Clar’s aromatic sextet rule

Armit and Robinson introduced the notion of six-member rings in unsaturated compounds associated with the k electron aromatic sextet that bestows on them unusual stability, for which they have introduced the circle notation. Robinson later abandoned this novel model to characterize benzenoid compounds. It was Clar who not only adopted the notion of aromatic sextets, but significantly developed the model into a theory, which could explain several regularities of selected properties of these compounds. Clar s aromatic sextet theory assigns to individual benzenoid hydrocarbons novel structural formulas, which are obtained by following the rules summarized in Table 11.1. 

The electronic properties and stability of PAHs depend not only on the size, but also markedly upon the arrangement of benzenoid rings. According to Clair s aromatic sextet rule, the stability of the isomers of certain polycyclic aromatics increases with the number of sextets. Thus, all-benzenoid polycyclic hydrocarbons (PBAHs) whose structure can be represented by a single Clar formula with no double bonds show extremely high stability, high melting point and low chemical reactivity. Some of them even exist in interstellar space [26]. A typical example is the hexa-peri-hexabenzocoronene (HBC, in Fig. 3.3) which has been studied since... 

Polycyclic aromatic hydrocarbons (PAHs) or polyacenes " like naphthalene, anthracene, phenanthrene etc. are aromatic systems with fused rings (Fig. 5). Since Hiickel s MO theory is mainly developed for monocyclic systems the (An +2)n electrons rule for aromaticity is not directly applicable to PAHs. Number of Clar s k sextet " (benzenoid rings) present in a PAH may give an indication towards its aromaticity. Although sometimes it is termed as superaromaticity (with more than one benzene ring) actually they are not so. Molecules like naphthalene and phenanthrene undergo addition reactions in addition to substitution reactions. The 3D aromatic system " " like fullerene exhibits exclusively addition reactions with hardly any signature of substitution reaction. 

M. Sola, Forty years of Clar s aromatic x-sextet rule. Front. Chem., Theor. Comput. Chem. 1 (2013) 22. 

Sola M (2013) Forty years of Clar s aromatic r-sextet rule. Frontiers Chem 1(22) 1-8... 

The parallelism between Clar s intuitive notion of aromatic sextets, migrating sextets, and anpty (of sextets) rings and the relative magnitudes of the numerical ring bond orders is so complete that there is not a single exception. So, in this respect, this theory is comparable with the Woodward-Hoffmann rules [28-31] initially formulated to explain the impressive stereospecificity of electrocyclic reactions under thermal and photochemical control. One lengthy review article on Woodward-Hoffmann rules ends with Exceptions None." The same is the case with our numerical characterization of k aromatic sextets with RBO. 

G. Portella, J. Poater, and M. Sola. Assessment of Clar s aromatie p-sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity, J. Phys. Org. Chem. 18 (2005) 785-791. 

In this chapter we review the results of our recent investigations on the abovementioned rales of aromaticity. It is worth noting that extension of these rales to aromatic polycyclic hydrocarbons (PAH) has led to the Clar n-sextet rule [20,21] and the GlideweU-Lloyd [22] extension of Clar s rale. These extensions will not be discussed in the present chapter. Neither we will comment on the 4m rale for Mobius aromaticity [23, 24] nor on its extension by Rzepa in what is known as the linking number rale [25]. 

Clar s Structure Rules Benzenoid having a single Clar structure with only aromatic Jt-sextets and empty rings, the fully benzenoid systems described by Clar, are the most stable benzenoids having several Clar structures are the next most stable and benzenoids having a single Clar structure with rings with isolated CC double bonds are the least stable. 

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