Wheland Intermediate on:
[Wikipedia]
[Google]
[Amazon]
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 pi system, as depicted on the following resonance structures:
:
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 H(CB11H(CH3)5Br6). The benzenium salt is crystalline with thermal stability up to 150 °C. 
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 science, 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.Clay ...
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 conve ...
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 ato ...
.
:
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, as depicted on the following resonance structures:
:
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 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 ...
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 angle ...
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 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. Man ...
2. Electrophilic attack of methyl triflate 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 angle ...
) positive charge located in aromatic para position 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
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom ma ...
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 b ...
group.
See also
* Aryl radical * Cyclopentadienyl anion * Meisenheimer complex, the analogous intermediate in nucleophilic aromatic substitution * Tropylium cationSome historic references
* *References
{{Reflist Reactive intermediates Carbocations