Huckel’s rules

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... 

Huckel proposed his theory before ideas of antiaromaticity emerged We can amplify his generalization by noting that among the hydrocarbons covered by Huckel s rule those with An) tt electrons not only are not aromatic they are antiaromatic... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

In the next section we 11 explore Huckel s rule for values of n greater than 1 to see how it can be extended beyond cyclobutadiene benzene and cyclooctatetraene... 

Use Frosts circle to construct orbital energy diagrams for (a) [lOjannulene and (b) [12]annulene Is either aromatic according to Huckel s rule ... 

Most of the synthetic work directed toward the higher annulenes was earned out by Franz Sondheimer and his students first at Israel s Weizmann Institute and later at the University of London Sondheimer s research systematically explored the chemistry of these hydrocarbons and provided experimental venfication of Huckel s rule... 

Huckel s rule is now taken to apply to planar monocyclic completely conjugated systems generally not just to neutral hydrocarbons... 

The article A History of the Structural Theory of Benzene—The Aromatic Sex tet and Huckel s Rule in the February 1997 issue of the Journal of Chemical Educa tion (pp 194-201) IS a rich source of additional informa tion about this topic... 

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

From among the molecules and ions shown all of which are based on cycloundecapentaene identify those which satisfy the criteria for aromaticity as prescribed by Huckel s rule... 

It is of interest to be able predict the stability of such fiised-ring compounds. Because Huckel s rule applies only to monocyclic systems, it cannot be applied to the fiised-ring compounds, and there have been many efforts to develop relationships which would predict their stabihty. The underlying concepts are the same as for monocyclic systems stabilization should result from a particularly stable arrangement of MOs whereas instability would be associated with unpaired electrons or electrons in high-energy orbitals. 

We saw in Chapter 12 that aromaticity reveals itself in various ways. Qualitatively, aromatic compounds are more stable and less reactive than alkenes. Quantitatively, their heats of hydrogenation are smaller than expected. Theory, especially Huckel s rule, furnishes a structural basis for aromaticity. Now let s examine some novel features of their NMR spectra. 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

Huckel s rule (Section 15.3) A rule stating that monocyclic conjugated molecules having 4n + 2 tt electrons n = an integer) are aromatic. 

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... 

According to Huckel s rule a monocyclic planer conjugated system having (4n + 2)n electrons is aromatic and hence stable in the ground state. A system having An n electrons is unstable and will be antiaromatic. 

We know that aromatic compounds, polynuclear and heterocyclic compounds follow Huckel s rule in which they have a (4n + 2) system of n electrons and the protons attached to such systems are extremely deshielded due to the circulating sextet of n electrons. As a result, the signals of aromatic protons appear at a very low field than those observed even for benzene. The tau value of benzene is T = 2.63. From this the aromaticity of a compound can be verified. 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

Tables 1 and 2 in combination delineate the extremely large variety of monocyclic heterocycles obeying Huckel s rule. If one adds to Table 1 heavier atoms (even including metals, as will be exemplified for osmabenzene), the diversity is even richer. Most of this diversity remains to be explained for example, very few of the possible combinations involv-...

HUckel s 4n -H 2 rule orgchem Aromatic (ring) compounds must have 4n + 2 pi-bonding electrons, where n is a whole number and generally limited to = 0 to 5. When M = 1, for example, there are six pi-electrons, as for benzene. Also known as Huckel s rule. hhk-alz jfor, en plos tu, rul ... 

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 ... 

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