Orbitals, molecular pentadienyl system

The interaction of atomic orbitals giving rise to molecular orbitals is the simplest type of conjugation. Thus in ethylene the two p orbitals can be described as being conjugated with each other to make the n bond. The simplest extension to make longer conjugated systems is to add one p orbital at a time to the n bond to make successively the n components of the allyl system with three carbon atoms, of butadiene with four, of the pentadienyl system with five, and so on. Hiickel theory applies, because in each case we separate completely the n system from the a framework, and we can continue to use the electron-in-the-box model. 

The molecular orbital diagrams for propenyl and pentadienyl systems are illustrated in Figure 1.7 in which the molecular orbitals for their corresponding... 

K Molecular orbitals of the pentadienyl system (for the radical, the HOMO is 3)... 

The hydrogen atom is simple to describe—it merely consists of a proton and a single electron in a It atomic orbital. The pentadienyl molecular orbital system isn t difficult either. There are five n molecular orbitals and these can be constructed by taking combinations of five carbon orbitals. We assume again that it is the highest energy... 

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. 

Pentadienyl System 2, 4-pentadienyl system possesses five molecular orbitals. Their symmetry properties and electron occupancies are given in Fig. 2.11. 

Qualitative representation of the molecular orbitals for linear 7t systems (a) pentadienyl and (b) hexatriene. (Adapted from reference 8.)... 

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. 

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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