4124
ARNOLDZWEIG, WILLIAMG. HODGSON, AND WALTERH. [CONTRIBUTION FROM
THE
VOl. 86
JURA
CHEMICAL DEPARTMENT, CENTRAL RESEARCHDIVISION,AMERICAN CYANAMID COMPANY, S T A M F O R D , COSNECTICUT]
The Oxidation of Methoxybenzenes' BY ARNOLDZWEIG, %
r G . HODGSOX, ~ ~ A N D L ~ A L T E R~H. JURA~
~
~
RECEIVED APRIL25, 1964 The half-wave polarographic oxidation potentials of the methoxybenzenes have been determined in acetonitrile a t a rotating platinum electrode. The e.s r. spectra of the methoxybenzene cation radicals and other radical oxidation products have been observed by means of controlled potential electrolysis in the microwave cavity of the e.s r. spectrometer and also by oxidation of the methoxybenzenes in concentrated sulfuric acid. The measured properties are compared with previously obtained charge-transfer absorption data and calculations based on molecular orbital methods.
Introduction In recent years magnetic resonance techniques have been successfully applied on a semiquantitative basis to the charting of a-electron distribution in conjugated molecules while charge-transfer absorption maxima, polarography in aprotic solvents, and ultraviolet spectroscopy have aided in determining the relative and absolute energies of molecular orbital levels.2 Some of these physical methods have also aided in uncovering new and previously unsuspected organic reactions and compound^.^ Polarographic oxidation and characterization of the resulting cation radicals by e.s.r. techniques has been limited to aromatic and to a few nitrogen and sulfur substituted Oneelectron oxidation of electron-rich aromatic substances is known to take place in concentrated sulfuric acid.'O In this medium, however, side reactions often result in the misleading occurrence of other paramagnetic species. 11*l 2 Methoxy-substituted aromatics comprise a type of compound with electron-rich molecular a-orbitals whose one-electron oxidations have not been previously investigated. The methoxybenzenes are a particularly attractive group of compounds for such study since all of the members of the set are known,I3 the substituent interacts strongly with the ring, and the high symmetry of benzene derivatives simplifies theoretical treatment of their properties. l y e report here a study of the polarographic oxidation of the methoxybenzenes in acetonitrile at a platinum electrode. Izlso reported are e.s.r. spectra which are obtained when controlled potential oxidation of the methoxybenzenes are performed in the microwave (1) Cumulative Influence of Conjugated Substituents o n the r-System Propeities of Aromatic Hydrocarbons. VII. Part V I . A Zweig, TeiraI i p d y m i L d l e r c , No. 2 , 89 (1964). ( 2 ) 4 . Streitrvieser, J r . , "Molecular Orhital Theory for Organic Chemi s t s , ' ' John Wiley and Sons, Inc., Kew York, S Y ,1961. ( 3 ) A . K . Hoffmann. W . G. Hodgson. D. IISTRIBUI'ION O F HIGHEST F I L L E D >I.O.'S METHGXYBENZENES 010 = cuc 2.0 p c c ; pcci = 0.8 6c.c
OF
+
Compound
Position
Methoxybenzene (anisole) 1 2,6 3,5 4 1,2-Dimethosybenzene 1,2
3,6 4,5 1,3-Dimethoxyberizene 1,3 2 4,6 5 1,4-Dirnethoxybenzene 1,4 2,3,5,6 1,2,3-Trirnethoxybenzene 1,3 2 4,6 5 1,2,4-Trirnethoxybenzene 1 2 4
Spin density on Carbon Oxygen
nil
ai, P,,
0 258 0 120 122 . . 048 ,282 . . . . i 3 3 0 092 ,012 162 . . ,161 0.069
0 0.270
. . . .
