Alkylperoxy

There are two especially important radical—radical reactions of alkylperoxy radicals (28) both beheved to proceed via formation of a transient tetroxide (eqs. 13 and 15). 

Reversibility of Equation 2. Notwithstanding the problems and conflicts, there is widespread agreement that the NTC phenomenon may well be related to the reversibiUty of equation 2 (13,60,63—67) R- + O2 ROO-. In the low temperature regime, the equiUbrium Hes to the right and alkylperoxy radicals are the dominant radical species. They form hydroperoxides, the chain-branching agent, by reaction 3. 

Alternatively, a number of investigators (69—73) have proposed, on the basis of plausible kinetic arguments, that the conjugate olefin is produced by a rearrangement of alkylperoxy radicals (eq. 24). 

As the temperature is increased through the NTC zone, the contribution of alkylperoxy radicals falls. Littie alkyl hydroperoxide is made and hydrogen peroxide decomposition makes a greater contribution to radical generation. Eventually the rate goes through a minimum. At this point, reaction 2 is highly displaced to the left and alkyl radicals are the dominant radical species. 

Higher Hydrocarbons. The VPO of higher hydrocarbons is similar to that of the lower members of the series with two significant additional comphcations (/) the back-bitiag reactions of alkylperoxy radicals (eq. 32), particularly at positions 2 or 3 carbons removed from the peroxy position, and (2) above the NTC region, radical fragmentation (eq. 28). 

An important function of manganous ions is the ready reduction of alkylperoxy radicals (181) (eq. 34). 

Mn (IT) is readily oxidized to Mn (ITT) by just bubbling air through a solution in, eg, nonanoic acid at 95°C, even in the absence of added peroxide (186). Apparently traces of peroxide in the solvent produce some initial Mn (ITT) and alkoxy radicals. Alkoxy radicals can abstract hydrogen to produce R radicals and Mn (ITT) can react with acid to produce radicals. The R radicals can produce additional alkylperoxy radicals and hydroperoxides (reactions 2 and 3) which can produce more Mn (ITT). If the oxygen feed is replaced by nitrogen, the Mn (ITT) is rapidly reduced to Mn (IT). 

The selectivity to alcohol in LPO may be significantly increased when boric acid, meta-hotic acid, or boric anhydride is present in stoichiometric amounts (2). The boron compounds appear to convert alkyUiydroperoxides to alkyl borates and may also intercept alkylperoxy radicals, converting them to alkylperoxyboron compounds these are later converted to alkyl borates. The alkyl borates are resistant to further oxidation they are hydrolyzed to recover alcohols. 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

By way of example, tert-huty peroxyacetate [107-71-1] is more thermally stable than 3-hydroxy-1,1-dimethylbutyl peroxyneoheptanoate [110972-57-1]. Although other factors affect thermal stabiUty, the trends shown can be used to quaUtatively predict peroxyester reactivity trends. The order of activity of the R group ia peroxyesters is also observed ia other / fZ-aLkylperoxy-containing compounds. 

Diperoxyketals. Some commercially available di(/ f2 -alkylperoxy)ketals and their corresponding 10-h half-life temperatures (deterrnined in dodecane) are hsted in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually foUowed by P-scission of the resulting alkoxy radicals (40). For acychc diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

Eurther reactions of the alkylperoxy radical (ROO-) depend on the environment but generally cause generation of other radicals that can attack undecomposed hydrosend peroxide, thus perpetuating the induced decomposition chain. Radicals also can attack undecomposed peroxide by radical displacement on the oxygen—oxygen bond ... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Many organic peroxides of metals have been hydrolyzed to alkyl hydroperoxides. The alkylperoxy derivatives of aluminum, antimony, arsenic, boron, cadmium, germanium, lead, magnesium, phosphoms, silicon, tin, and zinc yield alkyl hydroperoxides upon hydrolysis (10,33,60,61). 

