atoms. Consider now cyclopropenyl. It again has three carbon atoms but they now

Question

atoms. Consider now cyclopropenyl. It again has three carbon atoms but they now form a triangular ring by means of -bonds between them. In addition, Just like in the allyl case, a second bond forms between the carbon atoms. This is a -bond. It is generated by each carbon atom contributing one 2p atomic orbital (AO) per- pendicular to the plane of the molecule as shown: Find (a) the energies; (b) the wave functions of the possible molecular orbitals (MO) formed as linear combinations of the three 2p AOs contributed by the three carbons. Each of these MOs can be occupied by an electron 'shared by the three carbons in forming this type of bond

Explanation / Answer

In the cation, there are only two -electrons which go into the bonding MO. There are no electrons in the antibonding orbitals. The lowering of energy due to the electron pair in the bonding MO provides the stability or aromaticity. In the free radical, there is one additional electron in the antibonding orbital, tending to destabilize the system. In the cyclopropenyl anion, there are two electrons in the antibonding MO, which completely cancels the stabilizing effect of the electron pair in the bonding MO. Note that in the allyl system, the third and fourth electrons go into a nonbonding orbital, whereas in the cyclic system they go into antibonding orbitals. Thus the cyclopropenyl free radical and anion are much more unstable than their allyl counterparts

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