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Tropylium cation

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Tropylium cation top 6 facts


In organic chemistry, the tropylium ion is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropine (itself named for the molecule atropine). Salts of the tropylium cation can be stable, e.g., tropylium tetrafluoroborate. It can be made from cycloheptatriene (tropylidene) and bromine or phosphorus pentachloride.

Contents

Tropylium cation Illustrated Glossary of Organic Chemistry Tropylium cation

It is a regular heptagonal, planar, cyclic ion; as well, 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.

Tropylium cation Tropylium cation Wikipedia

The structure shown is a composite of seven resonance contributors in which each carbon atom carries part of the positive charge.

Tropylium cation Illustrated Glossary of Organic Chemistry Tropylium cation

In 1891 G. Merling obtained a water-soluble salt from a reaction of cycloheptatriene and bromine. The structure was elucidated by Eggers Doering and Knox in 1954.

Tropylium cation Resonance Tutorial

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:

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

Reduction by lithium aluminum hydride yields cycloheptatriene.

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

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:

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.

Tropylium cation

References

Tropylium cation Wikipedia


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