Why is phenyl carbocation unstable?

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Answer 1 · 2015-06-27T19:37:10

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.

chemed.chem.purdue.edu

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.