Symmetrical diaLkyl peroxides are commonly named as such, eg, dimethyl peroxide. For unsymmetrical diaLkyl peroxides, the two radicals usually are hsted ia alphabetical order, eg, ethyl methyl peroxide. For organomineral peroxides or complex stmctures, ie, where R and R are difficult to name as radicals, the peroxide is named as an aLkyldioxy derivative, although alkylperoxy is stUl used by many authors. CycHc peroxides are normally named as heterocychc compounds, eg, 1,2-dioxane, or by substitutive oxa nomenclature, eg, 1,2-dioxacyclohexane however, when the two oxygens form a bridge between two carbon atoms of a ring, the terms epidioxy or epiperoxy are frequendy used. The resulting polycycHc stmcture has been called an endoperoxide, epiperoxide, or transaimular peroxide. 

Thermally unstable cycHc trioxides, 1,2,3-trioxolanes or primary o2onides are prepared by reaction of olefins with o2one (64) (see Ozone). Dialkyl trioxides, ROOOR, have been obtained by coupling of alkoxy radicals, RO , with alkylperoxy radicals, ROO , at low temperatures. DiaLkyl trioxides are unstable above —30° C (63). Dialkyl tetraoxides, ROOOOR, have been similarly produced by coupling of two alkylperoxy radicals, ROO , at low temperatures. Dialkyl tetraoxides are unstable above —80°C (63). 

Miscellaneous OC-Substituted Peroxides. 3-Aryl-3-(/ i alkylperoxy)-phthaHdes (12) are prepared from the corresponding 3-chlorophthaHdes and alkyl hydroperoxide (156). 2-Methyl-2-(/ f2 -alkylperoxy)-l,3-benzodioxan-4-ones (13) are obtained from 0-acetylsaHcyloyl chloride and alkyl hydroperoxides (157). Trisubstituted 2-(/ f2 -alkylperoxy)-l,3-dioxolan-4-ones (14) are synthesized from stericaHy favored a-acyloxy acid chlorides and alkyl hydroperoxides (158). 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

Alkylperoxy (RO2) and peroxyacyi (RC(O)OO) radicals react with NO to form NO2. The alkylperoxy radicals (RO2) react with NO2 to form pemitric acid-type compounds, which decompose thermally as the temperature increases. The peroxyacyi radical reacts with NO2 to form PAN-type compounds, which also decompose thermally. 

The relative importance of the various pathways depends on the alkyl groups (R). The rate constants for scission of groups (R ) from /-aikoxy radicals (RR C-O) increase in the order isopropylalkyl radical is less important when R is methyl than when R is a higher alkyl group, if the pathway to alkylperoxy radicals is dominant, the resultant polymer is likely to have a proportion of peroxy end groups.200 211... 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Alkylperoxy radicals are generated by the reactions of carbon-centered radicals with oxygen and in the induced decomposition of hydroperoxides (Scheme 3.82). Their reactions have been reviewed by Howard452 and rate constants for their self reaction and for their reaction with a variety of substrates including various inhibitors have been tabulated.453... 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Alkyl radicals generated from azoalkanes as in (7) react with oxygen added to argon matrices giving alkylperoxy radicals. In this manner radicals... 

This is the main reaction for the formation of ozone although, under equilibrium conditions, the concentrations of NO2, NO, and O3 are interdependent and no net synthesis of O3 occurs. When, however, the equilibrium is disturbed and NO is removed by reactions with alkylperoxy radicals (reactions 1+2+3), synthesis of O3 may take place. 

The yield of alkylperoxy (RO2 ) radicals in reaction 6.8, on the contrary, provides a route for chain branching to take place and is both pressure and temperature... 

Thermally, alkylperoxy (RO2 ) radicals can become unstable as temperatures approach 600 K, since equilibrium would favor the alkyl radical and O2 as reactants. As temperatures increase further, high temperature oxidation mechanisms will begin to prevail. 

A number of alkylperoxy Co111 complexes including mainly polyimine ligands have been prepared, exemplified by (223).979 Hydroxylation of saturated hydrocarbons, preferentially at the more nucleophilic C—H bonds, yields alcohols, ketones, and t-butylperoxo products, whereas olefins form epoxides if they carry no allylic H atoms. 

Hendry, D.G., Mill, T., Piszkiewicz, L., Howard, J.A., Eigenman, H.K. (1974) A critical review of H-atom transfer in the liquid phase chlorine atom, alkyl, trichloromethyl, alkoxy and alkylperoxy radicals. J. Phys. Chem. Ref. Data 3, 944-978. 

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