Radicals, alkoxyl hydrogen

It is usually postulated that the final product in the accepted mechanism, the alkoxyl radical (4), cleaves (eqs. 14 and 15) before or after hydrogen abstraction, and that this accounts for the drop in molecular weight of the... 

Oxaziranes derived from isobutyraldehyde react with ferrous salts to give only substituted formamides fEq. (23)], The chain propagating radical 30 thus suffers fission with elimination of the isopropyl group. An H-transfer would lead to substituted butyramides, which are not found. Here is seen a parallel to the fragmentation of alkoxyl radicals, where the elimination of an alkyl group is also favored over hydrogen. The formulation of the oxazirane fission by a radical mechanism is thus supported. 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Among the most useful radical fragmentation reactions from a synthetic point of view are decarboxylations and fragmentations of alkoxyl radicals. The use of (V-hydroxy-2-thiopyridine esters for decarboxylation is quite general. Several procedures and reagents are available for preparation of the esters,353 and the reaction conditions are compatible with many functional groups.354 f-Butyl mercaptan and thiophenol can serve as hydrogen atom donors. 

Alkoxyl radicals are very active in reactions of hydrogen atom abstraction (see Table 2.20). The problems of their reactivity will be discussed in Chapter 6. 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Alkoxyl radicals are very active and rapidly enter into bimolecular reaction (see Chapter 2). Moreover, alkoxyl radicals with sufficiently long alkyl substituents react with intramolecular hydrogen atom transfer, for example [37] ... 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table 6.12. Isomerization proceeds via the formation of a six-membered activated complex, and the activation energy for the thermally neutral isomerization of alkoxyl radicals is equal to 53.4 kJ mol-1. These parameters were used for the calculation of the activation energies for isomerization of several alkoxyl radicals via Eqns. (6.7, 6.8, 6.12) (see Table 6.14). The activation energies for the bimolecular reaction of hydrogen atom (H-atom) abstraction by the alkoxyl radical and intramolecular isomerization are virtually the same. 

Alkoxyl radicals can result from the isomerization of peroxyl radicals of oxidized PP (see above 13.1.6). If alkoxyl radicals cause polymer destruction, then, as they are produced from alkyl radicals, their accumulation and quasistationary concentration must decrease with increasing p02. However, despite varying p02, vs = const, in the oxidized PE and PP and, therefore, alkoxyl radicals essentially do not contribute to the oxidative destruction of polymers. At moderate temperatures, alkoxyl radicals eliminate hydrogen atoms from PH more rapidly than they undergo degradation. 

The same effect is typical of the reactions of alkoxyl radicals with phenols, that is, these reactions are much slower in solvents capable of forming hydrogen bonds with O—H and N—H groups [50]. MacFaul et al. [50] proposed a universal scale for correlating the reactivities of phenols and the hydrogen-bonding abilities of solvents. 

As mentioned earlier, MPO-hydrogen peroxide-chloride system of phagocytes induces the formation of lipid peroxidation products in LDL but their amount is small [167-169], It was proposed that HOCL can decompose the lipid hydroperoxides formed to yield alkoxyl radicals [170]. It was also suggested that chloramines formed in this process decompose to free radicals, which can initiate lipid peroxidation [171]. 

More relevant to our consideration now is the radical addition of hydrogen bromide to an alkene. Radical formation is initiated usually by homolysis of a peroxide, and the resultant alkoxyl radical may then abstract a hydrogen atom from HBr. 

Figure 22.7. Schematic depiction of the TS for hydrogen atom abstraction from methylcuhane (6) hy an alkoxyl radical. The polarity of the TS, depicted in the bottom resonance stmcture, was confirmed by the results of population analyses. " ...

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table... 

Figure 10.10. Orbitals for a hydrogen atom abstraction reaction by a) alkoxyl radical from H—OR b) methyl radical from CH4 (one bond shown) (c) silyl radical from SiH4 (one bond shown).

The photolysis of n-octyl nitrite in n-heptane provides a favorable situation for comparing the tendency of an alkoxyl radical to undergo the Barton-type intramolecular reaction or, alternatively, the intermolecular reaction as indicated in eqs. 1 and 2. Calculations23 reveal that both intramolecular and intermolecular hydrogen abstraction involving a second-... 

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