This can also explain why allylic radicals are much more stable than secondary or even tertiary carbocations. Since every carbon center shown has two electrons in the lower energy, bonding π orbitals, the energy of each system is lowered overall (and thus more stable), regardless of cation, radical, or anion.Ĭonjugated dienes are more stable than non conjugated dienes (both isolated and cumulated) due to factors such as delocalization of charge through resonance and hybridization energy. The π bonding orbital is lower in energy than the nonbonding p orbital. This molecular orbital description can also illustrate the stability of allylic carbon centers in figure 7.įig.7 diagram showing how the electrons fill based on the Aufbau principle. Because of this, the charges of the molecule are mainly on the two terminal carbons and not the middle carbon. This figure also shows that π 2 is the only molecular orbital where the electrion differs, and it is also where a single node passes through the middle. Even though the nonbonding orbitals can be filled, the overall energy of the system is still lower and more stable due to the filled bonding molecular orbitals. The Bonding orbitals are the lowest energy orbitals and are favorable, which is why they are filled first. So no matter what kind of carbon center exists, no electron will ever go into the antibonding orbital. The last anion electron goes into the nonbonding orbital also. If it is a radical or anion, the next electron goes into the π 2 molecular orbital. The first two electrons will go into the π 1 molecular orbital, regardless of whether it is a cation, radical, or anion.

π 1 is bonding with no nodes, π 2 is nonbonding (In other words, the same energy as a regular p-orbital) with a node, and π 3 is antibonding with 2 nodes (none of the orbitals are interacting). If we just take the π molecular orbital and not any of the s, we get three of them. Fig.6įig.6 Shows the 3 possible Molecular orbitals of 2-propenyl Molecular orbital descriptions can explain allylic stability in yet another way using 2-propenyl. It all comes down to where the location of the electron-deficient carbon is. You can clearly see the interactions between all three of the p orbitals from the three carbons resulting in a really stable cation. Instead, it is a group of 3 adjacent, overlapping, non-hybridized p orbitals we call a conjugated π electron system. The stability of the carbocation of propene is due to a conjugated π electron system. Molecular Orbitals of an Allylic Carbocation

Meaning of resonance in chemistry