In organic chemistry, the tropylium ion is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropine (itself named for the molecule atropine). Salts of the tropylium cation can be stable, e.g., tropylium tetrafluoroborate. It can be made from cycloheptatriene (tropylidene) and bromine or phosphorus pentachloride.
The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge.
The tropylium ion is frequently encountered in mass spectrometry in the form of a signal at m/z = 91 and is used in mass spectrum analysis. This fragment is often found for aromatic compounds containing a benzyl unit. Upon ionization, the benzyl fragment is cleaved off as a cation (PhCH2+), which rearranges to the highly stable tropylium cation (C7H7+).
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