The tropylium ion or cycloheptatrienyl cation is an

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

species with a formula of ₇H₇sup>+. Its name derives from the molecule

tropine Tropine is a derivative of tropane containing a hydroxyl group at the third carbon. It is also called 3-tropanol. It is a poisonous white hygroscopic crystalline powder. It is a heterocyclic alcohol and an amine. Tropine is a central building bl ...

from which

cycloheptatriene Cycloheptatriene (CHT) is an organic compound with the chemical formula, formula C7H8. It is a closed ring of seven carbon atoms joined by three double bonds (as the name implies) and four single bonds. This colourless liquid has been of recurring ...

(tropylidene) was first synthesized in 1881. Salts of the tropylium cation can be stable, even with nucleophiles of moderate strength e.g.,

tropylium tetrafluoroborate Tropylium tetrafluoroborate is an organic compound with the formula . Containing the tropylium cation and the non-coordinating tetrafluoroborate counteranion, tropylium tetrafluoroborate is a rare example of a readily isolable carbocation. It is ...

and tropylium bromide (''see below''). Its bromide and chloride salts can be made from cycloheptatriene and

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

phosphorus pentachloride Phosphorus pentachloride is the chemical compound with the formula . It is one of the most important phosphorus chlorides/oxychlorides, others being and . finds use as a chlorinating reagent. It is a colourless, water-sensitive solid, althoug ...

, respectively. It is a regular

heptagon In geometry, a heptagon or septagon is a seven-sided polygon or 7-gon. The heptagon is sometimes referred to as the septagon, using ''Wikt:septa-, septa-'' (an elision of ''Wikt:septua-, septua-''), a Latin-derived numerical prefix, rather than ...

al, planar, cyclic ion. It has 6 π-electrons (4''n'' + 2, where ''n'' = 1), which fulfills

Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π-electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was f ...

of aromaticity. It can coordinate as a

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

atoms Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other ...

. 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

and

. 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 Doering is a surname. Notable people with the surname include: *Aaron Doering (born 1971), American professor * Bernd von Doering (1903–1944), German general * Charles R. Doering (born 1956), American professor * Chris Doering (born 1973), Americ ...

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 X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The bond length of the carbon-carbon bonds is longer (147 pm) than those of

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

(140 pm) but still shorter than those of a typical single-bonded species like

ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...

(154 pm).

Acidity

The tropylium ion is an acid in aqueous solution (i.e., an

Arrhenius acid Arrhenius may refer to: * Birgit Arrhenius (born 1932), Swedish archaeologist * Carl Axel Arrhenius (1757–1824), Swedish army lieutenant and amateur mineralogist who discovered ytterbite, a mineral that led to the discovery of yttrium by Johan ...

) 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 Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white ...

gives acidic aqueous solutions in much the same way.) The equilibrium constant is , making it about as acidic in water as

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

Mass spectrometry

The tropylium ion is frequently encountered in

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

in the form of a signal at ''m''/''z'' = 91 and is used in

mass spectrum analysis Mass spectral interpretation is the method employed to identify the chemical formula, characteristic fragment patterns and possible fragment ions from the mass spectra. Mass spectra is a plot of relative abundance against mass-to-charge ratio. It i ...

. This fragment is often found for aromatic compounds containing a benzyl unit. Upon

ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

, 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 Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula or . It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthe ...

yields

. Reaction with a cyclopentadienide salt of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

yields 7-cyclopentadienylcyclohepta-1,3,5-triene: : + → + When treated with oxidising agents such as

chromic acid Chromic acid is a chemical compound with the chemical formula . It is also a jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide. The term "chromic ...

, the tropylium cation undergoes rearrangement into

benzaldehyde Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is among the simplest aromatic aldehydes and one of the most industrially useful. It is a colorless liquid with a characteristic almond-li ...

: : + → + + Many metal complexes of tropylium ion are known. One example is o(''η''⁷-C₇H₇)(CO)₃sup>+, which is prepared by hydride abstraction from cycloheptatrienemolybdenum tricarbonyl.

References

{{Reflist Simple aromatic rings Carbocations Non-benzenoid aromatic carbocycles Cycloheptatrienyl complexes de:Tropylium

History

Acidity

Mass spectrometry

Reactions

See also

References