Butyl radicals

3 tert-Butoxyl radical. Radical transformation reactions are of interest for the interpretation of complex reaction systems. By TR EPR during intermittent photochemical radical production, a technique was applied to obtain absolute rate constants for the P-sdssion of the tm-butoxyl radical in various solvents and its hydrogen abstraction from cyclohexane, cyclopentane, ferf-butylbenzene and anisole, respectively. Kinetic studies of the neophyl and 2-methyloxiran-2-yl rearrangements and the decarboxylation of the lerl-butoxycarbonyl radical demonstrate the method s versatihty. Furthermore, rate constants are given for the addition of the methyl radical to benzene and fluorobenzene, as well as for the hydrogen abstraction of methyl from di-tm-butyl peroxide. 

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Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

The resulting free radicals react with chlorine to give the corresponding alkyl chlorides Butyl radical gives only 1 chlorobutane sec butyl radical gives only 2 chlorobutane... 

At combustion temperatures, the oxidation of butane [106-97-8] is similar to that of propane (153). This is because most butyl radicals are consumed by carbon—carbon bond scission (reaction 28). 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

RCF2 (20), and CF3 (300) show this trend. Further accumulation of fluorine enhances this effect still more, and the perfluoro-/-butyl radical is typically 8-10 times more reactive than CFa- toward aikenes. 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstraction, the n-butyl/5ec-butyl radical ratio and, therefore, the 1-chloro/2-chlorobutane ratio, would be given by the relative numbers of hydrogens in the two equivalent methyl groups of CH3CH2CH2CH3 (six) compared with those in the two equivalent methylene groups (four). The product distribution expected on a statistical basis would be 60% 1-chloro-butane and 40% 2-chlorobutane. The experimentally observed product distribution, however, is 28% 1-chlorobutane and 72% 2-chlorobutane. 5ec-Butyl radical is therefore formed in greater anounts, and n-butyl radical in lesser anounts, than expected statistically. 

Giese and Kretzschmar7j found the rate of addition of hexenyl radicals to methyl acrylate increased 2-fold between aqueous tetrahydrofuran and aqueous ethanol, Salikhov and Fischer74 reported that the rate constant for /-butyl radical addition to acrylonitrile increased 3.6-fold between tetradecane and acetonitrile. Bednarek et al75 found that the relative reactivity of S vs MMA towards phenyl radicals was ca 20% greater in ketone solvents than it was in aromatic solvents. 

Minato ct a/.1(12 proposed that the transition state for disproportionation has polar character while that for combination is neutral. The finding that polar solvents enhance kJkK for ethyl170 and /-butyl radicals (Section 2.5.3.5), the very high kjktc seen for alkoxy radicals with a-hydrogens,171 and the trend in kJkK observed for reactions of a scries of fluoroalkyl radicals (Scheme 1.13, Table 1.7) have been explained in these terms.141102... 

Figure 1.13 Temperature dependence of A U /Alc values for /-butyl radicals with dodecane (—) or 3-methyl-3-pentanol (---------------) as solvent.

A proof for the formation of alkyl radicals was found by their addition to the aci-nitromethane anion (CH2=N02 ) and by their reaction with p-benzoquinone to give the optically active nitroalkane radical-anion and the semiquinone radicals, respectively. In the case of di-r-butyl sulfoxide the f-butyl radical was observed directly by its absorption spectra. 

Di-f-butyl sulfone is different from the other dialkyl sulfones in that RH is mainly alkene and not alkane [G(isobutene) = 3.2 and G(isobutane) = 1.2]. The preference for isobutene over isobutane means that the formation of the alkene cannot be due to disproportionation of two t-butyl radicals but is due to a hydrogen atom expulsion as suggested by Bowmer and O Donnell70... 

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