Dibromobenzene meta isomer

The variation in the initial Tg s show the effect on processability of the structures of the diol and the dibromobenzene. Two of the systems, those based on thiodiphenol and dihydroxybiphenyl, gave crystalline products which could not be made amorphous upon heat treatment. The effect on chain flexibility of dibromobenzene structure could be seen with the two hydroquinone systems. In this case, the meta isomer gave an initial Tg of 12°C while the more rigid para system had a Tg of 59°C. Variation of diol structure shows a parallel effect as shown with the resorcinol/j>-dibromobenzene system. This material has a softening temperature of 20°C. 

Among benzene derivatives, halogen-substituted compounds have been extensively studied and in the structure-reactivity studies " carried out on the reaction of OH and SO with the ortho and meta isomers of dichloro and dibromobenzenes and mono-bromotoluenes, the formation of substituted hydroxycyclohexadienyl radical was observed to be the major reaction channel. The bimolecular rate constants obtained for the reaction of OH with substituted halobenzenes are in the range (1.7 to 9.3) x 10 dm mol s. The rate constants obtained are found to follow the Hammett relationship for the reaction of OH with substituted halobenzenes and the p was found to be -0.5, indicating that OH radicals react by addition to the benzene ting. 

The spectra of the intermediate transients formed in the reaction of OH with dichloro- and dibromo-benzenes and chloro- and bromo-toluenes exhibited absorption maxima around 325-330 nm with both ortho- and w to-isomers of dichlorobenzenes, dibromobenzenes, and bromotoluenes (Fig. 6). The transients were assigned to the isomeric OH adducts formed from the addition of OH to the benzene ring. A blue shift was observed in the absorption maximum of -bromotoluene (315 nm) when compared to its ortho- and meta-isomers which have maxima at 330 nm. Such a behavior was also seen in the absorption spectra of OH adducts of 0- and w-chlorotoluenes (325-330 nm) and w-xylenes (326-328 nm) as compared to their... 

The physical constants listed in Table 25.1 illustrate very well a point previously made (Sec. 12.3) about the boiling points and melting points of ortho, meta, and para isomers. The isomeric dihalobenzenes, for example, have very nearly the same boiling points between 173° and 180° for the dichlorobenzenes, 217° to 221° for the dibromobenzenes, and 285° to 287° for the diiodobenzenes. Yet the melting points of these same compounds show a considerable spread in each case, the para isomer has a melting point that is some 70-100 degrees higher than the ortho or meta isomer. The physical constants of the halotoluenes show a similar relationship. 

The bottle of dibromobenzene with the melting point of 87° is the para-isomer the bottle with the melting point of 6° is the ortho-isomer the bottle with the melting point of -7° is the meta-isomer. 

The second column in the table indicates whether the dibromobenzene used was the meta or the para isomer. In order to provide more flexibility for the rigid diols,... 

Explain how NMR spectroscopy could be used to distinguish the ortho-, meta-, and PRACTICE PROBLEM 14.14 /(tw-dibromobenzene isomers one from another. 

The isomer distribution for 5-ArDMU s was analogous to those typically observed in aromatic photosubstitution reactions. On the other hand, few reports have appeared on the meta-oriented aromatic photosubstitution reaction, the primary example being the photoinduced proton exchange in benzenes that proceeds via an electrophilic aromatic photosubstitution reaction. In the case of either the 5-chloro-or 5-fluoro-l,3-dimethyluracil (5-ClDMU, 5-FDMU), > addition of TFA simply promoted 5-arylation (photo-Friedel-Crafts reaction), while the addition of a bromide (p-dibromobenzene, 1,2-dibromoet-hane, benzyl bromide, HBr) to the solution induced the meta-oriented 6-arylation (Scheme 2)."... 

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