Aromatic compounds classification

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

Following the classification of Chapter 4.01, three classes will be considered, (a) Compounds isomeric with aromatic compounds (6), (7) and (8). The quaternary, non-aromatic salts (Scheme 7, Chapter 4.01) will be discussed only in connection with protonation studies which lead to the conclusion of their non-existence. The carbonyl derivatives (9), (10), (13) and (14) will also be included here because it is possible to write an aromatic tautomer for each one, (9 )-(14 ), even if it is energetically unfavoured, (b) Dihydro compounds. In this class not only pyrazolines (15), (16) and (17) but also pyrazolidinones (18) and pyrazolinediones like (1) are included, (c) Tetrahydro compounds. Besides the pyrazolidines (19), the pyrazolidinetriones (2) are included here. 

Most substances which appear in the examples of this chapter are analysed In Part Two and their enthalpy of decomposition determined experimentally. This is because most of them are considered hardly stable. This is one of the reasons for assigning no Tow risk in the suggested classifications. But it is also indisputable that criterion Cf overestimates the instability risk. It is the case for all aromatic compounds that are generally very stable. In the examples above, N-methylaniline, dichlorobenzene... 

Collie s hypothesis that aromatic compounds are made biologically from ethanoic acid was greatly expanded by A. J. Birch to include an extraordinary number of diverse compounds. The generic name acetogenin has been suggested as a convenient classification for ethanoate (acetate)-derived natural products, but the name polyketides also is used. Naturally occurring aromatic compounds and quinones are largely made in this way. An example is 2-hydroxy-6-methylbenzoic acid formed as a metabolite of the mold Penicillium urticae ... 

The side chains of the amino acids do not form a natural series, and thus, there is no easy way to learn their structures. It is useful to classify them according to whether they are polar or nonpolar, aromatic or aliphatic, or acidic or basic, although these classifications are not mutually exclusive. Tyrosine, for example, can be considered to be both aromatic and polar, although the polarity introduced by a single hydroxyl group in this aromatic compound is somewhat feeble. 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

It is important not to attach undue weight to the division between aliphatic and aromatic compounds. Although extremely useful, it is often less important than some other classification. For example, the similarities between aliphatic and aromatic acids, or between aliphatic and aromatic amines, are more important than the differences. 

Although, as we shall see, it is possible to account for these effects in a reasonable way, it is necessary for the student to memorize the classifications in Table 11.3 so that he mhy deal rapidly with synthetic problems involving aromatic compounds. 

In contrast to the expected classification, the obvious effect is the discrimination between aliphatic and aromatic compounds. We achieve a better result by excluding unnecessary information. A clear discrimination occurs by excluding hydrogen atoms from the calculation (Figure 6.11, right). Whereas the normal descriptor describes the aliphatic character of the compounds, the hydrogen-excluded descriptor emphasizes the differences in heteroatoms correctly. 

The structures of organic polynuclear aromatic compounds are not limited to planar systems of carbon and hydrogen atoms. A classification of three-dimensional aromatic compounds is proposed on the basis of the number of recognizable edges (boundaries) in the molecular structure. Aromatic structures with no edges are included in this classification an example is the recently proposed truncated icosahedral structure for C6o (Buckminsterfullerene). The current literature and activity in the subfield of nonplanar aromatic compounds is reviewed. Three-dimensional aromatic compounds are possible tools for use in studies of polynuclear aromatic chemistry, and some possible applications to the particular chemical topics presented in this book are outlined. 

J. Brunvoll, B. N. Cyvin, S. J. Cyvin, E. C. Kirby, and I. Gutman, Fully-naphthalenoid hydrocarbons Enumeration and classification, Polycyclic Aromatic Compounds 4, 219-229 (1995). 

Retention index data can be employed to recognize molecular structures of unknown monofunctional compounds. This was principally demonstrated by Huber and Reich C113, 2373. Retention data of 199 compounds on ten different stationary phases were utilized for cluster analysis, KNN--classifications and the computation of classifiers with the learning machine. The minimum number of stationary phases was only two for the classification of aromatic compounds, four for alcohols and 13 for aldehydes and ketones. A two-step classification procedure was developed. 

Hydrocarbons are compounds composed entirely of carbon and hydrogen atoms bonded to each other by covalent bonds. These molecules are further classified as saturated or unsaturated. Saturated hydrocarbons have only single bonds between carbon atoms. These hydrocarbons are classified as alkanes. Unsaturated hydrocarbons contain a double or triple bond between two carbon atoms and include alkenes, alkynes, and aromatic compounds. These classifications are summarized in Figure 19.3. 

---