Tropylium cation

The tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of [C₇H₇]⁺.^[4] Its name derives from the molecule tropine from which cycloheptatriene (tropylidene) was first synthesized in 1881. Salts of the tropylium cation can be stable, even with nucleophiles of moderate strength e.g., tropylium tetrafluoroborate and tropylium bromide (see below). Its bromide and chloride salts^[5] can be made from cycloheptatriene and bromine or phosphorus pentachloride, respectively.^[6]

Tropylium^[1]

skeletal formula
ball-and-stick model space-filling model
Names
Preferred IUPAC name Cycloheptatrienylium[2]
Other names cyc-C 7H+ 7, Cyclohepta-2,4,6-trienylium,[3][1] Cyclohepta-1,3,5-triene,[3] 2,4,6-Cycloheptatrienylium[1]
Identifiers
CAS Number	4118-59-6
3D model (JSmol)	Interactive image
Beilstein Reference	1902352[1]
ChemSpider	4394444 [1]
PubChem CID	5224206 [3]
CompTox Dashboard (EPA)	DTXSID10411305
InChI InChI=1S/C7H7/c1-2-4-6-7-5-3-1/h1-7H/q+1 Key: OJOSABWCUVCSTQ-UHFFFAOYSA-N[1] [3]: InChI=1S/C7H7/c1-2-4-6-7-5-3-1/h1-7H/q+1 Key: OJOSABWCUVCSTQ-UHFFFAOYSA-N[3]
SMILES c1=cc=c[cH+]c=c1
Properties
Chemical formula	C 7H+ 7[3]
Molar mass	91.132 g·mol−1
Structure
Point group	D7h
Molecular shape	regular heptagon
Related compounds
Other anions	Tropylium tetrafluoroborate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

It is a regular heptagonal, planar, cyclic ion. 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. 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.^[7] 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, C
₇H⁺
₇Br⁻
, by Doering and Knox in 1954 by analysis of its infrared and ultraviolet spectra.^[8][9] The ionic structures of tropylium perchlorate (C
₇H⁺
₇ClO⁻
₄) and tropylium iodide (C
₇H⁺
₇I⁻
) have been confirmed by X-ray crystallography.^[10] 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:

C
₇H⁺
₇ + 2 H
₂O ⇌ C
₇H
₇OH + H
₃O⁺

(Boric acid gives acidic aqueous solutions in much the same way.) The equilibrium constant is 1.8×10⁻⁵, making it about as acidic in water as acetic acid.^[8]

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 (PhCH⁺
₂), which rearranges to the highly stable tropylium cation (C
₇H⁺
₇).^[11]

Reactions

The tropylium cation reacts with nucleophiles to form substituted cycloheptatrienes, for example:^[12]

C
₇H⁺
₇ + CN⁻
→ C
₇H
₇CN

Reduction by lithium aluminium hydride yields cycloheptatriene.^[12]

Reaction with a cyclopentadienide salt of sodium or lithium yields 7-cyclopentadienylcyclohepta-1,3,5-triene:^[12]

C
₇H⁺
₇X⁻
+ C
₅H⁻
₅Na⁺
→ C
₇H
₇C
₅H
₅ + NaX

When treated with oxidising agents such as chromic acid, the tropylium cation undergoes rearrangement into benzaldehyde:^[12]

C
₇H⁺
₇ + HCrO⁻
₄ → C
₆H
₅CHO + CrO
₂ + H
₂O

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.^[13]

See also

• Azulene
• Borepin

References

1. "tropylium | ChemSpider". www.chemspider.com. p. Names. Retrieved 30 December 2018. tropylium
2. International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 1127. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
3. "Tropylium". pubchem.ncbi.nlm.nih.gov. Retrieved 30 December 2018. Chemical Names: Tropylium; Cycloheptatrienylium; Cyc-C
   ₇H⁺
   ₇; Cyclohepta-2,4,6-trienylium
4. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "molecule". doi:10.1351/goldbook.M04002
5. A mixture of [C₇H₇]⁺Cl⁻ and [C₇H₇]⁺[PCl⁻
   ₆] is produced by treatment of tropylidene with phosphorus pentachloride.
6. Tropylium fluoborate Organic Syntheses, Coll. Vol. 5, p.1138 (1973); Vol. 43, p.101 (1963). link Archived 2012-08-29 at the Wayback Machine
7. Merling, G. (1891). "Ueber Tropin". Berichte der Deutschen Chemischen Gesellschaft. 24 (2): 3108–3126. doi:10.1002/cber.189102402151.
8. Eggers Doering, W. von; Knox, L. H. (1954). "The Cycloheptatrienylium (Tropylium) Ion". J. Am. Chem. Soc. 76 (12): 3203–3206. doi:10.1021/ja01641a027.
9. Balaban, Alexandru T.; Oniciu, Daniela C.; Katritzky, Alan R. (2004). "Aromaticity as a Cornerstone of Heterocyclic Chemistry". Chem. Rev. 104 (5): 2777–2812. doi:10.1021/cr0306790. PMID 15137807.
10. Kitaigorodskii, A. I.; Struchkov, Yu. T.; Khotsyanova, T. L.; Vol'pin, M. E.; Kursanov, D. N. (1960). "Crystal structures of tropylium perchlorate and iodide". Bulletin of the Academy of Sciences of the USSR Division of Chemical Science. 9 (1): 32–36. doi:10.1007/bf01178699. ISSN 0568-5230.
11. Lifshitz, Chava (1994). "Tropylium Ion Formation from Toluene: Solution of an Old Problem in Organic Mass Spectrometry". Accounts of Chemical Research. 27 (5): 138–144. doi:10.1021/ar00041a004.
12. O. P. Agarwai (2009). Reactions and Reagents (46th ed.). Krishna Prakashan Media. pp. 614–615. ISBN 978-81-87224-65-5.
13. Green, Malcolm L. H.; Ng, Dennis K. P. (1995). "Cycloheptatriene and -enyl Complexes of the Early Transition Metals". Chemical Reviews. 95 (2): 439–473. doi:10.1021/cr00034a006.