Ortho-bromination

Dichloromethane (DC 9.10) is one of the most suitable solvent for ortho-bromination because of its easy handling, and high solvent power for NBS. 

However it is almost impossibly difficult to prepare a precursor diphenyldiazo-methane bearing four ortho CF3 groups. The precursor we were able to prepare is diphenyldiazomethane (1051), which has two ortho bromine groups in addition to two CF3 groups (Scheme 9.36). 

A-l. Write the three most stable resonance contributors to the cyclohexadienyl cation found in the ortho bromination of toluene. 

For chlorination, the formation of the (7-complex would be expected to be the rate-determining step since the aromatic chlorination of other substrates does not appear to give rise to deuterium isotope effects (Baciocchi et al., 1960 de la Mare and Lomas, 1967). The effect of the experimental conditions on the rate-determining step in the aqueous bromination of aromatic amines has been investigated in detail by Dubois and his co-workers (Dubois et al., 1968a, b, c Dubois and Uzan, 1968). This work suggests that, for tertiary aromatic amines, the proton loss is fast for para-bromination but partly or wholly rate-determining for ortho-bromination. There is however some... 

The para halogen atoms are less labile than those in the ortho positions about 20% of the molecules that yield diazo oxides lose an atom of bromine from the para position, whereas approximately 80% lose an ortho bromine.9 The more highly halogenated diazonium salts are more likely to undergo hydrolytic cleavage.109... 

The major utility of bromamines is in the selective ortho- bromination of phenols755. The high selectivity of N, A-dibromo-/ butylamine has been neatly demonstrated by Desmurs and colleagues756, who have selectively brominated eugenol at the 6 position without affecting the double bond (equation 101). 

Regioselectivity of aromatic bromination of co-phenylpolyoxaalkanols in aqueous solutions of SDS or CTAB are related to the concentration of the surfactant. At equal detergent/substrate ratio para bromination is preferred, and at higher surfactant concentration ortho bromination is promoted. These results are attributed to the orientation of the substrates in the micellar microenvironment which is also supported by -NMR studies1018. 

Earlier studies on photodegradation using lower brominated PBB congeners (TeBB and lower), reported a preferential loss of ortho bromines [47, 48]. On photolysis of Fire Master BP-6 (solvent cyclohexane), no preferential loss of or-... 

Brominated compounds, where the bromine is in the meta position to the phenolic hydroxyl, have been shown to be more hydrolytically and thermally stable than ortho-brominated compounds such as TBBA (4). These stable bromine compounds can be incorporated in the CEN molecule and formulated into a molding compound. These molding compounds provide increased device reliability without sacrificing any of the beneficial properties of present molding compounds. 

Photolysis of Related HaloaromatIcs. Many other classes of hazardous materials may be susceptible to In situ photochemical treatment. Polybromoblphenyls (PBB) photodecompose more readily than PCB due to the weaker C-Br bonds. The major reaction Is reductive debromlnatlon but quaterphenyls also are formed In low yield (30). Ortho-bromines are cleaved preferentially for pure Isomers but apparently not In commercial PBB mixtures (29). 

The effect is most pronoimced for the 2-Br analog. This could be due to a slight rotation of the phenyl ring out of the plane of pi-electron coiyugation caused by the steric hindrance of the bulky ortho bromine atom. 

The isotope effect in the bromination of 7V,iV-dimethylaniline in aqueous acid implies dissimilar mechanisms for the ortho and the para substitution. The rate-determining step could well involve, among other factors, fission or lengthening of the C—H bond in ortho bromination, but this is unlikely in para bromination. The di-methylamino group and bromine are believed to form an intermediate complex in these reactions, which appears to decrease the velocity of substitution with additional electrophile. Recent quantitative measurements, in electrophilic bromination , have reached the conclusion that the NMe2 group activates the para position of the ring ten times more than the ortho position. 

Phenyl-l,2,3-triazoles can be ortho-chlorinated in good yields by treatment of the heterocycle with tetrachloropalladate to give the cyclopal-ladated complex, followed by reaction with chlorine (Allison et al, 1975). Ortho-bromination of a cyclopalladated dithiocarbamate complex has also been described, but the yield was not given (Cornock et al., 1976). 

---