Resonance stabilized radical

The last comprehensive review of reactions between carbon-centered radicals appeared in 1973.142 Rate constants for radical-radical reactions in the liquid phase have been tabulated by Griller.14 The area has also been reviewed by Alfassi114 and Moad and Solomon.145 Radical-radical reactions arc, in general, very exothermic and activation barriers are extremely small even for highly resonance-stabilized radicals. As a consequence, reaction rate constants often approach the diffusion-controlled limit (typically -109 M 1 s"1). 

It has been noted by a number of workers that the presence of a-substituents which delocalize the free spin favors combination over disproportionation.127,14 175 For radicals of structure (CH )nC( )-X, kjk increases as shown in Figure 1.12. A correlation between the degree of exothennicity and the value of AU has also been found but only for the case of resonance stabilized radicals.144 176177... 

The following reasoning was used to eliminate the less probable mechanisms shown in Figure 4. A H atom is added to naphthalene to form an a-radical in reaction 1A and a /3-radical in reaction IB. Both are resonance-stabilized radicals. They can lose either a 2H atom or a H atom to regenerate naphthalene. We have shown a 2H atom lost to form a protium-enriched product in reactions 1A and IB. The fact that we observe a fourfold increase of protium in the a-position of spent naphthalene suggests that reaction IB is faster than reaction 1A and, therefore, is the predominant mechanism. 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

The OH can react with catechol, by hydrogen abstraction or addition to the aromatic ring, to produce the resonance-stabilized radical The latter could couple with other catechol molecules or oxygen to eventually form polymerized, highly colored materials, according to the scheme proposed for phenol by Voudrias (90) (Figure 6). 

Both these reactions would be very fast at 400° C and lead to the same resonance-stabilized radical. 

A benzylic radical is generated if a compound like toluene reacts with bromine or chlorine atoms. Hydrogen abstraction occurs from the side-chain methyl, producing a resonance-stabilized radical. The... 

Thus, the radical from the initiation reaction abstracts hydrogen from the allylic position of cyclohexene, as we have seen previously, to give the resonance-stabilized radical (see Section 9.2). 

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

Vitamin C (ascorbic acid) is also a well-known antioxidant. It can readily lose a hydrogen atom from one of its enolic hydroxyls, leading to a resonance-stabilized radical. Vitamin C is acidic (hence ascorbic acid) because loss of a proton from the same hydroxyl leads to a resonance-stabilized anion (see Box 12.8). However, it appears that vitamin C does not act as an antioxidant in quite the same way as the other compounds mentioned above. 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

The cyclopentadienyl radical, which is an important intermediate in the transition from cyclic to linear chain kinetics, is a resonance stabilized radical. It is presumably consumed mainly through fast radical-radical reactions,... 

In particular, crude polymerizates prepared in the presence of AIBN as initiator, which yield resonance stabilized radicals (17) that are unable to extract hydrogen from hydrocarbon supports (I, 18) show the same content of non-extractable rubber as that of the polymerizates prepared in the presence of active radicals in the hydrogen extraction from hydrocarbon polymers, such as those derived from the decomposition of benzoyl peroxide. 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

Yokono et al. [85] have suggested that the results obtained by Lewis and Edstrom [84] can be understood in terms of the maximum value of the index of free valence as calculated by the HMO method. However, as Herndon [30] has shown, some discrepancies occur when the free valence approach is applied to the experimental findings. He found that the structure count ratio for the single position in each compound that would give rise to the most highly resonance stabilized radical is a reliable reactivity index to correlate and predict the qualitative aspects of the thermal behaviour of benzenoid hydrocarbons. 

Heterocyclic thiazyl radicals hold considerable potential in the design of both conductive and magnetic materials. In the pursuit of improved conductivity, a series of resonance-stabilized radicals based on diselenadiazoles, sulfaselenazoles, and diselenazoles were obtained (185-188) (Fig. 17) [298-303], Structural analyses of 187 and 188 (R1 = Me, R2 = H) confirm that lattice and n-delocalization energies are sufficient to prevent solid state dimerization of the radicals. Incorporation of selenium leads to a dramatic increase in conductivity and reduction in thermal activation energy relative to sulfur-based radicals [300],... 

Clean, regioselective, monochlorinations can be achieved with hydrocarbons that react via resonance-stabilized radicals. 

A given molecular transformation, for example, the reaction C—H — C—Cl, is called regioselective when it takes place preferentially or exclusively at one place on a substrate. Resonance-stabilized radicals are produced regioselectively as a consequence of product development control in the radical-forming step. 

Copolymerization. The importance of VDC as a monomer results from its ability to copolymerize with other vinyl monomers. Its j2 value equals 0.22 and its e value equals 0.36. It most easily copolymerizes with acrylates, but it also reacts, more slowly, with other monomers, eg, styrene, that form highly resonance-stabilized radicals. Reactivity ratios ( and r2) with various monomers are listed in Table 2. Many other copolymers have been prepared from monomers for which the reactivity ratios are not known. The commercially important copolymers include those with vinyl chloride (VC),... 

Antioxidants, such as 2,6-di-te/ /-butyl-4-mcthylphenol (also known as butylated hydroxytoluene or BHT) and 2-/ert-butyl-4-methoxyphenol (also known as butylated hydroxyanisole or BHA), are added to many organic materials to prevent autoxidation. They function by interfering with the autoxidation chain reaction. When a radical encounters an antioxidant molecule, such as BHA, it abstracts a hydrogen to produce a resonance-stabilized radical ... 

The resonance-stabilized radical adds to 02. The resulting oxygen radical adds to the other double bond. 

Radicals prefer to add to C-I of isoprene because the resulting radical is the most stable of the four possibilities. It is stabilized by resonance, and the odd electron is on a tertiary carbon in one resonance structure and a primary carbon in the other. In contrast, addition at C-2 or C-3 produces less stable radicals because they have no resonance stabilization. Addition at C-4 produces a resonance stabilized radical, but the odd electron is on a secondary carbon and a primary carbon in the two resonance structures. 

Clean monochlorinations can be achieved only with hydrocarbons that react via resonance-stabilized radicals. They also exhibit high regioselectivity. This follows from the structure of these resonance-stabilized radicals. 

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