Benzene anion radical

Fig. 12.2. EPR spectra of small organic free radicals, (a) Spectrum of the benzene radical anion. [From J. R. Bolton, Mol. Phys. 6 219 (1963). Reproduced by permission of Taylor and Francis, Ltd.] (b) Spectrum of the ethyl radical. [From R. W. Fessenden and R. H. Schuler, J. Chem. Phys. 33 935 (1960) J. Chem. Phys. 39 2147 (1963). Reproduced by permission of the American Institute of Physics.]...

Arylsilane radical anions undergo cleavage and coupling reactions, usually under conditions where excess reducing agent is available. Reduction of phenylsilane, diphenylsilane, or triphenylsilane with sodium-potassium alloy under preparative conditions gives high yields of tetraphenylsilane (7). In the reduction of phenylsilanes, the appearance of 1,4-bis(silyl)benzene radical anions is frequently observed (135, 35, 86, 97, 75, 120, 100). Typical results are shown in Table II. 

This mechanism is reasonable as a) reduction of benzene occurs at a cathode potential of -2,5 V vs. S.C.E., roughly corresponding to the standard potential of the hydrated electron 293 while the potential for the direct electron transfer to benzene is more negative ( -3,0 V) and b) in situ electrolysis in the ESR cavity produces at -100 °C the characteristic singlet of the solvated electron 293a>, which changes to the septett of the benzene radical anion, when benzene is added to the solution. 

The radical anion can participate in electron transfer reactions, in which the electron is transferred to a compound with suitable electron affinity. The parent aromatic compound is regenerated. Such a reaction is shown in Eq. (83), where chlorobenzene is dechlorinated by electron transfer from the benzene radical anion ... 

Benzene Benzene radical anion Benzenated electron... 

EPR spectrum of electrochemically generated benzene radical anion, C6H6-. The hyperfine interaction between the free spin and the six H1 nuclei generates a seven-line spectrum of nominal relative intensities 1 6 15 20 15 6 1. The hyperfine splitting constant is 0.375 mT. 

For the benzene radical anion, one thus expects a seven-line pattern with intensity ratios of 1 6 15 20 15 6 1, in good agreement with the ESR spectrum shown in Fig. 2. 

For the molecular case, the essential conclusion is that the orbital must have some s (or cr) character for the impaired electron to interact with a magnetic nucleus. Consider however the case of the benzene radical anion, in which the electron is usually described as being in a tt orbital with a node in the molecular plane. As a consequence no coupling with the proton nuclei is expected, a prediction clearly in conflict with the hyperfme splitting of 3.75 gauss seen in the ESR spectrum of this species as shown in Fig. 2. Flow, then, does the unpaired tt electron density appear at the Ft nucleus ... 

Transfer of an electron to 1,4-dinitrobenzene gives a radical anion so stable that it is unlikely to transfer an electron to anything else. The reaction of benzene radical anion with 1,4-dinitrobenzene proceeds to the right, because the radical anion of the 1,4-dinitrobenzene is much more stable than the radical anion of benzene itself. 

Benzene is most difficult to convert to an anion radical nevertheless, with potassium at — 80°C in DME the anion radical of benzene can be identified by ESR. Spin-concentration measurements on benzene in 2 1 by volume of THF DME with Na-K alloy at — 83°C show that at equilibrium only ca. 0.1 % benzene is converted to radical anion. Benzene also forms an anion radicaF by ESR with Rb and Cs in THF and DME, but not with Na or Li in THF-DME. Alkyl substitution destabilizes the radical anions as prepared with Na-K in THF-DME at — 100°C, with destabilization increasing with size [CH3 < CH CHj < CH(CH3)2 < C(CH3)3] and with numbers of such groups . In contrast, Si(CH3)3, Ge(CH3)3, CN and NO2 groups stabilize their benzene radical anions. Radical-anion formation even in benzene, toluene, or mesitylene with potassium can be brought about by addition of dicyclohexyl-18-crown-6 , 18-crown-6 or [2.2.2]cryptate at RT or lower. Radical anions may be prepared similarly from Rb and Cs, but not from Na with 18-crown-6 or [2.2.2]cryptand . The yield or how long such solutions are stable is not known. 

Since only an s orbital has a finite density at the nucleus, the occurrence of hyperfine splitting requires that the unpaired electron is either in a pure a oribtal (as in the hydrogen atom, for which = 507 G) or in an orbital which has some s character. It is therefore not immediately apparent how splitting arises in a radical in which the unpaired electron is in a ir-orbital (e.g. the benzene radical-anion, for which as = 3-75 G). The mechanism by which it does so is described as spin polarization thus, in a fragment in which the unpaired electron is in a p orbital,... 

Bowers and Greene (1963) reported the e.s.r. spectrum of the radical-anion of cyclopropane and Bowers ealkali-metal reduction of the parent compound. However, Gerson et al. (1966) have found that none of these compoimds is reduced under these conditions (i.e. the e.s.r. signal due to the solvated electron is not quenched) and Jones (1966) has foimd that the signal from the supposed adamantane radical-anion is that of the benzene radical-anion. 

The hyperfine proton coupling constants aH are used for estimating local spin densities (at proton bearing centers p). The McConnell Equation 56 57) (Eq. 11) relates aH and qu 57). The proportionality constant Q was chosen for benzene radical-anion as —2.25 mT 56>. For larger jr-systems higher values of Q were used. An agreement between the local spin densities and the total one should be sought. The McLach-lan... 

Q The ESR spectrum of the benzene radical anion, [C6H(,], in I which the unpaired electron is in a molecular orbital delocalized round the benzene ring, shows a septet, with a coupling constant of 0.375 mT. Comment on the spectrum, in relation to the quartet I shown by the methyl radical, with n(C-H) = 2.30 mT. 

Unpaired electrons can be present in ions as well as in the neutral systems that have been considered up to this point. There are many such radical cations and radical anions, and we consider some representative examples in this section. Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are examples. The ESR spectrum of the benzene radical anion was shown earlier in Figure 11.2a. These reductions must be carried out in aprotic solvents, and ethers are usually used for that purpose. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representative compounds are given in... 

The theoretical aspects of long-range electron transfer have been reviewed in a recent monograph. Mikkelsen and Ratner have used their density matrix approach to examine the effects of a molecular species (H2O) placed between the benzene radical anion donor and the benzene molecule acceptor and of the relative... 

CF8-P-P(CF8) C(CF8) C(CF3) P CF8 [both from (CFs P) (x = 4 or 5)-l-CFs CiC CFs at l70°C i ] in which it is concluded that these substrates accommodate an entering electron in an MO primarily localized on the s/> -carbons. The radical anion of the triphosphacyclopentene decomposes slowly, even at —130 °C, giving hexakis(trifiuoromethyl)benzene radical anion, which is the sole radical... 

For a solution, the radial distribution function will typically have a structure as shown in Figure 14.6 for a simulation of a benzene radical anion in water. ... 

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