Ferf-Butoxy radicals

For the photolysis of ferf-butyl nitrite a possible reaction mechanism (Scheme 6) consists of the production of ferf-butoxy radicals (equation 3), followed by their decomposition to give acetone and methyl radicals (equation 4). The latter are trapped by the nitric oxide liberated in the first step (equation 5). However, the absence of ethane production in the actual experiments suggested that an intramolecular formation of nitrosomethane is unlikely ". ... 

The energy of activation for the addition of trifluoroinethyl radical to the C=0 double bond of hexafluoroaeetone was calculated to be 9.7 0.26 kcal. mol.-1 and that for the decomposition of the perfluoro tert-butoxy radical was found to be 30.6 1.3 kcal. mol.-1, so that AH for the formation of the perfluoro ferf-butoxy radical is —20.9 kcal. mol.-1. Thus at high light intensities and elevated temperatures, the contributions of these reactions cannot be neglected. 

Paul H, Small RD, Scaiano JC (1978) Hydrogen abstraction by ferf-butoxy radicals. A laser photolysis and electron spin resonance study. J Am Chem Soc 100 4520-4527... 

Scheme 10.14 /3-Scission of the ferf-butoxy radical for the measurement of the relative rates of hydrogen...

The first-order ft-scission of the ferf-butoxy radical is one of the oldest radical clock reactions and has been used for over 50 years for the measurement of the relative rates of hydrogen abstraction from organic compounds (AH) in solution (Scheme 10.14). At low conversions, when the concentration of AH has not appreciably changed, the ratio of the rate constants for hydrogen atom abstraction, kAH> and /3-scission, kp, can be determined simply by analysis for acetone and ferf-butyl alcohol formation in the reaction. This is most conveniently achieved by gas chromatography ... 

Experimental support for the given order of stability in the fluoromethyl series has been provided in a study by Jiang et al. of the radical fragmentation of the respective series of fluorinated ferf-butoxy radicals [32] ... 

Nucleophilic and electrophilic radicals abstract different hydrogen atoms from butyrolactone 7.10 the ferf-butoxy radical selectively abstracts a hydrogen atom from the methylene group adjacent to the oxygen atom, whereas a boryl radical abstracts a hydrogen atom from the methylene group a to the carbonyl group. 

Figure 24.1 Formation of ferf-butoxy radicals and their use to measure H-abstraction...

Figure 24.2 Effect of degree of polymerization (DP) on the ability of ferf-butoxy radicals to abstract H-atoms from the PS backbone to form ferf-butyl alcohol (f-BA) or fragment to form acetone...

A common feature of all radical initiators is a relatively weak bond that readily undergoes homolytic cleavage. In all but one of the radical initiators shown in Table 28.3, the weak bond is an oxygen-oxygen bond. Two factors enter into the choice of radical initiator for a particular chain-growth polymerization. The first is the desired solubility of the initiator. For example, potassium persulfate is often used if the initiator needs to be soluble in water, whereas an initiator with several carbons is chosen if the initiator must be soluble in a nonpolar solvent. The second factor is the temperature at which the polymerization reaction is to be carried out. For example, a ferf-butoxy radical is relatively stable, so an initiator that forms a fert-butoxy radical is used for polymerizations carried out at relatively high temperatures. 

Valuable insight of multiple additions of free radicals came from the ESR spectroscopic investigations of benzyl radicals, C-labeled at the benzylic positions [97,98]. These radicals can be prepared in situ by photolysis of saturated solutions of Cgo in labeled toluene containing about 5% di-ferf-butyl peroxide. Thereby, the photochemically generated ferf-butoxy radicals readily abstract a benzylic hydrogen atom from the toluene. Two radical species with a different microwave power saturation behavior can be observed. One radical species can be attributed to an allylic radical 63 and the other to a cyclopentadienyl radical 65 formed by the addition to three and five adjacent [5]radialene double bonds, respectively (Scheme 11). In these experiments no evidence for the radical 61 is found, which is very likely a short-lived species. 

Sample Solution (a) ferf-Butoxy radical adds to the CH2 group of vinyl chloride. The free radical formed in this process has its unpaired electron on the carbon bonded to chlorine. 

N2H3 radicals were generated by abstracting H atoms from hydrazine in benzene solution with ferf-butoxy radicals (formed by photolysis of di-ferf-butyl peroxide) [40]. 

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