Halogen substitution reactions

Alkyl and aryl derivatives of poly(dichlorophosphazene) are not efficiently synthesized by nucleophilic reaction of LXXXIV with metal alkyls or aryls. The halogen substitution reaction occurs but is accompanied by polymer chain cleavage. Use of poly(difluorophosphazene) or introduction of aryl and alkyl groups at the monomer stage offer some improvement, but neither method is fully satisfactory. The best route to alkyl and aryl derivatives is polymerization of A-(trimethylsilyl)-/).P-dialkyl-.P-halophosphoranimines at moderate temperatures (25-60°C) in the presence of a Lewis acid [Allcock et al., 1996, 2000, 2001a,b Neilson and Wisian-Neilson, 1988]. The reaction proceeds as a cationic chain polymerization ... 

Alkyl Aryl-Halogen Substitution. Reactions of (NPCla)a with MeLi are supposed to proceed via methyl-halogen substitution and not via metal-halogen exchange, thus giving rise to extensive ring cleavage. An important factor responsible for the low yields of reactions of (NPXa)s (X = Cl, F) with alkyl-lithium compounds... 

A halogen substitution reaction was performed according to ISSO3F- + Br2 - > IBrJS03F-. ... 

In halogen substitution reactions using free halogen, only half of this goes into the product, the remainder appearing as the halogen acid ... 

The role of crown ethers and their analogues as catalysts, co-catalysts, and substrates in reactions involving ions and ion pairs has been reviewed in Polish. States of solvation or association affect the reactivity of ionic reagents in spectacular ways. A series on mechanoactivated halogen-substitution reactions of alkyl halides has continued in a study of halogen substitution in ethyl bromide on the surface of mechanically... 

Conditions have been worked out for the synthesis of methyl polyfluoroalkyl sulphides and 2-bromo-l,l-difluoroethyI phenyl sulphide by halogen-substitution reactions involving sodium thiolates. / -Nitrothiophenol reacts with 1,2-dichlorohexafluorocyclopentene to give the l,2-bis(p-nitrophenylthio)-analogue. Aryl alkyl tellurides PhTeR and bis(phenyltelluro)methane PhTeCHgTePh can be obtained from PhTeLi and an alkyl halide, or di-iodomethane, respectively. 

CF3CO2H. Colourless liquid, b.p. 72-5 C, fumes in air. Trifluoroacetic acid is the most important halogen-substituted acetic acid. It is a very strong acid (pK = o y) and used extensively for acid catalysed reactions, especially ester cleavage in peptide synthesis. 

Nucleophilic substitution reactions of alkyl halides are related to elimination reactions m that the halogen acts as a leaving group on carbon and is lost as an anion The... 

Unlike elimination and nucleophilic substitution reactions foimation of oigano lithium compounds does not require that the halogen be bonded to sp hybndized carbon Compounds such as vinyl halides and aiyl halides m which the halogen is bonded to sp hybndized carbon react m the same way as alkyl halides but at somewhat slowei rates... 

Reactions. The CF O— group exerts predominant para orientation in electrophilic substitution reactions such as nitration, halogenation, acylation, and alkylation (350). 

Many of the common electrophilic aromatic substitution reactions can be conducted on indole. CompHcations normally arise either because of excessive reactivity or the relative instabiUty of the substitution product. This is the case with halogenation. 

Conjugate addition to a P-halogen substituted methaciylate results in an addition—elimination reaction which regenerates the tZ,/ -unsaturated moiety (19-21). 

For reaction with hydrogen haUdes, the substitution reaction with haUde ion easily occurs when a cuprous or cupric compound is used as the catalyst (23) and yields a halogenated aHyl compound. With a cuprous compound as the catalyst at 18 °C, the reaction is completed in 6 h. Zinc chloride is also a good catalyst (24), but a by-product, diaHyl ether, is formed. 

Control of addition vs substitution by free radicals can be effected by the reaction conditions, ie, radical concentration, temperature, and phase. Using halogens as propylene reactants, high temperatures and the gas phase favor high radical concentrations and substitution reactions cold, Hquid-phase conditions favor addition reactions. 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

In the case of substituted phenazine fV-oxides some activation of substituents towards nucleophilic substitution is observed. 1-Chlorophenazine is usually very resistant to nucleophilic displacements, but the 2-isomer is more reactive and the halogen may be displaced with a number of nucleophiles. 1-Chlorophenazine 5-oxide (56), however, is comparable in its reactivity with 2-chlorophenazine and the chlorine atom is readily displaced in nucleophilic substitution reactions. 2-Chlorophenazine 5,10-dioxide (57) and 2-chlorophenazine 5-oxide both show enhanced reactivity relative to 2-chlorophenazine itself. On the basis of these observations, similar activation of 5- or 6-haloquinoxaline fV-oxides should be observed but little information is available at the present time. 

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