Huckel rule polyenes

The next higher annulene, cyclooctatetraene, is readily determined to be nonaromatic. The bond lengths around the ring alternate as expected for a polyene, and thermochemical data provide no evidence of special thermodynamic stability. Neither is the molecule antiaromatic. It is readily isolated and has the chemical behavior of a polyene. The molecule has a tub shape,and therefore is not a planar system of the type to which the Huckel rule applies. 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]aimulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Huckel rule can be tested by considering the properties of members of the annulene series. 

Annulene offers a particularly significant test of the Huckel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

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... 

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (Huckel s rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when m = 1, i.e. that with 6k electrons. IOti electrons ( = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol ], and 4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mor respectively ... 

Planar cyclic polyenes containing (4rt + 2) rt-electrons obey Huckel s rule for aromaticity and show greater stability than that predicted from their classical structures. 

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. 

Huckel s rule (in its original form) stated that monocyclic polyenic molecules are aromatic only if their re-systems contain An + 2) re-el ec-trons, where n is an integer 1>. There have been many advances in LCAO-MO theory since Hiickel s original contributions (although the simplest approximation still bears his name, i.e. HMO), and today a more precise statement of the rule might read as follows. 

This is a 16-membered conjugated polyene, with 16 tt electrons spread over the carbons of the rin. Since its number of tt electrons does not satisfy Huckel s rule,... 

Oxonin (11) is a polyenic heterocycle, even though Huckel s 4n + 2 rule of aromaticity is formally obeyed. The naming of reduced derivatives of (11) follows a similar pattern to that of l f-azonine (1), but the fully saturated oxocycle (12) is given its own name, oxonane. The lactone (13) (caprylo-lactone) is named by lUPAC rules as 2-oxonanone. 

Prior to this discussion of experimental n.m.r. data, a brief theoretical outline of annulene chemistry will be included, which begins with a consideration of the phenomenon of bond alternation in the next section. An understanding of bond alternation is necessary as it augments HUckel s rule in delineating more precisely the relationship between the annulenes and the corresponding linear polyenes. A section will then be devoted to a consideration of some of the more important experimental parameters (other than magnetic properties) which have been studied in relation to aromaticity. 

This argument can obviously be extended to concerted pericyclic reactions of all kinds. The transition state for any such reaction will be isoconjugate with a normal Hiickel-type cyclic polyene or an anti-Hiickel analog of one. If the transition state is aromatic, the resulting stabilization will lower its energy and so accelerate the reaction. If it is antiaromatic, the converse will be true. Since, moreover, the rules for aromaticity in Huckel-type and anti-Huckel-type systems are diametrically opposite, in each case one will be aromatic and the other antiaromatic. If, then, a reaction can follow one of two alternative pericyclic paths, one involving a Hiickel-type transition state and the other an anti-Hiickel-type transition, the reaction will prefer to follow the path in which the transition state is aromatic. If, on the other hand, only one of the two alternatives is sterically possible, the reaction will take place relatively easily if the corresponding transition state is aromatic and with relative difficulty if it is antiaromatic. In the latter case, the antiaromatic transition state will, if possible, be bypassed by a two-step mechanism in which the transition state is linear instead of cyclic [e.g., equation (5.291)]. 

Other Cyclic Polyenes Huckel s Rule CHAPTER 15... 

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