Butoxy radicals, decomposition

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

The reactions of /-butoxy radicals are amongst the most studied of all radical processes. These radicals are generated by thermal or photochemical decomposition of peroxides or hyponitrites (Scheme 3.75). 

The f-butoxy radical can be produced from the radical decomposition of f-butylhypochlorite using azobisisobutyronitrile (AIBN) as catalyst(33) ... 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy 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. 

The rate constant for the decomposition of tert-butoxy radicals... 

Hoare and Wellington (22) produced CH3O radicals from the photochemical (50° and 100°C.) and thermal (135°C.) decompositions of di-terf-butyl peroxide in the presence of 02. The initially formed tert-butoxy radicals decomposed to acetone plus methyl radicals, and the methyl radicals oxidized to methoxy radicals. Formaldehyde and CH3OH were products of the reaction the formation of the former was inhibited, and the latter was enhanced as the reaction proceeded. If the sole fate of CH3O were either... 

A recent oommtiruo tion by Gritter and Wallace discloses initiation of a study of the free-radical chemistry of epoxides Under the influence of U t-butoxy radicals, formed by thermal decomposition of di-lerf-butyl peroxide, propylene oxide is believed to yield an epoxy radical as shown in Eq. (3). The latter undergoes Isomerization to CHsCOCH - and further reaction with unreaoted propylene oxide or other available substrates, such as 1-octene, toluene, oyolohexene, and ethanol,fl7a as shown in Eq. (3). 

Batt L, Hisham WWM, Mackay M (1998) Decomposition of the f-butoxy radical. II. Studies over the temperature range 303-393 K. Int J Chem Kinet 21 535-546 Bausch R, Schuchmann H-P, von Sonntag C, Benn R, Dreeskamp H (1976) CIDNP detection of the transient 4-benzyl-cyclohexa-2,5-dienone in the photorearrangement of benzyl phenyl ether. J Chem Soc Chem Commun 418-419... 

Fittschen C, Hippier H, ViskoIczB (2000) The (1 C-C bond scission in alkoxy radicals thermal unimo-lecular decomposition of f-butoxy radicals. Phys Chem Chem Phys 2 1677-1683 Gajewski E, Dizdaroglu M (1990) Hydroxyl radical induced cross-linking of cytosine and tyrosine in nudeohistone. Biochemistry 29 977-980... 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

The radical initially produced by homolytic decomposition of a dialkyl peroxide can undergo further scission. The rate of scission depends on the temperature and the stability of the resulting radical(s). For example, t-butoxy radicals decompose on heating to methyl radicals and acetone. 

Kinetics of thermal decomposition of dialkyl peroxides in solution as well as the gas phase have been reviewed by Molyneux and Frost and Pearson . The decomposition of dialkyl peroxides is moderately free from induced decomposition, compared to other types of peroxides. As seen from Table 65, the first-order rate coefficient increases by about 16 % when the initial peroxide concentration is increased about 5 fold at reasonably high peroxide concentrations. The increase in the rate coefficient is attributed to an induced decomposition where hydrogen atom abstraction generates the radical (I). Further reaction of (I) produces isobutylene oxide and the f-butoxy radical, viz. 

Although some termination may occur via free alkoxy radicals, kinetic and isotopic labeling data are most consistent with termination via the tetroxide in the case of r-butyl hydroperoxide . The chain length for the decomposition of t-butyl hydroperoxide is approximately 6-10 , Decomposition of the hydroperoxide is induced by t-butoxy radicals which are generated from di-t-butyl-... 

Decomposition. This peroxide undergoes smooth thermal decomposition in a first-order reaction to give t-butoxy radicals which, in a secondary reaction, decompose to methyl radicals and acetone. The extent of the secondary reaction is dependent upon the reactivity of the solvent toward the f-butoxy radical. The... 

By ESR spectroscopy Janzen measured the rate of formation of aminyloxides from benzoyloxy radicals and N-aryliden-tert-butylamin-N-oxides 81 and so determined the rate of thermal decomposition of benzoylperoxide90cl The value estimated in this manner is in accordance with those determined by other methods. In the same way the rate constants for spin trapping of tert-butoxy radicals by different spin traps were determined901. The relative values listed in the following table show how effectively the spin traps operate. 

Li believed that a cyclic alkane radical was generated from the hydrogen abstraction by a tert-butoxy radical that arose from the iron-catalyzed decomposition of the tert-butyl peroxide. The cyclic radical 62 was then believed to react with the iron enolate 63 to form the alkylated product 61 (Scheme 34). 

The right-hand side of Table 6-13 shows relative rates for alkoxy radical reactions in the atmosphere for boundary layer conditions. Comparison of the rates makes it immediately clear that reactions with N02 (or NO) are of little importance. For the smaller alkoxy radicals the reaction with oxygen is preponderant, whereas for alkoxy radicals largerthan butoxy, decomposition and isomerization reactions become competitive. Tertiary butoxy radicals have no abstractable hydrogen atom and thus cannot react with oxygen. In this case, decomposition is dominant. 

The rate of abstraction of H atoms from n-butane by lerl.-butoxy radicals was studied relative to the radical decomposition reaction... 

Another strategy involves decomposition of a peroxide or other initiator in the presence of a monomer. Conditions can be chosen such that only one unit of monomer is consumed. I hus, decomposition of UBPOX in S in the presence of DTBN provides 101 (Scheme 9.20). " The monomer initiator and/or combination should be chosen with care to obtain high yield of effective alkoxyamines. Many oxygen-centered radicals react with monomer by multiple pathways. Specificities shown by oxygen-centered radicals in their reaction with monomers have been studied extensively and are discussed in Section 3.4.2. Hydrogen abstraction, often by a source of f-butoxy radicals at low temperature [e.g. (/BuO)2/hv, DBPOX, " /BuOOH/Co(lI) "- j, in the presence of a nitroxide is another common method for generating benzylic and other alkoxyamines. 

Rate coefficients for thermal decomposition of di-t-butyl hypo-nitrite at 50° compared very well with those extrapolated from data in the literature, obtained by very different techniques, and reaction products contained one mole of base for each mole of PQ + consumed, strongly supporting the proposed electron transfer mechanism. It follows that, in 1 1 t-butyl alcohol-H20, t-butoxy radical must possess oxidizing power in excess of that of PQ2+, for which E0 = —446 mV (NHE), and may, therefore, function as a primary one-electron oxidant in reactions at present interpreted in other ways. 

---