Cyclic polyenes Cyclobutadiene

Both thermochemical and MO approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy which is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncydic polyene models are called antiaro-maticf ... 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

Cyclobutadiene, the simplest cyclic polyene, has long intrigued chemists however, the tt-electron molecule has... 

According to Hiickel s rule, annulenes with 4n tt electrons are not aromatic. Cyclobutadiene and cyclooctatetraene are [4n]-annulenes, and their properties are more in accord with their classification as cyclic polyenes than as aromatic hydrocarbons. Among higher [4n]-annulenes, [16]-annulene has been prepared. [16]-Annulene is not planar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene. 

First it provided an adequate explanation for the alternation in properties shown by the first three members of the cyclic polyene series, cyclobutadiene, benzene and cyclooctatetraene. 

Members of another set of cyclic polyenes have (4n) ii-electrons, such as cyclobutadiene (113 4 7i-electrons) and cyclooctatetraene (114 8 ir-electrons), so they do not adhere to the Hiickel rule. Compound 113 is a cyclic compound and every carbon is sp hybridized, but it has only 4 7t-electrons and does not satisfy the 4n -i- 2 rule (4 is not part of this series, so 113 does not have 2, 6, 10, 14, etc. 7t-electrons). Because 113 does not satisfy the Hiickel rule, it is not aromatic. Likewise, 114 is cyclic and has a continuous array of sp carbons, but 8 7t-electrons do not fit the 4n -H 2 series and thus do not satisfy the Hiickel rule, and 114 is not aromatic. These compounds are not aromatic and are also very unstable and difficult to prepare. Because they are so difficult to prepare and the ring system is so unstable, cyclic compounds such as this with 4n 7i-electrons are called antiaromatic compounds. For practical purposes, assume that such compounds cannot be prepared (although they can be if extremely low temperatures and specialized conditions are used). 

But not all ring compounds containing double bonds behave in this way. Some cycloalkenes react as expected cycloheptatriene and cyclooctatetraene are examples of this kind of cyclic molecule. Others are exceptionally unstable cyclobutadiene is the archetype of this kind of cyclic polyene (Fig. 13.3). 

Let us now examine tlie properties of the next higher cyclic polyene analog of benzene, 1,3,5,7-cyclooctatetraene, another 4n tt cycle (n = 2). Is it antiaromatic, like 1,3-cyclobutadiene First prepared in 1911 by Willstatter, this substance is now readily available from a special reaction, the nickel-catalyzed cyclotetramerization of ethyne. It is a yellow liquid (b.p. 152 °C) that is stable if kept cold but that polymerizes when heated. It is oxidized by air, eatalytically hydrogenated to cyclooctane, and snbject to electrophilic additions and to cycloaddition reactions. This chemical reactivity is diagnostic of a normal polyene (Section 14-7). 

Since the preparation of [I8]annulene, many other annulenes have been made. As long as they are (nearly) planar and delocalized, those with (4n + 2) tt electrons, such as benzene and [18]annulene, are aromatic, whereas those with 4n tt electrons, such as cyclobutadiene and [16]annulene, are antiaromatic. When cyclic delocalization is prohibited by angle or steric strain, such as in cyclooctatetraene or [10]annulene (Exercise 15-15), the systems are nonaromatic. Of course, cyclic polyenes in which there is no contiguous array of p orbitals are not annulenes and therefore also nonaromatic. 

Some cyclic conjugated polyenes do not satisfy the Hiickel rule and are not aromatic. Two examples are cyclobutadiene and cyclooctatetraene. Both are cyclic polyenes with alternating single and double bonds, but neither is aromatic. 

We do not need to look far to find the explanation. The four 71 electrons of cyclobutadiene do not satisfy the Hiickel rule. Cyclobutadiene is extremely unstable and has been isolated only at very low temperature. Its fleeting existence also has been inferred from the products of its reactions. Cyclooctatetraene has eight 71 electrons. This number that does not satisfy Hiickel rule either. Cyclic polyene that have 4 71 electrons, where is an integer, are unusually unstable they are antiaro-made. 

It is in this area that qualitative MO procedures have great success because there are general characteristics of the 77 molecular orbitals of mono-cyclic, conjugated polyene systems that predict differences in the properties of cyclobutadiene, benzene, cyclooctatraene, and other similar compounds that are not obvious from the simple VB method. 

More sophisticated calculations indicate that cyclic An systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17, 46]. 

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. 

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. 

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. 

UnUke cyclobutadiene and cyclooctatetraene, certain higher cyclic conjugated polyenes are aromatic. All of them have two properties in common They contain (4n + 2) tt electrons, and they are sufQciently planar to allow for delocahzation. 

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