Question

Why is phenyl carbocation unstable?

Answer 1

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

Explanation:

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.