Aromatic Compounds, definition heterocyclic

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

There has been no work published assessing the aromatic character of 1,2,3-thiadiazole. From the Huckel definition of aromaticity, (4n + 2) ir-electrons in a ring constitute an aromatic compound and by this criterion 1,2,3-thiadiazoles should be considered aromatic compounds. Chemical shifts for 1,2,3-thiadiazoles in both 13C and 1H NMR, which are related to the electronic environment, are in accord with an aromatic heterocyclic ring. 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

As a result of interaction of 843 and pyridine, the adduct 845 is formed [53], The structures of coordination compounds 844 and 845 were proved by x-ray diffraction. As shown above (Sec. 3.4.3.2), the direct ammonia synthesis [55,56] with participation of various ligands (especially aliphatic, aromatic, and heterocyclic amines, aminoalcohols), elemental metals (or their oxides), and NH4SCN in mostly non-aqueous media, opens definite possibilities for obtaining thiocyanate complexes. In this respect, transformation (4.9) should be mentioned [57] ... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure, and many physicochemical characteristics of heterocyclic compounds. However, there is no precise quantitative definition of aromaticity that is universally acceptable. Aromaticity is usually... 

Using benzene-like aromatic systems and pericyclic reactions with an even number of centers, the principles of graph-theoretical structure theory are described and extended to conjugated heterocycles and cyclic systems with an odd number of centres. With topological analysis of the graphs of these systems as a foundation, a graph-theoretical definition of the idea of aromaticity in regard to monocyclic compounds is presented. 

The results of pyrolysis experiments indicate that compound 1 decomposes with the formation of the pyrazyne 3. Pyrolysis in the presence of benzene results in a small but definite yield of 2-phenylpyrazine (8) from an insertion reaction with pyrazyne. This result is similar to results from pyrolysis experiments with various aromatic and heterocyclic anhydrides. Pyrazyne is also invoked to explain the formation of maleonitrile... 

Look back again at the definition of aromaticity in the previous section "... a cyclic, conjugated molecule containing 4n + 2 tt electrons. Nothing in this definition says that the number of tt electrons mnst he the same as the number of atoms in the ring or that all the atoms in the ring mnst he carbon. In fact, both ions and heterocyclic compounds, which contain atoms of different elements in their ring, can also he aromatic. The cyclopentadienyl anion and the cycloheptatrienyl cation are perhaps the best known aromatic ions, while pyridine and pyrrole are common aromatic heterocycles. 

---