• 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 now known to be less stable isomers (or merely a transition state rather than an energy minimum in the case of the cyclobutyl cation).

• This results in a species that contains a 3c-2e bond between a carbon and two hydrogen atoms, a type of bonding common in boron chemistry, though relatively uncommon for carbon.

• One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity in various experiments, while
  the NMR data point to a highly fluxional system that undergoes rapid rearrangement to give an averaged spectrum consisting of only two 13C NMR signals, even at temperatures as low as −132 °C.

• [3] Carbonium ions, as originally defined by Olah, are characterized by a three-center two-electron delocalized bonding scheme and are essentially synonymous with so-called
  ‘non-classical carbocations’, which are carbocations that contain bridging C–C or C–H σ-bonds.

• [2] In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom.

• They can, however, be generated radiochemically via the beta decay of tritium:[32] Order of stability of examples of tertiary (III), secondary (II), and primary (I) alkylcarbenium
  ions, as well as the methyl cation (far right).

• [44] Substituted cyclopropylcarbinyl cations have also been studied by NMR:[45][46] In the NMR spectrum of a dimethyl derivative, two nonequivalent signals are found for the
  two methyl groups, 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: 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.

• [4] Definitions According to the IUPAC, a carbocation is any cation containing an even number of electrons in which a significant portion of the positive charge resides on
  a carbon atom.

• As an alternative view point, the 3c-2e bond of carbonium ions could be considered as a molecule of H2 coordinated to a carbenium ion (see below).

• The stability order of carbocations, from most stable to least stable as reflected by hydride ion affinity (HIA) values, are as follows (HIA values in kcal/mol in parentheses):
  As noted in the history section, the tropylium cation (C7H+7) was one of the first carbocations to be discovered, due to its aromatic stability.

• [33] Relative formation energy of carbocations from computational calculation Carbocations typically undergo rearrangement reactions from less stable structures to equally
  stable or more stable ones by migration of an alkyl group or hydrogen to the cationic center to form a new carbocationic center.

• A carbocation with a two-coordinate positive carbon derived from formal removal of a hydride ion (H−) from an alkene is known as a vinyl cation.

• Oxocarbenium and iminium ions have important secondary canonical forms (resonance structures) in which carbon bears a positive charge.

• The effect of alkyl substitution is a strong one: tertiary cations are stable and many are directly observable in superacid media, but secondary cations are usually transient
  and only the isopropyl, s-butyl, and cyclopentyl cations have been observed in solution.

• Olah proposed a redefinition of carbonium ion as a carbocation featuring any type of three-center two-electron bonding, while a carbenium ion was newly coined to refer to
  a carbocation containing only two-center two-electron bonds with a three-coordinate positive carbon.

• In most, if not all cases, the ground state of alleged primary carbocations consist of bridged structures in which positive charge is shared by two or more carbon atoms and
  are better described as side-protonated alkenes, edge-protonated cyclopropanes, or corner-protonated cyclopropanes rather than true primary cations.

• Subsequently, others have used the term carbonium ion more narrowly to refer to species that are derived (at least formally) from electrophilic attack of H+ or R+ on an alkane,
  in analogy to other main group onium species, while a carbocation that contains any type of three-centered bonding is referred to as a non-classical carbocation.

• As such, they are carbocations according to the IUPAC definition although some chemists do not regard them to be “true” carbocations, as their most important resonance contributors
  carry the formal positive charge on an oxygen or nitrogen atom, respectively.

• Non-classical ions Some carbocations such as the 2-norbornyl cation exhibit more or less symmetrical three-center two-electron bonding.

• In the absence of geometric constraints, most substituted vinyl cations carry the formal positive charge on an sp-hydridized carbon atom of linear geometry.

• The trityl carbocation (shown below) is a stable carbocationic system that has been used as homogeneous organocatalyst in organic synthesis,[13] for example in the form of
  trityl hexafluorophosphate.

• [43][35] A variety of carbocations (e.g., ethyl cation, see above) are now believed to adopt non-classical structures.

• Indeed, carbonium ions frequently decompose by loss of molecular hydrogen to form the corresponding carbenium ion.

• Thus, in at least one of their resonance depictions, they possess a carbon atom bearing a formal positive charge that is surrounded by a sextet of electrons (six valence electrons)
  instead of the usual octet required to fill the valence shell of carbon (octet rule).

