Homolytic reactions elementary radicals

Addition of the silyl radical to carbon-carbon double bonds is an elementary reaction of radical hydrosilation (Scheme 1). Homolytic aromatic silationalso occurs involving silyl radicals. Silyl radicals are nucleophilic owing to the high SOMO energy, as evidenced by the directive effects in the hemolytic aromatic substitution. The intermediate cyclohexadienyl radicals have been observed by ESR. 

Notwithstanding the enormous diversity of free-radical processes, the following most common elementary reactions may be singled out 1) cleavage of the atom X, usually a hydrogen atom, by a radical from the R—X bonds 2) addition of the radical to the double bond and 3) reactions of radical (homolytic) substitution Sh2 proceeding without intermediates. Some other processes, for example, the widespread reactions of radical aromatic substitution and the type of Eq. (8.1) reactions occur as a combination of two elementary steps by the addition-elimination mechanism (AdE). 

Thermal reactions of light alkanes with oxygen in the combustion process have been studied extensively (6, 7). These studies were typically conducted at high temperatures—flame temperatures. The elementary reactions of the hydrocarbon species often involve reactions with atomic (H, O) or free radical species (OH, alkyl, etc.). The initiation step is the homolytic cleavage of C—C single bonds to form alkyl radicals. The C—C bonds are the weakest bonds in an alkane molecule (Table I). The chain-propagation step... 

In many cases both homolytic and heterolytic pathways afford the same products, e.g. alcohols and ketones from hydrocarbons, which means that results have to be interpreted with care. Certain elementary tests for homolytic pathways need to be performed, e.g. inhibition by a radical scavenger such as lonol indicates a free radical chain mechanism and loss of yield on flushing with an inert gas suggests the intermediacy of dioxygen in reactions with H2O2 or RO2H. More sophisticated reality tests can also be performed to demonstrate the intermediacy of alkoxy radicals in oxidations with RO2H [17]. 

Some of the elementary concepts of free radical mechanisms were presented in Chapter 1. Reactions following free radical mechanisms have reactive intermediates containing unpaired electrons which are produced by homolytic cleavage of covalent bonds. A method of detecting free radicals was published in 1929, and it is based on the fact that metals such as lead react with free radicals. When heated, tetramethyl lead decomposes. 

Radicals are generated, consumed, or propagated by a relatively few types of elementary reactions. Radical generation usually involves the homolytic dissociation of some covalent bond. 

We suggest the original method for evaluation of catalytic activity of complexes formed in situ at the beginning of reaction and in developed process, at elementary stages of oxidation process [33, 90-93] by simplified scheme assuming quadratic termination of chain and equality to zero of rate of homolytic decomposition of ROOH. In the If amework of radical-chain mechanism the chain termination rate in this case will be Eq. (1) ... 

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