Resonance stabilization free radicals

Rate constants for free radical propagation increase with decreasing polymer free radical resonance stabilization (Table 20-2). The activation energies, however, are more or less independent of the constitution. Consequently the rate constants are predominantly determined by the preexponential factors of the Arrhenius equation. In addition, they also depend on the viscosity of the reaction medium to a slight extent. 

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. 

Some compounds are capable of coupling after the initial abstraction of a hydrogen atom from one carbon, usually by a free-radical process. Such abstraction of hydrogen occurs easily when the remaining free radical is stabilized by resonance. 

We have said that benzyl and allyl free radicals are stabilized by resonance but we must realize, of course, that they are stable only in comparison with simple alkyl radicals like methyl or ethyl. Benzyl and allyl free radicals are extremely reactive, unstable particles, whose fleeting existence (a few thousandths of a second) has been proposed simply because it is the best way to account for certain experimental observations. We do not find bottles on the laboratory shelf labeled benzyl radicals or allyl radicals. Js there, then, any direct evidence for the existence of free radicals ... 

Allylic carbocations, free radicals, and carbanions are resonance stabilized. In each case the stabilization is the result of delocalization of the positive or negative charge or the free radical. Resonance forms differ in the position of electrons and charge but not atoms. Every atom in an allylic carbocation, free radical, or carbanion possesses a p-orbital and the pi-electrons and charges or unpaired electrons are delocalized throughout these orbitals. 

The reduction of diquat solutions gives a solution of an intense green colour. During the process a water-soluble, stable free radical is formed by the uptake of one electron. The free radical can be written in 18 possible resonance forms, and the odd electron can occupy any of the places in the nucleus. This delocalisation gives the free radical its stability. 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Copolymers of VF and a wide variety of other monomers have been prepared (6,41—48). The high energy of the propagating vinyl fluoride radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (49). The low value of is consistent with titde resonance stability and the e value is suggestive of an electron-rich monomer. 

Reaction Mechanism. High temperature vapor-phase chlorination of propylene [115-07-17 is a free-radical mechanism in which substitution of an allyhc hydrogen is favored over addition of chlorine to the double bond. Abstraction of allyhc hydrogen is especially favored since the allyl radical intermediate is stabilized by resonance between two symmetrical stmctures, both of which lead to allyl chloride. 

The Q-e Scheme. The magnitude of and T2 can frequentiy be correlated with stmctural effects, such as polar and resonance factors. For example, in the free-radical polymerization of vinyl acetate with styrene, both styrene and vinyl acetate radicals preferentially add styrene because of the formation of the resonance stabilized polystyrene radical. 

At first, these highly reactive free radicals react with the antioxidant, but as the antioxidant is consumed, the free radicals react with other compounds. Hydrogens on methylene groups between double bonds are particularly susceptible to abstraction to yield the resonance stabilized free radical ( ) ... 

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... 

It is pointed out that the dissociation of certain substituted ethanes into free radicals is due not to weakness of the carbon-carbon bond in the ethane but to the stabilization of the free radicals resulting from resonance among the structures in which the unpaired electron is located on the... 

With resonance possibilities, the stability of free radicals increases some can... 

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-with-drawing group.This is called the push-pull or captodative effect (see also pp. 159). The effect arises from increased resonance, for example ... 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

KRISTENSEN D, krOger-ohlsen m V and SKIBSTED L H (2002) Radical formation in dairy products Prediction of oxidative stability based on electron spin resonance spectroscopy, in Morello M J, Shahidi F Ho CT Free Radicals in Food, Chemistry, Nutrition and Health Effects, ACS Symposium Series 807, Washington D C, 114-25. 

II), and its formation therefore is more probable. If the substituent X possesses unsaturation conjugated with the free radical carbon, as for example when X is phenyl, resonance stabilization may be fairly large. The addition product (I) in this case is a substituted benzyl radical. Comparison of the C—I bond strengths in methyl iodide and in benzyl iodide, and a similar comparison of the C—H bond strengths in methane and toluene, indicate that a benzyl radical of type (I) is favored by resonance stabilization in the amount of 20 to 25 kcal. 

When studying the free-radical copolymerization of methacrylic and acrylic acids with vinyl monomers, it was established that the addition of catalytic amounts of SnCl and (C6Hs)3SnH has a marked effect on the copolymer composition. It was found that complexes are formed by charge transfer between unsaturated acids and the above tin compounds. It has been suggested that the change in polymer composition is caused by the interaction of the tin compounds with a transition complex resulting in a decrease of the resonance stabilization of the latter 94,). 

The opinion that stabilization of 1 by resonance was decisive, predominated for a long time and mastered the discussion of the relationship between structure and reactivity in free radical chemistry till quite recently5 Accordingly selectivity in free radical reactions was assumed to be mainly due to differences in the thermodynamic stability of the radicals taking part in a reaction or a potential competing reaction. 

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