Cyclopropenyl cations molecular orbitals

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system (C l I3). How ate these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical Which of the three substances is aromatic according to Hiickel s rule ... 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 15.2, draw the tt molecular orbital energy levels for the cyclohep-tatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the tt electrons. Which of the compounds are aromatic Which are andaromatic ... 

Cyclopropene has the correct number of it electrons to be aromatic, namely 4(0) + 2 = 2, but it does not have a continuous closed loop of 2p orbitals. If, however, the CH2 group becomes a CH+ group in which the carbon atom is sp hybridized and has a vacant 2p orbital, thus still containing only two electrons, then the overlap of orbitals is continuous, and according to molecular orbital theory, the cyclopropenyl cation should be aromatic. The cyclopropenyl cation can be drawn as a resonance hybrid of three equivalent contributing structures. The fact that we can draw three equivalent contributing structures is not the key to the aromaticity of this cation the key is that it meets the Hiickel criteria of aromaticity. 

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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