• These varying cation stabilities, depending on the number of π electrons in the ring system, can furthermore be crucial factors in reaction kinetics.

• For the same reasons, the partial p character of strained C–C bonds in cyclopropyl groups also allows for donation of electron density[38] and stabilizes the cyclopropylmethyl
  (cyclopropylcarbinyl) cation.

• However, some university-level textbooks continue to use the term carbocation as if it were synonymous with carbenium ion,[6][7] or discuss carbocations with only a fleeting
  reference to the older terminology of carbonium ions[8] or carbenium and carbonium ions.

• Although conjugation to unsaturated groups results in significant stabilization by the mesomeric effect (resonance), the benefit is partially offset by the presence of a more
  electronegative sp2 or sp carbon next to the carbocationic center.

• Computationally, it was confirmed that the energetic landscape of the C4H+7 system is very flat, and that the two isomers postulated based on experimental data are very close
  in energy, the bicyclobutonium structure being computed to be just 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure.

• The sp2 lone pair of molecule A is oriented such that it forms sufficient orbital overlap with the empty p orbital of the carbonation to allow the formation of a π bond, sequestering
  the carbonation in a contributing resonance structure.

• In contrast, at least in a formal sense, carbenium ions are derived from the protonation (addition of H+) or alkylation (addition of R+) of a carbene or alkene.

• However, others have more narrowly defined the term ‘carbonium ion’ as formally protonated or alkylated alkanes (CR+5, where R is H or alkyl), to the exclusion of non-classical
  carbocations like the 2-norbornyl cation.

• In this usage, 2-norbornyl cation is not a carbonium ion, because it is formally derived from protonation of an alkene (norbornene) rather than an alkane, although it is a
  non-classical carbocation due to its bridged structure.

• For the remainder of this article, the term carbonium ion will be used in this latter restricted sense, while non-classical carbocation will be used to refer to any carbocation
  with C–C and/or C–H σ-bonds delocalized by bridging.

• This overlap of the orbitals allows the positive charge to be dispersed and electron density from the π system to be shared with the electron-deficient center, resulting in
  stabilization.

• (For clarity, a dashed line is used to show that the hydrogen atom is still attached, although the formal C–H bond order in the hyperconjugative structure is zero.)

• The cyclopropenium cation (C3H+3), although somewhat destabilized by angle strain, is still clearly stabilized by aromaticity when compared to its open-chain analog, allyl
  cation.

• Hence, the structure of the ion is often described as fluxional.

• In especially favorable cases like the 2-norbornyl cation, hydrogen shifts may still take place at rates fast enough to interfere with X-ray crystallography at 86 K (−187
  °C).

• Given the role of carbocations in many reaction schemes, such as SN1 for example, choosing the conjugation of starting materials can be a powerful method for conferring kinetic
  favorability or unfavorability, as the rate constant for any given step is dependent on the step’s activation energy according to the Arrhenius equation.

• Based on hydride ion affinity, the parent vinyl cation is less stable than even a primary sp2-hybridized carbocation, while an α alkyl-substituted vinyl cation has a stability
  that is comparable to the latter.

• On opposing sides were Herbert C. Brown, who believed that what appeared to be a non-classical carbocation represents the average of two rapidly equilibrating classical species
  (or possibly two structures exhibiting some degree of bridging or leaning but is nevertheless not symmetric) and that the true non-classical structure is a transition state between the two potential energy minima, and Saul Winstein, who believed
  that a non-classical structure that possessed a plane of symmetry was the sole potential energy minimum and that the classical structures merely two contributing resonance forms of this non-classical species.

• Therefore, carbenium ions (and carbocations in general) are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge.

• [23] There is seldom any experimental support for primary carbocations in the solution phase, even as transient intermediates (the ethyl cation has been proposed for reactions
  in 99.9% sulfuric acid and in FSO2OH·SbF5), and methyl cation has only been unambiguously identified in the gas phase.

• Although there appear to be five bonds to carbon in carbonium ions, they are not hypervalent, as the electron count around the central carbon is only eight, on account of
  the 3c-2e bond.

• [41][42] At least for the 2-norbornyl cation itself, the controversy has been settled overwhelmingly in Winstein’s favor, with no sign of the putative interconverting classical
  species, even at temperatures as low as 6 K, and a 2013 crystal structure showing a distinctly non-classical structure.

