Up to the present, redox potentials of phenoxyl radicals could not be measured directly. Platinum is unsuitable as electrode material because it catalyses electrolytic oxidation of the solvent too strongly at high electrode potentials.We have found that sintered boron carbide electrodes [1,2] react reversibly to the redox equilibrium 11) (2) ( 3 ) 141 in acetate-buffered ethanolic solution (but not in 0.01 N HCI in ethanol). The e.m.f. of the cells 6 9 4 1 5 7 1 2 % 20 1.07 1203.6i 0.5 52 6 9 4 5 5 7955 40 0.50 1203.41 0.5 54 629+ 5 4 1 1 i 10 30 1023.2 J; 1.0 0.6 496i 2 2780i 50 m 1008.8 t 1.0 0.23 and Pt/Hg/[Hg2Acz] ... /.../Hz, Pt (electrolyte: CZHSOH/[N~AC], [HAc] < 0.1 mde/l) was measured at 15.0°C. The ditference E between the two e.m.f.'s is independent of the pH, provided the electrolyte composition is the same in both cells.Ek is the redox potentialThe dimeric phenoxyls ( I ) and (2) were prepared according to [3] and recrystallized under nitrogen at -40 OC from petroleum ether/benzene (1O:l). In solution, the blue-green monomeric phenoxyl radicals are present in equilibrium with their colorless dimers. The dissociation constants K = [C6H50-]2/ [(c6H&.),] and the molar extinction coefficient Emax were determined from the concentration dependence of the optical density A = ~. E ,~, . [ C~H @ * ] at the maximum of the longwave absorption band. The phenoxyls (3) and (4) were prepared by partial electrolytic oxidation of solutions of the corresponding phenols directly within the spectroscopy cell.The gradation of the redox potentials (see Table) cannot be expressed using the Hammett o constants.The decomposition of the phenoxyls is a first order reaction (rate constant k) relative to the concentration of the monomeric phenoxyl. Received: January 25th, 1965 [ Z 921/751 IE] German version: Angew. Chem. 77, 378 (1965) [I From Elektroschmelzwerk Kempten GmbH. ; Norton International Inc., Worcester 6 , Mass. (U.S.A.). [Z] T. R. Mueller and R. N. A d a m , Anal. Chirn. Acta 23, 467 (1960), describe the purification and installation of sinrered boron carbide electrodes but in a different connection.The fact that 1,6-methanocyclodecapentaene ( I ) [ I ] has a largely delocalized 10 x-electron system made it appear likely that the caibonium ion (3) [2] is capable of existence. The conversion of benzene via norcaradiene and cycloheptatriene into the tropylium cation served as a guide for the synthesis of ( 3 ) .When (I) is treated with diazomethane in the presence of CuCI, the bright yellow olefin (2) is obtained, possibly in admixture with other isomers; (2) has a b.p. of 98"C/l mrn, 17500), 250 (133001, and 335 (3500). The conversion of ( l ) into (2) is clearly shown by N M R spectroscopy (see Figure): owing to the loss of the cyclic conjugation, the resonance signals for thevinylic protons shift from 2.3-3.2~ for ( I ) to 3.3-4.7 T for (2), and the signal for the bridge protons, originally a singlet a t 10.5 7, splits into an AB system at 5.3 and 8.8 T with J = 11 cps. The remaining signal group in the N M R spect...
