Butylperoxy radicals, reactions

The proposed mechanism for producing ethanol [64-17-5] from butane involves -scission of a j -butoxy radical (eq. 38). The j -butoxy radicals are derived from j -butylperoxy radicals (reaction 14 (213)) and/or through some sequence involving reaction 33. If 25% of the carbon forms ethanol, over 50% must pass through the j -butoxy radical. Furthermore, the principal fate of j -butoxy radicals must be the P-scission reaction the ethoxy radical, on the other hand, must be converted to ethanol efficiently. 

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

Content of prime - tertiary peroxide groups was measured by the quantity of products of complete decay, which were measured by chromatography. It is known that the main contents in products of the complete decay of Oct-MA-TBPMM samples are acetone and 2,2-dimethylpropanol, which arise in reactions of chain fragmentation of tert-butylperoxy radical or in reaction of chain transfer of this radical. In this case the sum of acetone and 2,2-dimethylpropanol molecules is equal to the quantity of peroxide groups in polymer. As an internal standard we used chloroform. 

Epoxide formation may be a side reaction occurring during initiation by t-butylperoxy radicals. The mechanism proposed for this process is as follows (Scheme 3,831, 1... 

The tert-butylperoxy radical is known to decompose by various routes, of which Reaction 2 is more important than Reaction 3 by a factor of 3 to 1 (8). 

Howard, J.A. and E. Furimsky. 1973. Arrhenius parameters for reactions of tert-butylperoxy radicals with some hindered phenols and amines. Can.. Chem. 51 3738-3745. 

In Table III the rate constants, k p, for reaction of several substrates with tert-butylperoxy radicals are compared with the rate constants, kp, for reaction with their own peroxy radicals. 

Rate Constants per Labile Hydrogen for Reaction of Substrates with Their Own Peroxy Radicals, (kp) and with tert-Butylperoxy Radicals (k p) at 30°Ca... 

The e.s.r. spectrum of the peroxy-radical is very different from that of the parent radical, so that the extent of reaction can be determined directly from the composite e.s.r. spectrum of the deposit. For example the spectra of t-butyl and t-butylperoxy radicals and of a deposit containing a mixture of the two radicals are shown in Fig. 14. 

The important concentration-time profiles include those for 1,2- and 2,3-epoxybutane, 2-methyloxetan, tetrahydrofuran, butyraldehyde and 2-butanone, the relative concentrations of these products being determined by the relative rates of 1- and 2-butylperoxy radical isomerization and decomposition. The kinetic data used to represent these reactions were the parameters recommended by Walker [229] in 1961. The formation of butyraldehyde and 2-butanone require internal isomerizations of the form... 

In this case, the fe/t-butylperoxy radicals are the more persistent species. The authors also confirmed that the highest yield of the cross-reaction product is obtained when the cross-reacting radicals are formed with equal rates. Schiesser et al.76 introduced phenyl telluroformates as precursors of alkyl radicals (Scheme 19). In the absence of trapping agents, the parent molecules appeared stable, that is, the crossreaction between the oxyacyl and the tellurium-centered radical is even faster than the decarboxylation of the oxyacyl species, which normally takes only nano- to microseconds. Obviously, the tellurium-centered radical must be rather persistent and builds up in large concentrations. 

In the gas phase chain lengths are shorter, and yields of the reaction products reflect the increased importance of self-reactions of 5cc-butylperoxy radicals. ... 

Diacetoxyiodo)benzene in the presence of tert-butyl hydroperoxide readily oxidizes alkenes at the allylic position (Scheme 3.88) [269]. This reaction proceeds via initial formation of the tert-butylperoxy radical and it can be extended to the oxidation of unactivated C—H bonds in alkyl esters and amides to give the corresponding keto compounds under mild conditions [270]. 

No new data have been reported for tertiary alkylperoxy radicals. It has been confirmed that the rate constant of the t-butylperoxy radical self-reaction is very... 

Percent product distribution acetone 24.5 5.1, 2-methyl-2-propanol 18.8 4.0, 2-methyl-2-hydroperoxypropane 36.7 7.5, 2-methyl-propanal 14.0 3.9, 2-methyl-propanol 4.4 1.3, tertiary butylperoxide < 1.7. The peroxy radicals involved are primary 2-methyl-1-propylperoxy, primary methylperoxy and tertiary 2-methyl-2-propylperoxy. The relatively large yield of tertiary butanol is due to the interaction between CH3OO and tertiary butylperoxy radicals. Computer simulations based on the known rate coefficients for the self-reactions of these radicals [2] gave = 3 x 10" cm molecule s for the cross combination reaction. To simulate the observed ratio of primary alcohol and aldehyde requires a rate coefficient p 3 x 10" cm molecule s for the interaction between 2-methyl-1-propylperoxy and tertiary 2-methyl-2-propyl-peroxy radicals. The oxidation mechanism is quantitatively well understood. 

