Arenium ion
Forms during electrophilic substitution on benzene ring
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Through Organic chemistry, Benzene and Pi bond
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 lengths deduced from X-ray crystallography 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 is converted through dppe to metal complex 2. Electrophilic attack of methyl triflate forms methylene arenium ion 3 with (based on X-ray crystallography) positive charge located in aromatic para position and with the methylene group 6° out of the plane of the ring. Reaction first with water and then with triethylamine hydrolyzes the ether group.
Some historic references
- Olah, G. A. (1972). "Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- or tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions". J. Am. Chem. Soc. 94 (3): 808–820. doi:10.1021/ja00758a020.
- Wheland, G. W. (1942). "A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules". J. Am. Chem. Soc. 64 (4): 900–908. doi:10.1021/ja01256a047.