Tropylium cation
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| Names | |||
|---|---|---|---|
| Preferred IUPAC name
Cycloheptatrienylium[3] | |||
| Other names | |||
| Identifiers | |||
3D model (JSmol)
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| 1902352[1] | |||
| ChemSpider | |||
PubChemCID
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CompTox Dashboard(EPA)
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| Properties | |||
| C 7H+ 7[2] | |||
| Molar mass | 91.132g·mol−1 | ||
| Structure | |||
| D7h | |||
| regular heptagon | |||
| Related compounds | |||
Otheranions
|
Tropylium tetrafluoroborate | ||
Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa).
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Thetropylium ionorcycloheptatrienyl cationis anaromaticspecies with a formula of [C7H7]+.[4]Its name derives from the moleculetropinefrom whichcycloheptatriene(tropylidene) was first synthesized in 1881. Salts of the tropylium cation can be stable, even with nucleophiles of moderate strength e.g.,tropylium tetrafluoroborateand tropylium bromide (see below). Its bromide and chloride salts[5]can be made from cycloheptatriene andbromineorphosphorus pentachloride,respectively.[6]
It is a regularheptagonal,planar, cyclic ion. It has 6 π-electrons (4n+ 2, wheren= 1), which fulfillsHückel's ruleof aromaticity. It can coordinate as aligandtometalatoms.The structure shown is a composite of sevenresonance contributorsin which each carbon atom carries part of the positive charge.
History
[edit]In 1891 G. Merling obtained a water-soluble bromine-containing compound from the reaction ofcycloheptatrieneandbromine.[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−
,byDoeringand 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 byX-ray crystallography.[10]The bond length of the carbon-carbon bonds is longer (147 pm) than those ofbenzene(140 pm) but still shorter than those of a typical single-bonded species likeethane(154 pm).
Acidity
[edit]The tropylium ion is an acid in aqueous solution (i.e., anArrhenius 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+ 2H
2O⇌C
7H
7OH+H
3O+
(Boric acidgives acidic aqueous solutions in much the same way.) The equilibrium constant is1.8×10−5,making it about as acidic in water asacetic acid.[8]
Mass spectrometry
[edit]The tropylium ion is frequently encountered inmass spectrometryin the form of a signal atm/z= 91 and is used inmass spectrum analysis.This fragment is often found for aromatic compounds containing a benzyl unit. Uponionization,the benzyl fragment forms a cation (PhCH+
2), which rearranges to the highly stable tropylium cation (C
7H+
7).[11]
Reactions
[edit]The tropylium cation reacts with nucleophiles to form substituted cycloheptatrienes, for example:[12]
- C
7H+
7+CN−
→C
7H
7CN
Reduction bylithium aluminium hydrideyieldscycloheptatriene.[12]
Reaction with a cyclopentadienide salt ofsodiumorlithiumyields7-cyclopentadienylcyclohepta-1,3,5-triene:[12]
- C
7H+
7X−
+C
5H−
5Na+
→C
7H
7C
5H
5+NaX
When treated with oxidising agents such aschromic acid,the tropylium cation undergoes rearrangement intobenzaldehyde:[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 fromcycloheptatrienemolybdenum tricarbonyl.[13]
See also
[edit]References
[edit]- ^abcdefg"tropylium | ChemSpider".chemspider.p. Names.Retrieved30 December2018.
tropylium
- ^abcdef"Tropylium".pubchem.ncbi.nlm.nih.gov.Retrieved30 December2018.
Chemical Names: Tropylium; Cycloheptatrienylium; Cyc-C
7H+
7;Cyclohepta-2,4,6-trienylium - ^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.ISBN978-0-85404-182-4.
- ^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "molecule".doi:10.1351/goldbook.M04002
- ^A mixture of [C7H7]+Cl−and [C7H7]+[PCl−
6] is produced by treatment of tropylidene with phosphorus pentachloride. - ^Tropylium fluoborateOrganic Syntheses, Coll. Vol. 5, p.1138 (1973); Vol. 43, p.101 (1963).linkArchived2012-08-29 at theWayback Machine
- ^Merling, G. (1891). "Ueber Tropin".Berichte der Deutschen Chemischen Gesellschaft.24(2): 3108–3126.doi:10.1002/cber.189102402151.
- ^abEggers Doering, W. von; Knox, L. H. (1954). "The Cycloheptatrienylium (Tropylium) Ion".J. Am. Chem. Soc.76(12): 3203–3206.doi:10.1021/ja01641a027.
- ^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.PMID15137807.
- ^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.ISSN0568-5230.
- ^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.
- ^abcdO. P. Agarwai (2009).Reactions and Reagents(46th ed.). Krishna Prakashan Media. pp. 614–615.ISBN978-81-87224-65-5.
- ^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.