[[File:Tert-butyl cation resonance (cropped).svg|thumb|120px|The tert-butyl cation is a relatively stable carbenium ion.Thomas H. Lowery (1981). 006044083X, Harper and Rowe. 006044083X
Nomenclature
Reactivity
Carbenium ions are generally highly reactive due to having an incomplete
octet rule of electrons; however, certain carbenium ions, such as the
tropylium cation ion, are relatively stable due to the positive charge being delocalised between the carbon atoms.（It can even exist stably in aqueous solution.）
Rearrangements
Carbenium ions sometimes rearrange readily. For example, when pentan-3-ol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce 3-chloropentane and 2-chloropentane in a ratio of approximately 1:2.
[March] Migration of an alkyl group to form a new carbocationic center is also observed.
This often occurs with
in excess of 10
10 s
−1 at ambient temperature and still takes place rapidly (compared to the NMR timescale) at temperatures as low as −120 °C (
see Wagner-Meerwein shift). 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 .
Typically, carbocations will rearrange to give a tertiary isomer. For instance, all isomers of rapidly rearrange to give the 1-methyl-1-cyclopentyl cation. This fact often complicates synthetic pathways. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about one third 3-chloropentane and two thirds 2-chloropentane. The Friedel–Crafts alkylation suffers from this limitation; for this reason, the acylation (followed by Wolff–Kishner or Clemmensen reduction to give the alkylated product) is more frequently applied.
As electrophiles
Carbocations are susceptible to attack by
, like water, alcohols, carboxylates, azide, and halide ions, to form the addition product. Strongly basic nucleophiles, especially hindered ones, favor elimination over addition. Because even weak nucleophiles will react with carbocations, most can only be directly observed or isolated in non-nucleophilic media like
.
Types of carbenium ions
Stability
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):
| +Hydride ion affinity (HIA) as a measure of carbocation stability
! Carbocation | (most stable) | | | | | | | |
|
|
|
Since carbenium ions can be highly reactive, a major consideration is their stability. The stability of carbenium ions correlates with the electron-donating properties of the substituents. Trialkylcarbenium ions, such as , are isolable as salts, but cannot. An analogous situation applies to triarylcarbenium ions: salts of triphenylcarbenium are readily isolable (see trityl), and those with amine substituents so robust that they are used as dyes, e.g. crystal violet. Carbenium ions can also be stabilized by conjugation to double bonds giving allyl cations, which enjoy some resonance stabilization. This situation is illustrated by the isolation of protonated benzene.[Hansjörg Grützmacher, Christina M. Marchand (1997), "Heteroatom stabilized carbenium ions", Coord. Chem. Rev., 163, 287–344. ]
Alkylium ions
The stability of alkyl-substituted carbocations follows the order . This trend can be inferred by the hydride ion affinity values (231, 246, 273, and 312 kcal/mol for , , , and ).
The effect of alkyl substitution is a strong one:
-
tertiary cations are stable and many are directly observable in superacid media. The stabilization by alkyl groups is explained by hyperconjugation.
-
Secondary cations are usually transient. Only the isopropyl, s-butyl, and cyclopentyl cations have been observed in solution.
-
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 ),
Carbenium ions can be prepared directly from by removing a hydride anion, , with a strong acid. (Equivalently, the corresponding tend to eliminate Hydrogen gas.) For example, magic acid, a mixture of antimony pentafluoride () and fluorosulfuric acid (), turns isobutane into the trimethylcarbenium cation, .[George A. Olah and Joachim Lukas (1967), "Stable Carbonium Ions. XLVII. Alkylcarbonium ion formation from alkanes via hydride (alkide) ion abstraction in fluorosulfonic acid-antimony pentafluoride-sulfuryl chlorofluoride solution". J. Am. Chem. Soc. 89 (18), 4739–4744 ]
Extra stabilizing effects
A carbocation may be stabilized by resonance by a carbon–carbon double bond or by the lone pair of a
heteroatom adjacent to the ionized carbon. The
allyl cation and
benzyl cation are more stable than most other carbenium ions due to donation of electron density from π systems to the cationic center.
The doubly- and triply-benzylic carbocations, diphenylcarbenium and triphenylcarbenium (trityl) cation, are particularly stable. For the same reasons, the partial p character of strained C–C bonds in cyclopropyl groups also allows for donation of electron density
and stabilizes the
cyclopropylmethyl (cyclopropylcarbinyl) cation.
Oxocarbenium and iminium ions have important secondary canonical forms (resonance structures) in which carbon bears a positive charge. 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.
Aromatic carbenium ions
The
tropylium ion is an
aromatic species with the formula .
Its name derives from the molecule
tropine (itself named for the molecule
atropine). Salts of the tropylium cation can be stable, e.g. tropylium tetrafluoroborate. It can be made from
cycloheptatriene (tropylidene) and
bromine or phosphorus pentachloride.
["Tropylium tetrafluorate" Organic Syntheses, Coll. Vol. 5, p.1138 (1973); Vol. 43, p.101 (1963). link ]
It is a planar, cyclic, ion; it also has 6 π-electrons (4 n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms.
