Alkyl radicals radical reactions

The similarity of oxidation rates of different hydrocarbons in the higher temperature regions is probably related to the predominance of alkyl radical cracking reactions under these conditions (reaction 28). The products of such reactions would be similar for most common hydrocarbons (96). 

Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides (10 equiv) and (TMSlsSiH (5 equiv) in the presence of AIBN in refluxing benzene, for 18 h (Reaction 66). " These reactions are believed to proceed through intermediates in which the unpaired electron is interacting with the adjacent arene chromium tricarbonyl moiety since the analogous reaction with styrene affords only traces of addition products. 

The values of rate constants, activation energies, and geometric parameters of secondary alkyl peroxyl radical reaction with several aromatic amines are presented in Table 15.7. 

It has subsequently been shown (Maeda et ai, 1978) that the rates of these alkyl-radical trapping reactions show no significant solvent dependence. 

The field of alkyl radical macrocyclization reactions was further augmented with an n + 1) strategy, which incorporates a CO unit in the macrocycle [93], Thus, in the presence of highly diluted (0.005-0.01 M) (TMS)3SiH, co-iodoacrylates underwent an efficient three-step radical chain reaction to generate 10- to 17-membered macrocycles in 28-78% yields, respectively (Reaction 7.82). 

The mechanism of the photochemical alkylation shows particular characteristics as regards the formation of alkyl radicals, the reaction of these radicals with the heteroaromatic substrates, and the rearomatization of the intermediate products. A variety of alkylating agents (hydrocarbons, alcohols, amines, carboxylic acids, amino acids) have been used for photochemical and y-ray-induced alkylation. " ... 

Scheme 10.10. Alkylation of Alkyl Radicals by Reaction with Alkenes... 

It is clear that the aldehydes produced in the oxidation of butenes arise from addition reactions as already assumed by several workers (4, 16, 17). Nothing in the present work indicates whether the aldehydeforming addition product is a peroxy alkyl (17), (Reaction 2) or an alkoxy alkyl (16) radical (Reaction 3) as has been postulated, or both. 

Such relative lack of kinetic impact of olefinic fluorine substituents on alkyl radical addition reactions is consistent with Tedder s early studies on methyl affinities, the results of which are shown in Table 16, where the range of reactivities observed for the addition of methyl radical to ethylenes with varying fluorine content is seen to be relatively small [93,163],... 

Refluxing or photolytic treatment of O-acyl esters (2) in the presence of a hydrogen donor such as Bu3SnH or ter -BuSH, provides the corresponding reduction products via alkyl radicals. This reaction can be applied to primary-, secondary-, and tertiary-alkyl chained carboxylic acids, and can also be used for steroids, sugars, and peptides as shown in eq. 8.4 [9-14]. Racemization does not occur at other chiral centers. 

Scheffold and colleagues subsequently reported that alkyl iodides 249 add to xfl-unsaturated esters 248 catalyzed by 23 mol% hydroxycobalamine 247 (X=OH) and mediated by light to homolyze 253B [302]. The generation of the active Co(I) species 253A proceeded by electrochemical reduction at a potential of —1.2 V vs SCE. The transformation required a large excess of 248 to promote the addition of the initially generated alkyl radical. The reaction was applied to the total synthesis of the California red scale pheromone. The product... 

A certain kind of radical transfer can be modelled by the transfer of a hydrogen atom from an alkane molecule to a small alkyl radical. This reaction was studied in detail in the gas phase. With hydrocarbon partners, heats of reaction are a fairly safe measure of the relative rate of transfer, as the pre-exponential Arrhenius factors remain approximately constant for a series of transfers to a given radical. Tabulated thermodynamic data indicate, however, [31, 32] that the correlation between the heat of reaction and the transfer rate is not valid for reactions of a radical with polar substrates [32, 33], In condensed phases, transfer reactions have not been sufficiently studied. Polymerizations themselves are the source of the most valuable, though incomplete, information. 

Evidence for a radical pathway includes the observation that the reaction is accelerated by radical initiators (such as oxygen or peroxides) and the presence of UV light. Moreover, the order of reactivity for the R group is IIP > II0 > 1°, which is inconsistent with a direct displacement mechanism, but is in accord with the stability of alkyl radicals. Radical inhibitors (such as steri-cally hindered phenols) retard the rate of reaction with sterically-hindered alkyl halides, but not when R = methyl, allyl, and benzyl. When stereoisomerically pure alkyl halides are used, OA results in the formation of a 1 1 mixture of stereoisomeric alkyl iridium complexes, consistent with the formation of an intermediate radical R-. 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

The hydroperoxide-peroxy radical (I) reacts with hydrocarbons through inter-molecular hydrogen radical abstraction, resulting in an alkyl dihydroperoxide (II) and a chain-initiating alkyl radical. Radical (I) may also intramolecularly abstract a hydrogen radical, Reaction (4.5) ... 

Butyl radicals decompose quickly to form ethylene and propylene. At high temperatures, alkyl radical decomposition reactions constitute an important reaction class and the prevailing fate of alkyl radicals. Take 1-propyl and 1-butyl radicals, for example. These primary alkyl radicals give rise to the following / -decomposition reactions ... 

Alkyl radical decomposition reactions (to form primary radicals) ... 

As previously discussed, alkyl radicals decomposition reactions constitute an important fate and reaction path of alkyl radicals. Due to the very short lifetimes of alkyl radicals, Rice and Herzfeld (1933, 1934) suggested a complete decomposition mechanism where all the radicals larger than methyl were considered instantaneously decomposed into alkenes and H and CH3 radicals. In this mechanism, all the intermediate alkyl radicals decompose to directly form alkenes and smaller alkyl radicals. This would mean that the final ethylene production from a steam cracking process would be significantly overestimated when compared with the experimental measurements. For instance, the net and final result of the successive decomposition mechanism of 1-decyl radical would be 5 moles of ethylene and one H radical. 

In all the nitroxide regeneration mechanisms the nitroxide reacts with an alkyl radical, this reaction is in competition with the extremely fast reaction between an alkyl radical and oxygen (see Scheme 17.14)... 

For the alkyl peroxy radicals, reaction (45a) can form the corresponding alkoxy (RO) radical together with N02, or the corresponding alkyl nitrate,... 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

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