Free radicals resonance

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... 

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

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. 

Figure 2. Free radical resonance structures of monohgnols according to the resonance theory. Only coniferol is illustrated.

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. 

FID does not die away before the deadtime has elapsed. In die case of inliomogeneously broadened EPR lines (as typical for free radicals in solids) the dephasing of the magnetizations of the individual spin packets (which all possess slightly different resonance frequencies) will be complete within the detection deadtime and, therefore, the FID signal will usually be undetectable. 

Dinse K P, Biehl R and Mdbius K 1974 Electron nuclear triple resonance of free radicals in solution J. Chem. Rhys. 61 4335—41... 

Biehl R, Plato M and Mdbius K 1975 General TRIPLE resonance on free radicals in solution. 

Hyde J S, Chien J C W and Freed J 1968 Electron-electron double resonance of free radicals In solution J. Chem. Phys. 48 4211-26... 

Smaller B, Remko J R and Avery E C 1968 Electron paramagnetic resonance studies of transient free radicals produced by pulse radiolysis J. Chem. Rhys. 48 5174-81... 

Adrian F J 1971 Theory of anomalous electron spin resonance spectra of free radicals in solution. Role of diffusion-controlled separation and reencounter of radical pairs J. Chem. Rhys. 54 3918-23... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

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. 

When dissolved ia water, the solution is identical with that obtained by dissolving sodium carbonate ia aqueous hydrogen peroxide. There is some evidence for the presence of the traces of tme peroxocarbonate anion, HCO , ia these solutions (95). If the peroxohydrate is heated for about an hour at 100°C and then allowed to cool to room temperature, some decomposition occurs and the product effervesces when placed ia water. Electron spia resonance experiments (64) iadicate that free radicals are present ia this partially decomposed material, but the nature of these radicals is obscure. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

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. 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

The most promising approach to laboratory techniques for predicting performance is to understand the mechanism of failure and then use iastmmental methods to study the susceptibiUty of a coating to failure. The most powerful tool available now is the use of esr spectrometry to monitor the rate of free-radical appearance and disappearance (117—119) (see Magnetic spin resonance). 

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 ( ) ... 

This free radical (1) exists as three resonance contributors. It reacts with oxygen to yield a conjugated peroxy free radical such as (2). In a molecule, like trilinolein with its multiple functionaHty, there are many possible hydroperoxy products that are formed (21) radical (2) iUustrates the principal type of stmcture formed. 

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