nonclassical ion
{{short description|Type of molecule in organic chemistry}}
File:CSD CIF HIGNAOhires.png. C-C bond lengths not labeled are normal (ca. 1.5 Å).{{Cite journal|last1=Scholz|first1=F.|last2=Himmel|first2=D.|last3=Heinemann|first3=F. W.|last4=Schleyer|first4=P. v R.|last5=Meyer|first5=K.|last6=Krossing|first6=I.|date=2013-07-05|title=Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation|journal=Science|language=en|volume=341|issue=6141|pages=62–64|doi=10.1126/science.1238849|issn=0036-8075|pmid=23828938|bibcode=2013Sci...341...62S|s2cid=206549219 }}]]
In chemistry, a nonclassical ion usually refers to carbonium ions, a family of organic cations. They are characterized by delocalized three-center, two-electron bonds. The more stable members are often bi- or polycyclic.{{cite book|title=Mechanism and Theory in Organic Chemistry, Second Edition|author=Thomas H. Lowery|author2=Kathleen Schueller Richardson|publisher=Harper and Rowe|year=1981|isbn=0-06-044083-X|page=396}} Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005[http://www.uscibooks.com/adfm_new.pdf]
Examples
Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides.Sykes P. A guide book to mechanism in organic chemistry 6th Ed. , New Delhi: Orient Longman, 1986, p. 111 ff[http://about.mdma.ch/picproxie_docs/000536454-Guidebook_to_Mechanism_in_Organic_Chemistry.pdf] Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976 [https://link.springer.com/content/pdf/bfm%3A978-1-4684-0826-3%2F1.pdf] Explanations for these rates was once controversial.Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,[https://onlinelibrary.wiley.com/doi/10.1002/poc.3846]
The 2-norbornyl cation is one of the best characterized carbonium ions:
:{{chem2|C7H10 + H+ -> C7H11+}}
In fact, it has emerged as the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, it has a symmetric structure with an RCH2+ group bonded to an alkene group, stabilized by a bicyclic structure.
Solvolyses of cyclopropylcarbinyl, cyclobutyl, and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion.Saunders, M., Laidig, K.E., Wiberg, K.B. , Schleyer Structures, energies, and modes of interconversion of {{chem2|C4H7+}} ions J. Am. Chem. Soc., 1988, 110, 7652–7659 [https://pubs.acs.org/doi/pdf/10.1021/ja00231a012] Siehl , H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem. , 2018, 52, 1-47 [https://www.elsevier.com/books/advances-in-physical-organic-chemistry/williams/978-0-12-815211-9]
Further reading
- Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press: New York /Springer 1977 [https://www.springer.com/de/book/9781461341208]
- Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014 , 27, 374-379 [https://onlinelibrary.wiley.com/doi/full/10.1002/poc.3290]