Halogenation reactions free-radical

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. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

Abstraction of a halogen has been studied much less, but the order of reactivity is RI > RBr > RCl 3> RF. There are now many cases where free-radical reactions are promoted by transition metals. ... 

Diels-Alder adduct from cyclopentadiene, 8 222t Diels-Alder reactions of, 25 488-489 economic aspects of, 25 507-509 electrophilic addition of, 25 490 in ene reactions, 25 490 esterification of, 25 491 free-radical reactions of, 25 491 from butadiene, 4 371 Grignard-type reactions of, 25 491 halogenation of, 15 491—492 health and safety factors related to, 25 510-511... 

The Hunsdiecker reaction is a free-radical reaction for the synthesis of an alkyl halide. The starting material comes from the reaction of a silver carboxylate with a solution of a halogen in a solvent such as carbon tetrachloride (see Figure 12-44). The overall free-radical mechanism is shown in Figure 12-45. 

Mozurkewich, M Mechanisms for the Release of Halogens from Sea-Salt Particles by Free Radical Reactions, J. Geophys. Res., 100, 14199-14207 (1995). 

Gas phase free radical reactions are used in industry for pyrolysis, halogenation and combustion reactions. Nowadays, and probably for a long time to come, the thermal cracking of hydrocarbons constitutes the main production route for olefins, which are the basic feedstocks of the chemical industry around the world. Hydrocarbon pyrolysis is thus of considerable economic interest, as is shown by the very large amount of effort dedicated both to fundamental and applied research in this field (see, for example, refs. 35—37). 

Treatment of the tris(dialkyldithiocarbamato)iron(III) complexes in benzene with a controlled amount of concentrated hydrohalic acid affords the black bis(ligand) complexes [FeX(S2CNR2)2] (X = Cl, Br or I).306 For X = Cl the complexes may also be prepared by irradiation of the tris-(ligand) complex in a halogenated solvent. This free radical reaction is believed to proceed via excited-state labilization of one ligand followed by attack of solvent.307 Analogous complexes of pseudohalide ions (X = NCO, NCS- or NCSe ) have been obtained from reaction of the parent tris complex with the appropriate Ag+ salt.380 Representative complexes of this class have been shown by X-ray diffraction methods to have square pyramidal structures (71) in which the sulfur atoms of the two bidentate ligands comprise the basal plane (Fe—S 2.228 2.30 A) with the halide ion in the apical position (Fe—Cl 2.26-2.28 A).309 310 In the cases examined the metal atom sits 0.6 A out of the mean S4 plane in the direction of the apical halide ion. 

Competing free radical reactions during combustion of halogen (X)-containing material (M). R is the organic residue. 

Under certain conditions, benzene can react with halogens by addition rather than by substitution. In the presence of sunlight, a free-radicaL reaction takes place with chlorine that leads to addition products in which the aromatic character has been lost. The final product is hexa-chlorocyclohexane (benzene hexachloride), which can exist in eight possible stereoisomeric forms. The process starts with the photolytic dissociation of chlorine. Free-radical addition to the 7i-electron system of the aromatic ring follows and a chain reaction ensues (Scheme 9.1). 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

On an industrial scale alkyl halides—chiefly the chlorides because of the cheapness of chlorine—are most often prepared by direct halogenation of hydrocarbons at the high temperatures needed for these free-radical reactions (Secs. 3.19, 6.21, and 12.12-12.13). Even though mixtures containing isomers and compounds of different halogen content are generally obtained, these reactions are useful industrially since often a mixiure can be used as such or separated into its components by distillation. 

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