Sources and Names of Aromatic Compounds

The reactivity of substituted aromatic compounds, more than that ol any other class of substances, is intimately tied to their exact structure. As a result, aromatic compounds provide an extraordinarily sensitive probe for studying the relationship between structure and reactivity We ll examine that relationship in this and the next chapter, and we ll find that the lessons learned are applicable to all other organic compounds, including such particularly important substances as the nucleic acids that control our genetic makeup. 

Unlike coal, petroleum contains few aromatic compounds and consists largely of alkanes (Chapter 3 Focus On). During petroleum refining, however, aromatic molecules are formed when alkanes are passed over a catalyst at about 500 °C under high pressure. 

Aromatic substances, more than any other class of organic compounds, have acquired a large number of nonsystematic names. The use of such names is discouraged, but 1UPAC rules allow for some of the more widely used ones to be retained (Table 15.1). Thus, methylbenzene is known commonly as toluene hydroxybenzene, as phenol ami nobenzene, as aniline and so on. 

Monosubstituled benzenes are systematically named in the same manner as other hydrocarbons, with -benzene as the parent name. Thus, C6H5Br is bromo-benzene, C6H5NC 2 is nitrobenzene, and Q5H5CH2GII2CH3 is propylbenzene. 

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. 

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Benzene (which is the name that was given to the aromatic compound CeUe) is probably the most common and industrially important aromatic compound in wide use today. It was discovered in 1825 by Michael Faraday, and its commercial production from coal tar (and, later on, other natural sources) began in earnest about twenty-five years later. The structure ofbenzene emerged during the 1860s, the result of contributions from several chemists, most famously that of Kekule. 

A number of the common names for aromatic compounds refer to their fragrance and natural sources. Several of them have been accepted by lUPAC. As before, a consistent logical naming of these compounds will be adhered to as much as possible, with common names mentioned in parentheses. 

The common names of these intermediates derive from the sources from which they were first isolated. Geraniol, a component of rose oil, has the aroma of geraniums, and farnesol is an aromatic compound found in the flowers of the Farnese acacia tree. Many natural... 

In common with many other aromatic compounds, benzoic acid was first discovered in a renewable raw material, namely gum benzoin, by Blaise de Vigenere as early as in the 16th century. Carl Wilhelm Scheele further studied this raw material in 1755, and it remained the main source for medicinal benzoic acid until the mid-nineteenth century. The first technical synthesis of benzoic acid was based on naphthalene, via the intermediate phthalic anhydride this synthesis was introduced in 1863. In 1877, August Wilhelm von Hofmann reported the synthesis of benzoic acid from hippuric acid, which is present in the urine of herbivores. 

Structures of the presently known naturally occurring terpenoids and aromatic compounds are summarized in charts following the various sections. Structure numbers (number order), names, physical properties (molecular formula, m.p. [a]6) of terpenoids, steroids and aromatic compounds, plant sources and literature references are found in Table I. w-Alkanes, alkanoic acids, amino acids, carotenoids, carbohydrates and miscellaneous compounds, plant sources and literature references are also found in Table II. Plant names (in alphabetical order) and chemical constituents [terpenoids, steroids and aromatic compounds (in alphabetical order)] are found in Table III. 

Benzene is the parent of many aromatic compounds, which have both common and lUPAC names. The common names of substituted benzenes often came ftom their sources. One example is toluene, which used to be obtained ftom the South American gum tree, Toluifera balsamum. A few benzene compounds are shovm below. Their common names are shown in parentheses below their lUPAC names. The common names have been used for so long that they have become accepted by lUPAC. 

Pyrocatechase is an enzyme widely distributed among bacteria. It catalyzes an oxidation of o-dihydroxybenzene, and is named according to the traditional name of the substrate, pyrocatechol. Pyrocatechol, more commonly known by the abbreviated designation catechol, is an intermediate in the metabolism of many aromatic compounds, including mandelic acid, nitrobenzoic acid, anthranilic acid, and other compounds that may be converted to salicylic acid. Benzoic acid and phenol are also precursors of catechol. The best-studied enzyme is that obtained from a strain of Pseudomonas that can use tryptophan as a carbon source. The enzyme is formed adaptively when tryptophan, catechol, or any intermediate between the amino acid and catechol (Fig. 4) is used as a substrate for the cells (Suda et al., 1950). 

Aromatic hydrocarbons, which originally got their name from the distinctive odors many of them have, are called arenes. They all contain an aromatic ring, usually the six-membered ring of benzene, which was introduced in Sections 2.7, 3.7, and 3.12. An abundant source of arenes is coal, which is a very complex mixture of compounds, many of which consist of extensive networks containing aromatic rings (Section 18.10). 

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