Question

Why is phenyl carbocation unstable?

Answer 1

Hint: To solve this question we should know about:
A chemical that is stable is unreactive, while an unstable chemical is reactive. The Unequal Distribution of Electrons (UDED) in a chemical species determines reactivity (molecule, atom, and ion). This is how "unreactive" chemical species behave. Like the compressed atmosphere in a sealed cylinder, their electrons are securely confined by their nuclei. To make it explode, you'll probably need a big fire (a lot of energy) in the room (react)

Complete answer:
The creation of a phenyl cation might be thought of as
$C_6{H}_5-H\to C_6{H}_5+H+e^{-}$
The benzene $C-H$ bonds are $s{p}^2$ hybridized.
Because the electrons are closer to the nucleus with a high s character, we must use more energy to remove them and break the bond.
Ethane, for example, requires $$423KJmo{l}^{-1}$$ to break the C-H bond, but benzene requires $$470KJmo{l}^{-1}$$ .
Furthermore, the unoccupied $s{p}^2$ orbital is in the ring's plane.
Because it can't overlap with the system's $\pi$ orbitals, it can't be stabilized via resonance.
The phenyl cation is an unstable, high-energy species.

Because of the high bond energy of the aromatic $C-H$ bond, the phenyl carbocation is unstable.

Note:
Because the phenyl cation core has two ligands but no lone pairs, $sp$ is the most stable hybridization. Of course, it's not genuinely $sp$ because it can't take on a linear geometry, but it's also not $s{p}^2$ because the bond angle is greater than 120°.