Para-disubstituted benzenes

Chlorine or bromine react with benzene in the presence of carriers, such as ferric halides, aluminum halides, or transition metal halides, to give substitution products such as chlorobenzene or bromobenzene [108-86-17, C H Br occasionally para-disubstitution products are formed. Chlorobenzene [108-90-7] ... 

Disubstituted benzenes are named using one of the prefixes ortho- (o), meta- (in), or para- (p). An ortho-disubstituted benzene has its two substituents in a 1,2 relationship on the ring, a meta-disubstituted benzene has its two substituents in a 1,3 relationship, and a para-disubstituted benzene has its substituents in a 1,4 relationship. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

The para-disubstituted benzene derivative, 13, of Fig. 11 has two groups of equivalent bonds to hydrogen the ones to hydrogens 1 and 4, and those to hydrogens 2 and 3. If only one C—H bond from the first group in this molecule is to be broken, it... 

The appearance of a 4 proton symmetrical pattern in the aromatic region near 5 7.9 and 6.6 ppm is strongly indicative of a para disubstituted benzene ring. This is confirmed by the presence of two quaternary resonances at 5 152 and 119 ppm in the spectrum and two CH resonances at 5 131 and 113 ppm. 

Note that the presence of a para disubstituted benzene ring also accounts for the element of symmetry identified above. The triplet of 3H intensity at approximately 5 1.4 and the quartet of 2H intensity at approximately 5 4.3 have the same spacings of 1.1 mm. On this 100 MHz NMR spectrum, 100 Hz (1 ppm) corresponds to 16.5 mm so the measured splitting of 1.1 mm corresponds to a coupling of 6.7 Hz that is typical of a vicinal coupling constant. The triplet and quartet clearly correspond to an ethyl group and the downfield shift of the CH2 resonance (5 4.3) indicates that it must be attached to a heteroatom so this is possibly an -O-CH2-CH3 group. 

Para-disubstituted benzenes show a very strong peak between 800 and... 

The 5 ring H s of monosubstituted benzenes, C H G, are not equally reactive. Introduction of E into Cf,HjG rarely gives the statistical distribution of 40% ortho, 40% meta, and 20% para disubstituted benzenes. The ring substituent(s) determine(s) (a) the orientation of E (meta or a mixture of ortho and para) and (b) the reactivity of the ring toward substitution. 

The green and violet tetraammines have the same chemical composition, that is, they are isomers and are the only two isomers with this composition. Werner realized that this was possible only if the six ligands were deployed about the cobalt(III) center in an octahedral arrangement (cf. octahedral coordination in solids, Sections 4.3 and 4.4) for example, a flat hexagonal complex Co(NH3)4Cl2+ would have three isomers, like ortho-, meta-, and para-disubstituted benzenes. Werner correctly identified the green compound as the trans isomer (chloro ligands on opposite sides of the octahedron) and the violet as cis (same side), as in Fig. 13.1. 

With respect to unsaturated hydrocarbons we should note that compounds exhibiting one or several double bonds are often called alkenes or olefins. Finally, we need to add a brief note concerning the nomenclature in aromatic systems, particularly, in six-numbered rings such as benzene. Here the terms ortho-, meta-, and ara-substi-tution are often used to express the relative position of two substituents in a given ring system. Identically, we could refer to those isomers as 1,2-(ortho), 1,3-(meta), 1,4-(para) disubstituted compounds (see margin). 

After a paper devoted to GIAO/B3LYP calculations of 13 monosubstituted benzenes and 21 1-substituted pyrazoles [148] that allowed discussion of some structural problems (such as conformation, tautomerism, and structure of salts) we have continued to use this combination of experimental (in solution and in the solid state, CPMAS NMR) and calculated values as a very useful exploratory technique. For instance, we have used II, 13C, and 15NNMR spectroscopy to study compounds 149-154. In para-disubstituted derivatives 149,151,152,153, in the solid state (no free rotation) the signals of the ortho carbons are split (one is close to N2) and, thanks to the calculated values, they can be assigned [149],... 

The amine 74 certainly looks like a product of Birch reduction of the simple para-disubstituted benzene 76 via the enol ether 75. 

The melting points, boiling points, and densities of benzene and some derivatives are given in Table 16-1. Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, so they pack better into crystals and have higher melting points. For example, benzene melts at 6 °C, while hexane melts at -95 °C. Similarly, para-disubstituted benzenes are more symmetrical than the ortho and meta isomers, and they pack better into crystals and have higher melting points. 

We have already seen that identical protons do not couple with each other. The three protons in a methyl group may couple to some other protons, but never couple with each other. They are an A3 system. Identical neighbours do not couple either. In the para-disubstituted benzenes we saw on pp. 253-254, all the protons on the aromatic rings were singlets. 

We shall end this section with a final example illustrating para-disubstituted benzenes and roofing as well as an ABX system and an isopropyl group. 

Electron-donating substituents activate the benzene ring to electrophilic attack, which results in the formation of the ortho- and para-disubstituted benzene derivatives. 

Although benzene itself absorbs at 7.3 ppm in its H NMR spectrum, the protons on substituted benzenes absorb either upfield or downfield from this value, depending on the substituent. Explain the observed values for the para disubstituted benzene derivatives X and Y. Then explain why p-difluorobenzene shows a single peak in its NMR spectrum at 7.00 ppm. 

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