Tropylium cation

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Tropylium [1]
Tropylium-ion-2D-skeletal.png
Tropylium-ion-3D-balls.png
Tropylium-ion-3D-vdW.png
Names
Preferred IUPAC name
Cycloheptatrienylium [2]
Other names
cyc-C
7
H+
7
, Cyclohepta-2,4,6-trienylium, [3] [1] Cyclohepta-1,3,5-triene, [3] 2,4,6-Cycloheptatrienylium [1]
Identifiers
3D model (JSmol)
1902352 [1]
ChemSpider
PubChem CID
  • 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]
  • c1=cc=c[cH+]c=c1
Properties
C
7
H+
7
[3]
Molar mass 91.132 g·mol−1
Structure
D7h
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

In organic chemistry, 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]

Contents

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 hydrocarbyl 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
7
H+
7
Br
, by Doering and Knox in 1954 by analysis of its infrared and ultraviolet spectra. [8] [9] The ionic structures of tropylium perchlorate (C
7
H+
7
ClO
4
) and tropylium iodide (C
7
H+
7
I
) 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:

C
7
H+
7
+ 2 H
2
O
C
7
H
7
OH
+ H
3
O+

(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
7
H+
7
).

Reactions

The tropylium cation reacts with nucleophiles to form substituted cycloheptatrienes, for example: [11]

C
7
H+
7
+ CN
C
7
H
7
CN

Reduction by lithium aluminum hydride yields cycloheptatriene. [11]

Reaction with a cyclopentadienide salt of sodium or lithium yields 7-cyclopentadienylcyclohepta-1,3,5-triene: [11]

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

When treated with oxidising agents such as chromic acid, the tropylium cation undergoes rearrangement into benzaldehyde: [11]

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

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. [12]

See also

Related Research Articles

Bromine Chemical element, symbol Br and atomic number 35

Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest halogen and is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between those of chlorine and iodine. Isolated independently by two chemists, Carl Jacob Löwig and Antoine Jérôme Balard, its name was derived from the Ancient Greek βρῶμος, referring to its sharp and pungent smell.

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The SN1 reaction is a substitution reaction in organic chemistry, the name of which refers to the Hughes-Ingold symbol of the mechanism. "SN" stands for "nucleophilic substitution", and the "1" says that the rate-determining step is unimolecular. Thus, the rate equation is often shown as having first-order dependence on the substrate and zero-order dependence on the nucleophile. This relationship holds for situations where the amount of nucleophile is much greater than that of the intermediate. Instead, the rate equation may be more accurately described using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary and secondary alkyl halides, the alternative SN2 reaction occurs. In inorganic chemistry, the SN1 reaction is often known as the dissociative substitution. This dissociation pathway is well-described by the cis effect. A reaction mechanism was first proposed by Christopher Ingold et al. in 1940. This reaction does not depend much on the strength of the nucleophile, unlike the SN2 mechanism. This type of mechanism involves two steps. The first step is the ionization of alkyl halide in the presence of aqueous acetone or ethyl alcohol. This step provides a carbocation as an intermediate.

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double bond of an alkene functional group.

Carbocation Ion with a positively charged carbon atom

A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+
3
, methanium CH+
5
and vinyl C
2
H+
3
cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered.

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

Hydrogen bromide Chemical compound

Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C. Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.

Benzyl group

In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure C6H5CH2–. Benzyl features a benzene ring attached to a CH2 group.

In organic chemistry, an electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. This organic reaction is typical of aromatic compounds and a very useful method for adding substituents to an aromatic system.

<i>N</i>-Bromosuccinimide Molecule

N-Bromosuccinimide or NBS is a chemical reagent used in radical substitution, electrophilic addition, and electrophilic substitution reactions in organic chemistry. NBS can be a convenient source of Br, the bromine radical.

Diazonium compound Diazonium salts of formula R-N≡N+

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group R−N+2X where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halogen.

Cycloheptatriene (CHT) is an organic compound with the 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 theoretical interest in organic chemistry. It is a ligand in organometallic chemistry and a building block in organic synthesis. Cycloheptatriene is not aromatic, as reflected by the nonplanarity of the methylene bridge (-CH2-) with respect to the other atoms; however the related tropylium cation is.

Carbenium ion

A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with a trivalent carbon that bears a +1 formal charge.

Arenium ion Forms during electrophilic substitution on benzene ring

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland (1907–1976). They are also called sigma complexes. The smallest arenium ion is the benzenium ion, which is protonated benzene.

Neighbouring group participation (NGP) in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond contained within the parent molecule but not conjugated with the reaction centre. When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry this page is limited to neighbouring group effects seen with carbocations and SN2 reactions.

Antimony pentachloride Chemical compound

Antimony pentachloride is a chemical compound with the formula SbCl5. It is a colourless oil, but typical samples are yellowish due to dissolved chlorine. Owing to its tendency to hydrolyse to hydrochloric acid, SbCl5 is a highly corrosive substance and must be stored in glass or PTFE containers.

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Chlorine perchlorate Chemical compound

Chlorine perchlorate is a chemical compound with the formula Cl2O4. This chlorine oxide is an asymmetric oxide, with one chlorine atom in +1 oxidation state and the other +7, with proper formula ClOClO3. It is produced by the photolysis of chlorine dioxide (ClO2) at room temperature by 436 nm ultraviolet light :

References

  1. 1 2 3 4 5 6 "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. 1 2 3 4 5 6 7 "Tropylium". pubchem.ncbi.nlm.nih.gov. Retrieved 30 December 2018. Chemical Names: Tropylium; Cycloheptatrienylium; Cyc-C
    7
    H+
    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. 1 2 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.
  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. 1 2 3 4 O. P. Agarwai (2009). Reactions and Reagents (46th ed.). Krishna Prakashan Media. pp. 614–615. ISBN   9788187224655.
  12. Green, Malcolm L. H.; Ng, Dennis K. P. (1995). "Cycloheptatriene and -enyl Complexes of the Early Transition Metals". Chemical Reviews. 95: 439–473. doi:10.1021/cr00034a006.