Ring systems, aromatization

Thiadiazole is a planar, thermally stable, and weakly basic aromatic ring system. Aromatic forms of the 1,2,5-thiadiazole nucleus are generally stable to concentrated mineral acids and are only slightly sensitive to base. Base-catalyzed deuterium exchange of the ring protons can be effected... 

The aniline point of crude oil is the temperature at which equal parts of aniline and the oil are completely miscible. For oils of a given type, the aniline point increases slightly with molecular weight but increases markedly with paraffinic character and may therefore be used to obtain an approximate estimation of aromatics content. Aniline point determinations are only infrequently applied to heavy oils and residua since their very character, and the other evaluation methods outlined here, indicates them to be complex with high proportions of ring systems (aromatic constituents and naphthene constituents). 

Most of the work on such compounds has been directed toward their synthesis, with some emphasis on their conformations and reactions. Unlike the smaller ring systems, aromatic ring examples are relatively unknown. 

A simple way of constructing each molecule in the ensemble is to cormect ring systems (aromatic or naphthenoaromatic) with alkyl chains in a linear fashion subject to stoichiometric constraints. It is easier to construct an ensemble of liquid molecules than solid molecules. The latter requires modeling of three-dimensional structures. 

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

Rings can either stabilize or destabilize molecules beyond what is to be expected from a simple additivity scheme. Stabilization comes from aromatic ring systems... 

The organic chemical structural types believed to be characteristic of coals include complex polycyclic aromatic ring systems with connecting bridges and varied oxygen-, sulfur-, and nitrogen-containing functionalities. 

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the syn thesis of dyes The molecular formula of acndine is C13H9N and its ring system is analogous to that of anthracene except that one CH group has been replaced by N The two most stable reso nance structures of acridine are equivalent to each other and both contain a pyndine like struc tural unit Wnte a structural formula for acridine... 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

Amine oxides, known as A[-oxides of tertiary amines, are classified as aromatic or aliphatic, depending on whether the nitrogen is part of an aromatic ring system or not. This stmctural difference accounts for the difference in chemical and physical properties between the two types. 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Other Radioprotective Chemicals. The bis-methylthio- and methylthioamino-derivatives of 1-methylquinolinium iodide and l-methylpyridinium-2-dithioacetic acid provide reasonable protection to mice at much lower doses than the aminothiols, which suggests a different mechanism of action (139). One of these compounds, the 2-(methylthio)-2-piperidino derivative of the l-methyl-2-vinyl quinolinium iodide (VQ), interacts with supercoUed plasmic DNA primarily by intercalation. Minor substitutions on the aromatic quinolinium ring system markedly influence this interaction. Like WR-1065, VQ is positively charged at physiological pH, and the DNA-binding affinities of VQ and WR-1065 appear to be similar. 

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. 

A number of derivatives of antimonin are also known (141,142). The potential aromaticity of this ring system has aroused considerable interest and has been investigated with the aid of spectroscopy as well as ab initio molecular orbital calculations (143). There seems to be no doubt that antimonin does possess considerable aromatic character. 

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