Dibromobenzene para 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,... 

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. 

Since there is only one possible isomeric product, this dibromobenzene is the para isomer. 

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 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. 

Explain how NMR spectroscopy could be used to distinguish the ortho-, meta-, and para-dibromobenzene isomers one from another. 

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