0
0 241 081 113 ,316 025 201 262 143 191 3 009 5 ,155 6 035 1,3,5-Trirnethoxybenzene 1,3,5 107 2,46 180 1,2,3,4-Tetrarnethoxy1,4 ,247 benzene 23 056 5,6 093 1,2,3,5-Tetramethoxy1,3 ,087 benzene 2 303 5 ,192 4,6 055 1,2,4,5-Tetrarnethoxy1,2,4,5 ,191 benzene 3,6 0 Pentarnethoxybenzene 1,s 0 191 2,4 0.191 3 0 6 0 Hmarnethoxybenzene 1-6 0.127
3 45
38 3
0 084 0 019
2.7
32 1
.
2 7
0 090 0 013 0 120
0.090 ,049
,066 .. . . .
0 046 .. .
0.029 100 064 .
.
0.059 2 . 2 1 0 89 0 059 2 . 2 8 0.059 1 . 9 8 0 .. 0.60 0 040 1 . 3 0
37 5
38.6 33.6
32 5
same as those used to calculate the energy levels.16 Also reported are the tentative assignments of proton coupling constants and the resulting proportionality of coupling constants to spin density. Despite many complicating factors, these proportionality factors all have nearly the same value. Hexameth0xybenzene.-The e.s.r. spectrum of the hexamethoxybenzene cation radical is of particular interest as the simple molecular orbital picture of this molecule suggests an orbitally degenerate ground state. It has been suggested39that radicals of this type have broadened hyperfine lines due to Jahn-Teller distortions which modulate the spin densities. Examination of the e.s.r. spectrum of the coronene cation radical4" and the hexamethylbenzene cation radicals1' have borne this out. In the present study, the narrowest line width between derivative extremes for the hexamethoxybenzene cation radical was 0.23 gauss. The iso(39) M G. Townsend and S I. Weissrnan. J. ChPin P h y s , 32 909 (1960) (40) J . R. Bolton and A . Carrington I f o i . P h y s , 4, 271 (1961). 141) R . Hulme a n d M. C. R . Symons, Pvoc C h e m Soc , 211 11963).
Oct. 5, 1 9 N
OXIDATION 0 1 7 METHOXYBENZENES
tropic coupling constant of 1.4 gauss found for the hexarnethoxybenzene cation radical in sulfuric acid should be compared to the 6.45-gauss coupling constant reported for hexamethylbenzene in the same solvent.41 The molecular orbital calculations, using a0 = ac f B&- and Pco = 0.8 pee, suggest that 0.128 e is localized a t each ring carbon and t h a t 0.039 e is a t each oxygen in the degenerate h.f .m.o. of hexamethoxybenzene. If Pco is reduced to 0.4& to adjust for the probable nonplanarity of the p-orbitals on carbon and oxygen, the unpaired spin on the ring carbons is calculated to increase to 0.149 with concomitant decrease of e on oxygen to 0.018. The highest filled m.o.’s are thus weakly perturbed benzene orbitals. The experimental data and the molecular orbital picture are in agreement in suggesting t h a t most of the spin density is located on the benzene ring and little seeps out to the oxygens. Pentameth0xybenzene.-The e.s.r. spectrum of the electrolytically oxidized pentamethoxybenzene is in very good agreement with what would be expected from the m.0. calculations. The latter predict that the orbital occupied by the unpaired electron is antisymmetric with regard to the symmetry plane perpendicular to the benzene ring. Thus the protons of the 1-,2-,4-, and 5-methoxy groups should contribute most significantly to the hyperfine structure of the e.s.r. spectrum. The splitting due to the methoxy protons on positions 1 and 5 should be somewhat larger than those of positions 2 and 4 owing to inability of the latter methoxy groups to be planar with the benzene ring. On this basis we assign the 2.28-gauss splitting to the I- and 5-groups and the 1.98-gauss splitting to the 2- and 4-groups. Hiickel theory predicts no spin density a t the nodal 3- and 6-positions so that the small further splitting that is observed is probably caused by negative spin density a t one or both of these positions. 4 2 1,2,4,5-Tetramethoxybenzene.-This cation radical is also of particular interest because all of its magnetic nuclei are accounted for by the hyperfine structure and because the splitting caused by the protons a t the nodal 3- and 6-positions are not predicted by the Hiickel calculations (cf. Table 11). In this case and in hydrocarbon anions and cations where such nodes occur,42 it is necessary to improve the wave function t h a t describes the welectron distribution in order to account for the discrepancy. This can be accomplished either by including excited state configurations or by selfconsistent field methods. The McLachlan method43 derived from the latter theory, which may be formulated in terms of Hiickel molecular orbital quantities, has been widely used in such In this theory the spin density a t position r is given by pr in eq. 7
where Cor2and Cosz are the Hiickel spin densities at (42) For a discussion of negative spin density see ref 33. (43) A. D. McLachlan, 4101. P h y s . , 3 , 233 (1960). (44) C. S. Johnson, J r . , and H S. Gutowsky, J. Chem. P h y s . , 39, 58 (1963). (45) G. Vincow. ibid., 38, 917 (1963) (46) S. H . Glarum and L C Snyder, ibid., 36, 2989 (1962). (47) A . Carrington and J dos Santos-Veiga, MOL.P h y s . , 5 , 28.5 (1962) (48) I . Bernal, P. H. Rieger. and G . K . Fraenkel, J. Chem. Phys , 3 7 , 1489 (1962). (49) G Vincow, ibid.,57, 2484 (1962).
