile solution containing lithium perchlorate. By cyclic voltammetry and chronopotentiometry,it was shown that anodic oxidation of 2,6-xylenol at the platinum electrodeproceedsin two steps. 2 electronsare involved in the first step, as determined by cyclic voltammetry using hydroquinone and DPPH as a reference compound. Polarographicstudy suggestedthat the first step is an electrophilicattack on the aromatic nucleus of the non-ionized phenol. The electrolysisproducts were analysed by thin-layer chromatography, ir and uv spectrophotometryand mass spectrometry. It is estimated that dimer, trimer and tetramer are produced by the anodic oxidation of 2,6-xylenol and that the dimer is 2,2’-dihydroxyl-3,3’dimethylstilbene. The formation reactions of trimer and tetramer probably proceed by the same mechanismas dimer formation. R&urn&Etude dam l’ac&onitriIe, au moyen de diversesm&hodes dlectrochimiqueset chimiques, de l’oxydation anodiquedu 2,6-xylenol a l’blectrodede platine. Les mesuresde conductiviteindiquent que l’hydroxylephenoliquedu 2,6-xylenol n’est pas dissociedans une solution d’ac&onitrilecontenant du perchloratede lithium. Par voltam&ie cycliqueet chronopotentiometrie,on montre que l’oxydation anodique du 2,6-xylenol a l’electrode de platine se deroule en deux &apes. 2 electrons sont impliqu& dans la premierecomme il r&.ultede la voltam&rie cyclique, en utilisant l’hydroquinone et le DPPH comme r&f&ences. L%tude polarographique sugg&reque la premiere &ape est une attaque electropbiledu noyau aromatique du phenol non ionise. Les produits de l’tlectrolyse sont analyses par chromatographieen couche mince, spectrophotometriesIR et UV et spectrom&rie de masse. On estime alnsi que le dim&e, le trim&e et le t&ran&e sont produits par oxydation anodiquedu 2,6_xylenolet que le dim&e est le 2,2’-dihydroxy-13-3’-dim&hylstylb&ne.Les reactionsde formation des trim&e et t&ramere pro&dent probablement par le meme meCanismeque pour la formation du dim&e. Zusammenfassu~-Die anodischeOxydation von 2,6-Dimethylphenolan einerPlatinelektrodewurde mit Hilfe verschiedenerelektrochemischerund chemischer Methoden untersucht, Leifftiigkeitsmessungen zeigten, dass die phenolische Hydroxylgruppe in 2,dDimethylphenol in einer Acetonitrilliisung,welche Lithlumperchloratenthalt, nicht dissoziiertist. Durch zyklischeVoltametrie und Chronopotentiometrie wurde gezeigt, dass die anodische Oxydation von 2,6-Dimethylphenol in zwei Schritten erfolgt. Durch zyklische Voltametrie (Hydrochinon und DPPH als Referenzsubstanzen) konnte gezeigt werden, dass am ersten Schritt zwei Elektronenbeteiligtsind. Polarographische Studienweisendarauf hin, dass der ersteSchritt ein elektrophilerAngrlff auf den aromatischen Kern des nicht ionisiertenPhenolsist. Die ElektrolyseproduktewurdendurchDilnnschichtchromatographic, IR-und UV-Spcktrophotometrie sowie Masscnspektrometrieuntersucht. Man nimmt an, dass bei der anodischen Oxydation van 2,6-Dimethylphenol such Dimere, Trimere und Tetramere gebidet werden,wobei das Dimere 2,2’-dihydroxy-3,3’-dimethylstilbenist. Die Bildung der Trimeren und Tetramerenverlluft wahrscheinlichnach demselbcn Reaktionsschemawie die Dimerisierung. 1. INTRODUCTION
POLYMERIZATION by oxidative coupling reaction is an essentia1 process in biosynthesis of the important
natural
products
such as lignins
and alkaloids.
