Aromaticity reactivity criteria

Reactivity and aromaticity are directly related only when the energy level of the transition state remains constant, which is usually not the case. Only for a series of compounds where the changes in the energy of ground state and transition state are proportional would a reactivity criterion be straightforward. ... 

The bonding of all cycZo-M H molecules is characterized by a common ring-shaped electron density, which is constructed by highly delocalized a-, n-, and 8- type MOs and associated with their aromatic character. The aromaticity of all cyclo-M U (M = Cu, Ag, Au n = 3-6) molecules was estimated by making use of several criteria for aromaticity, such as the NICS(O) parameter, the relative hardness, At], and electrophilicity index, o). It was further verified on the grounds of a chemical reactivity criterion of aromaticity, that of the interaction of the aromatics with electrophiles, such as Li" ", T1+, or Ag" ". 

The reactivity criterion is less than satisfactory since many compounds that are generally recognized cis aromatic are quite reactive. For example, both phenanthrene and anthracene will add bromine, and anthracene will undergo Diels—Alder additions. The bond length criterion is also not quite satisfactory since compounds such a naphthalene, anthracene, and phenanthrene have a range of bond lengths. ... 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

An additional criterion, in our opinion a very important one, but which is not a property of the ground state, is reactivity. Generally, aromatic compounds undergo electrophilic substitution reactions (aromatic substitution) more easily than addition, which is often expressed as a typical tendency of these kinds of systems to retain their initial tr-electron structure [16, 17]. 

Quantum Mechanical Studies of Metabolism of Polycyclic Aromatic Hydrocarbons—Bay Region Reactivity as a Criterion for Carcinogenic Potency. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

Whereas strained ring systems are usually reactive and often unstable, molecules which satisfy the criteria for aromaticity exhibit enhanced stability. As is evident from the structural formula of 1, the cycloproparenes set these features in juxtaposition as they are strained molecules in which a single carbon atom is fused across adjacent centres of an aromatic system. The interest of the experimentalist in strained molecules has been matched by the theoretician in the search for suitable models for developing the concepts of chemical bonding and aromaticity. The cycloproparenes have been particularly important in this regard as they meet the criterion for partial aromatic bond localization and consequent bond length alternation in the aromatic ring as proposed by Mills and Nixon in 1930, viz. la vs lb. The cation 5, anion 6 and radical 7 derived from 1, and also the ketone 8 and exocyclic methylene derivative 9, are of interest in this respect. 

The discrepancy between physicochemical and reactivity criteria for aromaticity was known long ago. Yet the data procured by the studies of compounds like 186 or 189 shed a new light on this problem. The observed deviation from planarity did not cause any substantial changes in the spectral parameters of these compounds. Thus they still should be considered aromatic from the point of view of physical chemists. At the same time, this distortion of geometry so dramatically affected their reactivity pattern that, in accordance with this criterion, cyclophanes 186 or 189 should best be referred to as non-aromatic compounds, derivatives of 1,3,5-cyclohexatriene. ... 

As discussed in the Introduction, Huckel s rule is an electronic criterion for aromaticity, and is based on the configuration of the rr electrons. Another characteristic of aromatic compounds is a relatively large HOMO-LUMO gap, which indicates the absence of high-energy, reactive electrons, in agreement with the reduced reactivity of aromatic compounds to electrophilic reagents. This facet of electronic configuration can be expressed in terms of hardness (see p. 96 for the definition of hardness in terms of DFT theory). ... 

The reactions that have been illustrated should give an idea of the potential of a methodology which combines the criterion of multicomponent reactions with that of selectivity, usually difficult to reconcile. One key feature is the use of an olefin as a scaffold for the construction of a palladacycle that is able to direct aromatic functionalization selectively and can be easily removed at the end of the process. Another important feature is the use of different oxidation states of palladium to control reactivity. The combination of an inorganic catalyst (palladium) with an organic one (norbornene) leads to a variety of syntheses in one-pot reactions, which represent only the beginning of what may be expected to be a very fruitful development. Needless to say, any advancement in this area requires a thorough study of the reactivity of the organometallic species involved. 

In these cases, the chlorobenzenes act as electron acceptors while the benzidine molecule behaves as an electron donor. The effect of chlorine substitution on the aromaticity of the planar benzene ring of the chlorobenzene derivatives is assessed from the nucleus independent chemical shift (NICS) criterion proposed by Schleyer et al. [338]. Among the various global and local molecular descriptors, electrophilicity (m) is found to be the most appropriate reactivity parameter regarding toxicity... 

Reactivity is also a very important criterion of aromaticity, because that is what started the whole concept of aromaticity in chemistry. Indeed, it was the lack of reactivity toward addition reactions for benzene and other initially identified aromatic molecules that was recognized as a special property. Instead, aromatic molecules undergo electrophihc substitution rather than addition. Yet, this probe of aromaticity is the most difficult to use especially in inorganic chemistry. 

Ciesielski, A., Krygowski, T. M., Cyranski, M. K., Dobrowolski, M. A., Balaban, A. T. (2009). Are thermodynamic and kinetic stabilities correlated A topological index of reactivity toward electrophiles used as a criterion of aromaticity of polycyclic benze-... 

The term aromatic was coined in 1855 by August Wilhelm von Hofmann (1818-1892), before the physical mechanism determining so-called aromaticity was unraveled. Originally, this definition was derived from a characteristic property, namely a sweet scent that was associated with some aromatic compounds. Subsequently, it was shown that arenes are unsaturated compounds, but showed a chemical reactivity which differed from that of both aUcenes or aUcynes. This unique reactivity pattern of arenes was used at the end of the nineteenth century as the only criterion of aromaticity. At the start of the twentieth century, additional... 

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