Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Carbon-centered radicals, reactivity

Cycloalkoxy radical intermediates are readily generated from a parent alcohol by various methods (e.g., nitrite ester photolysis, hypohalite thermolysis, lead tetraacetate oxidation) (83MI1). Once formed, reactive cycloalkoxy radicals undergo /3-scission to produce a carbonyl compound and a new carbon-centered radical. [Pg.108]

In the absence of heteroatom containing substituents (e.g. halo-, cyano-), at or conjugated with the radical center, carbon-centered radicals have nucleophilic character. Thus, simple alkyl radicals generally show higher reactivity toward electron-deficient monomers (eg. acrylic monomers) than towards electron-rich monomers (e.g, VAc, S) - Table 3.6. [Pg.113]

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... [Pg.117]

Busfield and coworkers extended the technique to the study of less reactive carbon-centered radicals (e.g. cyanoisopropyl)353354 and short propagating radicals505 507. The very low concentration of nitroxide required to allow limited propagation was maintained by feeding with a syringe pump. [Pg.138]

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... [Pg.273]

In the co-end of the chain, the dissociation always occurs at the bond which is indicated by the arrow A. The dissociation of this C-S bond at the A position gives a more-reactive carbon-centered radical and a less-reactive polymer thiyl radical, which leads to the termination of the active chain ends. In the case of the a-chain end, however, there is a possibility that the bond at the C position dissociates to produce a diethylaminothiocarbonyl radical and a thiyl radical in addition to the preferable bond scission at B. Such dissociation at C may not induce living radical polymerization [76]. [Pg.98]

The dissociation of model compounds for co-chain ends of polymers obtained using iniferters with the DC group was examined by the spin-trapping technique, similar to the disso dation of 7 and 8 previously mentioned [174,175]. From the results of the trapping experiments, it was concluded that 46,47, and 48 as model compounds for poly(MA), poly(MMA), and poly(VAc), respectively, dissociated at the appropriate position to produce a reactive carbon-centered radical and a stable DC radical. In fact, these compounds were found to induce the living radical polymerization of St when they were used as photoiniferters. [Pg.103]

A one-electron reduction of the bond between an aliphatic carbon and a halogen leads to a halogen anion and a carbon-free radical. A good example is the reduction of carbon tetrachloride as discussed earlier in this chapter. The first product in the reduction is the trichloromethyl-free radical. Carbon-centered radicals are not very reactive with biological molecules, but they react very rapidly with molecular oxygen (a diradical) to form a peroxy-free radical (Fig. 5.15), which is quite toxic (10). [Pg.118]

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. [Pg.83]

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... [Pg.185]

There has been considerable effort directed towards obtaining a fundamental understanding of the factors that govern the reactivities of carbon-centered radicals in bimolecular reactions, particularly with respect to their addition to alkenes [84]. From early liquid and gas phase studies, reactivity in such addition reactions was concluded to derive from a complex interplay of polar, steric, and bond-strength terms [85], which is much influenced by the nature and position of substituents on both the radical and the alkene. [Pg.113]


See other pages where Carbon-centered radicals, reactivity is mentioned: [Pg.289]    [Pg.386]    [Pg.390]    [Pg.392]    [Pg.398]    [Pg.412]    [Pg.416]    [Pg.639]    [Pg.172]    [Pg.176]    [Pg.67]    [Pg.305]    [Pg.248]    [Pg.351]    [Pg.861]    [Pg.903]    [Pg.162]    [Pg.79]    [Pg.83]    [Pg.103]    [Pg.245]    [Pg.945]    [Pg.1203]    [Pg.45]    [Pg.940]    [Pg.945]    [Pg.1203]    [Pg.45]    [Pg.726]    [Pg.736]    [Pg.753]    [Pg.811]    [Pg.103]    [Pg.76]    [Pg.2]    [Pg.367]   
See also in sourсe #XX -- [ Pg.204 ]




SEARCH



Carbon centers

Carbon radicals

Carbon reactive

Carbon reactivity

Carbon-centered

Carbonate radical

Centered Radicals

Radical centers

Radical reactivity

Radicals reactive

Reactive center

© 2024 chempedia.info