Why is phenyl carbocation unstable?

The phenyl carbocation is unstable because of the high bond energy of the aromatic ##"C-H"## bond.

We can view the formation of a phenyl cation as

##"C"_6"H"_5"-H" → "C"_6"H"_5^+ +"H" + "e"^-##

The ##"C-H"## bonds of benzene are ##"sp"^2## hybridized.

The high ##"s"## character means that the electrons are closer to the nucleus, so we must add more energy to remove these electrons and break the bond.

For example, it takes 423 kJ/mol to break the ##"C-H"## bond in ethane, but 470 kJ/mol in benzene.

In addition, the vacant ##"sp"^2## orbital is in the plane of the ring.

It cannot overlap with the orbitals of the ##π## system, so it cannot be stabilized by resonance.

The phenyl cation is a high-energy, unstable species.