Tetra-bromobenzene

To elucidate the mechanism of the reaction transhalogenation of 1,2,4,5-tetra-bromobenzene (Br4-Bz) was investigated in detail. Subsequent halogen exchange yields tetrachlorobenzene (CI4-BZ) via the three intermediates B, C and D. 

Bromobenzenes are converted into the corresponding chloro compounds on reaction with aqueous sodium hypochlorite in the presence of tetra-n-buty lammoni um hydrogen sulphate [40]. The reaction is pH dependent. At pH > 10, the bromobenzenes are effectively inert, but over the pH range 7.5-9, conversion occurs into the chlorobenzenes without any side reactions and the reaction appears to be light-induced. At more acidic levels (pH 4-5), bromobenzene is converted quantitatively into chlorobenzene within one hour. No reaction occurs in the absence of the catalyst and yields from light and dark reactions are comparable. Side reactions are observed, however, with substituted bromobenzenes under these low pH conditions. 

An example of the Sn2 rate dependence on the nature of the counterion is given by the reaction of -butyl 4-bromobenzene sulfonate with lithium- and tetra-n-butylammonium halides in the weakly dissociating solvent acetone (cr = 20.6) [279]. 

Cahours (25) found that heating methyl iodide with arsenic gives tetra-methylarsonium iodide as the major product. More recent investigation has shown that other alkyl halides can be used, that copper can act as a catalyst, and that even bromobenzene and vinyl bromide react (26). The method has been developed as a means of preparing bromomethylarsines (27). 

Dibromo-9,9-dioctylfluorene (0.88 g, 1.61 mmol), 2,7-bis(4,4,5,5-tetra-methyl-l,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (1.06 g, 1.64 mmol or 1.03g, 1.61 mmol) and tetrakis(triphenylphosphine)palladium (13.4mg, lmol%) are dissolved in anhydrous toluene (16 mL). A phase-transfer catalyst, Aliquat 336 (65 mg, 0.16 mmol) is added to 2 M aqueous Na2C03 (2.6 mL, 5.2 mmol). The solution is degassed by sparging with nitrogen for 15 min and transferred to the toluene solution via cannula. The reaction mixture is stirred and heated at reflux for 60 h until the reaction mixture becomes viscous. The end group is capped by heating the mixture for 12 additional hours with bromobenzene... 

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