Alkoxy radicals tertiary

Under moderate conditions, primary alkoxy radicals tend to undergo reaction 12 whereas secondary and tertiary alkoxys tend to undergo -scission. In general, the alkyl group that can form the lowest energy radical tends to become the departing radical. The -scission of secondary alkoxy radicals yields aldehydes as the nonradical products tertiary alkoxy radicals yield ketones. 

Oxidation of tertiary alcohols by lead tetraacetate gives alkyl radicals by /3-scission of the initially formed alkoxy radicals. The reaction has been used to alkylate protonated heteroaromatic bases using 1-methyl-cyclohexanol. (Scheme 4). 

The mechanisms for ambient temperature oxidations become more complex as the alkyl radical R becomes more complex and as the reactions proceed. Thus, with azo-2-methylpropane (52) the pyrolysis of the tert-butoxy radical must be included, and in all but the initial stages, reactions of alkoxy radicals with products containing weak C—H bonds must be included. Numerous tertiary reactions can then occur. As with most free radical systems, useful information can be obtained only if the degree of conversion of the starting material is kept low. 

Tertiary peroxy radicals have no alpha hydrogen that can be abstracted by 02 in the second step. However, a unimolecular decomposition of the alkoxy radical results in the formation of a peroxy radical, which can be measured. 

Polypropylene, in which tertiary radicals predominate, nevertheless gives CL. This has been an argument against the validity of the Russell mechanism, which requires at least one of the peroxy radicals to be primary or secondary. However, Mayo and co-workers [15, 16] showed that termination reactions are accompanied by production of alkoxy radicals that will cleave to... 

Tertiary alcohols are oxidized in water-dioxane-NaOH to alkoxy radicals, wliich fragmentate to ketone and alkyl radicals R- (Eq. (216) ). The relative rate of cleavage decreases with R in the order sec -butyl > isopropyl > ethyl > propyl > pentyl > isobutyl > methyl 46 8). Likewise, the bisulfite adduct of cyclohexanone is converted in 20% yield to 4-hydroxyhexanoic acid lactone (160) and 3-hydroxycyclohexanoic acid lactone (161) by anodic fragmentation (Eq. (222) ) 469 ... 

Tertiary alcohol 293, when reacted with iodobenzene diacetate and iodine, underwent a formal alkoxy radical fragmentation and provided the nine-membered diketone 294 in 80% yield as a separable 1.2 1 mixture of epimers (Equation 40) <1999JOC4576>. 

The abstraction of a hydrogen atom occurs preferentially at the tertiary carbon of the structure, leading to a polystyryl radical. This radical adds to oxygen to form a peroxy radical. By abstraction of another hydrogen atom, the peroxy radical leads to a hydroperoxide. Hydroperoxides have an IR absorption at 3450 cm-1. The decomposition of the hydroperoxide either by photolysis or by thermolysis gives an alkoxy radical that may react in several ways ... 

By -scission, the alkoxy radical may give a chain ketone (detected at 1725 cm-1) and an end-chain aromatic ketone (1690 cm-1). The -scissions are accompanied by the formation of a benzyl and an alkyl radical. The former is the precursor of benzene and the latter isomerizes to a tertiary radical. This last radical is the precursor, after several reactions, of acetophenone (1690 cm-1), end-chain aliphatic ketone (1725 cm-1), end-chain aliphatic acid (1710-1753cm-1) and acetic and formic acids (1710cm-1). 

Alkoxy radicals produced by homolysis of hypohalites can attack non-activated C-H bonds, and lead to tetrahydro-furanoid structures after basic treatment of the intermediate halo-alcohols. Hypochlorites of [2y,2g] and C(20)-tertiary alcohols [28,2gJ gave 6 8,19- and 18,20-oxido derivatives respectively on irradiation and hydrolysis, but the more reactive hypobromites and hypoiodites are now preferred. Akhtar ef al. [30] distinguish between two modes of reaction of a 6jS-hypobromite (2), generated in situ by treating the alcohol with bromine and silver acetate. Experiments with... 

This is not a termination reaction. It is one means of converting alkylperoxy radicals to alkoxy radicals. It is the dominant reaction when neither peroxy radical contains an a-hydrogen, but it even occurs to a significant extent (in one report about 40% of the time [17]) with peroxy radicals that do contain a-hydrogens. Alkoxy radicals are vigorous hydrogen abstractors [12]. This appears to be the main reaction for primary alkoxy radicals the products are primary alcohols. Secondary and tertiary alkoxys, however, tend to undergo a competitive 6-scission reaction to a major extent [18] ... 

Reaction (D) will for example provide one possible route to alcoholic groups, while an alternative reaction, 6-scission of a secondary alkoxy radical, will give aldehyde, reaction (H), which rapidly oxidizes further to peracid. 6 scission of a tertiary alkoxy radical result in ketones. Aldehydes and ketones may react further with peracid to give acid and ester groups. 

Secondary and tertiary alkoxy radicals prefer to form aldehydes, Reaction (4.11), and ketones, Reaction (4.12) ... 

Tertiary alkoxy radicals are not expected to react with Oz because of the absence of a readily available hydrogen atom. Assuming a ground-level 02 ... 

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. 

Reaction 6 is the hydrogen atom abstraction from PVA, an important propagation reaction. Since it was shown by the spin-trapping technique using 2-methyl-2-nitrosopropane that only the tertiary hydrogen of polypropylene was abstracted by the alkoxy radical (28), it was assumed that the first attack was also exclusively on the tertiary hydrogen of PVA. The a-hydroxyperoxy radical formed by the addition of oxygen (Reaction... 

The data provide an estimate of the ratio of reactivity to oxidative attack of branch points compared with linear hydrocarbon chains. The butyl C2 carbon resonance intensity at 23.4 ppm (Figure 2) decreases from 9.7 to 6.6 per 1000 CH2 upon absorption of 53 ml-g"1 of oxygen. Oxidative cleavage at an n-butyl (or longer) branch point occurs as follows (15,16,17) (the tertiary alkoxy radical having been generated by steps parallel to those shown above) ... 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

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