Pentadienyl cation molecular orbitals

Fig. 10.1. TT-Molecular orbitals and energy levels for the pentadienyl cation.

Sketch the nodal properties of the highest occupied molecular orbital of pentadienyl cation (CH2=CHCH=CHCH2 ). 

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

In order for a substitution to occur, a cr complex must be formed. The term a complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the group to be displaced. The term implies that a a bond has been formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a complex is a four-7r-electron delocalized system which is electronically equivalent to a pentadienyl cation. The 7r-molecular orbitals and energy levels for the pentadienyl cation are shown in Fig. 10.1. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive charge of the cation is located at the positions ortho and para to the site of substitution. 

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. 

Reaction of the pentadienyl cation with a nucleophile occurs by interaction with an empty molecular orbital. Which orbital ... 

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. 

---