Tropylium cation

In organic chemistry, the tropylium ion is an aromatic species with a formula of ⁺. 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 Tropylium tetrafluorate Organic Syntheses, Coll. Vol. 5, p.1138 (1973); Vol. 43, p.101 (1963).

It is a heptagonal, planar, cyclic ion; as well, it has 6 π-electrons (4n+ 2, where n=1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms.

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The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge.

In 1891 G. Merling obtained a water soluble salt from a reaction of cycloheptatriene and bromine Merling, G. (1891), Ueber Tropin. Berichte der deutschen chemischen Gesellschaft, 24: 3108–3126. . The structure was elucidated by von Eggers Doering and Knox in 1954 The Cycloheptatrienylium (Tropylium) Ion W. Von E.Doering, L. H. Knox J. Am. Chem. Soc., 1954, 76 (12), pp 3203–3206 Aromaticity as a Cornerstone of Heterocyclic Chemistry Alexandru T. Balaban, Daniela C. Oniciu, Alan R. Katritzky Chem. Rev., 2004, 104 (5), pp 2777–2812

Mass spectrometry

The tropylium ion is...
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