Carbocation

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The tert-butyl cation is a relatively stable carbenium ion. Tert-butyl cation resonance (cropped).svg
The tert-butyl cation is a relatively stable carbenium ion.

A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+
3, methanium CH+
5, acylium ions RCO+, and vinyl C
2H+
3 cations. [2]

Contents

Until the early 1970s, carbocations were called carbonium ions. [3] 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 are further classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions. This nomenclature was proposed by G. A. Olah. [4] 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. 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. [5]

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. [6] Prior to the observation of five-coordinate carbocations by Olah and coworkers, carbocation and carbonium ion were used interchangeably. 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. 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. 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. The IUPAC acknowledges the three divergent definitions of carbonium ion and urges care in the usage of this term. 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.


Structure and properties

Carbonium ions

Structure of the 2-norbornyl carbonium ion. All other C-C bond lengths are normal (ca. 1.5 A). CSD CIF HIGNAOhires.png
Structure of the 2-norbornyl carbonium ion. All other C-C bond lengths are normal (ca. 1.5 Å).

Carbonium ions can be thought of as protonated or alkylated alkanes. They feature delocalized 3c-2e bonds. For this reason, they are often referred to as non-classical ions. A well studied example, albeit of no practical value, is the 2-norbornyl cation. Like carbenium ions, carbonium ions are often invoked as intermediates in the upgrading of hydrocarbons in refineries.

Carbenium ions

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. 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). 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. In accord with VSEPR and Bent's rule, unless geometrically constrained to be pyramidal (e.g., 1-adamantyl cation), 3-coordinate carbon in carbenium ions are usually trigonal planar, with a pure p character empty orbital as its lowest unoccupied molecular orbital (LUMO) and CH/CC bonds formed from C(sp2) orbitals. A prototypical example is the methyl cation, CH+3.

History

The history of carbocations dates back to 1891 when G. Merling [8] reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material, C
7H
7Br. He did not suggest a structure for it; however, Doering and Knox [9] convincingly showed that it was tropylium (cycloheptatrienylium) bromide. This ion is predicted to be aromatic by Hückel's rule.

In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep-yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes when treated with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the salt-like character of the compounds formed. He dubbed the relationship between color and salt formation halochromy, of which malachite green is a prime example. The trityl carbocation (shown below) is indeed a stable carbocationic system, for example in the form of trityl hexafluorophosphate. [10]

reaction of triphenylmethanol with sulfuric acid TriphenylmethanolCarbocationFormation.svg
reaction of triphenylmethanol with sulfuric acid

Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in 1899, [11] was further developed by Hans Meerwein in his 1922 study [12] [13] of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, the E1 reaction, and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them.

An NMR spectrum of a carbocation was first reported by Doering et al. [14] in 1958. It was the heptamethylbenzenium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960 [15] by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established that it was non-classically bridged (the first stable non-classical ion observed).

In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert-butyl fluoride in magic acid. The NMR spectrum of the norbornyl cation was reported by Schleyer et al. [16] It was shown to rapidly undergo proton-scrambling . [17]

See also

Related Research Articles

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double bond of an alkene functional group.

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

<span class="mw-page-title-main">George Andrew Olah</span> Hungarian-American chemist (1927–2017)

George Andrew Olah was a Hungarian-American chemist. His research involved the generation and reactivity of carbocations via superacids. For this research, Olah was awarded a Nobel Prize in Chemistry in 1994 "for his contribution to carbocation chemistry." He was also awarded the Priestley Medal, the highest honor granted by the American Chemical Society and F.A. Cotton Medal for Excellence in Chemical Research of the American Chemical Society in 1996.

<span class="mw-page-title-main">Adamantane</span> Molecule with three connected cyclohexane rings arranged in the "armchair" configuration

Adamantane is an organic compound with formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the most stable isomer of C10H16. The spatial arrangement of carbon atoms in the adamantane molecule is the same as in the diamond crystal. This similarity led to the name adamantane, which is derived from the Greek adamantinos (relating to steel or diamond). It is a white solid with a camphor-like odor. It is the simplest diamondoid.

A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a net bonding effect and constituting a chemical bond among all three atoms. In many common bonds of this type, the bonding orbital is shifted towards two of the three atoms instead of being spread equally among all three. Example molecules with 3c–2e bonds are the trihydrogen cation and diborane. In these two structures, the three atoms in each 3c-2e bond form an angular geometry, leading to a bent bond.

<span class="mw-page-title-main">Magic acid</span> Superacid system prepared from a Brønsted and a Lewis superacid

Magic acid (FSO3H·SbF5) is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfuric acid (HSO3F) and antimony pentafluoride (SbF5). This conjugate Brønsted–Lewis superacid system was developed in the 1960s by the George Olah lab at Case Western Reserve University, and has been used to stabilize carbocations and hypercoordinated carbonium ions in liquid media. Magic acid and other superacids are also used to catalyze isomerization of saturated hydrocarbons, and have been shown to protonate even weak bases, including methane, xenon, halogens, and molecular hydrogen.

<span class="mw-page-title-main">Wagner–Meerwein rearrangement</span> Organic reaction

A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. They can be described as cationic [1,2]-sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention. As such, a Wagner–Meerwein shift is a thermally allowed pericyclic process with the Woodward-Hoffmann symbol [ω0s + σ2s]. They are usually facile, and in many cases, they can take place at temperatures as low as –120 °C. The reaction is named after the Russian chemist Yegor Yegorovich Vagner; he had German origin and published in German journals as Georg Wagner; and Hans Meerwein.

