What is a Cycloheptatrienyl Cation?

It is an aromatic carbocation having a general formula, [C₇ H₇]⁺. It is also known as the aromatic tropylium ion. Its name is derived from the molecule tropine, which is a seven membered carbon atom ring. Cycloheptatriene or tropylidene was first synthesized from tropine.

Structure of a Cycloheptatrienyl Cation

In a cycloheptatrienyl cation, seven carbon atoms are arranged in a cyclic manner so as to give it a structure similar to a regular heptagonal one. All the carbon atoms are sp² hybridized. In a tropylium ion or a cycloheptatrienyl cation, there are six electrons that take-up the three bonding  pi-molecular orbitals, thus, there are three double bonds present in the structure and the seventh carbon atom has a positive charge on it. Since it can undergo resonance, its hybrid structure consists of seven resonating structures in which each carbon atom carries part of the positive charge.

The C-C bond-length of the cycloheptatrienyl cation is 147 pm while the C-C bond-length of benzene is found to be 140 pm and that of ethane is 154 pm. Therefore, we can conclude that the C-C bond length of cycloheptatrienyl cation is longer than benzene but shorter than that of ethane.

Aromaticity

It is a property by which the stability of a compound is increased. It is described by four major components; we check either the compound has the following to be called as an aromatic compound:

• Cyclic.
• Planar.
• Conjugation in the system.
• Presence of [4n + 2] π electrons.

Huckel's rule of aromaticity

According to this rule, for a compound to be aromatic it must have an [4n + 2] π electrons creating an uninterrupted π electron cloud. Also it has to have a planar and cyclic structure. Furthermore, they must have conjugated bonds.  

How to distinguish whether a compound is an aromatic, non-aromatic or anti aromatic?

Aromaticity of cycloheptatrienyl cation

The "aromaticity" of tropylium carbocation like for any other compound can be determined by the Huckel's rule of aromaticity. It can be seen that it obeys Huckel’s rule as it has six electrons that falls under the category of (4n+2) π-electrons forming an uninterrupted electron cloud. Furthermore, it is a planar and a cyclic structure. Also, it has got three bonding π-MO (molecular orbitals) which are fully filled by the six pi electrons. Hence, all of this observations show that the tropylium cation is an aromatic ion or compound.

Characteristics and Properties

The molecular mass of a cycloheptatrienyl cation is 91.132g/mol. It has a melting point of -89.4° F and a boiling point of 256.96° F.  It can act as a ligand that can coordinate with a metal. 

A salt of tropylium cation is very stable when bonded with a nucleophile even of a moderate strength, for example, the compounds tropylium bromide and tropylium tetrafluoroborate are compounds formed by tropylium cation. To make a bromide or chloride salt of this aromatic compound, cycloheptatriene can be treated with bromine or phosphorous pentachloride respectively.

This ion is highly soluble in H₂O (water) and insoluble in hydrocarbons like ethers, C₆H₆ (benzene) and CHCl₃ (chloroform). It can be purified by carrying out crystallization with hot C₂H₅OH (ethanol).

The cycloheptatrienyl cation or the tropylium ion reacts with a nucleophile like cyanide ion (CN^-), we get a substituted cycloheptatriene as a product.

C₇H₇⁺ + CN^- → C₇H₇CN

When the ion reacts with an oxidizing agent like chromic acid (H₂CrO₄), then it will undergo rearrangement to give benzaldehyde (C7H6O).

C₇H₇⁺ + HCrO₄^- → C₆H₅CHO + CrO₂ +H₂O

 In an aqueous solution, this carbocation acts as an acid, i.e., an Arrhenius acid due to its Lewis acidity; where it first behaves as a Lewis acid to give an adduct with water. Chemically, it is represented as below:

C₇H₇⁺ + 2 H₂O ⇌ C₇H₇OH + H₃O⁺

It can be reduced by lithium aluminium hydride (LiAlH₄) to give us cycloheptatriene. 

On treating with cyclopentadiene salt of sodium or lithium, it forms 7-cyclopentadienylcyclohepta-1,3,5-triene.

C₇H₇X^- + C₅H^-₅Na⁺ → C₇H₇C₅H₅ + NaX

It obeys Huckel's rule of aromaticity due to the presence of 6 π- electrons (4n + 2, where n=1). Since the ion follows all the rule of aromaticity, it is an aromatic compound. 

The process of X-ray crystallography has confirmed the ionic structures of tropylium iodide or tropylium perchlorate.

The cation is often seen in mass spectrometry in the form of a signal that appears at mass by charge ratio of 91 in the mass chromatogram. This is used in analysis of mass spectrums. This fragment is commonly seen for aromatic compounds containing a benzyl unit (C₆H₅CH₂–). The benzyl fragment forms a cation, [PhCH₂]⁺ on ionization which then rearranges to tropylium cation [C₇H₇]⁺ which has higher stability. 

Cycloheptatrienyl Anion

The cycloheptatrienyl anion is planar, cyclic and has 8 electrons in its pi system which makes it antiaromatic and highly unstable. 

Cyclopentadienyl Anion

A cyclopentadienyl anion is a planar cyclic anion with a general formula C₅H^-₅. It is derived from cyclopentadiene. It is aromatic and forms a sandwich compound complex with metal atoms where there are two parallel cyclopentadienyl rings bonded with a metal atom between them.  

Context and Applications

This topic is significant in the professionals exams of both undergraduate and postgraduate courses, specifically for Bachelors and Masters in Chemistry and science courses at high school level.