The oxidative
coupling reaction of phenolic compounds has particularly important significance.14 In this case, the reactions are often catalysed by metallic compounds in higher * Presentedat the 38th Aunual Meeting of the ElectrochemicalSociety of Japan, Osaka, May 1971; manuscriptreceived5 June 1971. 1391
1392
C. IWAKUK&, M. TSUNAGA
and H. TAMURA
oxidation numbers and in general two types of oxidation coupling reaction are well known; one-electron-transfer and two-electron-transfer reactions. PPO resin, which was first prepared by Hays and is now produced industrially by G. E. Company, is a polymer with excellent properties prepared by oxidative coupling polymerization of 2,6-xylenol. 6.7 Depending on the ratio of pyridine to cuprous chloride used as catalyst in this reaction, diphenoquinone or polyphenylene ether is predominantly produced, by C-C or C-O coupling respectively. In the present work, replacement of the catalytic oxidation reaction of 2,6xylenol with oxygen by anodic oxidation was tried. Preliminary experiments with tic* revealed that the products were not diphenoquinone and polyphenylene ether, -which might be formed by the oxidative coupling reaction. To clarify the differences between the two cases, the electrode reaction of 2,6-xyIeno1 was analysed by the various electrochemical and chemical methods. The electrolytic oxidation of phenolic compounds has been investigated by many workers.s-12 Vermillion et all2 have reported that two entirely different anodic reactions are possible with phenolic compounds. The first is an electrophilic attack on the aromatic nucleus of the non-ionized phenol with the irreversible removal of two electrons to give a mesomeric phenoxonium ion. The second is the reversible removal of one electron from the phenoxide anion to give a phenoxy free radical. 2.
EXPERIMENTAL
TECHNIQUE
2.1 Preparation and purification of chemical reagents Commercial grade 2,6-xylenol obtained from Wako Pure ChemicaI Ind. Ltd. and Nakarai Chemicals Ltd. was purified by recrystallization from n-hexane for up to three times.s No contaminants could be detected in purified 2,6-xylenol by ir analysis and gas chromatography. Commercial acetonitrile used as solvent was commercially available from the same sources. It was refluxed over phosphorus pentoxide for more than 8 h and then distilled at atmospheric pressure.13 Lithium perchlorate used as supporting electrolyte was dried at 150°C under reduced pressure for over 12 h. The test solutions prepared were agitated and de-aerated by bubbling argon for 10 min, just before each measurement. 2.2 Electrochemical measurements Polarization measurement by the stationary method, cyclic voltammetry, chronocontrolled-potential electrolysis, controlled-potential coulometry, potentiometry, polarography and conductivity measurement were employed. The cell assembly mainly used is shown in Fig. 1. The working electrode was platinum wire, O-10 cm dia., 0.18 cm length, plate, 1 cm x 1 cm or net, 100 mesh, 1 cm x 2 cm, and the counter-electrode was a smooth platinum plate of large surface area. Prior to each experiment, these platinum electrodes were polished with emery paper and then sufficiently washed with water and further treated with acetone for degreasing and drying. Ag/O. 1 M AgNO, was used as reference electrode.? Unless * Thin-layer chromatography. t The single electrode potential of this reference electrode is 0.337 V(~c.e).~~
hodic
oxidation of 2,6-xylenolin acetonitrileon platinum
1393
Prc. 1. Electrolytic cell. WE, working electrode (Pt); CE, counter electrode(I%); RE, reference electrode (A&O-l M AgNO,); G, glassfilter; L, Luggin capillary; P, potentiostat. otherwise stated, electrode potentials are referred to this electrode in the present paper. All instruments used were conventional. Temperature was kept constant at 30°C. 2.3
Chemical analysis of products
After the controlled-potential electrolysis, the electrolyte was chemically analysed, sometimes with pre-treatment for separation of the products. The separating procedure was as follows: after removal of lithium perchlorate from the electrolyte by salting-out techniques, the solution was condensed with an evaporator, and developed it with silica-gel column. The following solvents were used as developing agent in turn ; n-hexane, carbon tetrachloride, benzene, chloroform, ether and methanol. Each fraction was dried out and then analysed by tic, and by ir, uv and mass spectrometry. 3. RESULTS
AND
DISCUSSION
3.1 Analysis of electrode reaction The anodic polarization characteristic was measured by the stationary method in acetonitrile containing 0.5 M lithium perchlorate. The current values observed at 2 min after setting at each potential were recorded. Figure 2 shows the current/ potential results obtained for three different concentrations of 2,6-xylenol. It was observed that the sohttion in the immediate neighbourhood of the electrode was coloured reddish-brown and the coloured substance fell toward the bottom of the cell in the potential region where the current rose (Fig. 2). For each curve a Tafel line, slope 120 mV, was obtained in the potential range between O-6 and 0.8 V, from which value an, was caIculated to be O-50. The anodic currents had a minimum near 1-OV, and reached to the so-called limiting current when the potential was made more anodic. The limiting current was nearly proportional to 2,6-xylenol concentration.
