What Is Aromaticity

Hiickel s criteria are summarized as follows. To be aromatic, a ring must 

Be planar or nearly planar, so that there is continuous overlap or nearly continuous overlap of all 2/ orbitals of the ring. 

Benzene meets these criteria. It is cyclic, planar, has one 2p orbital on each carbon atom of the ring, and has 6 pi electrons (an aromatic sextet) in the cyclic arrangement of its 2p orbitals. 

Let us apply these criteria to several heterocyclic compounds, all of which are aromatic. Pyridine and pyrimidine are heterocyclic analogs of benzene. In pyridine, one CH group of benzene is replaced by a nitrogen atom, and in pyrimidine, two CH groups are replaced by nitrogen atoms  

An organic compound that contains one or more atoms other than carbon in its ring. 

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Here, then, is the first difficulty about the problem we are trying to solve a scientist who discovers — as Kekule did — a fundamental new concept must surely introduce a new an well-chosen word to define it. If he does not do so, the risk is that complete confusion very soon occurs both in books and in the minds of men. We are of the opinion that one cause of the confusion which reigns today concerning the question What is aromaticity lies in Kekule s original choice of word, but it is also clear that the chemists who have worked... 

So apparent confusion about what is aromaticity is not due to a lack of explanation of aromaticity in terms of the presence, or lack of the presence, of conjugated circuits of 4 -I- 2 and 4n size in the set of Kekule valence structures of compounds, but due to (i) lack of attention given to articles describing conjugated circuits aud their use for characterization of aromaticity and (ii) not recognizing that these explanations, in fact, represent a generalization of the famous Hiickel rule to polycycUc systems, which Hellwinkel recognized just as the paper was published, almost 40 years ago. 

What is aromaticity This fundamental and widely used chemical concept is nevertheless one of the most controversial concepts in modem chemistry . Many definitions and criteria for characterizing and estimating the degree of aromaticity have been considered, and they are discussed extensively and critically in a recently published comprehensive book by Minkin and coworkers . Schleyer and Jiao have recently emphasized the importance of magnetic properties for estimating aromaticity. We will summarize here briefly the most widely used computational criteria for estimating the degree of aromaticity whilst the reader is referred to References 13, 14 and 16 and the papers cited therein for a more comprehensive discussion. 

Stanger A (2009) What is aromaticity a critique of the concept of arotnaticily—can it really be defined . Chem Commun 1939-1947... 

Ragu6 von Schleyer P, Jiao H (1996) What is aromaticity. Pure Appl Chem 68 209-218... 

What is aromaticity then As stated above, there is actnaUy no unique, generally applicable definition for it Instead, the coexistence of several properties of a given compound points toward its aromaticity. Several of these properties can be evalnated, which thns gives the possibility to qnantify the degree of aromaticity. Based on the... 

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