• 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

  • (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

  • 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

  • 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.


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