Aromatic compound heteroatoms

Chapter 21 introduces aromatic hydrocarbons and their unique chemistry. In another class of aromatic compounds, heteroatoms replace one or more of the ring carbons. These compounds are collectively known as heterocycles or heterocyclic aromatic compounds, and they comprise a class of compounds so large that an entire course is easily built around their chemistry. Heterocycles are seen in several places in this book, including a brief introduction to their nomenclature in Chapter 5 (Section 5.6) and in Chapter 8 (Section 8.9). The most common heterocycles include five- and six-membered monocyclic derivatives that contain nitrogen, oxygen, or sulfur. Several important bicyclic derivatives contain nitrogen. The use of heterocycles in medicine and industry is extensive. This chapter will expand the aromatic chemistry from Chapter 21 and introduce the world of heterocyclic chemistry. 

The NMR spectra of thiazoles show the same behavior as those of aromatic compounds, but the chemical shifts also depend on the two heteroatoms. 

A large group of heterocyclic aromatic compounds are related to pyrrole by replacement of one of the ring carbons p to nitrogen by a second heteroatom Com pounds of this type are called azoles... 

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

Coke-oven tar is an extremely complex mixture, the main components of which are aromatic hydrocarbons ranging from the monocyclics benzene and alkylbenzenes to polycycHc compounds containing as many as twenty or more rings. HeterocycHc compounds containing oxygen, nitrogen, and sulfur, but usually only one heteroatom per ring system are present. Small amounts of paraffinic, olefinic, and partly saturated aromatic compounds also occur. 

Discussion of these compounds is divided into isomers of aromatic compounds, and dihydro and tetrahydro derivatives. The isomers of aromatic azoles are a relatively little-studied class of compounds. Dihydro and tetrahydro derivatives with two heteroatoms are quite well-studied, but such compounds become more obscure and elusive as the number of heteroatoms increases. Thus dihydrotriazoles are rare dihydrotetrazoles and tetrahydro-triazoles and -tetrazoles are unknown unless they contain doubly bonded exocyclic substituents. 

Aromatic character in isoxazoles has been studied from a number of viewpoints, and these studies indicate that although isoxazole may be formally considered an aromatic system, the disposition of the ring heteroatoms modifies this character to an appreciable extent. From a qualitative viewpoint, thermal stability and electrophilic attack at the 4-position may be considered consistent with an aromatic character. Furthermore, NMR chemical shifts of the ring protons are consistent with those of an aromatic compound. References related to these studies may be found in Section 4.16.2.3.4. 

The 1,2,3-thiadiazole 1 possesses three contiguous heteroatoms in a five-membered ring and exists as a remarkably stable neutral aromatic compound. It is isomeric with the ring-opened a-diazothioketone 2 (Equation 1) although there is evidence that it reacts through this intermediate, all structural methods, including X-ray diffraction, point to 1 as the structure for a 1,2,3-thiadiazole. 

A variety of aromatic compounds can be polymerized electrochemically to form a thin polymer filmf 1 -41. Many films involving heteroatom aromatics can be made electrically conducting... 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

Vijayaraj. M. Heteroatom alkylation of aromatic compounds over metal oxides, Ph. D Thesis, University of Pune, 2006. 

Two additional examples of aromatic compounds containing heteroatoms eire shown in Figure 6-14. In both compounds, the heteroatom has two lone pairs. However, only one of the pairs is in a p-orbital perpendiculcir to the plane of the ring. The other electron pair is in the plane of the ring. 

Figures 13 and 14 also show that hydrotreating the catalytic cracker feedstock increases the zeolite cracking. C3, and C5+ compounds are possible products of primary zeolite cracking. These figures show that hydrotreating of the feedstock results in larger yields of these primary cracking products and hence more valuable products. This improvement is most likely due to the heteroatom removal and the saturation of aromatic compounds during hydrotreating which tend to block active sites and reduce the activity of the catalyst.

When we have obtained a good correlation for normal paraffins, we would naturally want to know if we can extend this to the branched paraffins, and onward to the population of all the saturated hydrocarbons (by including the cyclic paraffins), and onward to the population of all hydrocarbons (by including olefins, acetylenes, and aromatic compounds), and then onward to the population of all organic compounds (by including compounds with heteroatoms, such as O, N, Cl). A correlation that applies accurately to a larger domain is more useful than one that works only for a smaller domain. 

Asphaltenes These are complex high-molecular-weight polycyclic aromatic compounds which may contain oxygen, sulfur, or nitrogen heteroatoms. They are found in crude oil and in certain heavy fuel oils in micellar form. Their dispersion throughout oil can be stabilized by asphaltene precursors called resins and maltenes. 

Reaction of nucleophiles with the polarized N=C bond of azines proceeds via dearomatization and formation of the corresponding 1,2-adduct. With alkyllithiums, for example, it is possible to isolate the dihydro products by careful neutralization of the reaction mixtures these are, in general, rather unstable, however, and can easily be reoxidized to the fully aromatic compounds (Scheme 4). The dihydro adducts formed in these direct nucleophilic addition reactions can also be utilized for the introduction of substituent groups /3 to the heteroatom. Thus, reaction of (35) with one of a number of electrophiles, followed by oxidation of the intermediate dihydro product, constitutes a simple and, in many cases, effective method for the introduction of substituent groups at both the 2- and 5-positions of the pyridine ring (Scheme 4). Use of LAH in this sequence, of course, results in the formation of 3-substituted pyridines. 

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