Radicals relative stabilities

Cadet J, Voituriez L, Berger M, Myers LS Jr (1983) Radiation-induced degradation of purine and pyrimidine 2 -deoxyribonudeosides in agueous KBr solutions. Z Naturforsch 38b 1643-1651 Champagne MH, Mullins MW, Colson A-O, Sevilla MD (1991) Electron spin resonance evidence for intra- and intermolecular oo bonding in methionine radicals relative stabilities of S-CI, S-Br, S-N, and S-S three electron bonds. J Phys Chem 95 6487-6493 Chawla OP, Fessenden RW (1975) Electron spin resonance and pulse radiolysis studies of some reactions of S04. J Phys Chem 79 2693-2700... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

A similar explanation lies behind the diminished strength of the sp —sp carbon-carbon bond in ethylbenzene. The general trend toward weaker C—C bonds with increased substitution that can be recognized in Table 1.3 reflects the increased stability of substituted radicals relative to primary radicals. 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

With unsymmetrical ketones two different bonds are available for photolytic cleavage the actual cleavage pathway depends on the relative stability of the possible radical species R and R. ... 

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

All this evidence suggests that the radical produced from 2-vinylfuran is a rather strongly stabilized entity, compared with those of more common monomers, and is therefore, not very active in homopolymerization. On the other hand, because of its relative stability, it does not add easily to monomers like styrene, vinylidene chloride or butadiene, and thus the copolymerization rates are also low. Aso and Tanaka86) calculated the values of Q and e as 2.0 and 0.0, respectively. 

The data of Table 1.2 and Table 1.3 clearly cannot be rationalized purely in terms of the relative stabilities of the product radicals. Rather, a complex interplay of polar, steric, and bond strength terms5 must be invoked.13 In the following sections, each of these factors will be examined separately to illustrate their role in determining the outcome of radical addition. 

The success of the multifunctional initiators in the preparation of block and graft copolymers depends critically on the kinetics and mechanism of radical production. In particular, the initiator efficiency, the susceptibility to and mechanism of transfer to initiator, and the relative stability of the various radical generating functions. Each of these factors has a substantial influence on the nature and homogeneity of the polymer formed. Features of the kinetics of polymerizations initiated by multifunctional initiators have been modeled by O Driscoll and Bevington 64 and Choi and Lei.265... 

The first conclusive qualitative evidence for the relative stability of aryldiazenyl radicals was the isolation of phenylazotriphenylmethane (8.50) in the thermolysis of a-(phenylazo)cumene in the presence of triphenylmethyl radicals by Porter et al. (1978), as shown in Scheme 8-32. 

Stepwise decomposition of thietane oxides should be influenced by the relative stabilities of the developing radical centers, whereas the subsequent selection between... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

These reactions occur easily because of the relative stability of the radicals involved. The SET mechanism, which we have met several times (e.g., p. 402) is an important case. 

Product studies show (i) that the ease of elimination of Rp from R1R2R3COH depends on the relative stabilities of Rp, R2 and R3- and (//) that Rp can be eliminated from RjRjHCOH, i.e. that secondary alcohols can undergo a measure of C-C fission via an alkoxyl radical. The extent of C-C fission increases when RjRjDCOH is employed, but the h/ d values for several alcohols do not accord with the observed products . This paradox has been discussed at length . 

MORTENSEN A, SKIBSTED L H (1997) Relative stability of carotenoid radical cations and homologue tocopheroxyl radicals. A real time kinetic study of antioxidant hierarchy. FFBS Letters, 417, 261-6. 

The relative rates of these alternative processes should depend on the relative stabilities of the product radicals (I) and (II) In the product... 

The relative stability of simple alkyl radicals is found to follow the sequence ... 

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

B Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals ... 

Bond dissociation energies can be used to eatimate the relative stabilities of radicals. 

Figure 13.1 The relative stability of the allyl radical compared to 1°, 2°, 3°, and vinyl radicals. (The stabilities of the radicals are relative to the hydrocarbon from which was formed, and the overall order of stability is allyl > 3° > 2° > 1° > vinyl).

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