carbonium ion

{{Short description|Any cation that has a pentavalent carbon atom}}

File:CSD CIF HIGNAOhires.png. All other C-C bond lengths 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 carbonium ion is a cation that has a pentacoordinated carbon atom.{{GoldBookRef|title=Carbonium ion|file=C00839}} They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized in 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}}

2-Norbornyl cation

The 2-norbornyl cation is one of the best characterized carbonium ions. It is 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.

Cyclopropylmethyl cation

A non-classical structure for {{chem|C|4|H|7|+}} is supported by substantial experimental evidence from solvolysis experiments and NMR studies. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity. he NMR spectrum consists of two ¹³C NMR signals, even at temperatures as low as −132 °C. Computations suggest that the energetic landscape of the {{chem|C|4|H|7|+}} system is very flat. The bicyclobutonium structure is computed to be 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure. In the solution phase (SbF₅·SO₂ClF·SO₂F₂, with {{chem|SbF|6|–}} as the counterion), the bicyclobutonium structure predominates over the cyclopropylcarbinyl structure in a 84:16 ratio at −61 °C. Three other possible structures, two classical structures (the homoallyl cation and cyclobutyl cation) and a more highly delocalized non-classical structure (the tricyclobutonium ion), are less stable.{{Cite journal|last1=Olah|first1=George A.|last2=Surya Prakash|first2=G. K.|last3=Rasul|first3=Golam|date=July 2008|title=Ab Initio/GIAO-CCSD(T) Study of Structures, Energies, and ¹³C NMR Chemical Shifts of {{chem|C|4|H|7|+}} and {{chem|C|5|H|9|+}} Ions: Relative Stability and Dynamic Aspects of the Cyclopropylcarbinyl vs Bicyclobutonium Ions|url=https://pubs.acs.org/doi/10.1021/ja802445s|journal=Journal of the American Chemical Society|language=en|volume=130|issue=28|pages=9168–9172|doi=10.1021/ja802445s|pmid=18570420 |issn=0002-7863}}

File:Ch47cations.png

The low temperature NMR spectrum of a dimethyl derivative shows two methyl signals, indicating that the molecular conformation of this cation is not perpendicular (as in A), which possesses a mirror plane, but is bisected (as in B) with the empty p-orbital parallel to the cyclopropyl ring system:

:File:Cyclopropylcarbinyl_bisected_conformation.svg

In terms of bent bond theory, this preference is explained by assuming favorable orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital.{{cite book|first1=F.A. |last1=Carey |first2=R.J. |last2=Sundberg |title=Advanced Organic Chemistry Part A |edition= 2nd}}

Methanium and ethanium

The simplest carbonium ions are also the least accessible. In methanium ({{chem2|CH5+}}) carbon is covalently bonded to five hydrogen atoms.{{cite journal | doi = 10.1063/1.470138| title = Infrared spectroscopy of the molecular hydrogen solvated carbonium ions, {{chem|CH|5|+|(H|2|)|n}} (n = 1–6)| journal = The Journal of Chemical Physics| volume = 103| issue = 2| pages = 520| year = 1995| last1 = Boo| first1 = Doo Wan| last2 = Lee| first2 = Yuan T| bibcode = 1995JChPh.103..520B|url = https://zenodo.org/record/1232938}}{{ cite journal | journal = Science | volume = 309 | issue = 5738 | pages = 1219–1222 | title = Understanding the Infrared Spectrum of Bare CH₅⁺ | first1 = O. | last1= Asvany | first2= P. | last2= Kumar P | first3= B. | last3= Redlich | year = 2005 | first4= I. | last4= Hegemann | first5= S. | last5= Schlemmer | first6= D. | last6= Marx | doi = 10.1126/science.1113729 | pmid=15994376|bibcode = 2005Sci...309.1219A | s2cid = 28745636 | doi-access= free }}{{cite journal | author1 = Xiao-Gang Wang |author2 = Tucker Carrington Jr | title = Calculated rotation-bending energy levels of CH₅⁺ and a comparison with experiment | journal = Journal of Chemical Physics | year = 2016 | volume = 144 |issue = 20 | pages = 204304 | doi=10.1063/1.4948549 |pmid = 27250303 |bibcode = 2016JChPh.144t4304W }}{{cite journal | author1 = H. Schmiedt |author2 = Per Jensen | author3 = S. Schlemmer | title = Rotation-vibration motion of extremely flexible molecules - The molecular superrotor | journal = Chemical Physics Letters | year = 2017 | volume = 672 | pages = 34–46 | doi = 10.1016/j.cplett.2017.01.045|bibcode = 2017CPL...672...34S | doi-access = free }}

