Illustrated Glossary of Organic Chemistry

Tropylium cation: The carbocation of molecular formula [C₇H₇]⁺ with the molecular structure shown below. Tropylium cation is aromatic, and so has much greater stability than most other typical carbocations. Tropylium salts such as tropylium bromide can be prepared and stored.

Molecular Structure of the Tropylium Cation

Lewis structure		Resonance hybrid		Ball and spoke		Molecular model kit

The tropylium cation (or a derivative) is a common fragment in the mass spectra of molecules having a carbon atom bonded to a benzene ring (i.e., a benzyl moiety). Shown here is the electron impact mass spectrum of ethylbenzene (PhCH₂CH₃). The peak at m/z = 106 is M, and the peak at m/z = 91 is due to the tropylium cation. Click on the mass spectrum image to see a larger version.