Hydrocarbon structures free radical reactions

The free-radical chemistry of fluoroalkanesulfenyl chlorides with hydrocarbons was also investigated [S, 9], Depending upon the structures of the sulfenyl chloride and the hydrocarbon, these reactions yield as major products up to three of the following four types of organic compounds thiols, disulfides, sulfides, and chlorohydrocarbons (equation 6), Perfluoroisobutanesulfenyl chloride is unique m that the only major products detected are the thiol and chlorohydrocarbon [ ] (equation 6) (Table 3). 

Irons-phenyl, alkyl diazenes (2), peresters (3) and hydrocarbons (4). These equations are intended to be used for their predictive value for applications especially in the area of free radical polymerization chemistry. They are not intended for imparting deep understanding of the mechanisms of radical forming reactions or the properties of the free radical "products". Some interesting hypotheses can be made about the contributions of transition state versus reactant state effects for the structure activity relationships of the reactions of this study, as long as the mechanisms are assumed to be constant throughout each family of free radical initiator. 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

Atoms of S and Se can sufficiently structurally influence fragments of CH3 that are frequently located on the ends of hydrocarbon chains or in the form of free radicals. The data given confirm high reactivity of sulfur and selenium atoms as retardants of chain reactions of free radicals as elements drawing back impaired valence electrons of free radicals, but at the same time preserving the basic structure of hydrocarbon chain. 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

Atmospheric molecules such as 02, Os, NO and NOz are inherently reactive because of the free radical nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a clean fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. 

Paramagnetic centers containing a sulfur atom in different oxidation states, (=Si-0)3Si-0-S = O, (=Si-0)3Si-0-S 02, (=Si-0)3Si-0-S02-0, and (=Si-0)3Si-0-S02-0-0, were obtained in Ref. [118]. Their radio-spectroscopic parameters were determined, and the mechanism of free radical oxidation of S02 molecules in this system was established. The mechanism of the initial steps of free radical polymerization and copolymerization of hydrogen- and fluorine-substituted unsaturated hydrocarbons was studied in Ref. [117]. The pathways were found and the kinetic parameters were determined for reactions of intramolecular H(D) atom transfer between r (CH3, CD3, CH2-CH3) and r (CH2-CH2, CD2-CD2), in the structure of (=Si-0)2Si(r)(rI) [120]. 

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8, 167 Fluorocarbon Derivatives of Metals, I, 143 Free Radicals in Organometallic Chemistry, 14, 345 Heterocyclic Organoboranes, 2, 257... 

This resemblance is highly significant if one considers that 10,359 structural isomers exist for saturated hydrocarbons with 16 C atoms (Lederberg, 1972). Apparently the meteoritic hydrocarbons were made by FTT reactions, or some other process of the same extraordinary selectivity. The Miller-Urey reaction, incidentally, shows no such selectivity. Gas chromatograms of hydrocarbons made by electric discharges in methane show no structure whatsoever in the region around Cjg (Ponnamperuma et al., 1969). Apparently all 10 possible isomers are made in comparable yield, as expected for random recombination of free radicals. 

The two coupling reactions appear to have a common free-radical intermediate. Functional groups already in the aromatic compound, Axil, orient ortbo-para regardless of their nature. The reactions are most valuable for the preparation of biaryls of unequivocal structure when the hydrocarbon, Ar H, is unsubstituted. Good directions are given for the synthesis of p-bromobiphenyl (35%), and the literature of the reaction has been reviewed. Among the hydrocarbons prepared in this way are a- and yS-phenylnaphthalenes, o-, m-, and p-methylbiphenyls and m- and p-terphenyls. Thiophene and pyridine nuclei also have been aryl-ated. ... 

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