Flash photolysis study of the spectra and self-reactions of neopentylperoxy and /-butylperoxy radicals,... 

The substrate scope and mechanism of Rh2(cap)4-catalysed TBHP oxidation of phenol and aniline was discussed. The rate of oxidation of para-substituted phenols to 4-(f-butyldioxy)cyclohexadien ones increased significantly in aromatic hydrocarbon solvents. Comparative results with RuCl2(PPh3) and Cul were provided. The results were consistent with hydrogen atom abstraction by the f-butyl peroxy radical followed by combination of the phenoxy and the f-butylperoxy radicals. Under similar reaction conditions,para-substituted anilines were oxidized to the corresponding nitroarenes, and primary amines were oxidized to carbonyl compounds in moderate to good yields. ... 

L. B. Gan, S. Huang, H. X. Zhang, A. X. Zhang, B. C. Cheng, H. Cheng, X. L. Li, and G. Shang, Fullerenes as a tcrt-butylperoxy radical trap, metal catalyzed reaction of ferf-butyl hydroperoxide with fullerenes, and formation of the first fullerene mixed peroxides C6o(0)(OOfBu)4 and C7o(OOfBu)10. Journal of the American Chemistry Society, 124 (2002), 13384-5. 

The existence of di-t-butyl tetroxide was deduced from studies of the influence of temperat ire on the concentration of t-butylperoxy radicals by electron spin resonance spectroscopy (13 iH). Thus it was shown that at temperatures below 193K the radical concentration can be increased by raising the temperature and decreased by lowering the temperature with no apparent loss in radical concentration. The influence of temperature on the concentration of t-Bu02 is shown in Figure 1. If the tetroxide is completely dissociated at the highest temperature equilibriijm constants for reaction (2), K2, can be calculated from... 

There have been several reports of t-butylperoxy radicals undergoing self-reaction with first-order kinetics (21,26). Now it is well known that decay of certain radicals can be first-order if the radical is in equilibrium with a diamagnetic dimer and substantial concentrations of dimer are present at the decay temperature (27-28). t-Butylperoxy radicals do not fall into this category because the tetroxide is completely dissociated before irreversible radical decay occurs. Other less persistent t-ROa do, however, decay irreversibly in the presence of tetroxide and in these cases first-order decay kinetics are observed (29) because radical decay monitors tetroxide decomposition. 

Small radicals such as tert-butylperoxy and ethylperoxy can, however, react via 1,4 H-transfer only the strain energy involved in O-heterocycle formation is 28 kcal. per mole. In this case, k.4(x — 106 sec."1 whereas krta = 10r> 4 sec. 1 and when [02] = 200 mm. of Hg, ko[02] = 105,3 sec. 1, so that k.4ct < < (tkr,a + k [02]). The result is that in the oxidation of small alkyl radicals, the route via alkylperoxy radicals will be blocked because reverse Reaction —4 competes successfully with Reaction 5. Reaction 2 will thus be a more effective mode of reaction of alkyl radicals with oxygen and the conjugate alkene will be a major product. 

Negative reaction constants p1 for the oxidation of sulfides by [10-1-3]—(r-butylperoxy)iodanes are consistent with a mechanism involving rate-limiting formation of a sulfonium species by nucleophilic attack of sulfide on the iodine(III) atom followed by attack of water to give sulfoxide.151 However, in dichloromethane, inhibition by galvinoxyl implicates a free radical mechanism perhaps by homolytic cleavage of the weak iodine(III)-peroxy bond. 

On the other hand, the Gif-tert-butyl hydroperoxide (TBHP) systems seem to be much less complicated. These have been extensively studied by many groups and all workers agree that the reaction proceeds via radical pathways based on the reactivity of tert-butylperoxy and tert-butyloxy radicals [22-24]. Minisci et al. [22] suggested a Haber-Weiss radical chain mechanism [25] that accounts for the observed selectivities. 

By-products are formed in subsequent reactions of the terf-alkoxy and tert-alkylperoxy radicals with the hydroperoxide, solvent or olefin. For example, in the metal-catalyzed epoxidation of cyclohexene with ferf-butyl hydroperoxide in benzene, the main by-product was 3-rert-butylperoxy-l-cyclohexene, formed via the sequence433 shown in Eq. (314) [cf. reactions (89)-(94)] ... 

Further to its ability to perform ally lie and benzyhc oxidations, " t-butylperoxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields. o,o-Couphng dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

---