Polar effects decomposition

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Polar effects in decomposition of peroxidic compounds and related reactions... 

Sphingomyehn, IR spectrophotometry, 683-4 Spin delocahzation, polar effects in decomposition, 903 Spin labels, free radicals, 665 Spin-lattice relaxation oxidized functional groups, 695 poly (methylstyrene peroxide), 709 Spin trapping artemisinin ESR, 1291 free radicals, 665... 

The present volume comprises 17 chapters, written by 27 authors from 11 countries, and deals with theoretical aspects and structural chemistry of peroxy compounds, with their thermochemistry, O NMR spectra and analysis, extensively with synthesis of cyclic peroxides and with the uses of peroxides in synthesis, and with peroxides in biological systems. Heterocyclic peroxides, containing silicon, germanium, sulfur and phosphorus, as well as transition metal peroxides are treated in several chapters. Special chapters deal with allylic peroxides, advances in the chemistry of dioxiranes and dioxetanes, and chemiluminescence of peroxide and with polar effects of their decomposition. A chapter on anti-malarial and anti-tumor peroxides, a hot topic in recent research of peroxides, closes the book. 

Ethyl peracetate was the first ester of a peroxy acid, and was characterized by Baeyer and Villiger in 1901. Kinetic studies of perester decomposition were reported by Blomquist and Ferris in 1951, and in 1958 Bartlett and Hiatt proposed that concerted multiple bond scission of peresters could occur when stabilized radicals were formed (equation 46). As noted below (equation 57), polar effects in perester decomposition are also significant. 

The thermal decomposition of 1,2-oxathietane 2-oxides invariably leads to loss of sulfur dioxide and accompanying alkene formation (Scheme 17) (73JA3420, 78BAU142). The lack of any solvent polarity effect on rate (75CC724) as well as supporting theoretical calculations favor a concerted loss of sulfur dioxide without requiring a strained [[Pg.459]

Comparison of the cyclic systems in Table 17 leads to the opposite conclusion, however the destabilization of the 1-norbornyl radical relative to the 1-adamantyl is less for the azo decompositions. Perhaps the mechanism of the azo decompositions of the more unreactive systems is different from that of, for example, the f-butyl azo compound (i.e. the rate determining step of the 1-norbomyl azo compound may be a one bond homolysis rather than the synchronous two bond fission of the f-butyl system312, 315)). Also, the smaller 1-norbornyl/1-adamantyl rate ratio for the f-butyl perester decompositions may be due to a greater influence of polar effects in these reactions 309a). This problem is under active investigation 309a). 

Although peroxide-decomposition data have to be used with great caution, it can be concluded that double bonds are consumed by radical addition reactions. One can debate whether the unsaturations are consumed by multiple radical addition reactions or via consecutive radical addition/radical transfer sequences. The latter seems most likely, considering the low tendency of alkyl radicals for addition to alkyl-substituted double bonds under these relatively mild conditions. In radical addition reactions of this kind, the stabilisation of radicals due to polar effects is negligible. Experimental studies show that the reactivity is mainly controlled by steric effects [96]. The order of reactivity MNB > DCPD ENB > HD towards radical addition reactions as found by Fujimoto and coworkers [73, 74] is in line with these considerations. 

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