Ring compounds cyclic polyenes

Hiickel s rule (presented in Section 21.1) states that for planar, monocyclic hydrocarbons containing completely conjugated sp hybridized atoms, the presence of(4n + 2) K-electrons leads to aromaticity ( n is an integer in the series 0, 1, 2, 3, etc.). This rule is associated with particularly stable molecules. Acyclic molecules with a continuous array of sp hybridized carbon atoms are known as polyenes. For aromaticity, there must be a ring, so cyclic polyenes are required. If the number of 71-electrons does not equal 4n + 2, then aromaticity does not exist and the system is rather unstable (this is sometimes called an antiaromatic system). If HiickeVs rule (or the 4n + 2 rule, as it is called) is used, the presence of 2, 6, 10, 14, 18, etc. K-electrons in a ring where every atom is Sff hybridized is characteristic of an aromatic compound. Benzene clearly fits this rule and so does compound 111, which has 14 71-electrons. 

Later in this chapter we 11 explore the criteria for aromaticity in more detail to see how they apply to cyclic polyenes of different ring sizes The next several sections intro duce us to the chemistry of compounds that contain a benzene ring as a structural unit We 11 start with how we name them... 

Conjugated linear trienes were reported to undergo cyclization to form seven-membered ring systems.112 Cyclic polyenes, in turn, isomerize to yield bicyclic compounds. In the transformation of nonatriene (10), the role of the base was demonstrated to catalyze the formation of the conjugated triene [Eq. (4.23)], which... 

We can define an aromatic compound - - as a compound which will sustain an induced (diamagnetic) ring current" [35]. Evidence for ir-electron delocalisation in cyclic polyenes is also forthcoming from measures of coupling constants, especially the differences between the coupling constants across neighbouring bonds. If these are markedly different... 

Compound 111 is a cyclic compound with 14 7i-electrons, and every carbon in the ring is sp hybridized. Because it fits the Hiickel rule, it is aromatic. Larger cyclic polyenes may also be aromatic, and by definition they must be planar. Cyclic polyene 112 is a cyclic compound with 18 7i-electrons every carbon is sp hybridized, so it also satisfies the Hiickel rule and is aromatic. Compounds such as these are termed annulenes (completely conjugated aromatic hydrocarbons) and this becomes part of the name. Benzene could be named [6]-annulene, 111 is named [14]-annulene, and 112 is [18]-annulene. 

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). 

FIGURE 3.39 Some cyclic polyenes, ring compounds containing more than one double bond. 

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). 

Five and Six Ring Sulphur Systems.—The addition of sulphur dichloride to unsubstituted cyclic polyenes to give bridged bicyclic compounds has been known for a... 

This type of reaction, whether it involves the cyclisation of a polyene, as here, or the ring-opening of a cyclic compound to form a polyene, is known as an electrocyclic reaction. 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

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