1394
C.
IWAKURA,
M.
TSUNAGA
and H. T-
Potential,V(Ag/O-I M AgNO*) FIG. 2. Current/potentialcurves of 2,&xylenol at various concentrations. a, 0 M; b, O-01 M;
c, 0.05 M;
d, 0.10 M.
A typical voltammogram of 2,6-xylenol obtained with the multi-sweep method is given in Fig. 3. The anodic reaction evidently occurs in two steps, giving two anodic peak currents at 1-Xand 1.5 V. These are designated peak 1 and 2, respectively. The chronopotentiograms also revealed two well-defined waves at the corresponding potentials. The small anodic peak at ca 0.65 V in Fig. 3 might be caused by the further oxidation of the oxidized product of 2,6-xylenol because it has disappeared
Potential, V(Ag/O*I M AgNOJ FIG. 3. Typical cyclic vdtammogram of 2,6-xyknol
(multi-sweepmethod).
1395
Anodic oxidation of 2,6-xylenolin acetonitrileon platinum
with the single sweep, as indicated in Fig. 4. Since it can be presumed that the oxidation of 2,6-xylenol occurs at peak 1 and that the product, probably dimer, is further oxidized at peak 2, some detailed discussion follows.
Potential, V(Ag/O*I M AgNOJ FIG. 4. Typical
cyclic voltammograms (single-sweep method). hydroqtinone; c, DPPH.
According to the theory of cyclic voltammetry, the peak EP and the half-peak I&, is given by % -
E,/2
=
a, 2,6-xylenol;
b,
the potential increment between
0*0485~cW~v,
(1)
for a totally irreversible system at 30X!,” where ocis the transfer coefficient and n, is the number of electrons involved in the rate-determining step. Substituting avz, = 0.50 in (I), we obtain ED - Ep lz= O-097V. The experimental value is in good agreement; for instance, the potential increment was 0.10 V at u = 0.024 V/s for concentration 0.01 M. Therefore, the reaction is considered to be a totally irreversible reaction. For such a system, the current can be expressed as i=
nFAc"diix(/?t>,
(3
where i is the current (A), n the number of electrons, A the surface area of the electrode (cm2), co the bulk concentration of the species (mole/cm3), D,, the diffusion coefficient (cma/s) and B = cu@~/RTwith u the potential sweep rate w/s). The other notations have their usual significance. Here the current exhibits a peak at x(Bt) = O-282. Cyclic voltammetry was carried out in the sweep-rate range between O-024 and l-47 V/s with varying the 2,6-xylenol concentration from 0*01-O-05 M. Straight lines with good linearity were obtained by plotting ip against co or 2/;; on the basis of the voltammograms. The slope of the ip vs co curve (v = 0.11 V/s) was calculated to be 33 and therefore ?l(az2,)““D;~” = 4-93 x 10-S. (3) The following value was also obtained by calculating in the same way from the slope of ip vs 2/;; curve (c” = 1 x 1O-6 mole/cm3), IZ(cW~)1’2D;‘a = 5.91 x 10-s. Eggins and Chambers I6 have reported that hydroquinone and then reduction with an irreversible two electron-transfer
(4) undergoes oxidation at l-1 and O-3 V(sce).