The ethanium ion {{chem2|C2H7+}} has been characterized by infrared spectroscopy.{{Cite journal | doi = 10.1021/ja00197a015| title = Infrared spectroscopy of the pentacoordinated carbonium ion {{chem|C|2|H|7|+}}| journal = Journal of the American Chemical Society| volume = 111| issue = 15| pages = 5597| year = 1989| last1 = Yeh| first1 = L. I| last2 = Price| first2 = J. M| last3 = Lee| first3 = Yuan T}} The isomers of octonium (protonated octane, {{chem2|C8H19+}}) have been studied.{{Cite journal | doi= 10.1021/jp021724v| title= Isomers of Protonated Octane, {{chem|C|8|H|19|+}}| journal= The Journal of Physical Chemistry A| volume= 106| issue= 47| pages= 11653| year= 2002| last1= Seitz| first1= Christa| last2= East| first2= Allan L. L| bibcode= 2002JPCA..10611653S}}

Pyramidal carbocations

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! align = "center" | Pyramidal Carbocations

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One class of carbonium ions are called pyramidal carbocations. In these ions, a single carbon atom hovers over a four- or five-sided polygon, in effect forming a pyramid. The square pyramidal ion will carry a charge of +1, the Pentagonal pyramidal ion will carry +2.

X-ray crystallography confirms that hexamethylbenzene dication ([C₆(CH₃)₆]²⁺) is pentagonal-pyramidal.{{Cite journal|last1=Malischewski|first1=Moritz|last2=Seppelt|first2=K.|date=2016-11-25|title=Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication {{chem|C|6|(CH|3|)|6|2+}}|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=1|pages=368–370|doi=10.1002/anie.201608795|pmid=27885766|issn=1433-7851}}

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| width = "150px" | An example of the monovalent carbocation

width = "150px" | An example of the divalent carbocation

Applications

Carbonium ions are intermediates in the isomerization of alkanes catalyzed by very strong solid acids.{{Cite journal | doi = 10.1351/pac200072122309| title = Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes| journal = Pure and Applied Chemistry| volume = 72| issue = 12| pages = 2309| year = 2000| last1 = Sommer| first1 = J| last2 = Jost| first2 = R| doi-access = free}} Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).Office of Energy Efficiency and Renewable Energy, U.S. DOE (2006). [http://www1.eere.energy.gov/manufacturing/resources/petroleum_refining/pdfs/bandwidth.pdf "Energy Bandwidth for Petroleum Refining Processes"]{{Cite journal|last1=Kotrel|first1=S.|last2=Knözinger|first2=H.|last3=Gates|first3=B.C.|date=April 2000|title=The Haag–Dessau mechanism of protolytic cracking of alkanes|url=https://linkinghub.elsevier.com/retrieve/pii/S1387181199002048|journal=Microporous and Mesoporous Materials|language=en|volume=35-36|pages=11–20|doi=10.1016/S1387-1811(99)00204-8|bibcode=2000MicMM..35...11K }}{{cite journal |doi=10.1007/s11144-016-0978-9 |title=Pentane Transformations over Sulfated Alumina catalyst |date=2016 |last1=Marczewski |first1=M. |last2=Popielarska |first2=D. |last3=Marczewska |first3=H. |journal=Reaction Kinetics, Mechanisms and Catalysis |volume=118 |pages=267–280 |doi-access=free }}

References

Category:Reactive intermediates

Category:Carbocations