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 smallest arenium ion is the benzenium ion (C 6H+ 7), which is protonated benzene.
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring.[4] 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).[5] 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:[6]
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.
References
^Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 658, ISBN978-0-471-72091-1
^Smith, Michael B. (18 October 2010). Organic Chemistry: An Acid—Base Approach. CRC Press. ISBN9781439894620 – via Google Books.
^Reed, C. A.; Kim, K.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.; Mueller, L. J.; Boyd, P. D. W. (2003). "Isolating Benzenium Ion Salts". J. Am. Chem. Soc. 125 (7): 1796–804. doi:10.1021/ja027336o. PMID 12580605.
^Poverenov, E.; Leitus, G.; Milstein, D. (2006). "Synthesis and Reactivity of the Methylene Arenium Form of a Benzyl Cation, Stabilized by Complexation". J. Am. Chem. Soc. (Communication). 128 (51): 16450–1. doi:10.1021/ja067298z. PMID 17177364.
December 29, 2022
arenium, arenium, organic, chemistry, cyclohexadienyl, cation, that, appears, reactive, intermediate, electrophilic, aromatic, substitution, historic, reasons, this, complex, also, called, wheland, intermediate, after, american, chemist, george, willard, whela. 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 smallest arenium ion is the benzenium ion C6 H 7 which is protonated benzene Ball and stick model of the benzenium ion Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring 4 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 5 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 6 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 See also EditAryl radical Cyclopentadienyl anion Meisenheimer complex the analogous intermediate in nucleophilic aromatic substitution Tropylium cationSome historic references EditOlah 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 References Edit Smith Michael B March Jerry 2007 Advanced Organic Chemistry Reactions Mechanisms and Structure 6th ed New York Wiley Interscience p 658 ISBN 978 0 471 72091 1 Smith Michael B 18 October 2010 Organic Chemistry An Acid Base Approach CRC Press ISBN 9781439894620 via Google Books More commonly sigma complex usually refers to a type of organometallic complex Sykes Peter A Guidebook to Mechanism in Organic Chemistry p 130 133 ISBN missing Reed C A Kim K Stoyanov E S Stasko D Tham F S Mueller L J Boyd P D W 2003 Isolating Benzenium Ion Salts J Am Chem Soc 125 7 1796 804 doi 10 1021 ja027336o PMID 12580605 Poverenov E Leitus G Milstein D 2006 Synthesis and Reactivity of the Methylene Arenium Form of a Benzyl Cation Stabilized by Complexation J Am Chem Soc Communication 128 51 16450 1 doi 10 1021 ja067298z PMID 17177364 Retrieved from https en wikipedia org w index php title Arenium ion amp oldid 1037789265, wikipedia, wiki, book, books, library,