1396
C. IWAKURA, M. TSUNAQA and H. TAMURA
respectively, under similar conditions as the present experiment. This was again confirmed in our laboratory. Good agreement was obtained for all sweep rates studied by a comparison of the peak height of 2,6-xylenol with that of hydroquinone. A typical result is given in Fig. 4. A similar curve for 1,l-diphenyl-2-picrylhydrazyl (DPPH) is also shown: DPPH is known to undergo a pseudo-reversible reaction with one-electron transfer .lG20 The peak height of 2,6xyIenol is nearly double the peak height of DPPH. Consequently, we infer that 2,6-xylenol is oxidized at peak 1 with a two-electron transfer.* This is supported by controlled-potential coulometry. The coulometry was made at l-1 V for an acetonitrile solution containing 5 ml of 0.01 M 2,6-xylenol and O-5 M lithium perchlorate. The electrolysis was stopped when the current reduced to about 5 per cent of the initial value. The observed charge passed was lo-17 C and the calculated 9.65 C assuming n = 2. Using n = 2 and ouz, = 0.50, D, can be calculated with (3) and (4) as 1.22 x 1O-s and l-75 x 1O-5 cm2/s respectively, which are reasonable values. Do’s of quinone and hydroquinone, which have similar structure to 2,6-xylenol, have been reported to be 2.9 x 10N5and (3.16 -& O-15) x 1O-5 cm2/s respectively.16 Theelectrical conductivity of 2,6-xylenol in acetonitrilesolutioncontaining 0.001 M lithium perchlorate was measured by an ac method. A 1 kc/s sine wave was put into the impedance bridge and the equilibrium point was detected with an oscilloscope. O-1 M potassium chloride aqueous sohrtion was used as the standard solution for determining the cell constant. 21 The results are summarized in Table 1. The specific TABLE 1. E~rzr~o~rrrc. CONDUCTANCE OF ~,~-XYLENOL M ACETONITRILE SoLuTIoN CONTAINING 0.~1 M LITHILW PERCHLORATE Concentration of 2,6-xylenol M
Resistance c-2
specifk resistance R-cm
specific conductivity mho/cm x lo-’
:X)0156 ow313 000625 0.0125 00250 0~0500 0-1000
2316 2310 2310 2310 2310 2315 2330 2334
6401 6385 6385 6385 6385 6399 6440 6451
1562 1.566 1.566 l-566 1.566 1,563 1.553 1.550
resistance and the specific conductance remain in constant over a wide range of 2,6-xylenol concentration and furthermore are approximately the same as those of the blank solution without added 2,6-xylenol. This indicates that 2,6_xylenol is scarcely dissociated in the solution. The half-wave potentials for a totally irreversible electrode reaction at 25°C may be correlated by the Hammett12 equation -0*059 1 E l/2 - El”l2 = UP, (5) BnB
where Ella, ET/s are the half-wave potentials of substituted and non-substituted l Under the same experimental conditions, ip varies depending on n, aur, and D, with the different compounds. Here ovl, can be determined also from (1) and D, can be estimated by the similarity of the structure of the compounds.
