Pentadienyl anion, molecular orbitals

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... 

Nonetheless, some useful information regarding methylation has been obtained from molecular orbital calculations. One CNDO/2 study on the allyl and pentadienyl anions indicated that in all cases the methyl group would serve to withdraw net electron density from the anions196. For the allyl anion, a 1-methyl substituent would withdraw a substantial 0.102 electrons, while at the formally uncharged 2 position the effect was consider-... 

Molecular orbital (MO) calculations indicate that the intermediate radical anions have more anionic character at the positions ortho or meta to a TT-donating substituent and para or ipso. ipso means at the substituent-containing carbon) to a 7r-accepting substituent. (See Birch, A. J. Hinde, A. L. Radom, L. J. Am. Chem. Soc. 1980, 102, 3370-3376). The pentadienyl anions formed after transfer of the second electron have more anionic character on the central carbon, and that is where the second protonation occurs. 

Molecular Orbitals and Organic Chemical Reactions Student Edition by Ian Fleming, Wiley, Chichester, 2009 on pentadienyl anions,... 

FIGURE 2.18 Molecular orbitals and electron occupancy in pentadienyl cation and anion. 

Sketch the molecular orbitals for the pentadienyl system in order of ascending energy (see Figures 14-2 and 14-7). Indicate how many electrons are present, and in which orbitals, for (a) the radical (b) the cation (c) the anion (see Figures 14-3 and 14-7). Draw all reasonable resonance forms for any one of these three species. 

The highest energy 7i electrons of the pentadienyl anion are in the tIj molecular orbital. Draw this orbital, and predict the location of the negative charge for the ion. 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

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