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An arenium ion in 
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 3 carbon atoms by the 
A complexed electrophile can contribute to the stability of arenium ions.
Salts of benzenium ion can be isolated when benzene is protonated by the 
In this reaction sequence the R–Pd(II)–Br starting complex 1 stabilized by
An arenium ion in organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
is a cyclohexadienyl cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
that appears as a reactive intermediate in electrophilic aromatic substitution
Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic ni ...
.
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
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
.
:
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 3 carbon atoms by the pi system
In mathematics, a -system (or pi-system) on a set \Omega is a collection P of certain subsets of \Omega, such that
* P is non-empty.
* If A, B \in P then A \cap B \in P.
That is, P is a non-empty family of subsets of \Omega that is closed under ...
, as depicted on the following resonance structures
In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...
:
:
A complexed electrophile can contribute to the stability of arenium ions.
Salts of benzenium ion can be isolated when benzene is protonated by the carborane superacid
Carborane acids (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (''H''0 ≤ –18) ...
H(CB11H(CH3)5Br6). The benzenium salt is crystalline with thermal stability up to 150 °C. Bond length
In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
s deduced from X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
are consistent with a cyclohexadienyl cation structure.
In one study a methylene arenium ion is stabilized by metal complexation:
:
In this reaction sequence the R–Pd(II)–Br starting complex 1 stabilized by TMEDA
Tetramethylethylenediamine (TMEDA or TEMED) is a chemical compound with the formula (CH3)2NCH2CH2N(CH3)2. This species is derived from ethylenediamine by replacement of the four amine hydrogens with four methyl groups. It is a colorless liquid, ...
is converted through dppe
1,2-Bis(diphenylphosphino)ethane (dppe) is an organophosphorus compound with the formula (PhPCH) (Ph = phenyl). It is a commonly used bidentate ligand in coordination chemistry. It is a white solid that is soluble in organic solvents.
Preparation ...
to metal complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
2. Electrophilic
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 carri ...
attack of methyl triflate
Methyl trifluoromethanesulfonate, also commonly called methyl triflate and abbreviated MeOTf, is the organic compound with the formula CF3SO2OCH3. It is a colourless liquid which finds use in organic chemistry as a powerful methylating agent. T ...
forms methylene arenium ion 3 with (based on X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
) positive charge located in aromatic para position
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.
''Ortho'', ''meta'', and ''para'' substitution
* I ...
and with the methylene group
In organic chemistry, a methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by two single bonds. The group may be represented as , where the '< ...
6° out of the plane of the ring. Reaction first with water and then with triethylamine
Triethylamine is the chemical compound with the formula N(CH2CH3)3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine or tetraethylammonium, for which TEA ...
hydrolyzes the ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
group.
See also
*Aryl radical An aryl radical in organic chemistry is a reactive intermediate and an arene compound incorporating one free radical carbon atom as part of the ring structure. As such it is the radical counterpart of the arenium ion. The parent compound is the phen ...
* Cyclopentadienyl anion
In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene.
Properties
The cyclopentadienyl anion i ...
* Meisenheimer complex A Meisenheimer complex or Jackson–Meisenheimer complex in organic chemistry is a 1:1 reaction adduct between an arene carrying electron withdrawing groups and a nucleophile. These complexes are found as reactive intermediates in nucleophilic aroma ...
, the analogous intermediate in nucleophilic aromatic substitution
* Tropylium cation
In organic chemistry, the tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of 7H7sup>+. Its name derives from the molecule tropine from which cycloheptatriene (tropylidene) was first synthesized in 1881. Salts of t ...
Some historic references
* *References
{{Reflist Reactive intermediates Carbocations