Bromination mechanism

The objective of this chapter is not to repeat the reviews of others (Freeman, 1975 De la Mare and Bolton, 1982 Schmid, 1989 Ruasse, 1990) of the large body of relevant data, but to analyse the present status of the bromination mechanism (Scheme 2) and how it depends on the substituents and on the... 

In what follows, we present first the experimental conditions under which it is possible to obtain data relevant to the study of the bromination mechanism, and the present evidence for the occurrence of charge transfer complexes and cationic intermediates on the reaction pathway. Once these... 

The bromination of ethylenic compounds is in most cases a very fast reaction. Half-lives of typical olefins are given in Table 1. Most of them are very short. In order to obtain extended and meaningful kinetic data, it has been necessary to find suitable reaction conditions and to design specific kinetic techniques. This was not done until 1960-1970. As a consequence, kinetic approaches to the bromination mechanism are relatively recent as compared with those to solvolytic reactions, for example. 

In addition to the fact that steric crowding can slow the reaction by hindering bromine approach to the double bond, it appears now that bulky substituents can modify the bromination mechanism by inhibiting nucleophilic solvent assistance to ionization of the CTC and/or nucleophilic trapping of the ionic intermediates. Assistance to the rate-limiting ionization step by... 

Kinetic solvent isotope effect as a measure of electrophilic assistance to bromide ion departure limiting values rate data in ethanol, methanol and their aqueous mixtures using Bentley s TBr scale its decrease corresponds to the involvement of nucleophilic assistance. R = (/caqhtOII//cAcoH)r as a measure of nucleophilic solvent assistance. Model for a limiting bromination mechanism. Ruasse et al. (1991). /Ruasse and Zhang (1984). 9Argile and Ruasse (to be published). Modro et al. (1979). 

In the bromination mechanism different reaction pathways may compete68, as shown in Scheme 18. 

The regiochemistry of the bromination (Scheme 20) of a,/l-unsaturated ketones101 (in methanol) by bromine is affected by the presence of A-bromosuccinimide, which also depresses the overall rate of bromination. A possible explanation of the observed behaviour is the presence of a complex (50) involving the C=C double bond of olefin, TV-bromosuccinimide and bromine, as shown in Scheme 21. Probably, NBS acts as a hydrogen bromide scavenger, thereby causing a change in the bromination mechanism from an acid-catalysed pathway to a bromonium ion mechanism. 

Jensen and Rickbom (2) discussed the possibility of transition states in which the electrophilic species was partially bound to the o- orbital of carbon-mercury bonds. Thus the fourth "center of the four-center bromination mechanism was pictured as the carbon-mercury bond itself, or more specifically, the carbon sps orbital (2). 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

Methane, bromination mechanism, 60 Methide shift, 94 Methylene in synthesis, 67 Methyl salicylate, 440 Michael addition, 385, 459 Microscopic reversibility, 98 Migratory aptitude, 293 Molecular orbital, 14 Molecularity, 40 Molecules, geometry of, 18 polar, 27... 

Chlorination of benzene occurs by the same mechanism as bromination. mechanism for chlorination... 

SCHEME 2. Bromination mechanism invoiving an enoi intermediate. 

An understanding of the detailed mechanism of each step is not vital for explaining the specific features of oscillations. However, in certain suggested classroom experiments, attention has been focused on the bromination mechanism of methylene group of malonic acid, which proceeds by enolization mechanism. 

This reaction proceeds via a radical bromination mechanism and two radicals are possible A (2" and benzylic) and B (E). Since B (which leads to QH5CH2CH2Br) is much less stable, this radical is not formed so only C6H5CH(Br)CH3 is formed as product. 

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