Aromatics, base properties

The use of light olefins, diolefins, and aromatic-based monomers for producing commercial polymers is dealt with in the last two chapters. Chapter 11 reviews the chemistry involved in the synthesis of polymers, their classification, and their general properties. This book does not discuss the kinetics of polymer reactions. More specialized polymer chemistry texts may be consulted for this purpose. 

The central role played by DNA in cellular life guarantees a place of importance for the study of its chemical and physical properties. It did not take long after Watson and Crick described the now iconic double helix structure for a question to arise about the ability of DNA to transport electrical charge. It seemed apparent to the trained eye of the chemist or physicist that the array of neatly stacked aromatic bases might facilitate the movement of an electron (or hole) along the length of the polymer. It is now more than 40 years since the first experimental results were reported, and that question has been answered with certainty. 

The tertiary aromatic bases share with the phenols the property of taking up a nitroso-group in the p-position when treated with nitrous acid in acid solution. As was mentioned elsewhere (p. 307), this reaction may be compared to the coupling of these compounds with diazobenzene. 

Thus, transition metal cations in the lower valence state may also act as Lewis bases. Factors that affect the reactions promoted by Lewis acidity are listed in Table I. Lewis acid sites reversibly adsorb water (6s 9, 42), which may thus strongly compete with organic compounds that have weaker Lewis base properties, such as aromatic hydrocarbons. Lewis acidity depends on the degree of hydration and is strongest under desiccating conditions. Examples of reactions that are promoted by Lewis acidity are summarized in Table II. Other examples have been reviewed by Solomon and Howthorne (37). 

This is a good time to have another brief look at Sections 4.3.5 and 4.5.4, and compare how we used similar reasoning to consider the likely stability, or otherwise, of anions and cations in order to predict the acid-base properties of aromatic amines... 

Amino Acids Have Both Acid and Base Properties Aromatic Amino Acids Absorb Light in the Near-Ultraviolet... 

Armarego109 has determined the pAa values of indolizines and aza-indolizines from their ultraviolet spectra. He found that indolizine itself has the same basic strength as a-naphthylamine (pAa 3.9) and that a methyl substituent increases the pAa by two to three units. Mason and Smith,110 studying the fluorescence spectra of various aromatic nuclei including indolizine, found no evidence to support the prediction that acid-base properties of aromatic hydrocarbons in the first electronic excited state should differ from those in the ground state. 

Interest in (5,5)-fused ring systems developed in separate, albeit closely related, directions. In the first, chemists continue their quest for new compounds having unique biological, chemical and physical properties. In the second, emphasis is on the classification of aromaticity based upon molecular conformity with Hiickel s rule. An increasing understanding of the fundamental chemistry of heterocyclic ring systems has led to considerable advances in this field, as evidenced by the growing list of applications as biocides and in industrial processes. 

Some 60 dyes have been selected as possible photovoltaic materials their electrochemical redox potentials, surface adsorption, spectroscopic properties, fluorescence yields, and acid-base properties have been measured. The aim of this work is to produce a low-cost panel for harvesting solar energy as electrical power. The physical principles of fluorescent solar collectors have been discussed by Raue and Harnisch and several classes of dyes examined. Coumarin dyes are suitable convertors, particularly if the amino-group is fixed by ring closure to the aromatic system. 

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