Alkyl radicals small

Consideration of data on the reactions for small radicals (Section 2.3) suggest that the primary alkyl radical (H ) is more likely to give head addition than the normal propagaLing species (T-) for three reasons ... 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Ayscough and coworkers studied the ESR spectra of y-irradiated sulfones and polysulfones. Irradiation at 77 K leads mainly to alkyl radicals R formed by rupture of the C—S bonds. Small amounts of RS02 were also observed in some cases as well as radicals formed by loss of an a-hydrogen from the parent molecule—R". On warming the irradiated samples the simple alkyl radicals disappear first. At room temperature only the RS02 radicals have any appreciable stability. In some cases the yield of RS02 radicals increases on warming of the sample. 

Provided that the value d is small enough, there is a simple linear relation between the initial rate of variation in conductivity of oxide film (e.g., ZnO) and concentration of free radicals in the space adjacent to the film surface. When adsorbing the simplest alkyl radicals as well... 

In order to verify the properties of alkyl radicals adsorbed on semiconductor ZnO film, we conducted experiments, in which methyl radicals were adsorbed from the vapour phase of acetone at room temperature. Under these conditions, the radicals were formed via photolysis of acetone vapours (for details, see Chapter 3) at small pressure of acetone (of about 0.5 Torr). 

Small amounts of cyclized products are obtained after the preparation of Grignard reagents from 5-hexenyl bromide.9 This indicates that cyclization of the intermediate radical competes to a small extent with combination of the radical with the metal. Quantitative kinetic models that compare competing processes are consistent with diffusion of the radicals from the surface.10 Alkyl radicals can be trapped with high efficiency by the nitroxide radical TMPO.11 Nevertheless, there remains disagreement about the extent to which the radicals diffuse away from the metal surface.12... 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

The available kinetic and thermochemical data are summarized in Table 7. Based on the approximate equality of E and D1+D2 and on the magnitude of the frequency factor, Billinge and Gowenlock98 would place dimethyl mercury, di-B-propyl mercury, di-isopropyl mercury (above 230 °C) and (on the basis of the frequency factor only, since thermochemical data are not available) di-n-butyl mercury in class II (simultaneous rupture into mercury and two alkyl radicals). If the high frequency factors are simply due to a general softening of the vibrations in the activated state, then in the case of di-isopropyl mercury D2 — 0, while for dimethyl and di-B-propyl mercury D2 is small but finite (2-3 kcal.mole" ). However, within the limits of experimental error all of these alkyls for which thermochemical data are available may have E = Dl+D2, and thus all may belong to class II. At the same time it must be noted that some metal alkyls which are... 

Equation 5.7 indicates that l)//°(A B) = Z)//s°n(A-B) when the solvation terms cancel out. This is seldom observed in practice, but there is some experimental evidence that the net solvation effect on the enthalpy of reaction 5.6 may be small. For instance, it has been shown that the solvation enthalpies of an alkyl radical (R) and the parent hydrocarbon (RH) are similar both in polar and in nonpolar solvents [72], Hence, the solvation terms in equation 5.7 are approximated by Asoiv77°(RH) - A T/ R) - A /rdl) s -Asolv/7°(H). Surprisingly, the solvation enthalpy of the hydrogen atom is not readily available in the literature. Estimates based on the relationship Asoiv//°(H) Asoiv//0(H2) yield Asoiv °(H) 5 kJ mol-1 in most organic solvents and Asoiv7f°(H) -4 kJ mol-1 in water [73]. 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

The above considerations should bear some relationship with the stereochemistry of the reaction. As indicated earlier (8ection 2 Hebert et ai, 1985), in the reaction of anthracene anion radicals with optically active 2-octyl bromide, racemization is mostly observed together with a small but distinct amount (ca. 10%) of inversion. In the context of the ET-8n2 mixed mechanism sketched above, this can be rationalized in terms of a minor contribution of the latter pathway that would not detectably affect the overall rate constant of the reaction. The weakness of the bonded interactions in the transition state derives from the relatively poor affinity of the alkyl radical for the aromatic hydrocarbon. This is consistent with the fact that in those of the radical-anthracene pairs that were not favourably oriented for the 8, 2 reaction to occur, the alkyl radical escapes from the... 

Triethylborane in the presence of very small amounts of oxygen is an excellent initiator for radical chain reactions. For a long time it has been known that trialkylboranes R3B react spontaneously with molecular oxygen to give alkyl radicals (Reaction 4.7), but only recently has this approach successfully been applied as the initiation [22]. The reactions can be run at temperatures as low as — 78 °C, which allow for a better control of stereoselectivity (see below). 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

A quite different reaction course was observed with benzoyl peroxide. The increase in the decomposition rate on going from nonprotonated to protonated quinoline is relatively small. The high decomposition rate of decanoyl peroxide in the presence of protonated heteroaromatic bases was mainly ascribed to the nucleophilic character of the alkyl radicals, which allows the complete capture of the nonyl radicals escaping from the solvent cage and the consequently rapid induced decomposition. The... 

The development of an ethane combustion mechanism provides a historical context for understanding some overall trends of alkyl radical combustion. An understanding of the likely pathways for this small system is useful in modeling chemistry of larger systems, as can be observed from an examination of some other reactive radical intermediates. 

Some homolytic fragmentation reactions are driven by formation of small, stable molecules. Alkyl acyloxyl radicals (RCOp decarboxylate rapidly (fe > 1 x 10 s ) to give alkyl radicals, and even aryl acyloxyl radicals (ArCOp decarboxylate to aryl radicals with rate constants in the 10 s range." Azo radicals produced in the homolysis of azo initiators eliminate nitrogen rapidly. Elimination of carbon monoxide from acyl radicals occurs but is slow enough (fe 10" -10 such that the acyl radical can be trapped in a bimolecular process,... 

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