Tropylium

The tropylium cation, with formula C7H7+, is a seven-membered ring carbocation derived from cycloheptatriene. The ring is typically described as planar and fully conjugated, with the positive charge delocalized around the ring. The system contains six π electrons, satisfying Hückel’s rule for aromaticity (4n + 2, with n = 1). This aromatic stabilization renders the tropylium cation unusually stable for a carbocation, and it is commonly encountered as salts such as tropylium tetrafluoroborate (C7H7+ BF4−).

The structure corresponds to a cycloheptatriene framework in which hydride abstraction or related oxidative processes generate the cation. In solution, the tropylium ion is understood as a delocalized positive charge over the seven-membered ring, rather than a single localized carbocation center.

In organic chemistry, tropylium salts serve as electrophiles and sources of the C7H7+ unit in a variety of transformations, including transfer to substrates and participation in rearrangements and cycloaddition-type processes. The tropylium cation is also a classic example used to illustrate non-benzenoid aromaticity and the stabilizing influence of aromatic delocalization in seven-membered rings. While primarily of interest for synthetic and mechanistic studies, tropylium salts are widely used laboratory reagents. Tropylium is not known to occur as a natural species.