Arenium ion
Encyclopedia
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 or a sigma complex or σ-complex.
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 5 carbon atoms via the pi system, as depicted on the following resonance structures:
Another contribution to the stability of arenium ions is the energy gain resulting from the strong bond between the benzene and the complexed electrophile.
The smallest arenium ion is protonated benzene
C6H7+. The benzenium ion can be isolated as a stable compound 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
s deduced from X-ray crystallography
are consistent with a cyclohexadienyl cation structure.
More methods exist for the generation of stable arenium ions. In one study a methylene arenium ion is stabilized by metal complexation:
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution
Electrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
. For historic reasons this complex is also called a Wheland intermediate or a sigma complex or σ-complex.
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 5 carbon atoms via the pi system, as depicted on the following resonance structures:
Another contribution to the stability of arenium ions is the energy gain resulting from the strong bond between the benzene and the complexed electrophile.
The smallest arenium ion is protonated benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
C6H7+. The benzenium ion can be isolated as a stable compound 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
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
s deduced from X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
are consistent with a cyclohexadienyl cation structure.
More methods exist for the generation of stable arenium ions. In one study a methylene arenium ion is stabilized by metal complexation:
See also
- List of publications in chemistry: Jerry March Advanced Organic Chemistry
- Aryl radicalAryl radicalAn Aryl radical in organic chemistry is an 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 phenyl radical C6H5....