Electrophilic Bromination of Benzene

The first step in electrophilic bromination of benzene involves addition of Br, leading to an intermediate bromobenzenium ion. This is then rapidly followed by loss of a proton to give bromobenzene. 

Electrostatic potential map for bromobenzenium ion shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

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Figure 16.3 An energy diagram for the electrophilic bromination of benzene. The overall process is exergonic.

Interestingly, Demets et al. prepared bromine- and iodine-CB[6] complexes for use as nanoreactors [157]. These complexes are quite different from the two transition-metal complexes desalbed previously. Instead of a metal cation coordinated to one of the portals, in this case a Br2 or I2 molecule is included within the cavity, and is in place to catalyze or react with other molecules. The authors chose reactions that were known to be catalyzed by or involve as a reagent one of these halogen molecules. For example, Br2-CB[6] was effectively applied to the electrophilic bromination of benzene. Such a nanoreactor which is preloaded with a catalyst or reagent could have signihcant and interesting potential applications. 

Electrophilic Bromination of Benzene in the Gas Phase and in Carbon Tetrachloride Solvent... 

A quick calculation confirms that electrophilic bromination of benzene is exothermic. A phenyl-hydrogen bond (approximately 112 kcal mol Table 15-1) and a bromine molecule (46 kcal moF ) are lost in the process. Counterbalancing this loss is the formation of a phenyl-bromine bond DH° = 81 kcal moF ) and an H-Br bond (DH° = 87.5 kcal moF ). Thus, the overall reaction is exothermic by 158 — 168.5 = —10.5 kcal moF (43.9 kJ moF ). 

Another application of diazotization in synthetic strategy is illustrated in the synthesis of 1,3-dibromobenzene (m-dibromobenzene). Direct electrophilic bromination of benzene is not feasible for this purpose after the first bromine has been introduced, the second will attack ortho or para. What is required is a meta-directing substituent that can be transformed eventually into bromine. The nitro group is such a substituent. Double nitration of benzene furnishes 1,3-dinitrobenzene (m-dinitrobenzene). Reduction (Section 16-5) leads to the ben-zenediamine, which is then converted into the dihalo derivative. 

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