Toluene radical

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

By contrast, Co(III) oxidation gives oligomeric products that result from further reaction of the toluene radical cation with toluene ... 

The occurrence of intramolecular electron transfer in radical ion bond cleavage reactions is probably not straightforward and in this respect, two examples may help the reader—C-H deprotonation in the toluene radical cation and C-Cl bond cleavage in the 4-nitrobenzyl chloride radical anion (Scheme 25). 

In the toluene radical cation the electron hole is delocalized over the re-system and the deprotonation reaction is coupled with intramolecular electron transfer from the scissible bond to the aromatic ring, leading to the benzyl radical. In the 4-nitrobenzyl chloride radical anion, the unpaired electron resides in a re orbital that does not belong to the leaving group and C-Cl bond cleavage occurs simultaneously with an intramolecular electron transfer from this orbital to the a orbital of the scissible bond. 

When considering the rate constants for deprotonation reported in Table 2 one should, however, take into account that in methylbenzene radical cations with < 3 methyl groups, nucleophilic attack of water on the aromatic ring can compete efficiently with side-chain deprotonation. An elegant explanation which accounts for this competition has been provided for the toluene radical cation on the basis of the three-electron three-orbital three-configuration approach [135, 136]. Three electrons are involved in the deprotonation reaction—the unpaired electron delocalized over... 

As previously discussed, removal of an electron from the 7r-system of alkylaromatic substrates leads to a dramatic increase in the acid strength of the C -H bonds, as clearly shown by the pA a between -11 and —13 estimated for the deprotonation of toluene radical cation in acetonitrile [130]. The presence of a positive charge on the aromatic ring can also influence, however to a much smaller extent, the acidity of groups which are further spaced from the aromatic ring such as OH and CO2H as... 

In principle, the pK of the toluene radical cation can be estimated from the one-electron reduction potential of the radical cation and the C-H bond dissociation enthalpy for toluene (368 kj mor )[62] using equation (5). The resulting pR is ca -10, i.e., considerably more acidic than the phenol radical cation. Nicholas and Arnold have estimated the pfC of the toluene radical cation to between -9 and -13 in acetonitrile which is weU in line with the estimate given here. [32] Since the C-H bond dissociation enthalpies of substituted toluenes seem to be almost invariant with substituent,[63-66] the substituent effect on the pK of... 

The strong substituent effect on the acidity of toluene radical cations is also reflected by the effect of substitution on the rate of deprotonation from the radical cations. [8]... 

Clearly, there is no direct correlation between the gas-phase acidity and the pfC s in aqueous solution. This is mainly due to differences in solvation, especially for radical cations which can form strong hydrogen bonds to the solvent. Judging from the gas-phase properties, the phenol radical cation should be more acidic than the other three types of radical cations. The fact that it is much less acidic than the toluene radical cation can be accounted for by the stabilization of the phenol radical cation due to the strong solvation. 

For a number of different substituted benzenes, the substituents are involved themselves in the redox chemistry. Among these substituent-active compounds, we find phenolic substances, aromatic amines, chalcogenide substituted benzenes, benzyUc substances, benzoic adds, thiobenzoic adds and benzyl alcohols. As has already been discussed, phenol radical cations, aniline radical cations and toluene radical cations are more or less acidic and can deprotonate to form the corresponding neutral radical. Chedcogenide substituted benzenes (S, Se and Te) are usually characterized by the fact that their redox properties are determined by the chalcogenides rather than by the substituent pattern. 

ABSTRACT. Toluene radical anion, generated by dissolving potasssium metal in toluene by the assistance of dicyclohexano-18-crown-6, has been proved to be especially effective for reductive removal of fluorine atom from unactivated alkyl fluorides that resist common reduction conditions. Stereochemical and mechanistic aspects of the present method is discussed. In connection with the preparation of substrates the effect of dipolar aprotic solvents on the nucleophilic fluorination with potassium fluoride/dicyclohexano-18-crown-6 system was also examined, and sulfolane or N,N-dimethylformamide was shown to be a solvent of choice. 

These curious chemical entities led us to study the reactivity of potassium anion (K ) generated from K/crown ether/diglyme (or Et20, THF) system and toluene radical anion from K/crown ether/toluene system. The effectiveness of the former system for reductive cleavage of sulfonamide was reported elsewhere- -. In the present paper, reductive defluorination with the latter system is reported- -. ... 

It can be reasonably stated from the result in Table II that i) toluene is a better solvent than diglyme, proving the effectiveness of toluene radical anion for this type of reduction, ii) the presence of protic solvent depressed the reduction and iii) slightly more than one molar equivalent amount of crown ether is required for the completion of the reduction. 

Conclusively, K/DC-18-C-6/toluene was confirmed to be a stronger reducing system for this type of reduction than Li/liq NH, K/HMPT (with or without H" " and/or THF) or Na naphthalenide in dimethoxyethane. The interesting point is that the rather stable radical anion system (Na naphthalenide) is weaker than the labile counterpart (toluene radical anion). 

K solution is black-blue and toluene radical anion solution is brown-red in color. 

SCHEME 14.5 Possible transformations of toluene radical cation. 

The absorption spectrum of 7 in aqueous acidic solutions has also been detected. The vertical gas-phase ionization potential /p of benzene was measured from the He(I) photoelectron spectrum as 9.23 ev.37e standard anodic peak potential Ep could not be measured directly but was estimated from data for alkyl-substituted benzenes as 2.86 V vs NHE. Jahn-Teller distortion in 7 has been reviewed, and the high-resolution state-to-state threshold photoionization spectrum of benzene gives the shape of 7 8 and diminished significantly the mystery regarding the structure. The geometries, hyperfine structure, and relative stabilities of the two mono-deuterated Jahn—Teller-distorted ions CeHsD were examined theoretically and experimentally. EPR and ENDOR studies showed the toluene radical cation possessed the B2g structure. The IR spectra of the two Jahn—Teller forms of 7 were also calculated. On the basis of the calculated energy levels, both 7 and 8 have been classified as antiaromatic. ... 

The toluene radical cation provides a good example of how difficult converging a calculation to the electronic ground state of an open-shell molecule can be. If one does a single-point UHF/3-21G calculation at the equilibrium geometry of the neutral molecule, in which one hydrogen atom of the methyl group lies... 

Hitchcock PB, Lappert MF, Protchenko AV. The first crystalline alkali metal salt of a benzenoid radical anion without a stabilizing substituent and of a related dimer X-ray strucmres of the toluene radical anion and of the benzene radical anion dimer potassium-crown ether salts. Am Chem Soc. 2001 123 189-190. 

Aromatic donors form the largest class of substrates fragmentable by electron transfer in view of their accessible oxidation potential and the easy cleavage at the benzyhc position. Pioneering work by Nicholas and Arnold demonstrated the high acidity of the toluene radical cation. The process has been extended... 

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