The Concept of Aromaticity

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.) 

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of 

Simple Hiickel calculations on benzene, in contrast, place all the n electrons in bonding MOs. The 7t-electron energy of benzene is calculated by summing the energies of the six 71 electrons, which is 6a -F 8/S, lower by 2/S than the value of 6a -F 6/S for three isolated double bonds. Thus, the HMO method predicts a special stabilization for benzene. 

The pattern of two half-filled degenerate levels persists for larger rings containing 4n 71 electrons. In contrast, all 4 -F 2 systems are predicted to have all electrons paired in bonding MOs with net stabilization relative to isolated double bonds. This pattern provides 

Orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding 

For a historical account of early consideration of aromaticity, see J. P. Snyder, Nonbenzenoid Aromatics, Vol. 1, Academic Press, New York, 1969, Chapter 1. 

Cyclobutadiene has two bonding electrons, but the other two electrons are unpaired because of the degeneracy of the two nonbonding orbitals. The two electrons in the 

As indicated in Chapter 1, the simple HMO theory is based on rather drastic assumptions. More elaborate MO treatments indicate that the most stable geometry for cyclobutadiene is rectangular. Although several derivatives of cyclobutadiene are known and will be discussed shortly, cyclobutadiene itself has been observed only as a matrix-isolated species. Several compounds when photolyzed at very low temperature ( 10K) in solid argon release cyclobutadiene. Analysis of the infrared spectrum of the product and the tetradeutero analog generated from deuterated compounds is consistent with the theoretical conclusion that cyclobutadiene is a rectangular molecule.  

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There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid ( 2.5 fig. 2.5) rates relative to benzene vary in the range 68-80% sulphuric acid, and at the higher end of this range are not entirely measures of relative reactivity. For deactivated compounds this limitation is not very important, but for activated compounds it is linked with a fundamental limit to the significance of the concept of aromatic reactivity as already discussed ( 2.5), nitration in sulphuric acid cannot differentiate amongst compounds not less than about 38 times more reactive than benzene. At this point differentiation disappears because reactions occur at the encounter rate. 

Porphyrin is a multi-detectable molecule, that is, a number of its properties are detectable by many physical methods. Not only the most popular nuclear magnetic resonance and light absorption and emission spectroscopic methods, but also the electron spin resonance method for paramagnetic metallopor-phyrins and Mossbauer spectroscopy for iron and tin porphyrins are frequently used to estimate the electronic structure of porphyrins. By using these multi-detectable properties of the porphyrins of CPOs, a novel physical phenomenon is expected to be found. In particular, the topology of the cyclic shape is an ideal one-dimensional state of the materials used in quantum physics [ 16]. The concept of aromaticity found in fuUerenes, spherical aromaticity, will be revised using TT-conjugated CPOs [17]. 

Despite nearly two centuries of intense scrutiny, aromaticity remains a unique research stimulus in chemistry. The concept of aromaticity is elusive it is not directly observable. Numerous indirect measures have been devised, based on the manifestations and ramifications of aromaticity. One of the most recent and widely accepted definitions [2] described aromaticity as a manifestation of electron delocalization in... 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

Because of the conventional upbringing of organic chemists under the auspices of Hiickel theory53 and the concepts of aromaticity and antiaromaticity54-58, it is convenient to examine how the nature of pi nonbonded interactions is dependent... 

Aromaticity relates fundamentally to chemical reactivity from both the thermodynamic and kinetic standpoints.65 From the experimental chemist s point of view, the energetic implications of aromaticity dominate. Whereas the geometric and magnetic effects of aromaticity are of undoubted theoretical interest, it is the energy differences between a molecule, its reaction products, and the transition state which leads to the reaction products that governs the stability and the reactivity of that molecule.65 From a practical standpoint, the concept of aromaticity is thus of critical importance, as follows. 

The concept of aromaticity is important in the rationalization of chemical reactivity and, by extension, in the understanding of biological properties and the prediction of technological behavior. In heteroaromatic chemistry, the degree of aromaticity is of particular importance in guiding our understanding of aromaticity. 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

Finally, the concept of aromaticity in heterocyclic chemistry is the subject of an overview by Professor V. I. Minkin and Drs. B. Ya. Simkin and M. N. Glukhovtsev of Rostov State University in Russia. This new review shows just how wide the aromaticity concept has become. The present authors have successfully extended its range to the consideration of numerous very diverse heterocyclic systems. 

The relative stability of 1,2,3-triazole and benzotriazole tautomers can be rationalized based on the concept of aromaticity. The aromaticity of both 1,2,3-triazole tautomers probably being similar, the lone-lone pair repulsion accounts for the lower stability of 1//-1,2,3-triazole. In the case of benzotriazole, the aromaticity of the benzenoid 1//-benzotriazole has been considered greater than that of the quinonoid 2//-benzotriazole <89JA7348>. However, great care must be taken in measuring the difference in aromaticity between the two tautomers because this will depend strongly on the dielectric constant of the medium. 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

The concept of aromaticity has been linked to those of tautomerism and equilibrium by using KT, or an equilibrium constant as a measure of the binding energy difference between the pyridinoid and pyridonoid forms, and comparing this to the corresponding quantities for saturated derivatives (Scheme 12). 

In the discussion of structural and spectroscopic aspects of thiophenes to follow, the reader does well to bear in mind that in fact aromaticity is a hallmark of thiophenes that dominates their properties and governs their reactions. Moreover, thiophene stands as uniquely aromatic among the five-membered heterocycles. In concrete terms this means that not only does thiophene undergo the reactions and have the physical and spectral properties associated with the concept of aromaticity, but that this aromatic character is steadfastly maintained through all manners of transformations including fusion with other aromatic rings. This attachment of substituents or fusion with other rings spawns thousands of aromatic derivatives. 

Homoaromaticity as a chemical concept is based on the concepts of aromaticity and homoconjugation. In its simplest form aromaticity is an electron counting concept. Thus if there are 4q + 2 7t-electrons in a planar (or nearly planar) cyclic system, then this is considered to be aromatic. ... 

The concept of aromaticity has been extremely fruitful for both theoretical and experimental organic chemists. Aromatic compounds are cyclic unsaturated molecules characterized by certain magnetic effects and by substantially lower chemical reactivity and greater thermodynamic stability than would be expected from localized bond models. 

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