Radicals resonance

Sjsj2 reaction and, 377-378 Benzylic radical, resonance in, 578 spin-density surface of, 578 Benzylpenicillin, discovery of, 824 structure of, 1 Benzyne, 575... 

From Table III we see that the difference between the free radical resonance energies of tribiphenylmethyl and triphenylmethyl is 0.07a. Hence X]/X2 = 37 = 2.2 X103. Ziegler and Ewald8 found that at 20°C the value of the dissociation constant for hexaphenylethane in benzene solution is 4.1 X10-4 and consequently we calculate for hexabiphenylethane a value of X = 2.2X103 X4.1 X 10 4 = 0.90. This value is probably too low as the compound is reported to be completely dissociated the error may not be large, however, since a dissociation constant of 0.90 would lead to 91 percent dissociation in 0.05M solution. 

The discussion in section (c), page 373, regarding the quantitative agreement of the calculations with experiment has to be altered somewhat in accordance with the corrected values of the free radical resonance energies. The calculated dissociation constant of hexabiphenylethane now becomes... 

On the basis of this mechanism the difference in the energy of activation for the decomposition to isoprene and that for the decomposition of isopropylcyclobutane to 3-methylbutene-l should equal the allyl radical resonance energy. The value obtained in this fashion is 11-6 +1 kcal mole in close agreement with the values obtained by other methods. 

Hydrogen abstraction from a position a to the oxygen of alcohols and ethers provides a simple route to a-oxyalkyl radicals. Resonance stabilization and polar factors have been used to explain the ease of radical attack on these substrates. Recent studies appear to exclude the possibility that the oxygen atom in position a to the free C-radical may cause stabilization by resonance. The ease of hydrogen abstraction Avould be determined only by polar factors, arising with electrophilic radicals (X ) in contributions from the polar forms 13-15 to the transition state. 

In the other type of fragmentation, a bond is cleaved so that the positive charge remains with one fragment and the odd electron goes with the other. Only the positive fragment is detected and appears in the mass spectrum. The stability of the product cation and radical determine the favorableness of this type of cleavage. You are already quite familiar with the factors that affect the stability of cations, especially carbocations. Although radicals are inherently more stable than carbocations because they are less electron deficient, they are stabilized by the same factors that stabilize carbocations. Thus, tertiary radicals are more stable than secondary radicals, and secondary radicals are more stable than primary radicals. Resonance stabilization is also important. 

Isomeric Hydroperoxides Formed from Methylene the Structures Contributing to the Group Intermediate Free Radical Resonance Aldehydes Formed by Decom-... 

Allylie Bromination of ABtMtes 363 L0.6 Stability of the AJIyI Radical Resonance Revisited 10.7 Preparing Alkyl i (altdes from Alcohols 366... 

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