Para-Dibromobenzene

The most convenient method of preparing the flexible (low Tg) system is to employ the Ullmann ether reaction of dibromobenzene and aromatic bis-diols followed by catalytic replacement of the bromine atoms by terminal acetylene groups. A host of commercially available bis-diols have been used in the synthesis with both meta and para dibromobenzene. Low Tg arylether oligomers have been prepared containing sulfone, sulfide, carbonyl, isopropyl and perfluoroisopropyl groups in the backbone (9). 

PPP was first synthesized by Goldschmiedt (1886). He applied the Wurtz-Fittig reaction which coupled para-dibromobenzene using sodium, and the product was determined to be a tridecamer (13 subunits) by elemental analysis. In 1936, the same monomer was used and a PPP containing up to 16 benzene rings was obtained (Busch et al. 1936). 

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

Electrophilic Bromination of Bromobenzene Results in ortho- and para-Dibromobenzene... 

An innovative recent synthetic development is the use of electroreduction of various substrates in the presence of CO2. Electrocarboxylations have been accomplished on dibromobenzenes [167] and anthrone [168]. When meta- and para-dibromobenzenes were reduced on glassy carbon and silver electrodes in DMF, in the presence of CO2, ortho-, meta-, and para mono-carboxylation products of methyl bromobenzoate were obtained, while orffeo-dibromobenzene was converted into a dehalogenated carboxylation product of methyl benzoate. Moderate to good electrocarboxylation yields... 

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 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. 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

Brandsma and coworkers were able to show that meta- (59a) and para-dilithiated benzene (59b) are accessible via iodine-lithium exchange, using n-butyllithium. Therefore, 1,3- and 1,4-diiodobenzene (58a and 58b) respectively were reacted with two equivalents of n-butyllithium in diethyl ether at low temperatures (Scheme 21). The polyUthiation was confirmed by trapping reactions with dimethyl disulphide. The same reactions, starting from the corresponding dibromobenzenes, had led only to monolithiated benzenes. 

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 prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote the relative positions of the two substituted groups, as shown above for the dibromobenzenes. Compounds in which the two substituted groups are different are named accordingly. Thus,... 

SOLUTION TO 23a A bromo substituent is an ortho-para director, so halogenation cannot be used to introduce both bromo substituents of m-dibromobenzene. Knowing that a bromo substituent can be placed on a benzene ring with a Sandmeyer reaction and that the bromo substituent in a Sandmeyer reaction replaces what originally was a meta-directing nitro substituent, we have a route to the synthesis of the target compound. 

Two mononitroxylenes can be derived, theoretically, from orthoxylene, three from metaxylene, and but one from paraxylene. It follows, therefore, that the xylene boiling at 142°, from which two mononitro derivatives can be prepared, is orthoxylene, and that the ones boiling at 139° and 138° have the meta and para structure, respectively. The application of the so-called absolute method leads to the establishment of the structure of the three xylenes, which can serve, therefore, as reference-compounds. The three hydrocarbons yield three dibasic acids on oxidation orthoxylene gives phthalic acid, metaxylene gives isophthalic acid, and paraxylene gives terephthalic acid. The three phthalic acids become, thus, reference-compounds. Other compounds can be readily converted into xylenes or phthalic acids, and can, therefore, be used in the determination of structure. Thus, the three dibromobenzenes can be converted into the three xylenes by Fittig s synthesis. The transformations establish the structure of the halogen derivatives. From these other compounds may be prepared. An example will be instructive. The dibromobenzene which melts at 89° is converted into paraxylene when treated with methyl iodide and sodium —... 

As terephthalic acid, which has been shown to be a para-compound, is formed, the assumption that the ethylbenzoic acid belongs to this series is justified. The transformations which lead to the view of the structure of dibromobenzene, diethyl benzene, ethylbenzoic acid, and terephthalic acid are indicated by the following formulas. The conclusions rest upon the determination of the structure of paraxylene by the absolute method —... 

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. 

Selective monoamination of ortho-, para-, and weta-dibromobenzenes 33 with the polyamine 34 took place using DPPF (XI-1) as a ligand to give 35 in a reasonable yield even in the presence of an excess of the polyamine without giving the diamination product. In addition, selective monoarylation of the primary amine moiety in the polyamine 34 was observed [33],... 

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