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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 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 −.
This reaction type is linked to many forms of neighbouring group participation, for instance the reaction of the sulfur or nitrogen lone pair in sulfur mustard or nitrogen mustard to form the cationic intermediate. This reaction mechanism is supported by the observation that addition of pyridine to the reaction leads to inversion. The reasoning ...
For certain aryl substituted alkenes, the anti stereospecificity is diminished or lost, as a result of weakened or absent halonium character in the cationic intermediate. In 1970 George A. Olah succeeded in preparing and isolating halonium salts [ 6 ] by adding a methyl halide such as methyl bromide or methyl chloride in sulfur dioxide at −78 ...
An unusually shifted product is formed because the intermediate undergoes heterolysis at the C 5-Cl position. [24] Electrophilic attack to allene groups. Adapted from. In the electrophilic attack of allenes, it takes place in a manner that prefers to form a terminal adduct and the vinyl cation at the central carbon.
In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall ...
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]
When water is absent, the cationic intermediate loses a proton to give an allylic alcohol (4). With an excess of formaldehyde and a low reaction temperature the reaction product is a dioxane (5). When water is replaced by acetic acid the corresponding esters are formed. Scheme 1. The Prins reaction