HiickeFs rule

Bridged [10]- and [14]anulenes8 are of considerable interest not only because of their aromaticity (complying with HiickeFs rule) but also because of their (rigid) molecular geometry. The most simple representative l,6-methano[10]anulene (95) first obtained in 1964 U5), as well as others with oxido or imino bridges (cf. 103), have C2v-symmetry, but become chiral (C,) by substitution in any position. Such chiral anulenes can be classified as planar chiral under the assumption that the ten-... 

An aromatic molecule, according to HiickeFs rule, contains a cyclic array of orbitals in which there are [4n -f 2] electrons. Since the [44-2]-cycloaddition reaction involves the four TT-electrons of 1,3-butadiene and the two TT-electrons of ethene, there must be six electrons involved in the transition state. Therefore, the [4-1-2]-cycloaddition reaction involves an aromatic transition state. 

HiickeFs rule also pertains to charged cyclic conjugated systems. The cyclo-propenyl (2 tt electrons), cyclopentadienyl anion (6 tt electrons), and cycloheptatrienyl (tropylium) cation (6 tt electrons) are examples of stabilized systems. We say much more about the relationship between MO configuration and aromaticity in Chapter 9. 

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 HiickeFs rule. Benzene, with six tt electrons, satisfies HiickeFs rule iov n = 1. Square cyclobutadiene (four tt electrons) and planar cyclooctatetraene (eight tt electrons) do not. Both are examples of systems with An TT electrons and are antiaromatic. 

Erich Hiickel developed a shortcut for predicting which of the annulenes and related compounds are aromatic and which are antiaromatic. In using Hiickefs rule, we must be certain that the compound under consideration meets the criteria for an aromatic or antiaromatic system. 

Hiickefs Rule If the number of pi electrons in the cyclic system is ... 

The German physicist Erich Hiickel was the first to recognize that an aromatic compound must have an odd number of pairs of ir electrons. In 1931, he described this requirement in what has come to be known as HiickeFs rule, or the 4h + 2 rule. The rule states that for a planar, cychc compound to be aromatic, its unintenupted jT cloud must contain (4n + 2) jt electrons, where n is any whole number. According to Hiickel s rule, then, an aromatic compound must have 2 (n = 0),6 (n = 1), 10 (n = 2), 14 (n = 3), 18 (n = 4), and so on -jt electrons. Because there are two electrons in a pair, Hiickel s rule requires that an aromatic compound have 1,3,5,7,9 and so on pairs of jt electrons. Thus, Hiickel s rule is a mathematical way of saying that an aromatic compound must have an odd number of pairs of tt electrons. 

Table 9.1. HiickeFs Rule Relationships for Charged Species... 

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