Cyclopentadienyl cation, and

Figure 32, The main symmetric conical intersection of cyclopentadienyl cation, and five secondary conical intersections around it.

Repeat Problem 16-10 for the cyclopentadienyl ions. Draw one all-bonding MO, then a pair of degenerate MOs, and then a final pair of degenerate MOs. Draw the energy diagram, fill in the electrons, and confirm the electronic configurations of the cyclopentadienyl cation and anion. 

In practice, both the cyclopentadienyl cation and the radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-rr-electron cyclopentadienyl anion, by contrast, is easily prepared and remarkably stable. In fact, cyclopentadiene is one of the most acidic hydrocarbons known, with pKg = 16, a value comparable to tliat of water Cyclopentadiene is acidic because the anion formed by loss of H" is so stable (Mglire 15.5). 

While there is no doubt that as the degree of conjugation in the polymer increases, stable polyenic carbenium ions are formed in the acidic medium, the nature of the chain carriers involved in the formation of the dimeric ester and in the early sta s of its polymerisation is worth discussing. The authors invoked the formation of carbenium ions from the first step of the process but no proof of their presence, or of the presence of the dimer cation and dication was offered. An alternative mechanism based on activated ester molecules would be more plausible to us in view of the hi reactivity of the cyclopentadienyl cation and the low likelyhood that it would be formed in such mildly acidic conditions as those employed in these experiments. 

Although five resonance structures can also be drawn for both the cyclopentadienyl cation and radical, only the cyclopentadienyl anion has six ji electrons, a number that satisfies Huckel s rule. The cyclopentadienyl cation has four ji electrons, making it antiaromatic and especially unstable. The cyclopentadienyl radical has five ji electrons, so it is neither aromatic nor antiaromatic. Having the right number of electrons is necessary for a species to be unusually stable by virtue of aromaticity. 

Use the inscribed polygon method to show why the cyclopentadienyl cation and radical are not aromatic. 

The distribution of electrons in the tt molecular orbitals of (a) benzene, (b) the cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. The reiative energies of the tt molecular orbitals in a cyclic compound correspond to the relative levels of the vertices. Molecular orbitals below the midpoint of the cyclic structure are bonding, those above the midpoint are antibonding, and those at the midpoint are nonbonding. 

PRACTICE PROBLEM 14.4 Apply the polygon-and-circle method to cyclopentadienyl cation and explain whether it... 

Presumably, this transformation proceeds via initial protonation of divinylketone 49 by phosphoramide 50, resulting in the formation of adduct 53, which consists of a cyclopentadienyl cation and a phosphoramide anion. Subsequent conrotatory 4n electrocyclization leads to an oxyal-lyl cation 54 that, through the elimination of a proton, forms 55. Subsequent protonation results in the formation of cyclopentenone 51 or 52, regenerating Br0nsted acid catalyst 50 (Figure 8). 

Describe the ground-state electron configuration of the cyclopentadienyl cation and radical. Assuming each species is planar, would you expect it to be aromatic or antiaromatic ... 

Gallup and Norbeck have made valence bond ab initio calculations on the electronic structure of cyclopentadienyl cation and anion with Dj symmetry. The lowest singlet- and triplet-state energies were calculated for each ion, from which it was concluded that, as expected, the cation has a 2 ground state and the anion ground state, i.e. antiaromatic and aromatic respectively. 

The distribution of electrons in the rr molecular orbitals of (a) benzene, (b) the cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. 

Antiaromatic annulenes are cyclically conjugated hydrocarbons that contain 4nn electrons. Examples of antiaromatic annulenes containing 4n electrons are cyclopropenyl anion, cyclobutadiene, cyclopentadienyl cation, and benzene dication. At its geometry of highest symmetry an antiaromatic annulene... 

In antiaromatic annulene ions with 4m 1 or 4 2 atoms the two degenerate MOs have atoms in common. Consequently, because these MOs are nondisjoint, at the geometry of highest symmetry the triplet is calculated to lie well below the singlet in energy. Thus, as predicted by Hund s rule, many antiaromatic annulene ions (e.g., cyclopentadienyl cation and hexachlorobenzene dication) have been found experimentally to have triplet ground states. 

Ionization of 5-chloro-l,3-cyclopentadiene would produce a cyclopentadienyl cation, and the cyclopentadienyl cation (see Problem 14.8) would be highly unstable because it would be antiaromatic. 

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