Radical dissociation

The radical dissociation of the Gomberg dimer , [3-(diphenyl-methylidene)-6-(triphenylmethyl)-l,4-cyclohexadiene] [48], is familiar to organic chemists as the original observation of carbon-carbon a bond dissociation in a solution (Gomberg, 1900 Lankamp et al., 1968). The yellow colour of the triphenylmethyl radical in the benzene solution should have been an observation convincing synthetic organic chemists of the stable existence of the triphenylmethyl radical [8-j. 

Another means of observation of such radical dissociation of hydrocarbons, to a lesser extent, is achieved in modern esr spectroscopic studies. Dissociation of bitropyl [49] into the tropyl radical [3 ] was observed in m-xylene solution (11). The radical is a long-lived species but only at low concentration, 10 -10 m (Vincow et al., 1969). The radical dissociation (12) of 7-(triphenylmethyl)-l,3,5-cycloheptatriene [50] has been found to be more facile than that of bitropyl (Okamoto et al., 1970). 

The cleavage of carbon-carbon o- bonds is the main reaction in thermal cracking of naphtha, one of the commonest processes in the hydrocarbon conversion industry. Thus, radical dissociation or cleavage of carbon-carbon cr bonds has been quite familiar to synthetic and physical organic chemists. 

These empirical linear relations should be applied to groups of compounds with a common steric background. Thus, a negative deviation from these linear relations most probably indicates specific steric congestion assisting the dissociation. In this context, it is known that, in radical dissociation of carbon-carbon cr bonds, steric congestion is a major enhancing factor (Riichardt and Beckhaus, 1980). 

The semiquinone radical dissociates to a proton and semiquinone radical anion in the polar solvent (water). 

Table 5. Radical Dissociation Energy for Alkoxyamines (unit in kJ/mol)a...

In the limiting case of Eq. (18), if radical dissociation of the iniferter bond, addition of one monomer molecule, and reproduction of the iniferter site by the... 

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

Although the reaction with thiosulfate and with iodide ions may be a mere reduction of the halide, the reaction with sodium benzoate would appear to be a radical dissociation induced by the attack of a negative ion. The fate of the benzoate ion is unknown. Tris-( -nitrophenyl)-methyl benzoate is a stable substance which does not dissociate into radicals.23... 

The corresponding hyperconjugative interactions in an open-shell singlet description of radical dissociation would similarly promote homolytic bond cleavage. 

As an extension of this work, Atkinson and co-workers (123) prepared l-dibenzylamino-l,2-dihydro-2-quinolone (78) and 1 -(/V-benzy l-N-carboxy-methyl)amino-l, 2-dihydro-2-quinolone (79). The benzylic protons of 78 showed an AB quartet that did not coalesce up to 180°C, and 79 was resolved into optical isomers. The E, for racemization was 26.2 0.4 kcal/mol. Various attempts were made to elucidate the possible pathways for isomerization in these quinolone derivatives (123). Radical dissociation, a sigmatropic shift followed by rotation, and restricted rotation about the S—N bond were excluded. The aforementioned authors (123) also excluded the possibility of nitrogen inversion and preferred restricted rotation about the N—N bond as an explanation for the existence of stereoisomers. They supported this explanation by examining the steric effects... 

The normal internalization of the wild-type receptor, defined as a loss of cell surface receptors (measured by decreased maximal binding or B ), was unaffected for the desensitization-deficient Thr mutant (see Fig. 6.4C,D) but may have been affected when distal carboxyl terminal residues were mutated (see Fig. 6.3). Therefore some, although not all, GPCRs show radical dissociation between desensitization and internalization. This is found not only in the dopamine Dj receptor (122) but also in the N-formyl peptide (134), the CBl cannabinoid (17), and the M2 muscarinic (155) receptors. 

In solution the colorless 2,4,6-triphenylphenoxyl dimer attains a rapid equilibrium with its red monomer radical (dissociation constant in benzene 4 X 10 at 20°). The radical is surprisingly stable toward oxygen and can be stored in solution for a long time when it is protected from light. The stability of the 2,4,6-triphenylphenoxyl radical is ascribed to steric and mesomeric effects. The e.s.r. spectrum and an ENDOR-spectrum of the radical are described. 

It is generally accepted that the aza-[l,2]-Wittig rearrangement proceeds via a radical dissociation-recombination mechanism, whereas the aza-[2,3]-rearrangement proceeds... 

Flanagan and Rabinovitch were able to establish another point of general interest. They deduced from the relative rates of exchange and isomerization of isotope effect in the rupture of the carbon- hydrogen bond when adsorbed ethyl radicals dissociate to form adsorbed ethylene molecules. The ratio of the rupture probabilities of C—H and C—D decreased from 15.9 at —78° to 1.4 at 429°. More evidence of this kind would obviously be valuable because it suggests that some revision may be necessary of the theory for calculating initial distributions of... 

Here R denotes the primary alkyl radical derived from the alkane RH, and ROO is a peroxy radical, where the O2 may be bound at a primary, secondary, or tertiary site in the alkyl radical.2 Formation of ROO is thermodynamically favored in the low-temperature regime, while at higher temperatures the equilibrium is shifted to the left, and the ROO radical dissociates rapidly back to reactants. 

The fraction of the available energy residing in the CH2I radicals is much larger than that in the CH3 radicals dissociated from CH3I, which is only 12% (834). Qualitatively, this difference in the energy partitioning can be understood from (11-23) based on the impulsive model (see p. 93). 

Figure 4.85 Example of a radical dissociation reaction yielding a smaller free radical and a closed-shell fragment...

Considering nitration with the help of N02/N204 in an aprotic medium, one should avoid a simplified approach to its mechanism. Of course, radical dissociation of dinitrogen tetroxide is clear and usual. However, two ionic routes of dissociation of N204 are also possible in aprotic media ... 

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