Hydrocarbons Organic compounds that cyclic

Most organic compounds that show absorption in the visible or in the near-UV region have a linear or cyclic n system as the chromophoric system. Therefore, the results of the previous sections may be used and extended to discuss light absorption of all those compounds that can be derived from linear and cyclic hydrocarbons by including the influence of substituents in an appropriate way. (Cf. Michl, 1984.) A complete theory of substituent effects comprises all areas of organic chemistry. Here, only the fundamental concepts of the influence of inductive and mesomeric substituents will be considered. In order to simplify the discussion, substituent effects will be called inductive if in the HMO model they can be represented by a variation of the Coulomb integral of the substituted n center p. If they are due to an extension of the n system they will be called mesomeric. 

One of the reasons that such a variety of organic compounds exists is that carbon atoms can form ring structures. An organic compound that contains a hydrocarbon ring is called a cyclic hydrocarbon. 

Organic compounds that contain only carbon and hydrogen are called hydrocarbons. Hydrocarbon molecules may be divided into the classes of cyclic and open-chain depending on whether they contain a ring of carbon atoms. Open-chain molecules may be divided into branched or straight-chain categories. 

One of the reasons that such a variety of organic compounds exists is that carbon atoms can form ring structures. An organic compound that contains a hydrocarbon ring is called a cyclic hydrocarbon. To indicate that a hydrocarbon has a ring structure, the prefix cyclo- is used with the hydrocarbon name. Thus, cyclic hydrocarbons that contain only single bonds are called cycloalkanes. 

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. 

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

An important class of model compound study that we have not discussed here is the determination of transfer free energies mentioned above. Though the study of transfer of amino acids and their analogs from water into various organic compounds provides a wealth of information about various interactions, the current data base includes only values AG° (usually relative to glycine) and not values for AH0, AS0, and ACp, and thus is not suitable for the temperature-dependent information required within the context of this review. The thermodynamics from liquid hydrocarbon, crystalline cyclic dipeptide, and alkane gas dissolution are summarized in Table I. 

The electronic spectra of cyclic conjugated n systems depend inherently on the number of n electrons. Closed-ring systems with AN+l n electrons in the perimeter are aromatic compounds, of which benzene is the most important representative. Benzenoid hydrocarbons constitute a class of compounds whose UV spectra have been investigated most extensively both experimentally and theoretically. The fact that the spectra of aromatic compounds are so characteristic meant that formerly they were of considerable importance in the structure determination of organic compounds. However, these spectra cannot be explained in terms of the simple HMO model. If one seeks a theoretical basis for an understanding, one has the choice between the perimeter model and the Pariser-Parr-Pople or a more complicated numerical method. Before discussing these theoretical models, some empirical relations will be presented. Finally, cyclic systems derived from a perimeter of 4N Jt electrons will be considered. 

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