Anodic oxidation of 2,6-xylenol in acetonitrife on platinum
1397
compounds, respectively. p is the transfer coefficient for irreversible oxidation and ng is the number of electrons transferred in the potential-determining step of the electrode process. 0 is Hammett’s substituting constant derived by Gould22 and p is Hammett’s reaction constant. According to Vermillion ef aZ,12 (5) is in agreement for the hindered phenols of formula
(R = CH,, t-butyl, CHO, COOH) R and plots of El12 ag ainst d for the R group gave a straight line plot. In this case, the oxidation reaction is an electrophilic attack on the aromatic nucleus of a non-ionized phenol. On the other hand, for a reversible electrode reaction at 25”C, E l/2 --
E& = -o-0591ap,
where the reaction is a reversible removal of one electron from the phenoxide anion. The half-wave potential of 2,6-xylenol was determined by polarography at a rotating platinum electrode as l-1 to 1-3 V. The b+-value of 2,6-xylenol was calculated as -0.138 with the assumptions proposed by Vermillion et uLX2 The necessary values for this calculation are in the literature.= The Elf2 [14-l-6 V(sce)] and o+ (-0.138) values are plotted in Fig. 5, from which it can be seen that the point for 2,6-xylenol
3 8 >
I
1
-0.8
I
-0-4
I
I
0
0.4
u+
Hammett b+ plot. a, 2,6-dimethoxyphenol; b, 2,4,6-tri-tert-butylphenol; c, 2,6-di-tert-butyl-p-01; d, 2,6-di-tert-butylphenol; e, 3,5-di-tert-butyI4hydroxybenzaldehyde; f, 2,6-xylenoI. 0, data of Vermilion et al;” 0, present study. FIG. 5.
is on the same line as found by Vermillion et al. On the other hand, the calculated point was largely deviant from the line based on (6). Consequently the anodic oxidation of 2,6-xylenol in acetonitrile is presumed to be an electrophilic attack on the aromatic nucleus of the non-ionized phenol. 3.2 Analysis of electrode reaction by chemical approaches A spectral change by electrolysis of 2,6-xylenoi is shown in Fig. 6. It is clear from the similarity between (b) and (c) that 2,6-xylenol does not react with lithium
C. IWAKURA, M. TSUNAGA and H. TAMURA
1398
Wave
length,
tam
FIO. 6. Spectral change by electrolysis of 2,6-xyleno1 (qualitativeanalysis). a, blank solution (acetonitrile-O~S M LiClOJ; b, blank solution, 2,6-xylenol added after ektrolysis; c, before electrolysis with 2,6_xyienoI; d, after ekctrolysis with 2,6-xykllo1.
perchlorate and acetonitrile. And it can be found from the difference between (c) and (d) that new absorption peak appear at 253 and 260 nm and the peak of 2,6-xylenol at 270-280 nm is reduced by electrolysis. The products of electrolysis at 1-l V for 24 h were separated by column chromatography. Their fractions were analysed by mass spectroscppy and the following relations between the developing agents and the fractionated substances were obtained. (mw 240) n-hexane, carbon tetrachloride: dimer : trimer (mw 358) benzene : tetramer (mw 476) chloroform, ether : mixture (mean mw 450). methanol Here, the mean molecular weight of the methanol fraction was measured by the VP0 method, using acetonitrile as a solvent. Absorption bands observed in ir and uv spectra of the dimer were assigned to Ir spectrum stretching vibration of phenol. 3380 cm-l: O-H stretching vibration 1650 cm-l : &C C-O stretching vibration of phenol. 1190 cm-l: Uv spectrum 210 nm (strong Et absorption band): aromatic w 4 m* transition. 260 nm (strong K absorption band): presence of chlomophore conjugated with a ring. 280 nm (weak K absorption band): characteristic absorption of ctistilbene. Furthermore,
the ir spectrum of the product was simiIar to that of cis-stilbene. From
Anodic oxidation of 2,6-xylenol in acetonitrileon platinum
1399
the above facts, the dimer was estimated as 2,2’-dihydroxyl-3,3’-dimethylstylbene, OH
OH CH=CH,/!,/
Cl%
.
It is presumed that trimer and tetramer are produced by further polymerizatiqn in the same way, since the peak fragment of dimer (mw = 240) could be observed in their mass spectra. 3.3 Reaction mechanism On the basis of the above discussion, we propose the following mechanism for the anodic oxidation of 2,6-xylenol at the platinum electrode in acetonitrile. Since the phenol& hydroxyl is undissociated, the non-ionized 2,6-xylenol is presumed to form the resonance OH
r
@OH
@OH
1
H,C
H’ (III)
(11)
_I
Two electrons are withdrawn from (II) or (III) by electrochemical oxidation with the simuhaneous removal of hydrogen ion, and it forms a mesomeric phenoxonium ion according to ,CH.
