Halogen, radical substitution

In contrast to the free radical substitution observed when halogens react with alkanes halogens normally react with alkenes by electrophilic addition... 

Free-radical substitutions of heterocyclic compounds have been carried out with alkyl, aryl, and hydroxyl radicals in solution and with halogen atoms in the gas phase. Of these, arylations have been the most extensively investigated. 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

Figure 8-10 shows another pair of reactions for the halogenation of an aromatic compound. The reaction of the side chain is a free-radical substitution. Figure 8-11 shows the mechanism of this free-radical substitution. 

These halogenations are like free-radical substitutions of alkanes (see Section 4.4). The order of reactivity of H-abstraction is... 

With Fe, the products are o-, p-, and some m-BrCftH4CH,. In light the product is benzyl bromide, PhCHjBr. Like allylic halogenation (Section 6.5), the latter reaction is a free-radical substitution ... 

A free radical chain reaction is also called a radical substitution reaction, because radicals are involved as intermediates, and the end result is the substitution of a halogen atom for one of the hydrogen atoms of alkane. 

Under high temperature or UV light and in the gas phase, cyclohexene can undergo free radical substitution by halogens. A common reagent for allylic... 

Halogen functionality in heteroaromatics undergoes a variety of transformations by nucleophilic substitution metal-halogen exchange, radical substitution, and cross coupling reactions. The connecting link between these processes and the DoM reaction is therefore a powerful tool in heterocyclic chemistry. 

The two propagation steps, 2 and 3, are SH2 substitutions. Note that the substitutions occur by attack of the radical on a terminal, univalent atom, in one case H, in the other halogen. This feature is characteristic of bimolecular radical substitution steps attack at multiply bonded sites tends to be by addition (Equation 9.65), and attack at saturated carbon occurs only in highly strained molecules. Thus since terminal singly bonded centers in organic compounds are nearly always hydrogen or halogen, it is at these atoms that most SH2 substitutions occur. 

Direct radical substitutions can occur at atoms other than hydrogen, halogen, and carbon.143... 

Halogenation of heterocyclic compounds, 7, 1 Hammett equation, applications to heterocyclic compounds, 3, 209 20, 1 Hetarynes. 4, 121 Heleroadamanlane, 30, 80 Heleroannulenes, medium-large and large n-excessive 23, 55 Heteroaromalic compounds JV-aminoazonium salts, 29, 71 free-radical substitutions of, 2, 131... 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

Fredricks, P. S., and /. Tedder Free-radical substitution in aliphatic compounds. Part II. Halogenation of the n-butyl halides. J. chem. Soc. [London] 1960, 144. 

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