Reaction pathways, halogenated

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Quinones may react with carbon-centered radicals by addition at oxygen or carbon, or by electron transfer (Scheme 5.]6).l74, fi2 195 201 202 The preferred reaction pathway depends both on the attacking radical and the particular quinone (halogenated quinones react preferentially by electron transfer). The radical formed may then scavenge another radical. There is also evidence that certain quinones e.g. chloranil, benzoquinone (38)] may copolymerize under some conditions. ... 

First of all, the reaction pathways shown in Scheme 1 involve the formation of charge transfer complexes (CTC) between olefin and Br2- The formation of molecular complexes during olefin bromination had been hypothesized often (ref. 2), but until 1985, when we published a work on this subject (ref. 3), complexes of this type had been observed only in a very limited number of circumstances, all of which have in common a highly reduced reactivity of the olefm-halogen system, i.e. strongly deactivated olefins (ref. 4), or completely apolar solvents (ref. 5) or very low temperatures (ref 6). 

The formation of halogenation products from Grignard reagents and sulfonic acid anhydrides is the result of an oxidative reaction pathway . This side-reaction can be reduced by using sulfonic acid esters, however, in these cases alkylations as well as twofold sulfonylations (cf. corresponding results with sulfonyl fluorides ) are competing (equations 64 and 65). 

Nucleophilic substitutions are in many cases facile processes in heterocyclic chemistry. Also, in the area of the present chapter, many such routine transformations have been carried out. Such transformations are summarized in Table 6, where the structures of the starting compounds, products, the reagents, yields, and references are listed. These include reactions of halogen, methoxy, and methylsulfanyl derivatives with amines or alkoxides. One exceptional case (Table 6, entry 9) should be pointed out this exchange reaction, unlike the others in this table, proceeds via an elimination-addition mechanism. A few related transformations that follow more complicated pathways and therefore could not be classified unambiguously into this table, can be found in Table 7 in Section 11.17.5.6.5. 

Scheme 10.9. Anti-S l Allylation reaction with competing Srg2 reaction pathway. X = halogen, OAc, OP(0)Y2.

Oxidizers may not themselves be combustible, but they may provide reaction pathways to accelerate the oxidation of other combustible materials. Combustible solids and liquids should be segregated from oxidizers. Certain oxidizers undergo dangerous reactions with specific noncombustible materials. Some oxidizers, such as calcium hypochlorite, decompose upon heating or contamination and self-react with violent heat output. Oxidizers include nitrates, nitric acid, nitrites, inorganic peroxides, chlorates, chlorites, dichromates, hypochlorites, perchlorates, permanganates, persulfates and the halogens. 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

The complex is a brick red to brown, highly insoluble solid believed to have a polymeric, halogen-bridged structure.10 The principle reaction pathways of this complex involve chloride-bridge cleavage leading to a range of ruthenium(II) products.11... 

Bromoethylamine may undergo two reaction pathways in its modification of sulfhydryl groups in proteins (Fig. 84). In the first scheme, the thiolate anion of cysteine attacks the No. 2 carbon of 2-bromoethylamine to release the halogen and form a thioether bond (Lindley, 1956). This straightforward reaction mechanism is similar to the modification of sulfhydryls with iodoacetate (Section 4.2). In a two-step, secondary... 

When mechanisms involving radicals can be discounted, the main and general reaction pathway of addition of halogens to alkenes is shown in Scheme 10, where X is mainly Cl and Br69-70. 

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