Unit 6.3 Aromatic Organic Chemistry

Mine Safety Appliances Co. (MSA) manufactures potassium superoxide in the United States for use in self-contained breathing equipment. There are several published uses for potassium superoxide in organic chemistry, e.g., for oxidizing aromatic compounds and for initiating anionic polymerization. 

Methods based on classical organic chemistry led to the conclusion, already by 1940, that lignin is built up of phenylpropane units. Examples of typical reactions used in these studies are illustrated in Fig. 4-1. However, the concept of a phenylpropanoid structure failed to win unanimous acceptance, and as late as 30 years ago, some scientists were not convinced that lignin in its native state was an aromatic substance. Finally, the problem was solved by Lange in 1954, who applied UV microscopy at various wavelengths directly on thin wood sections, obtaining spectra typical of aromatic compounds. 

In summary the results observed in these studies [160] of poly(Sty-co-DVB) swelling in aromatic liquids serve to show that the method of measuring a is so sensitive that it can detect an effect caused by even the smallest modification in the molecular geometry of attached substituents, and that these differences correlate qualitatively with expectation based on the known principles of physico-organic chemistry of aromatic compounds. Since the observed a is the net effect of electronic attraction and steric hindrance between the sorbed molecule and the adsorption site, i.e. the monomer unit of the polymer, it would be impossible to separate quantitatively the electronic and steric contributions of a particular substituent. The ability to make such a differentiation, however, appears to be more promising with liquids that comprise homologous series of the type Z(CH2)nH (where Z is a phenyl, chloro, bromo or iodo substituent), since the added electronic contribution to Z by each additional methylene group is well known to be extremely small when n becomes >3 [165],... 

Internal addition of the thiol unit to the iminium salt generates the five-membered ring in 141, and aromatization (see Chapter 21, Section 21.1) leads to the thiazole unit in 142. Other enzyme reactions use 132 to prepare 131. Most of this sequence is complicated, and many subtle changes in the structure are brought about by the enzymes and genes used in the biosynthesis. However, each individual reaction may be correlated, more or less, with organic chemistry reactions found in other chapters. 

In ordinary conversation, the word "aromatic" conjures pleasant associations—the odor of freshly prepared coffee, or of a cinnamon bun. Similar associations occurred early in the history of organic chemistry, when pleasantly "aromatic" compounds were isolated from natural oils produced by plants. As the structures of these compounds were elucidated, a number of them were found to contain a highly unsaturated six-carbon structural unit that is also found in benzene. This special ring structure became known as a benzene ring, and the aromatic compounds containing a benzene ring became part of a larger family of compounds now classified as aromatic on the basis of their electronic structure rather than their odor. 

A large part of organic and macromolecular chemistry starts with the chemical functionalization of benzene, and benzene units serve us building blocks for important polymers. Naturally, benzene-based aromatic materials also represent an important subclass of jt-conjugaled architectures. Despite some synthetic difficulties related to the generation of structurally well-defined oligo- and poly(phenyl-... 

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