Bromine , halogenation reactions

Halogen donors are chemicals that release active chlorine or bromine when dissolved in water. After release, the halogen reaction is similar to that of chlorine or bromide from other sources. SoHd halogen donors commonly used in cooling water systems include l-bromo-3-chloro-5,5-dimethyIhydantoin, l,3-dichloro-5,5-dimethyIhydantoin, and sodium dichloroisocyanurate. 

Halobutyls. Chloro- and bromobutyls are commercially the most important butyl mbber derivatives. The halogenation reaction is carried out in hydrocarbon solution using elemental chlorine or bromine (equimolar ratio with enchained isoprene). The halogenation is fast, and proceeds mainly by an ionic mechanism. The stmctures that may form include the following ... 

The presence of an TV-oxide group activates the 1,2,4-triazine ring toward electrophilic attack, for instance, in halogenation reactions. Thus, 3-methoxy- and 3-amino(alkylamino)-1,2,4-triazine 1-oxides 16 react easily with chlorine or bromine to form the corresponding 6-halo-1,2,4-triazine 1-oxides 17 (77JOC3498, 78JOC2514). 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

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... 

It reacts violently with halogens it combusts in fluorine, chlorine and gaseous bromine. With liquid bromine the reaction is more violent and can lead to a detonation as with iodine. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

On the other hand, a proton directly bound to the 1,2,4-triazine ring can be replaced by halogen. This is an electrophilic substitution reaction at carbon, and, as expected for such a heavily aza-substituted ring system, it needs considerable activation by electron-donating substituents. The halogenation reaction is best known in bromination at the 6-position. From the published data it seems that either an amino group in the 3-position or an oxo group in the 5-position is necessary. The formation of 6-halo-substituted compounds has been reported for l,2,4-triazin-5-ones, l,2,4-triazine-3,5-diones, 3-amino-l,2,4-triazines and 3-amino-l,2,4-triazin-5-ones. 

In view of the importance of preparing well characterized, halogen terminated polymers, there is an obvious need for a careful examination of the direct halogenation reaction. Optimization of the halogenation reaction, however, may not be straightforward, since it has been observed by ESR spectroscopy that radicals are formed in the reaction of simple alkyllithiums with bromine or iodine in the presence of equimolar amounts of Lewis bases such as N,N,N ,N -tetramethylethylenediamine or ether 328),... 

Bromine follows reaction (i) with 1,1-dimethyl-2,5-diphenylsilole. If one equivalent of bromine is used, a mixture of dibromocyclopentenes in which the two halogen atoms are in the trans position is obtained (equation 47). 

The correlations for the extensively studied halogenation reactions, bromination, and chlorination are shown in Figs. 11 and 12, respectively. The excellence of these correlations extending over reactivity factors of over 1012 for bromination and 1011 for chlorination is a good test of the application of the cr+-constants to substitution reactions. Several of the values for log ( / H) or cr+ are uncertain. This uncertainty... 

In the field of asymmetric organocatalytic alkylation (see also Section 3.1) impressive examples with enantioselectivity > 99% ee have been reported by the Corey group, the Park and Jew group, and the Maruoka group [15-17]. Different types of catalyst have been used, with amounts of catalyst in the range 0.2 to 10 mol%. High enantioselectivity (99%) has also been achieved for asymmetric halogenation reactions (see also Section 3.4). This has been demonstrated for chlorination and bromination reactions by Lectka and co-workers [18]. 

The reactions described below refer to halogenation in aqueous solution under conditions where the halogenating agent is the molecular halogen with a possible contribution from the trihalide ion. Almost all the work comes from the research groups of Bell and Dubois. The majority of the studies refer to bromination, but comparisons are made with the other halogenation reactions where the results are available. 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

The main electrophilic reagents concerned in halogenation reactions are the neutral molecules X2 (or XY), positively charged species, such as X+ or XOH2+, and the hypohalous acids,68 HOX. Since different mechanisms involving different electrophilic species can operate in the same system, a subdivision of this section based on the nature of the halogen (chlorination, bromination, iodination) has been preferred to that based on the type of electrophilic species. 

Heat or light is usually needed to initiate this halogenation. Reactions of alkanes with chlorine and bromine proceed at moderate rates and are easily controlled. Reactions with fluorine are often too fast to control, however. Iodine reacts very slowly or not at all. We will discuss the halogenation of alkanes in Chapter 4. 

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