Antiaromatic conjugated circuit

However, in the 4, 8 semiregular planar lattice (Fig. 2) both the angle strain and the antiaromaticity contribute to destabilization in the conjugated circuits model there are 10- and 14-membered circuits with small positive contributions R2, Rsas well as 4-, 8- and 12-membered rings with appreciable negative coefficients Qi, Q2, 03. 

At the very beginning of this review, in Figure 2, we illustrated a sequence of structurally related compounds starting with aromatic benzene and ending with hypothetical anti-aromatic (planar) cyclooctatetraene. A gradual variation of the relative content of 4/7 4-2 and 4n conjugated circuits is expected as we move from the top to the bottom, from benzene to the planar hypothetical cyclooctatetrane. In Table 34 we show the aromaticity and antiaromaticity indices for the compounds in Figure 2. 

In all cases shown in Figure 90, we have only two Kekule valence structures. However, fully antiaromatic systems can have more than two Kekule valence structures, as illustrated in Figure 91. The expressions for their anti-aromatic destabilization energy are listed in Table 38. We see that the presence of only An conjugated circuits is a necessary condition for a structure to be a candidate for being anti-aromatic, or fully anti-aromatic . However, this is not sufficient. For such compounds to be truly antiaromatic, they must be planar and have all CC bonds of approximately equal length. 

The [nlannulenes may be defined as cyclic polyaUcenes possessing a closed circuit of n Ti-conjugated pz-orbitals. The first three small annulenes are shown in Fig. 1 and include [4]annulene (cyclobutadiene), [6]annulene (benzene), and [8]annulene (cyclooctatetraene). The bracketed number, n, can be classified as either a 4n+2 (Hiickel aromatic/Mobous antiasomatic) or 4n (Huckel antiaromatic/ Mobius aromatic) delocalized n-electron species. That said, it is not surprising that the concept of aromaticity is closely associated with the annulenes. Indeed, the first three annulenes listed cover the concepts of Hiickel aromatic, antiaromatic, and nonaromatic properties (see Fig. 1). 

The special stability and reactivity associated with cyclic delocalization is not unique to benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes can be aromatic, but only if they contain (An + 2) tt electrons (n = 0, 1, 2, 3,. . . ). In contrast, An tt circuits may be destabilized by conjugation, or are antiaromatic. This pattern is known as Hiickel s rule. Nonplanar systems in which cyclic overlap is disrupted sufficiently to impart alkene-like properties are classified as nonaromatic. Let us look at some members of this series, starting with 1,3-cyclobutadiene. 

---