Aromatic sextet rule

Clar 1957-1960 Spectra of acenes, aromatic sextet rules... 

We see, therefore, from the arguments in 5-2, that although there are difficulties with the simple picture in the case of monocyclic hydrocarbons, C H , when n = 4p, the basis of the aromatic-sextet rule has been well rationalised. Benzene is stable as a neutral species, the cyclopentadienyl system is more stable as an anion, (C5H5), and the cycloheptatrienyl system is more stable as a cation, (C7H7)+, and these observations are all well substantiated experimentally. We now attempt to extend the rule, in a heuristic way, to systems other than monocyclic hydrocarbons and consider some examples of such extensions. 

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... 

The article "A History of the Structural Theory of Benzene—The Aromatic Sextet and Huckel s Rule" in the February 1997 issue of the Journal of Chemical Education (pp. 194-201) is a rich source of additional information about this topic. 

The orbital symmetry rules also help us to explain, as on pages 1083 and 1433, the unexpected stability of certain compounds. Thus, 102 could, by a thermal [1,3] sigmatropic rearrangement, easily convert to toluene, which of course is far more stable because it has an aromatic sextet. Yet 102 has been prepared and is stable at dry ice temperature and in dilute solutions. ... 

Benzene conforms to Hiickel s ruie, which predicts that planar cyclic polyenes containing 4 -I- 2 7t electrons show enhanced stability associated with aromaticity (see Section 2.9.3). Pyridine is also aromatic nitrogen contributes one electron in a orbital to the Jt electron system, and its lone pair is located in an sp orbital that is in the plane of the ring and perpendicular to the n electron system. It also conforms to HtickeTs rule, in that we still have an aromatic sextet of Jt electrons. 

In the theory known nowadays as the Clar theory of the aromatic sextet [12] a benzenoid system is represented by a Clar structure which is obtained by drawing circles in some of the hexagons of the corresponding benzenoid graph. These circles represent the aromatic sextets in the hydrocarbon. We consider here only Clar structures containing the maximum number of circles which are some times referred to as proper (or correct) Clar formulas. The rules for constructing such Clar structures are as follows [13]. 

Aromaticity is one of the great unifying principles of chemistry and although it was devised for benzene and other hydrocarbons it was soon extended to heterocycles. For instance, the Hiickel rule was published in 1931 [1, 2], while Robinson introduced the aromatic sextet concept in 1925 [3-5] and extended it to five-membered heterocycles, considering that a C-C double bond can be replaced by an heteroatom bearing a lone pair [3-5] (see however [6]). In this review we will discuss only heteroaromaticity, leaving aside most aspects concerning aromaticity. Even so, the number of literature references is enormous so we will restrict ourselves to our own contributions and to some recent and important papers by others. 

The effect of the biphenyl rule is small and is usually screened by much stronger conjugation modes (e.g. those taken into account by resonance, conjug-ated-circuit and/or Clar-aromatic-sextet theories [64]), In some exceptional cases, however, the biphenyl rule can completely invert the conjugation pattern anticipated by the classical theories. This particularly occurs in benzenoid... 

Let us look at some of the evidence supporting the Hiickel rule. Benzene has six 7T electrons, the aromatic sextet six is, of course, a Hiickel number, corresponding to /f = 1. Besides benzene and its relatives (naphthalene, anthracene, phenanthrene. Chap. 30), we shall encounter a number of heterocyclic compounds (Chap. 31) that are clearly aromatic these aromatic heterocycles, we shall see, are just the ones that can provide an aromatic sextet. 

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