Radicals, from halogens

Whittle and co-workers have carried out a number of studies of halogen-abstraction reactions of CF3 radicals from halogen molecules and alkyl halides (see Table 26). The exothermic reactions between CF3 radicals and CI2, Br2 and I2 all have low activation energies, whereas the thermoneutral reactions between CF3 and alkyl halides have activation energies of about 10 kcal mole . ... 

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. 

TABLE 4. Absolute kinetic data for halogen abstraction by some radicals from sulfonyl halides... 

The first step involves a reduction of the diazonium ion by the cuprous ion, which results in the formation of an aryl radical. In the second step, the aryl radical abstracts halogen from cupric chloride, reducing it. The CuX compound is regenerated and is thus a true catalyst. 

An alternate positive ion approach, similar to that in Eq. 5.4a is to obtain a carbon-halogen BDE, R X, from which it is possible to obtain the enthalpy of formation of the radical from which the hydrocarbon BDE can be derived. The advantage of this approach is that it is easier to measure the R appearance energy from RX than it is from RH because of the weaker RX bond. However, a limitation of the approach is that the enthalpies of formation of organic halides, required to determine the enthalpies of formation of the cations, are generally not known as accurately as those for hydrocarbons. 

The possibility that substitution results from halogen-atom transfer to the nucleophile, thus generating an alkyl radical that could then couple with its reduced or oxidized form, has been mentioned earlier in the reaction of iron(i) and iron(o) porphyrins with aliphatic halides. This mechanism has been extensively investigated in two cases, namely the oxidative addition of various aliphatic and benzylic halides to cobalt(n) and chromiumfn) complexes. 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

Stratospheric Ozone depletion is largely due to chlorine and bromine radicals released from halogenated hydrocarbons. This paper describes properties, emission histories and budgets of relevant substances and outlines the pertinent photochemical processes, along with a comprehensive presentation of halocarbon measurements and global distributions. 

The survey is intended to cover the published literature on the subject as fully as possible. Although the four halogens are well known as constituents of a wide range of carbohydrate compounds, radical-mediated halogenations have, in most cases, been realized only with bromine. There are, however, some examples of chlorinations to the best of our knowledge, there are no reported fluorinations or iodinations which occur by direct, radical-mediated processes. The radical-mediated reactions by which bromodeoxy carbohydrates are obtainable from benzylidene acetals17-19 are not considered. 

Lemmes R, von Sonntag C (1982) On the formation of deoxy sugars from halogenated carbohydrates by a-hydroxyalkyl radicals considerations for the optimisation of reaction conditions. Carbohydr Res 105 276-282... 

The cathodic removal of one halogen radical from perhalogenated quinolines and isoquinolines 450) occurs with high selectivity ... 

Primary chlorides reacted predominantly by a direct mechanism (Sn2 or a multicentered process). Isobutyl or neopentyl halides led to contributions from electron transfer (free radicals) and halogen-metal exchange (anionoid) mechanisms. 

When the same authors studied the reaction in detail, it was found to proceed as a nucleophilic displacement on a halide atom by the R group from the accompanying reagent, which was essential for the reaction.634 A radical type process could not be excluded, however, as some halogen was eliminated from halogenated 1-alkylpyrazoles yielding 12-15% of dipyrazolyls. 

The P-cleavage of halogen atoms and thiyl radicals from biradicals appears to follow a simple enough mechanism, as shown in Sch. 2. After being formed, the 5-substituted 1,4-biradical can undergo radical cleavage in competition with its normal decay reactions. That leaves an... 

Bottoni also reported the results of computational studies into the abstraction of hydrogen atom by silyl and trichlorosilyl radicals from chloromethane, dichloromethane and chloroform (equation 15)42, and concluded that hydrogen abstraction does not effectively compete with halogen abstraction in the systems investigated. B3LYP/6-31G calculated energy barriers for these reactions fall in the approximate range of 46-72 kJmol-1. 

Radical anions formed from halogen-containing aromatic systems. . 321... 

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