The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge.
In 1891 G. Merling obtained a water-soluble salt from a reaction of cycloheptatriene and bromine.[Merling, G. (1891), "Ueber Tropin". Berichte der deutschen chemischen Gesellschaft, 24: 3108–3126. ] The structure was elucidated by Eggers Doering and Knox in 1954.["The Cycloheptatrienylium (Tropylium) Ion" W. von E. Doering, L. H. Knox J. Am. Chem. Soc., 1954, 76 (12), pp 3203–3206 ]["Aromaticity as a Cornerstone of Heterocyclic Chemistry" Alexandru T. Balaban, Daniela C. Oniciu, Alan R. Katritzky Chem. Rev., 2004, 104 (5), 2777–2812 ]
On the other hand, the antiaromatic cyclopentadienyl cation () is destabilized by some 40 kcal/mol.
Another aromatic carbenium ion is the cyclopropenyl or cyclopropenium ion, .["Cyclopropenyl Cation. Synthesis and Characterization." R. Breslow and J. T. Groves J. Am. Chem. Soc., 1970, 92 (4), 984–987 [2]] Although less stable than the tropylium cation, this carbenium ion can also form salts at room temperature. Solutions of such salts were exhibit conventional spectroscopic and chemical properties. The cyclopropenium cation (), although somewhat destabilized by angle strain, is still clearly stabilized by aromaticity when compared to its open-chain analog, allyl cation.
These varying cation stabilities, depending on the number of π electrons in the ring system, can furthermore be crucial factors in reaction kinetics. The formation of an aromatic carbocation is much faster than the formation of an anti-aromatic or open-chain carbocation.
Arenium ions
An
arenium ion 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,
["A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules" G. W. Wheland J. Am. Chem. Soc.; 1942; 64(4) 900–908; ] or a
σ-complex.
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring.
[ A guidebook to mechanism in organic chemistry, Peter Sykes; pp 130–133] The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 5 carbon atoms. Also contributing to the stability of arenium ions is the energy gain resulting from the strong C-e bond (E = electrophile).
The smallest arenium ion is protonated benzene, . The benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid, H(CB11H(CH3)5Br6).["Isolating Benzenium Ion Salts" Christopher A. Reed, Kee-Chan Kim, Evgenii S. Stoyanov, Daniel Stasko, Fook S. Tham, Leonard J. Mueller, and Peter D. W. Boyd J. Am. Chem. Soc.; 2003; 125(7) 1796–1804; ] The benzenium salt is crystalline with thermal stability up to 150 °C. deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure.
Acylium ions
An
acylium ion is a cation with the formula RCO
+.
[ Compendium of Chemical Terminology, acyl groups] The structure is described as R−C≡O
+ or R−=O. It is an acyl carbocation, but the actual structure has the oxygen and carbon linked by a triple bond. Such species are common reactive intermediates, for example, in the Friedel−Crafts acylations also in many other
such as the Hayashi rearrangement. Salts containing acylium ions can be generated by removal of the halide from
:
- RCOCl + SbCl5 → RCO+
The C–O distance in these cations is near 1.1 ångströms, even shorter than that in
carbon monoxide.
Acylium cations are characteristic fragments observed in EI-
mass spectra of
.
Vinyl and alkynyl carbenium ions
Based on hydride ion affinity, the parent vinyl cation (HIA = 287kJ/mol) is less stable than even a primary carbocation (HIA = 266kJ/mol), while an α alkyl-substituted vinyl cation (HIA = 258kJ/mol) has a stability that is between a primary and secondary carbocation.
Hence, vinyl cations are relatively uncommon intermediates. They can be generated by the ionization of a vinyl electrophile, provided the leaving group is sufficiently good (e.g., , IPh, or ). They have been implicated as intermediates in some vinyl substitution reactions (designated as SN1(vinyl)) and as intermediates in the electrophilic addition reactions of arylalkynes. With the exception of the parent vinyl cation, which is believed to be a bridged species, and geometrically constrained cyclic vinyl cations, most vinyl cations take on sp hybridization and are linear.
Aryl cations are less stable than vinyl cations due to the ring-enforced distortion to a nonlinear geometry and approximately sp2-character of the unoccupied orbital. Only in aryldiazonium salts is a good enough leaving group for the chemical generation of aryl cations.
Alkynyl cations are extremely unstable, much less stable than even (hydride ion affinity 386 kcal/mol versus 312 kcal/mol for ) and cannot be generated by purely chemical means. They can, however, be generated radiochemically via the beta decay of tritium:
Selected applications
Carbenium ions are so integrated into organic chemistry that a full inventory of their commercially useful reactions would be long. For example, catalytic cracking, a major step in petroleum refining involves carbenium ion intermediates.
The alkylation of benzene with to give linear alkylbenzene (LABs) illustrates the behaviour of secondary carbenium ions. The alkylation is initiated by strong acids. LABs are a key precursor to .
Derivatives of the triphenylcarbenium are the triarylmethane dyes.
Acylium ions are intermediates in Friedel-Crafts acylations and .
See also