,CH*
-20
(II),
cm
z
* (7)
+-+
H 09
A similar kind of phenoxonium ation,ls
+ CH,OH
+
\ H,C-
KC
0-1
ion as (V) and (VI) has been isolated by methoxyl-
0
/R
/@
0
‘OCH,
(V) undergoes the further removal of hydrogen ion from a methyl group to produce the intermediate 0
cv)
_H+*Ha% II FHa
0\I WI)
,
(9)
C. IWAKURA, M. TSUNAGA and H. TAMURA
1400
followed by its dimerization
to form (VIII),
2
H2C\
OH
OH
0
II
0\’ (VII)
(VIII)
involves an electrophilic attack on the aromatic nucleus at a position to the phenolic hydroxyl, the intermediate may be (IX), which produces (X) by dimerization,
If the reaction
para
..
CHa 0
ox>
It was confirmed by tic and ir spectrophotometry that diphenoquinone (X) was not produced. Though the same process is likely for trimer and tetramer formation, detailed discussion cannot be made without further investigations on the second step of the It should, however, be noted that the proportion of dimer in the electrode reaction. products is usually larger with decreased concentration of 2,6-xylenol and with shorter This may be related to the observed facts that the current duration of electrolysis. smoothly decreased with the electrolysis time at low concentration of 2,6-xylenol, whereas a minimum in the current/time curves was observed at high concentration. Acknowledgement-The authors wish to express their deep gratitude to Assistant Professor Toshiyuki Shono, Osaka University for his helpful suggestions in the course of this work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
W. I. TAYLOIZ and A. R. BA~ERSBY, Oxidatiue Coupling of Phenols. Dekker, New York (1968). A. I. Scorn,, Quart. Rev. 19, 1 (1965). K. FREUDE~ERC+, Science 148,595 (1965). B. FRANR, 0. BLA~CHKE and G. SCHLING~FP, Angew. Chem. 75,957 (1963). A. S. HAY, J. Am. them. Sot. 81, 6335 (1959). T. SHONO. K. YAMANOI, T. MATSUSK-XITA and K. SHINRA, Kogyo Kagaku Zasshi 70,2062 (1967). (1969). T. SHONO, M. MORI and K. SHINRA, Kogvo Kagaku Zasshi 72,178l J. F. HEDENB~R~and H. FREISER,Ana&. Chem. 25,1355 (1963). V. I. GINZBERC),Zh. fiz, Khim. 33, 1504 (1959). V. F. GAYLOR, A. L. CONRAD and J. H. LEUDERL,Analyt. Chem. 25,107s (1953). V. F. GAYLOR, A. L. CONRAD and T. H. L~UDERL, Analyt. Chem. 29,224 (1957). F. J. VERMILLION,JR. and I. A. PEARL, J. ebctrochem. Sot. 111, 1392 (1964). A. J. BARD, Electroanalytical Chemistry, Vol. 3, p. 57. Dekker, New York (1969). P. DEL-Y, New Instrumental Methods in Electrochemistry, p_ 115. Interscience, New York (1954). B. R. E-INS and J. Q. CHAMBERS,J, electrochem. Sot. 117, 186 (1970). E. SOLON and A. J. BARD, J. Am. them. Sot. 86,1926 (1964). E. SOLON and A. J. BARD, J.phys. Chem. 68,1144 (1964). D. A. HALL and P. J. ELVING, Electrochim. Acta 12, 1363 (1967). B. L. FUNT and D. G. GRAY, Can. J. Chem. 46,1337 (1968). C. IWAKURa and H. T AMURA, unpublished work (1970). H. C. PARKERand E. W. PARKER,J. Am. them. Sot. 46,3 12 (1924). N. S. GOULD, Mechanism and Structure in Organic Chemistry, p. 790. Holt, Rinehart & Winston, New York (1959). H. H. J&, Chem. Rev. 53, 191 (1953).