• Such structures, referred to as non-classical carbocations, involve the delocalization of the bonds involved in the σ-framework of the molecule, resulting in C–C and C–H bonds
  of fractional bond order.

• [28] Neopentyl derivatives are thought to ionize with concomitant migration of a methyl group (anchimeric assistance); thus, in most if not all cases, a discrete neopentyl
  cation is not believed to be involved.

• History The history of carbocations dates back to 1891 when G. Merling[11] reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to
  obtain a crystalline, water-soluble material, C7H7Br.

 

Works Cited

[‘Grützmacher, Hansjörg; Marchand, Christina M. (1997). “Heteroatom stabilized carbenium ions”. Coord. Chem. Rev. 163: 287–344. doi:10.1016/S0010-8545(97)00043-X.
2. ^ Robert B. Grossman (2007-07-31). The Art of Writing Reasonable Organic Reaction Mechanisms.
Springer Science & Business Media. pp. 105. ISBN 978-0-387-95468-4.
3. ^ Olah, George A. (1972). “Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center
bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions”. Journal of the American Chemical Society. 94 (3): 808–820. doi:10.1021/ja00758a020.
4. ^ Sommer, J.; Jost, R. (2000-01-01). “Carbenium
and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes”. Pure and Applied Chemistry. 72 (12): 2309–2318. doi:10.1351/pac200072122309. ISSN 1365-3075.
5. ^ “Carbocation”, IUPAC Compendium of Chemical Terminology,
International Union of Applied Chemistry, 2009, doi:10.1351/goldbook.C00817, ISBN 978-0967855097, retrieved 2018-11-03
6. ^ McMurry, John (August 1999). Organic chemistry (5th ed.). Brooks Cole. ISBN 978-0-534-37617-8.
7. ^ Vollhardt, K. Peter
C.; Schore, Neil Eric (2018). Organic chemistry: Structure and function (8th ed.). New York. ISBN 9781319079451. OCLC 1007924903.
8. ^ Yurkanis Bruice, Paula (2004). Organic Chemistry (4th ed.). Pearson/Prentice Hall. ISBN 978-0-13-140748-0.
9. ^
Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
10. ^ Fox, Marye Anne; Whitesell, James K. (1997). Organic Chemistry. Jones and Bartlett. ISBN
978-0-7637-0413-1.
11. ^ Merling, G. (1891). “Ueber Tropin”. Berichte der Deutschen Chemischen Gesellschaft. 24 (2): 3108–3126. doi:10.1002/cber.189102402151. ISSN 0365-9496.
12. ^ Doering, W. von E.; Knox, L. H. (1954). “The Cycloheptatrienylium
(Tropylium) Ion”. Journal of the American Chemical Society. 76 (12): 3203–3206. doi:10.1021/ja01641a027.
13. ^ “Discovery of an in situ carbocationic system using trityl chloride as a homogeneous organocatalyst”. Tetrahedron. 69: 212–218. 2013.
doi:10.1016/j.tet.2012.10.042.
14. ^ Urch, C. (2001). “Triphenylmethyl Hexafluorophosphate”. Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rt363f. ISBN 0471936235.
15. ^ “On the Constitution of the Salts of Imido-Ethers
and other Carbimide Derivatives”. American Chemical Journal. 21: 101. ISSN 0096-4085.
16. ^ Meerwein, H.; Emster, K. van (1922). “About the equilibrium isomerism between bornyl chloride isobornyl chloride and camphene chlorohydrate”. Berichte. 55:
2500.
17. ^ Rzepa, H. S.; Allan, C. S. M. (2010). “Racemization of Isobornyl Chloride via Carbocations: A Nonclassical Look at a Classic Mechanism”. Journal of Chemical Education. 87 (2): 221. Bibcode:2010JChEd..87..221R. doi:10.1021/ed800058c.
18. ^
Doering, W. von E.; Saunders, M.; Boyton, H. G.; Earhart, H. W.; Wadley, E. F.; Edwards, W. R.; Laber, G. (1958). “The 1,1,2,3,4,5,6-heptamethylbenzenonium ion”. Tetrahedron. 4 (1–2): 178–185. doi:10.1016/0040-4020(58)88016-3.
19. ^ Story, Paul
