tropylium cation

The tropylium cation is a seven-membered carbon ring carrying a positive charge, and like the cyclopentadienyl anion it is a striking case of aromaticity arising in an unexpected ring size. It proves once more that the count of pi electrons, not the number of ring atoms, is what decides aromaticity, and that even a positively charged ring can be remarkably stable when the count comes out right.

Begin with cycloheptatriene, a neutral seven-membered ring with three double bonds and one sp3 CH2 carbon that breaks the loop. Remove a hydride (a hydrogen with its bonding electron pair) from that CH2 carbon. The carbon is left short one electron pair, becoming a carbocation, and it rehybridizes to sp2 so its empty p orbital joins the ring. Now all seven carbons offer a p orbital to a continuous loop. Count the pi electrons: the three double bonds give six, and the cationic carbon contributes an empty p orbital (zero electrons). Six pi electrons is 4n+2 with n=1, so the flat ring is aromatic, and the positive charge is spread evenly over all seven carbons.

Because this cation is aromatic and so well stabilized, it forms unusually readily and persists where ordinary carbocations would not; tropylium salts can even be isolated as stable solids. The pair of the cyclopentadienyl anion (negatively charged, aromatic) and the tropylium cation (positively charged, aromatic) makes the same memorable point from opposite directions: aromaticity is about achieving a 4n+2 pi count, and a ring will gladly gain or lose a charge to reach it.