Radical halogenations hydrocarbons

The presence of halogen additives substantially increases the tendency of all fuels to soot under diffusion flame conditions [69], The presence of H atoms increases the soot pyrolysis rate because the abstraction reaction of H + RH is much faster than R + RH, where R is a hydrocarbon radical. Halogenated compounds added to fuels generate halogen atoms (X) at modest temperatures. The important point is that X + RH abstraction is faster than H + RH, so that the halogen functions as a homogeneous catalyst through the system... 

A more energy-efficient variation of photohalogenation, which has been used since the 1940s to produce chlorinated solvents, is the Kharasch process (45). Ultraviolet radiation is used to photocleave ben2oyl peroxide (see Peroxides and peroxide compounds). The radical products react with sulfuryl chloride (from SO2 and CI2) to Hberate atomic chlorine and initiate a radical chain process in which hydrocarbons become halogenated. Thus, for Ar = aryl,... 

One important prerequisite to the application of this reaction in hydrocarbon synthesis is the selective monochlorination of methane. Usual radical chlorination of methane is not selective, and high CH4 CI2 ratios are needed to minimize formation of higher chlorinated methanes (see Section 10.2.5). In contrast with radical halogenation, electrophilic halogenation of methane was shown to be a highly selective process.412... 

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

An unusual radical halogenation of hydrocarbons by phase-transfer catalysis may be performed by reacting alkanes with tetrahalomethanes. The reaction is initiated by single-electron oxidation of OH- by CHlg4. The tetrahalomethane radical anion formed decomposes to the 12 trihalomethyl radical [Eq. (10.56)], which is then involved in C—H activation and propagation steps 286... 

SALT. A compound formed by replacement of part or all of the hydrogen of an acid by one (or more) element(s) or radrcal(s) that are essentially inorganic. Alkaloids, amines, pyridines, and other basic organic substances may be regarded as substituted ammonias in this connection. The characteristic properties of salts are the ionic lattice in the solid state and the ability to dissociate completely in solution. The halogen derivatives of hydrocarbon radicals and esters are not regarded as salts in the strict definition of the term,... 

Stepwise decarboxylation also occurs, particularly in reactions in which the carboxylate radical (RC02 ) is formed. This radical usually decomposes to a hydrocarbon radical (R-) and C02- The overall decarboxylation product is determined by what R- reacts with If a good hydrogen donor is present, RH is formed if a halogen donor such as Br2 is present, RBr is formed ... 

Metal alkyl. A compound of a metal with directly linked aliphatic or aromatic hydrocarbon radicals, as in zinc dimethyl, Zn(CHs)2, or mercury diphenyl, Hg(C6H5)2. A normal alkyl has sufficient organic groups to satisfy the normal valence of the metal and therefore contains no halogen or other substituent in place of organic radicals. 

This suggested that if all the halogen atoms in the complex compounds of the transition metals could be replaced by hydrocarbon radicals, the resultant complex, for example, K2PtR4, should be stable. The main problem is to find a method of synthesis whereby one does not go through an unstable [PtR2Cl2] type of intermediate on the way, and we have not been able to achieve this. 

Hydroxyl Substitution Products.— All of the preceding facts lead to the conclusion that alcohol is a compound in which the ethyl radical is linked to the hydroxyl radicalj i.e, it is the hydroxyl substiltUion product of ethane or hydroxy ethane. Alcohols thus belong to the same general class of compounds as the halogen substitution products. The relationship between the hydrocarbons, the halogen substitution products, alkyl halides and the hydroxyl substitution products, alcohols, may be shown as follows ... 

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