Isomerism classification

Figure 2-67. Classification of isomeric structures of organic compounds.

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. 

Another type of isomerism arises when a molecule contains a chiral center or is chiral as a whole. Chirality (from the Greek cheir, hand) leads to the appearance of structures that behave like image and mirror-image and that cannot be superimposed ( mirror isomers). The most frequent cause of chiral behavior is the presence of an asymmetric C atom—i.e., an atom with four different substituents. Then there are two forms (enantiomers) with different configurations. Usually, the two enantiomers of a molecule are designated as L and D forms. Clear classification of the configuration is made possible by the R/S system (see chemistry textbooks). 

G.l.c. papers of interest include the classification of 22 acyclic monoterpenoid alcohols according to retention indexes, resolution of cyclic ketones [e.g. ( )-menthone, ( )-isomenthone] as diethyl (+)-tartrate acetals, and the use of lanthanide shift reagents to resolve non-terpenoid racemic epoxides.The occurrence and prevention of monoterpenoid hydrocarbon isomerization during silica gel chromatography has been examined and the separation of monoterpenoids and sesquiterpenoids by gel permeation chromatography is reported. Monoterpenoid hydrocarbons have been selectively extracted from essential oils using dimethylsilicone. ... 

The single and double scan methods allow estimation of the value of the characteristic parameters for both ionic and nonionic surfactants (see Table 1). The extrapolation of Eqs. 8 and 9 to SACN = 0 allows the classification of the head groups in some hydrophihcity scale. Inspection of Table 1 data indicates that the branching and isomeric structure does have quite an influence on the characteristic parameter, as it has been reported in the Hter-ature [12,27-32]. 

Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. 

In 1893 Werner founded his new constitutional formula for inorganic compounds, applied the theory to the systematic classification of the chromi-ammines, and found that all the chromi-ammines which had been investigated could be fitted in to his system of classification. Since then the chemistry of the chromi-ammines has been further developed hv Werner, Pfeiffer, and many others relationships have been traced between chromi-ammines, complex salts, and chromic salt hydrates, and numerous cases of isomerism have been discovered in this series of ammines. 

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

To examine selectivity relationships, the following classification of o-ethyltoluene products is proposed for simplification ethylbenzene, toluene, and benzene are dealkylation products p- and m-ethyltoluene are isomerization products. Formation of m- and p-xylenes must have involved skeletal rearrangements to have been produced from o-ethyltoluene. Accordingly, these compounds are counted as isomerization products. In all cases studied, toluene was the major dealkylation product, and m- and... 

Ionization, hydrate and coordination isomerism are classifications of constitutional isomerism that originated with Werner.27,28 Ionization and hydrate isomerism (equation 1) apply to cases in which there is a ligand exchange between primary and outer coordination spheres, whereas coordination isomerism (equation 2) arises in systems containing at least two metal ions, so that alternative primary coordination spheres are available. 

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