Monocyclic conjugated polyenes

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Among planar monocyclic fully conjugated polyenes only those possessing (4n + 2) TT electrons where n is a whole number will have special stability that be aromatic... 

The An + 2) tt electron standard follows from the pattern of orbital energies m monocyclic completely conjugated polyenes The tt energy levels were shown for ben zene earlier m Figure 114 and are repeated m Figure Figure 11 13a and 11 13c... 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

As Hiickel formulated it, the An + 2 rule applies only to monocyclic systems. However, as a practical matter it can be used to predict the properties of polycyclic conjugated polyenes, provided the important VB structures involve only the perimeter double bonds, as in the following examples ... 

Next we apply this simple model to the annulenes. These compounds are monocyclic conjugated polyenes with the general molecular formula (CH) with n even. Thus benzene may be considered as [6]-annulene. As n increases, essentially all [ ]-annulenes are non-planar. For instance, cyclooctatetraene, (CH)s, has the well-know tub structure. However, [18]-annulene is nearly planar, as shown below (bond lengths in picometers are displayed in bold italic font). 

On the basis of his analysis Hiickel proposed that only certain numbers of tt electrons could lead to aromatic stabilization. Only when the number of tt electrons is 2, 6, 10, 14, and so on, can a closed-shell electron configuration be realized. These results are summarized in Hiickel s rule Among planar, monocyclic, fully conjugated polyenes, only those possessing (4 + 2) ti electrons, where n is an integer, will have special aromatic stability. 

Section 11.19 An additional requirement for aromaticity is that the number of tt electrons in conjugated, planar, monocyclic species must be equal to An + 2, where n is an integer. This is called Hiickel s rule. Benzene, with six TT electrons, satisfies Huckel s rule for n = 1. Cyclobutadiene (four tt electrons) and cyclooctatetraene (eight tt electrons) do not. Planar, mono-cyclic, completely conjugated polyenes are called annulenes. 

For monocyclic conjugated polyenes, high stabilization is found for systems with (4n + 2) TT electrons but not for systems with (4n) tt electrons. The relationship is formulated as Hiickel s rule, which states that completely conjugated planar hydrocarbons are strongly stabilized (aromatic) when they have 4n -I- 2) tt electrons. Benzene (6 tt electrons) is aromatic but cyclobutadiene (4 tt electrons) and cyclooc-tatetraene (8 tt electrons) are not. 

Among planar, monocyclic, fully conjugated polyenes, only those possessing... 

Section 11.20 Annulenes are monocyclic, completely conjugated polyenes synthesized... 

E. Huckel was the first to show by the molecular orbital theory that the monocyclic conjugated polyenes have filled stable shells of ir electrons when the number of such electrons was 4n + 2, where n is a positive integer. When... 

The success of simple HMO theory in dealing with the relative stabilities of cyclic conjugated polyenes is impressive. Simple resonance arguments lead to confusion when one tries to compare the unique stability of benzene with the elusive and unstable nature of cyclobutadiene. (Two apparently analogous resonance structures can be drawn in each case.) This contrast is readily explained by Hiickel s rule, which states that a species composed of a planar monocyclic array of atoms. 

---