Free radical reactivity, comparison with

D. Comparison of Free-Radical Reactivity with Theoretical Calculations... 

The reaction enthalpy is known as a very important factor that determines the reactivity of reactants in free radical abstraction reactions [71]. The IPM method helps to calculate the increment of AEfi that enthalpy determines in the activation energy of the individual reaction. This increment can be estimated within the scope of IPM through the comparison of activation energy Ee of the chosen reaction and activation energy of the thermoneutral reaction Ee0 (see Equation [6.18] in Chapter 6). This increment was calculated for several reactions of different peroxyl radicals with ethers (Table 7.19). 

Armstrong D, Sun Q, Schuler RH (1996) Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals comparisons with phenoxyl radicals. J Phys Chem 100 9892-9899 Asmus K-D (1979) Stabilization of oxidized sulfur centers in organic sulfides. Radical cations and odd-electron sulfur-sulfur bonds. Acc Chem Res 12 436-442 Asmus K-D (1990a) Sulfur-centered free radicals. Methods Enzymol 186 168-180 Asmus K-D (1990b) Sulfur-centered three-electron bonded radical species. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 155-172... 

Acrylonitrile (AN) is a particular case among vinjd monmners reprding the solvent dependence of the propagation rate constant (kp) in homogeneous free radical polymerization. Whereas in dimethylformamide and dimethylsulfoxide kp has values which might be expected from a comparison with other monomers and from the reactivity of the growing radical (400 and 1910 1 mol s , respectively, at 25 the propagation rate constant is considerably hig ier in water... 

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 ... 

By extrapolating data of Lattimer and coworkers (24, 40), it may be anticipated that the ozonation of DPPD proceeds most probably via nitroxide and aminyl formation pathways. Dinitrone and N-N or N-C and C-C coupling products should therefore be considered as ozonation products formed via free radical intermediates. The reactivity of DPPD with aldehydes will be very limited. All these mechanistic features together with the low ozonation rate of DPPD in comparison with N,N -disec.alkyl PD (26) may be responsible for the generally reported poor antiozonant efficiency of N,N -diaryl PD in comparison with N-sec.alkyl-N -aryl and N,N -disec.alkyl PD. 

To study bridgehead reactivity, our group (89) prepared a cyclophane of C3 symmetry 74 a comparison with triphenylmethyl chloride indicated that the bridgehead chloride of 74 is conspicuous in its striking lack of reactivity in SnI displacement reactions and in free radical formation. 

The increasing reactivity with alkyl substitution in the acetylenes is very pronounced something in excess of a factor of 100 in relative rates at 298°K. This, it may be observed, is substantially greater than the factor of 20 between the reactivities of ethylene and 2-butene. Such large relative reactivity differences are rare in free radical reactions, and one realizes that the triplet sulphur atom must be very selective in its reaction with different olefins. A comparison of the behaviour of the triplet state atoms of oxygen, sulphur and selenium will be deferred until the end of the discussion of this whole group of atom additions. 

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