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QUESTION

# 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.