4129
positions r and s, X is an empirical constant, the value for which 1.2 has proved to be experimentally successful,45-49and rrSis the atom-atom polarizability calculated from the Hiickel theory.50 In Table I11 is reported the calculated spin densities for the cation radical of 1,2,4,5-tetramethoxybenzene CALCULATED S P I N
TABLE I11 DENSITIES1,2,4,5-TETRAMETHOXYBENZENE Spin density-------. McLachlan (A = 1.2)
I---
Position
Hiickel
1,2,4,5 3,6 Oxygens
0.1912
0.2265
0 0.0588
- ,0675 .05i3
by the Hiickel and the McLachlan methods. The McLachlan method results in a negative spin density a t the nodal positions. 1,2,3,5-Tetramethoxybenzene.-The molecular orbital calculations indicate t h a t the h.f.m.0. of this molecule has about twice as much spin localized on oxygen at position 5 as on the oxygens a t positions 1 and 3. The protons of the methyl groups attached t o these oxygens can therefore account for the major features of the e.s.r. spectrum. Although t h e calculations indicate a still larger unpaired spin localization a t the oxygen on the 2-position of the benzene nucleus, steric inhibition of coplanarity of the pa-orbital on this oxygen with the benzene ring probably greatly diminished the spin density and also decreases the hyperfine coupling owing to the methyl protons. This interpretation must, however, be regarded as tentative. 1,2,3,4-Tetramethoxybenzene.-The hyperfine splitting in the e.s.r. spectrum of the cation radical obtained from electrolytic oxidation of this compound is in reasonable agreement with the simple m.0. picture, which predicts that the major couplings will be with the two ring protons and the methoxy protons a t the 1and 4-positions. The splitting due to the methoxy protons a t the 2- and 3-positions is expected to be even smaller than suggested by Table I1 because of steric hindrance. 1,4-Dirnethoxybenzene.-Oxidation of this compound either by sulfuric acid or electrolytically in acetonitrile apparently gives two radical species. The seven-line A species, which seems to appear first electrolytically, has a coupling constant of 3.45 gauss, apparently due to six equivalent protons. T h a t these may be the six methoxy protons of the lj4-dimethoxybenzene cation radical (the oxygen analog of Wurster’s Blue) is strongly suggested, although not proved by the molecular orbital calculations. The calculated spin densities on oxygen in both 1,2,4,5-tetramethoxybenzene and 1,4-dimethoxybenzene are in the same proportions as the methyl proton coupling constants of the former cation radical and this coupling constant obtained by oxidation of the latter. The species responsible for the “B” spectrum is not known, although a report of dimerizations of dimethoxybenzenes by electrochemical oxidations in methanol may prove a clue to its i d e n t i f i ~ a t i o n . ~ ~ Acknowledgment.-We wish to thank J. E. Lehnsen and S. M. Kern f o r computational assistance. ( 5 0 ) Ref 2, p 112 (51) B Belleau and N I
