Aromatic Compounds—Substituted Benzene Rings

In the sections that follow, we will attempt to cover some of the most important types of benzene ring substitution. In many cases, it will be necessary to examine sample spectra taken at both 60 and 300 MHz. Many benzenoid rings show second-order splittings at 60 MHz but are essentially first order at 300 MHz or higher field. 

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FIGURE 7.59 The aromatic ring portions of the H NMR spectrum of ethylbenzene at (a) 60 MHz 

At 60 MHz, this chemical shift difference results in a complicated second-order splitting pattern for anisole (methoxybenzene), but the protons do fall clearly into two groups, the ortfio/para protons and the meta protons. The 60-MHz NMR spectrum of the aromatic portion of anisole (Fig. 7.60) has a complex multiplet for the o, p protons (integrating for three protons) that is upfield from the meta protons (integrating for two protons), with a clear separation between the two types. Aniline (aminobenzene) provides a similar spectrum, also with a 3 2 split, owing to the electron-releasing effect of the amino group. 

In two earlier seetions, we introduced the speetra of some other commonly encountered types of systems. The NMR speetra of both styrene oxide (Fig. 5.10) and vinyl acetate (Fig. 5.17) contain examples of AMX systems, where M, being in the middle of the alphabet, indicates a chemical shift intermediate between A and X. That is, aU three ehemical shifts (A, M, and X) are separated widely. trans-Cmnarmc acid (Fig. 5.16) is an example of an AX system. 

NMR spectram. Later you will see that some positions are affected more strongly than others in systems with substimtion patterns different from this one. Table 6.3 in Appendix 6 enables us to make rough estimates of some of these chemical shifts. 

FIGURE 5.53 Allyloxyanisole. (a) Expansion of Ha. (b) Peak positions (Hz) and selected frequency differences, (c) Splitting tree diagram showing the origin of the splitting pattern. 

286 Nuclear Magnetic Resonance Spectroscopy Part Three Spin-Spin Coupling 

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Catalytic hydrogenations over CojfCOjg (using Hj and CO) or with stoichiometric quantities of preformed hydridocarbonyl complex CoH(CO)4 are useful for the partial selective reductions of polycyclic aromatic compounds. Isolated benzene rings are not affected. Naphthalene is reduced to tetralin, at 200°C under a pressure of 20 X 10 kPa and anthracene to 9,10-dihydroanthracene (99%). The substituted phenanthrene nucleus is stable under these conditions as illustrated by hydrogenation of perylene 1 and pyrene 2. ... 

Aromatic compounds are benzene and its derivatives and compounds that resemble benzene in their behaviour in a chemistry dominated by ionic substitution. Benzene has the formula commonly written as the ring ... 

Moving on to multisubstituted aromatic systems, the real value of Table 5.4 soon becomes apparent. In dealing with such systems, it will not be long before you encounter a 1,4 di-substituted benzene ring. This substitution pattern (along with the 1,2 symmetrically di-substituted systems) gives rise to an NMR phenomenon that merits some explanation - that of chemical and magnetic equivalence and the difference between them. Consider the 1,4 di-substituted aromatic compound shown in Structure 5.1. 

Competitive consecutive reactions are combinations of parallel and series reactions that include processes such as multiple halogenation and nitration reactions. For example, when a nitrating mixture of HN03 and H2S04 acts on an aromatic compound like benzene, N02 groups substitute for hydrogen atoms in the ring to form mono-, di-, and tri-substituted nitro compounds. 

The classes of compounds that must contain an aromatic group include fused ring compounds, substituted benzenes and phenols. 

A functional group is a set of atoms that occurs in a wide range of compounds and confers upon them a common kind of reactivity (see Table 3.2). Phenols are generally represented by Ar-OH, in which Ar- represents an aromatic skeleton, composed of benzene rings or substituted benzene rings. Enols are molecules in which the -OH group is linked to an atom that is also engaged in a double bond. 

HO -Addition to Monocyclic Aromatic Compounds and Biphenyls. The rate constants for HO addition to aromatic rings are simply calculated by using the rate constant for the unsubstituted compound and the Hammett-cr+ constants. Thus, kHO, (Ar) for addition to a substituted benzene ring can be estimated by ... 

Ansamycins are a class of macrocyclic compounds in which non-adjacent positions on an aromatic ring system are spanned by the long aliphatic bridge (Latin ansa = handle). The aromatic portion may be a substituted naphthalene or naphthaquinone, or alternatively a substituted benzene ring. The macrocycle in the ansamycins is closed by an amide rather than an ester linkage, i.e. ansamycins are lactams. The only ansamycins currently used therapeutically are semi-synthetic naphthalene-based macrocycles produced from rifamycin B. 

The third is partial or total reduction of an aromatic ring. Any catalogue lists a vast number of available substituted benzene rings. Saturated compound 8 can obviously be made by total reduction of 9 but it may not be obvious that partial reduction (Birch) allows the enone 11 also to be made from 9. Birch reduction is the only new method here so we shall revise the Robinson and the Diels-Alder and concentrate on Birch. 

Besides the benzo derivatives just described, a series of compounds with a substituted benzene ring as well as those condensed with other aromatic or heteroaromatic systems is known. They all are accessible by the same or similar methods (66HC(2i-i)i). 

The simplest aromatic compound is benzene. It has a ring current of tt electrons, which shows strong ti -> ti absorptions at 184 nm 60,000), and at 204 nm 7900). (This is called a primary band.) Benzene exhibits a low intensity band at 256 nm 200) (Known as a secondary or fine-structure band), with a series of fine-structue bands between 230 and 270 nm). Any substitution n the benzene ring, irrespective of its electronic character... 

An empirical steric index, denoted as 2D, for substituted benzene rings and defined as the 6-term sum of the distances, given by the L Sterimol length parameter, between the six atoms bonded to the benzene carbon atoms, i.e. the value of the external perimeter of the benzene ring [Taillander et ai, 1983 Ravanel et al., 1985]. It represents the perimeter of the efficacious section and describes the steric properties of aromatic compounds. 

These 2 bonds are characteristic of the C— C bonds in aromatic rings. These 2 bands have variable intensities and a weak band should not be overlooked (see the weak band in Fig. Id). The sharp bands near 3000 cm are characteristic of aromatic C-H bonds. The bands between 1650 cm and 2000 cm between 1225 cm" and 950 cm , and below 900 cm are correlated with the number and position of the substituent groups on the benzene ring. Notice the difference between the peaks of Fig. la - d (at 1650 - 2000 cm M/ corresponding to the mono-substituted benzene ring, and the ortho-, meta-, and para-isomers respectively. Perhaps the most helpful bands in this respect are the ones between 690 cm and 840 cm . For many aromatic compounds, absorption occurs at ... 

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