R.; Saunders, Martin (1960). “The 7-norbornadienyl carbonium ion”. Journal of the American Chemical Society. 82 (23): 6199. doi:10.1021/ja01508a058.
20. ^ Schleyer, Paul von R.; Watts, William E.; Fort, Raymond C.; Comisarow, Melvin B.; Olah, George
A. (1964). “Stable Carbonium Ions. X.1 Direct Nuclear Magnetic Resonance Observation of the 2-Norbornyl Cation”. Journal of the American Chemical Society. 86 (24): 5679–5680. doi:10.1021/ja01078a056.
21. ^ Saunders, Martin; Schleyer, Paul von R.;
Olah, George A. (1964). “Stable Carbonium Ions. XI.1 The Rate of Hydride Shifts in the 2-Norbornyl Cation”. Journal of the American Chemical Society. 86 (24): 5680–5681. doi:10.1021/ja01078a057.
22. ^ Anslyn, Eric V.; Dougherty, Dennis A. (2000).
Modern Physical Organic Chemistry. Sausalito, CA: University Science Books. ISBN 978-1891389313.
23. ^ Carroll, Felix A. (2010). Perspectives on structure and mechanism in organic chemistry (2nd ed.). Hoboken, N.J.: John Wiley. ISBN 9780470276105.
OCLC 286483846.
24. ^ Olah, George A.; O’Brien, Daniel H.; White, Anthony Mallinson. (October 1967). “Stable carbonium ions. LII. Protonated esters and their cleavage in fluorosulfonic acid-antimony pentafluoride solution”. Journal of the American
Chemical Society. 89 (22): 5694–5700. doi:10.1021/ja00998a036. ISSN 0002-7863.
25. ^ Carey, Francis A. (2007). Advanced organic chemistry. Sundberg, Richard J. (5th ed.). New York: Springer. ISBN 9780387448978. OCLC 154040953.
26. ^ Lowry, Thomas
H. (1987). Mechanism and theory in organic chemistry. Richardson, Kathleen Schueller. (3rd ed.). New York: Harper & Row. ISBN 0060440848. OCLC 14214254.
27. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms.
Sundberg, Richard J. (5th ed.). New York: Springer. p. 300-301. ISBN 9780387448978. OCLC 154040953.
28. ^ Schultz, Jocelyn C.; Houle, F. A.; Beauchamp, J. L. (July 1984). “Photoelectron spectroscopy of 1-propyl, 1-butyl, isobutyl, neopentyl, and
2-butyl radicals: free radical precursors to high-energy carbonium ion isomers”. Journal of the American Chemical Society. 106 (14): 3917–3927. doi:10.1021/ja00326a006. ISSN 0002-7863.
29. ^ Yamataka, Hiroshi; Ando, Takashi; Nagase, Shigeru; Hanamura,
Mitsuyasu; Morokuma, Keiji (February 1984). “Ab initio MO calculations of isotope effects in model processes of neopentyl ester solvolysis”. The Journal of Organic Chemistry. 49 (4): 631–635. doi:10.1021/jo00178a010. ISSN 0022-3263.
30. ^ Carey,
Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Sundberg, Richard J. (5th ed.). New York: Springer. p. 300-301. ISBN 9780387448978. OCLC 154040953.
31. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part
A: Structure and Mechanisms. Sundberg, Richard J. (5th ed.). New York: Springer. p. 436. ISBN 9780387448978. OCLC 154040953.
32. ^ Angelini, Giancarlo; Hanack, Michael; Vermehren, Jan; Speranza, Maurizio (1988-02-17). “Generation and trapping of
an alkynyl cation”. Journal of the American Chemical Society. 110 (4): 1298–1299. doi:10.1021/ja00212a052. ISSN 0002-7863.
33. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Sundberg, Richard J. (5th ed.).
New York: Springer. p. 436. ISBN 9780387448978. OCLC 154040953.
34. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Sundberg, Richard J. (5th ed.). New York: Springer. p. 440. ISBN 9780387448978. OCLC 154040953.
35. ^
Jump up to:a b Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v R.; Meyer, K.; Krossing, I. (2013-07-05). “Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation”. Science. 341 (6141): 62–64. Bibcode:2013Sci…341…62S. doi:10.1126/science.1238849.
ISSN 0036-8075. PMID 23828938. S2CID 206549219.
36. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Sundberg, Richard J. (5th ed.). New York: Springer. p. 300-301. ISBN 9780387448978. OCLC 154040953.
37. ^
Aue, Donald H. (2011). “Carbocations”. WIREs Computational Molecular Science. 1 (4): 487–508. doi:10.1002/wcms.12. ISSN 1759-0884. S2CID 222190636.
38. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Sundberg,
Richard J. (5th ed.). New York: Springer. p. 426-427. ISBN 9780387448978. OCLC 154040953.
39. ^ Strictly speaking, the hyperconjugative stabilization of alkyl-substituted carbocations is a type of three-center bonding. Geometrically, the C–H bonds
involved in hyperconjugation are observed (or computed) to “lean” slightly toward the carbocationic center as a result (that is, the +C–C–H bond angle decreases somewhat). Nevertheless, the hydrogen atom is still primarily bonded to the carbon α to
the cationic carbon. To qualify as a non-classical carbocation, the two-electron three-center bond needs to feature a group equally (or nearly equally) bonded to two electron-deficient centers. In practice, there is a continuum of possible bonding
schemes, ranging from slight involvement of a neighboring group (weak hyperconjugation) to equal sharing of a group between adjacent centers (fully non-classical bonding).
40. ^ Carey, Francis A. (2007). Advanced Organic Chemistry: Part A: Structure
and Mechanisms. Sundberg, Richard J. (5th ed.). New York: Springer. p. 447-450. ISBN 9780387448978. OCLC 154040953.
41. ^ Olah, George A.; Prakash, G. K. Surya; Saunders, Martin (May 2002). “Conclusion of the classical-nonclassical ion controversy
based on the structural study of the 2-norbornyl cation”. Accounts of Chemical Research. 16 (12): 440–448. doi:10.1021/ar00096a003.
42. ^ George A. Olah – Nobel Lecture
43. ^ Yannoni, C. S.; Myhre, P. C.; Webb, Gretchen G. (November 1990). “Magic
angle spinning nuclear magnetic resonance near liquid-helium temperatures. Variable-temperature CPMAS spectra of the 2-norbornyl cation to 6 K”. Journal of the American Chemical Society. 112 (24): 8991–8992. doi:10.1021/ja00180a060. ISSN 0002-7863.
44. ^
Olah, George A.; Surya Prakash, G. K.; Rasul, Golam (July 2008). “Ab Initio/GIAO-CCSD(T) Study of Structures, Energies, and 13C NMR Chemical Shifts of C
4H+
7 and C
5H+
9 Ions: Relative Stability and Dynamic Aspects of the Cyclopropylcarbinyl
vs Bicyclobutonium Ions”. Journal of the American Chemical Society. 130 (28): 9168–9172. doi:10.1021/ja802445s. ISSN 0002-7863. PMID 18570420.
45. ^ Kabakoff, David S.; Namanworth, Eli (1970). “Nuclear magnetic double resonance studies of the dimethylcyclopropylcarbinyl
cation. Measurement of the rotation barrier”. Journal of the American Chemical Society. 92 (10): 3234–3235. doi:10.1021/ja00713a080.
46. ^ Pittman Jr., Charles U.; Olah, George A. (1965). “Stable Carbonium Ions. XVII.1a Cyclopropyl Carbonium Ions
and Protonated Cyclopropyl Ketones”. Journal of the American Chemical Society. 87 (22): 5123–5132. doi:10.1021/ja00950a026.
47. ^ Carey, F.A.; Sundberg, R.J. Advanced Organic Chemistry Part A (2nd ed.).
48. ^ Wang, George; Rahman, A. K. Fazlur;
Wang, Bin (May 2018). “Ab initio calculations of ionic hydrocarbon compounds with heptacoordinate carbon”. Journal of Molecular Modeling. 24 (5): 116. doi:10.1007/s00894-018-3640-9. ISSN 1610-2940. PMID 29696384. S2CID 13960338.
49. ^ Malischewski,
Moritz; Seppelt, K. (2016-11-25). “Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C
6(CH
3)2+
6”. Angewandte Chemie International Edition. 56 (1): 368–370. doi:10.1002/anie.201608795. ISSN 1433-7851. PMID
27885766.
Photo credit: https://www.flickr.com/photos/39908901@N06/7055500907/’]