Halogenation bromination reaction mechanism

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. " Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), " and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step " " " instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide. " ... 

If. instead of CC14. water is used as the solvent in a halogen addition reaction, a halohydrin is formed. The student proposed that such a reaction would follow Mechanism A with water replacing bromine as the nucleophile. If this hypothesis is correct, which of the following is the most likely product for the addition of bromine to propene in water ... 

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. 

NCS, NBS and NIS are the best reagents for halogenation of metal / -diketonates , the rate of bromination being much faster than that of iodination and chlorination. Two reaction mechanisms have been proposed to explain the halogenation an ionic mechanism, which is operative in polar solvents like chloroform, methanol and acetic acid, and a free radical one occnrring in non-polar solvents like carbon tetrachloride . 

Step (3), attack of methyl radicals on halogen, is exothermic for all four halogens, and for chlorine, bromine, and iodine it has very nearly the same A/f. For these reactions. Fact could be very small, and does indeed seem to be so probably only a fraction of a kcal. Even iodine has been found to react readily with methyl radicals generated in another way, e.g., by the heating of tetramethyllead. In fact, iodine is sometimes employed as a free-radical trap or scavenger in the study of reaction mechanisms. The third step, then, cannot be the cause of the observed relative reactivities. 

Many classical reactions have been recognized to proceed according to this halogen-ATC catalysis mechanism examples are the dehydroiodination of secondary iodides by Pt° [10], the bromination of [Mn2(CO)io], and the exchange of carbonyls by phosphines or isonitriles [50]. 

Halogenation of Lactams. Selective and high yielding iodination and bromination of lactams occurs with Iodine or Bromine, respectively, in the presence of TMS-I and a tertiary amine base (eq 24). The proposed reaction mechanism involves intermediacy of the silyl imino ether. 

Quinolines and isoquinolines can also react with electrophiles at the pyridine side. This can be rationalized by a different reaction mechanism involving the prior introduction of a nucleophile in the heterocyclic quinoline/isoquinoline ring followed by an electrophilic substitution involving attack on the intermediate enamine. Notable is the electrophilic bromination of isoquinoline hydrobromide in a solvent like nitrobenzene that provides 81% yield of 4-bromoisoquinoline, in contrast to the bromination or chlorination of an isoquinoline-aluminum chloride complex that affords 78% of 5-bromoisoquinoline. Exhaustive bromination or chlorination under Lewis acid conditions usually yields mixtures of 5,8-halogenated isoquinolines along with 5,7,8-trisubstituted derivatives. ... 

Following the F -F H2O and Cl 4- H2O reactions, the Br 4- H2O reaction would appear to come next in the quest to understand the reaction mechanisms of the halogen atom-water systems, X 4- H2O HX 4- HO (X = F, Cl, Br) reactions. The Br 4- H2O reaction has been widely studied because Br atoms destroy ozone in the atmosphere [1,2] and because some brominated compounds play important roles in combustion chemistry acting as lire retardants [3]. 

So, a modification of our earlier addition mechanism suffices to explain the reaction. In contrast to the addition to aikenes we saw in Chapter 9 (HX, carbocations, and boranes), in alkene halogenation (bromination and chlorination) a three-membered ring is produced as an intermediate. In some, very spedalized cases the bromonium ion can even be isolated, and it seems its intermediacy in most brominations is assured. 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane 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 hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

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