<span class="mw-page-title-main">Carbenium ion</span> Class of ions

A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic carbon-based cations. In parallel with carbenium ions is another subset of carbocations, the carbonium ions with the formula R5+. In carbenium ions charge is localized. They are isoelectronic with monoboranes such as B(CH3)3.

<span class="mw-page-title-main">Arenium ion</span> Forms during electrophilic substitution on benzene ring

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland (1907–1976). They are also called sigma complexes. The smallest arenium ion is the benzenium ion, which is protonated benzene.

In organic chemistry, neighbouring group participation has been defined by the International Union of Pure and Applied Chemistry (IUPAC) as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma or pi bond contained within the parent molecule but not conjugated with the reaction centre. When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry this page is limited to neighbouring group effects seen with carbocations and SN2 reactions.

<span class="mw-page-title-main">Carbonium ion</span> Any cation that has a pentavalent carbon atom

In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom. 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.

<span class="mw-page-title-main">2-Norbornyl cation</span> Term in organic chemistry

In organic chemistry, the term 2-norbornyl cation describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography confirmed the non-classical structure.

<span class="mw-page-title-main">Homoaromaticity</span> Organic molecular structure

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

<span class="mw-page-title-main">Methanium</span> Ion of carbon with five hydrogens

In chemistry, methanium is a complex positive ion with formula [CH5]+ or [CH3(H2)]+, bearing a +1 electric charge. It is a superacid and one of the onium ions, indeed the simplest carbonium ion.

<span class="mw-page-title-main">Halonium ion</span> Any onium ion containing a halogen atom carrying a positive charge

A halonium ion is any onium ion containing a halogen atom carrying a positive charge. This cation has the general structure R−+X−R′ where X is any halogen and no restrictions on R, this structure can be cyclic or an open chain molecular structure. Halonium ions formed from fluorine, chlorine, bromine, and iodine are called fluoronium, chloronium, bromonium, and iodonium, respectively. The 3-membered cyclic variety commonly proposed as intermediates in electrophilic halogenation may be called haliranium ions, using the Hantzsch-Widman nomenclature system.

<span class="mw-page-title-main">Nonclassical ion</span> Type of molecule in organic chemistry

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.

<span class="mw-page-title-main">Methenium</span> Ion of carbon with three hydrogens

In organic chemistry, methenium is a cation with the formula CH+
3. It can be viewed as a methylene radical with an added proton, or as a methyl radical with one electron removed. It is a carbocation and an enium ion, making it the simplest of the carbenium ions.

Hydrogen-bridged cations are a type of charged species in which a hydrogen atom is simultaneously bonded to two atoms through partial sigma bonds. While best observable in the presence of superacids at room temperature, spectroscopic evidence has suggested that hydrogen-bridged cations exist in ordinary solvents. These ions have been the subject of debate as they constitute a type of charged species of uncertain electronic structure.

In chemistry, the decay technique is a method to generate chemical species such as radicals, carbocations, and other potentially unstable covalent structures by radioactive decay of other compounds. For example, decay of a tritium-labeled molecule yields an ionized helium atom, which might then break off to leave a cationic molecular fragment.

<span class="mw-page-title-main">Triphenylcarbenium</span> Chemical compound

In chemistry, triphenylcarbenium, triphenylmethyl cation, tritylium , or trityl cation is an ion with formula [C19H15]+ or (C6H5)3C+, consisting of a carbon atom with a positive charge connected to three phenyl groups. It is a charged version of the triphenylmethyl radical (C6H5)3C•. The name is often abbreviated to triphenylmethyl or trityl in salts, although these names also denote the chemical group in compounds like triphenylmethyl chloride that do not contain the cation.

References

  1. ↑ Scholz, Franziska; Himmel, Daniel; Scherer, Harald; Krossing, Ingo (2013). "Superacidic or Not…︁? Synthesis, Characterisation, and Acidity of the Room-Temperature Ionic Liquid [C(CH3)3]+[Al2Br7]−". Chemistry – A European Journal. 19 (1): 109–116. doi:10.1002/chem.201203260. PMID   23180742.
  2. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 235, ISBN   978-0-471-72091-1
  3. ↑ 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.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ "Carbocation", IUPAC Compendium of Chemical Terminology, International Union of Applied Chemistry, 2009, doi: 10.1351/goldbook.C00817 , ISBN   978-0967855097 , retrieved 2018-11-03
  7. ↑ 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.
  8. ↑ Merling, G. (1891). "Ueber Tropin". Berichte der Deutschen Chemischen Gesellschaft. 24 (2): 3108–3126. doi:10.1002/cber.189102402151. ISSN   0365-9496.
  9. ↑ 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.
  10. ↑ Urch, C. (2001). "Triphenylmethyl Hexafluorophosphate". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rt363f. ISBN   0471936235.
  11. ↑ "On the Constitution of the Salts of Imido-Ethers and other Carbimide Derivatives". American Chemical Journal. 21: 101. ISSN   0096-4085.
  12. ↑ Meerwein, H.; Emster, K. van (1922). "About the equilibrium isomerism between bornyl chloride isobornyl chloride and camphene chlorohydrate". Berichte. 55: 2500.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ Story, Paul R.; Saunders, Martin (1960). "The 7-norbornadienyl carbonium ion". Journal of the American Chemical Society . 82 (23): 6199. doi:10.1021/ja01508a058.
  16. ↑ 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.
  17. ↑ 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.
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