Anyone know why a benzene cation is unstable?

Question

Why is a benzene cation (1 H removed) unstable? Even though a benzylic cation is VERY stable. Isn't a benzene cation still aromatic? Yet apparently it is a "horrible" molecule.

Answer 1

You are right that the "phenylium" cation is aromatic, however, first of all the atom carrying the positive charge _should_ become sp with a linear geometry, and hence a strain exists within the ring (which should become distorted in an energy compromise between sp2 and sp); second of all sp carbons are more electronegative than sp2 carbons and less able to stabilize a positive charge.

On the other hand, the benzylic cation has the positive charge located on a primary sp2 carbon (which would be sp3 if it were a neutral molecule), and this positive charge can also become delocalized into the ring. In these reasonance forms the aromaticity is broken, but the cation is still a good deal more stable than a phenylic cation.