Aromatic compounds structural isomers

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

Ortho-Effect. The ortho-effect is one of the most widely known structural phenomena in organic chemistry. It is widely used in organic chemistry for synthetic purposes. The mass spectra of the majority of ort/jo-substituted aromatic compounds possess significant differences in comparison with the spectra of their meta- and para-isomers. A classic example of the ortho-effect in mass spectrometry involves fragmentation of alkylsalicylates. The intense peaks of [M - ROH]+ ions dominate in the El spectra of these compounds. These peaks are absent in the spectra of their meta- and para-isomers. The reaction leading to these ions may be represented by Scheme 5.12. 

Draw and name three aromatic isomers with the molecular formula C10H14. [Isomers are compounds that have the same molecular formula, but different structures. See the Concepts and Skills Review for a review of structural isomers.)... 

A closely related compound to the xylenes is ethylbenzene. Ethylbenzene is a structural isomer with the xylenes, but its properties distinguish it from the three xylene isomers. Ethylbenzene is part of xylene mixtures and is also one of the principal aromatic components in BTEX associated with petroleum products. 

Basically, three experimental problems are involved in the substitution reactions of aromatic compounds (1) proof of structure of the isomers that are formed (2) determination of the percentage of each isomer formed, if the product is a mixture and (3) measurement of the reactivity of the compound being substituted relative to some standard substance, usually benzene. 

Unlike phenols (Section 26-l), structural analysis of many of the hydroxy-substituted aza-aromatic compounds is complicated by isomerism of the keto-enol type, sometimes called lactim-lactam isomerism. For 2-hydro xypyrimidine, 19, these isomers are 19a and 19b, and the lactam form is more stable, as also is true for cytosine, 15, thymine, 16, and the pyrimidine ring of guanine, 18. 

The position of the nitro group is also an important structural factor in determining the biological activity in nitro-aromatic compounds. This is evident from studies that show markedly different mutagenic potency in structural isomers of nitro-aromatic compounds [3,15, 24, 26, 30, 31,44,46]. Structural isomers like 1-, 2-, 3-, and 6-nitrobenzo[a]pyrene have dramatically different mutagenic potency [47—49] 6-nitrobenzo[a]pyrene is a weak mutagen, while 1-, 2-, and 3-nitrobenzo [a]pyrene are potent mutagenes [47 -9]. 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekule structures in drawing reaction mechanisms, however, to show the movement of individual pairs of electrons. 

All possible interactions between the K and L groups were taken into account and Akk = 0. The definition contains a minimum number of groups and is satisfactory for most of the binary systems studied. However, it cannot take into account the structural differences which exist between position isomers. This is the case of polycyclic aromatic compounds presenting cycle position isomers or substitute position isomers. Structural differences of this type determine the gaps between the values of certain thermophysical properties of isomers, such as, for example, the fusion temperature or sublimation enthalpy. The further the temperature falls, the more these differences are accentuated. The representation of the solid-fluid (low temperature) equilibria is consequently more difficult and the model must take into account the existing structural differences. We came across this problem in the compounds such as anthracene, phenanthrene, pyrene, methylated naphthalenes, hexamethylbenzene and triphenylmethane. As it was out of the question to increase the number of groups because... 

Content of the light aromatic compounds such as benzene, toluene and indene in the gas was determined on-line with both the spectrophotometric and chromatographic methods. The constituents of the tars collected from the gas or extracted from the fly ash were identified by GC-MS analysis. The presence of the compounds having large quantities was confirmed by doping the samples with pure compounds. Doping of the sample was also used in identification of the structural isomers having retention times close to each other. 

Capillary electrophoresis has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear-aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various... 

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