The tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of C7H7+.[A mixture of C7H7+Cl− and C7H7+ is produced by treatment of tropylidene with phosphorus pentachloride.] can be made from cycloheptatriene and bromine or phosphorus pentachloride, respectively.[ Tropylium fluoborate Organic Syntheses, Coll. Vol. 5, p.1138 (1973); Vol. 43, p.101 (1963). link ]
It is a regular , planar, cyclic ion. It has 6 π-electrons (4 n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms. The structure shown is a composite of seven resonance contributors in which each carbon atom carries part of the positive charge.
History
In 1891 G. Merling obtained a water-soluble bromine-containing compound from the reaction of
cycloheptatriene and
bromine.
Unlike most alkyl bromides, this compound, later named tropylium bromide, is water-soluble but insoluble in many organic solvents. It is purified by crystallization from hot ethanol. Reaction with aqueous silver nitrate immediately gave silver bromide, indicating labile bromide. Tropylium bromide was deduced to be a salt, , by Doering and Knox in 1954 by analysis of its infrared and ultraviolet spectra.
The ionic structures of tropylium perchlorate () and tropylium iodide () have been confirmed by X-ray crystallography.
The bond length of the carbon-carbon bonds is longer (147 pm) than those of
benzene (140 pm) but still shorter than those of a typical single-bonded species like
ethane (154 pm).
Acidity
The tropylium ion is an acid in aqueous solution (i.e., an Arrhenius acid) as a consequence of its Lewis acidity: it first acts as a Lewis acid to form an adduct with water, which can then donate a proton to another molecule of water, therefore indirectly acting as an Arrhenius acid:
- + 2 +
(
Boric acid gives acidic aqueous solutions in much the same way.) The equilibrium constant is , making it about as acidic in water as
acetic acid.
Mass spectrometry
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 forms a cation (), which rearranges to the highly stable tropylium cation ().
Reactions
The tropylium cation reacts with nucleophiles to form substituted cycloheptatrienes, for example:
- + →
Reduction by lithium aluminium hydride yields cycloheptatriene.
Reaction with a cyclopentadienide salt of sodium or lithium yields Sesquifulvalene:
- + → +
When treated with oxidising agents such as chromic acid, the tropylium cation undergoes rearrangement into benzaldehyde:
- + → + +
Many metal complexes of tropylium ion are known. One example is Mo( η7-C7H7)(CO)3+, which is prepared by hydride abstraction from cycloheptatrienemolybdenum tricarbonyl.
See also
