Primary and secondary alkoxy radical

Primary and secondary alkoxy radicals generally show a reduced tendency to abstract hydrogen or to undergo p-scission when compared to the corresponding /-alkoxy radical.This has been correlated with the lesser nucleophilicity of these radical 

It has been suggested that primary and secondary alkoxy radicals may react with S by donation of a hydrogen atom to the monomer and production of an aldehyde. 

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The termination reaction proceeds through primary and secondary peroxy radicals according to Reaction (4.14), but at temperatures above 120°C these peroxy radicals also interact in a non-terminating way to give primary and secondary alkoxy radicals. Reaction (4.21) [6]. These radicals again contribute to the formation of cleavage products via Reactions (4.10) and (4.11) ... 

Relatively few studies have dealt with the reactions of primary and secondary alkoxy radicals (isopropoxy, methoxy, etc.) with monomers, fhese radicals are conveniently generated from the corresponding hyponitrites (Scheme... 

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. 

A considerably higher ratio of disproportionation occurs with more polar radicals such as primary or secondary alkoxy radicals and halogen alkyl radicals. The predominance of disproportionaticHi over coml ation for pdar radicals could be explained by electrostatic r ulsion of the radicals with electric dipoles functioning against dimerization. The polar structure of radu facilitates the proper orientation at the formation of an activated complex of disproprationation. 

In the framework of laboratory studies of elementary reactions relevant to tropospheric chemistry, we think that very interesting results have been achieved in the field of alkoxy radicals, in which exist a lot of theoretical speculations and estimations but only a few reliable experimental results. It seems to us that investigations on reactions of primary and secondary butoxy radicals can serve as models to understand the behaviour of higher RO radicals. Based on this assumption, investigations are in progress on reactions of n-CsHuO radicals in oxygen, which will soon, as we are convinced, lead to interesting conclusions. 

Two mechanisms have been considered for the self-reaction of primary and secondary alkylperoxy radicals, the Russell mechanism (lO) reactions (26), (27), and (28) and a mechanism involving the intermediacy of alkoxy radicals ( 3, kk) reactions (29), (30), (31), and (32). The reactions involved in these two mechanisms are presented in Scheme II. 

There is no doubt that all alkylperoxy radicals interact to give a tetroxide which decomposes to give either radical or nonradical products. Furthermore, it would appear that the structure of the tetroxide determines the overall rate and mechanism of the reaction. Di-t-alkyl tetroxides decompose either by a concerted or two step process to give t-alkoxy radicals, a fraction of which combine in the cage. This reaction pathway is also available to primary and secondary alkylperoxy radicals but seems to be preferred at higher temperatures. At temperatures below 373K these radicals appear to react principally by a non-radical... 

Primary and secondary peroxy radicals are more reactive in hydrogen abstraction than analogous tertiary radicals [64, 65]. The secondary peroxy radicals can abstract a labile hydrogen atom from another polyethylene molecule to form hydroperoxides species (Reaction 3 from Scheme 2.2), which are the compoxmds with higher contribution to the oxidation cycle, as well as another radical, through which the process can continue. In most polymers, the rate of this step in the chain reaction determines the overall rate of oxidation [66]. Finally, due to the thermal instability of the 0-0 bonds of these species (bond energy = 40 kcal/mol), they readily decompose into hydroxyl (OH ) and alkoxy (RO ) radicals, which ultimately result in the appearance of final... 

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. 

Dialkyl peroxydicarbonates have been reported as low temperature sources of alkoxy radicals (Scheme 3.30)lfMJfb and these radicals may be formed in relatively inert media. However, it is established, for primary and secondary peroxydicarbonates, that the rate of loss of carbon dioxide is slow compared to the rate of addition to most monomers or reaction with other substrates.186,187 Thus, in polymerizations carried out with diisopropyl peroxydicarbonate (47), chains will be initiated by isopropoxycarbonyloxy (48) rather than isopropoxy radicals (49) (see 3.4.2.2).188... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

Recently, Kabasakalian et al.138-140 have reported the nitroso dimer formation in the photolysis of primary and secondary nitrites. Both this reaction and the Barton reaction16 are explained in terms of reactions of alkoxy radicals. 

Chemical Properties. Acyclic di-ferf-alkyl peroxides efficiently generate alkoxy free radicals by thermal or photolytic homolysis. Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions. Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

The reaction of alkoxy radicals, as the intermediates of hydrocarbon oxidation, with molecular oxygen takes place in the case of primary and secondary radicals... 

TEMPO is a commercially available nitroxyl radical-containing reagent that catalyzes the oxidation of primary and secondary alcohols in conjunction with co-oxidants (oxygen, hypochlorite, bromite, hypervalent iodine, or peroxy acids).The catalyst is particularly useful for the oxidation of optically active a-alkoxy- or a-amino alcohols to the corresponding aldehydes without loss of enantiomeric purity. ... 

Alkylperoxy radicals are selective, and abstraction of a secondary H-atom is favored over abstraction of a primary H-atom. Alkoxy radicals, on the other hand, are less selective and can abstract primary hydrogens ... 

Peroxyesters derived from secondary (e.g. peroxyisobutyrate esters) and tertiary acids (e.g. peroxypivalate esters) are believed to undergo concerted 2-bond cleavage leading to direct production of an alkoxy and an alkyl radical and a molecule of carbon dioxide. " On the other hand, primary (e.g. peroxyacetate and peroxypropionate esters) and aromatic peroxyesters (t. g. BPB, Scheme 3.32) are thought to undergo I-bond scission to generate an acyloxy and an alkoxy radical. " " Evidence for the transient existence of acyloxy radicals includes the observation of substantial cage return. 

Using the e.s.r.-ENDOR technique. X-ray irradiation of single crystals of a-D-glucopyranose at 12 and 77 K has been shown to yield four free-radical species two secondary alkoxy-radicals centred at 0-2, a primary hydroxyalkyl radical at C-6, and a secondary hydro xyalkyl radical at C-3. ... 

Many types of peroxides (R-O-O-R ) are also utilized, including diacyl peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, and inorganic peroxides such as persulfate, the latter being used mainly in water-based systems. The rate of peroxide decomposition as well as the subsequent reaction pathway is greatly affected by the nature of the peroxide chemical structure, as illustrated for fert-butyl peroxyesters in Scheme 4.2. Pathway (a), the formation of an acyloxy and an alkoxy radical via single bond scission, is favored for structures in which the carbon atom in the a-position to the carbonyl group is primary (for example, terf-butyl peroxyace-tate, R = CHg). Pathway (b), concerted two-bond scission, occurs for secondary and tertiary peroxyesters (for example, terf-butyl peroxypivalate, R = C(CH3)3) [1, 2]. The tert-butoxy radical formed in both pathways may decompose to acetone and a methyl radical, or abstract a hydrogen atom to form tert-butanol. 

The rate constants for reaction of Bu3SnH with the primary a-alkoxy radical 24 and the secondary ce-alkoxy radical 29 are in reasonably good agreement. However, one would not expect the primary radical to react less rapidly than the secondary radical. The kinetic ESR method used to calibrate 24 involved a competition method wherein the cyclization reactions competed with diffusion-controlled radical termination reactions, and diffusional rate constants were determined to obtain the absolute rate constants for the clock reactions.88 The LFP calibrations of radical clocks... 

Their rate coefficients were determined and showed that the primary alkoxy radicals have slightly higher rate coefficients for the reaction with O2 than the secondary... 

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