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An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. [1] For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland (1907–1976). [2] They are also called sigma complexes. [3]
The carbonyl carbon is typically electrophilic. A qualitative order of electrophilicity is RCHO (aldehydes) > R 2 CO (ketones) > RCO 2 R' (esters) > RCONH 2 (amides). A variety of nucleophiles attack, breaking the carbon-oxygen double bond. Interactions between carbonyl groups and other substituents were found in a study of collagen. [3]
An arenium ion is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. [28] For historic reasons this complex is also called a Wheland intermediate, [29] or a σ-complex. Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. [30]
Electrophilic aromatic substitution (S E Ar) 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 nitration , aromatic halogenation , aromatic sulfonation , alkylation Friedel–Crafts ...
The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a π-complex 1. Forming of a three-membered bromonium ion The alkene is working as an electron donor and bromine as an electrophile. The three-membered bromonium ion 2 consisted of two carbon atoms and a bromine atom forms with a release of Br −.
A ball-and-stick model of a bromonium ion formed from cyclopentene. A halonium ion is any onium ion containing a halogen atom carrying a positive charge. This cation has the general structure R− + −R′ where X is any halogen and no restrictions on R, [1] this structure can be cyclic or an open chain molecular structure.
The driving force for the actual migration of a substituent in step two of the rearrangement is the formation of a more stable intermediate. For instance a tertiary carbocation is more stable than a secondary carbocation and therefore the S N 1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether.
These intermediate species' concentrations can be calculated by integrating the system of kinetic equations. The full reaction is a free radical propagation reaction which is filled out in detail below. Initiation: This reaction can occur by thermolysis (heating) or photolysis (absorption of light) leading to the breakage of a molecular ...