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Carbocation

A carbocation is a reactive intermediate species in organic chemistry consisting of a trivalent carbon atom bearing a formal positive charge and only six valence electrons, resulting in an incomplete octet.^[1] These ions are highly electrophilic due to their electron deficiency and typically exist transiently during reactions rather than as stable, isolable compounds.^[2] Carbocations are classified according to the number of alkyl groups (or carbon atoms) attached to the positively charged carbon: primary (one attached carbon), secondary (two), tertiary (three), or methyl (none).^[1] Their stability follows the order tertiary > secondary > primary > methyl, primarily due to hyperconjugation—where adjacent C-H or C-C sigma bonds donate electron density to the empty p-orbital—and inductive effects from alkyl substituents that disperse the positive charge.^[2] Resonance stabilization further enhances stability in cases like allylic or benzylic carbocations, where the charge can delocalize over multiple atoms.^[3] In organic synthesis and mechanisms, carbocations serve as key intermediates in processes such as the SN1 nucleophilic substitution, E1 elimination, and electrophilic addition to alkenes or alkynes, often dictating regioselectivity via Markovnikov's rule.^[2] Due to their tendency to rearrange via 1,2-hydride or alkyl shifts to form more stable isomers, carbocations can lead to unexpected products in reactions involving protonation or ionization steps.^[4] Historically termed "carbonium ions," the modern nomenclature distinguishes classical trivalent carbocations from pentacoordinate "nonclassical" variants, though the former predominate in most solution-phase chemistry.^[1]^[5]

Fundamentals

Definition and Nomenclature

A carbocation is defined by the International Union of Pure and Applied Chemistry (IUPAC) as a cation containing an even number of electrons in which a significant portion of the positive charge resides on one or more carbon atoms.^[6] This distinguishes carbocations from odd-electron species such as carbon-centered radicals, which possess an unpaired electron and are thus neutral overall, and from carbanions, which are anions with an even number of electrons and a negative charge localized on a carbon atom bearing an unshared electron pair.^[7]^[8] The term "carbocation" serves as the general nomenclature for these species, encompassing a range of structures. Specifically, "carbenium ion" refers to trivalent carbocations of the form \ce{CR3^{+}}, where the carbon atom has three substituents and an empty p-orbital, while "carbonium ion" denotes pentacoordinate carbocations of the form \ce{CR5^{+}} or those involving multicenter bonding beyond simple two-center two-electron bonds.^[9]^[10]^[11] For example, the methyl carbocation, denoted \ce{CH3^{+}}, is a carbenium ion in which the central carbon is sp² hybridized, resulting in a trigonal planar geometry.^[12] The nomenclature derives from combining "carbon" and "cation," with the term "carbocation" introduced by George A. Olah in the early 1970s to unify and clarify the classification of these reactive intermediates.^[13]

Historical Development

The concept of carbocations traces its origins to 1891, when German chemist Gustav Merling reported the addition of bromine to cycloheptatriene, yielding a water-stable bromide salt that was later recognized as the first isolated example of a stable carbocation—the tropylium ion. Although Merling did not identify the ionic nature of the product at the time, this serendipitous discovery laid the groundwork for recognizing positively charged carbon species as viable chemical entities.^[14] In the early 20th century, the idea of trivalent carbon ions gained traction through mechanistic proposals in organic reactions. In 1922, British chemist Robert Robinson advanced the understanding of electrophilic aromatic substitution and related processes by invoking trivalent positively charged carbon intermediates in his electronic theory of organic chemistry, building on Arthur Lapworth's concepts of "kationoid" reagents. During the 1930s, Romanian chemist Costin D. Nenitzescu further proposed trivalent carbon ions as key intermediates in Friedel-Crafts alkylations and related rearrangements, providing experimental evidence through studies on hydrocarbon reactions catalyzed by aluminum chloride. These contributions shifted perceptions from transient radicals to stable ionic intermediates, though direct observation remained elusive due to their high reactivity under normal conditions.^[14] A major breakthrough occurred in the late 1950s and early 1960s with the advent of spectroscopic techniques and superacid media. In 1958, William von E. Doering and colleagues reported the first NMR spectrum of a stable carbocation, the heptamethylbenzenium ion, prepared under controlled conditions. Shortly thereafter, George A. Olah extended this work dramatically; between 1958 and 1962, he demonstrated the generation and NMR characterization of simple alkyl carbocations, such as the tert-butyl cation, in superacids like fluorosulfonic acid-antimony pentafluoride mixtures at low temperatures. Olah's key publications in the 1960s and 1970s, including studies on stable ions in superacid media, provided irrefutable evidence for carbocations as discrete species and revolutionized mechanistic organic chemistry.^[15] In 1972, Olah proposed a seminal redefinition of terminology to clarify structural distinctions: "carbenium ions" for classical trivalent species and "carbonium ions" for pentacoordinate or bridged forms, with "carbocation" as the overarching term.^[16] This nomenclature, which addressed earlier ambiguities where "carbonium" was used generically for all such ions prior to the 1970s, was adopted by the IUPAC and remains standard today.^[9] Olah's comprehensive body of work earned him the 1994 Nobel Prize in Chemistry for enabling the preparation and study of carbocations, fundamentally advancing understanding of reaction mechanisms.^[15] During this period, non-classical carbocation structures were also proposed, though their detailed elucidation came later.^[14]

Classification and Structure

Classical Carbenium Ions

Classical carbenium ions, also known as trivalent carbocations, have the general formula \ce{CR3+}, where the central carbon atom is bonded to three substituents (R) and possesses an empty p-orbital, resulting in a positively charged species with only six electrons in the valence shell of the carbon atom.^[16] These ions exhibit a trigonal planar geometry around the charged carbon, with bond angles approximately 120° and sp² hybridization of the carbon atom.^[13] The empty p-orbital lies perpendicular to the plane formed by the three substituents, allowing for potential π-interactions with adjacent groups.^[16] Representative examples of classical carbenium ions include the methyl cation (\ce{CH3+}), ethyl cation (\ce{CH3CH2+}), isopropyl cation (\ce{(CH3)2CH+}), and tert-butyl cation (\ce{(CH3)3C+}).^[13] In these structures, the Lewis depiction shows the central carbon with three bonds and a formal positive charge, represented as a sextet (six valence electrons) on the carbon atom.^[16] Due to the positive charge, bond lengths in classical carbenium ions are typically shorter than in corresponding neutral hydrocarbons; for instance, the C-H bonds in the methyl cation are approximately 1.08 Å, compared to 1.09 Å in methane.^[17] Similarly, C-C bonds, such as those in the tert-butyl cation, measure about 1.45 Å, shorter than the standard 1.54 Å single bond length.^[18] The stability of these ions follows the order tertiary > secondary > primary > methyl, reflecting increasing alkyl substitution.^[13]

Carbonium Ions and Non-Classical Carbenium Ions

Carbonium ions constitute a distinct class of carbocations featuring pentacoordination at the central carbon atom, following the general formula CR₅⁺. The archetypal member of this family is the methanium ion, CH₅⁺, which possesses a trigonal bipyramidal geometry with the carbon atom at the center and five hydrogen ligands arranged in axial and equatorial positions.^[19] This structure enables the carbon to exceed the traditional octet rule through hypervalent bonding. The unusual stability of carbonium ions arises from three-center two-electron (3c-2e) bonds, wherein a single pair of electrons is shared among three atomic centers, typically the carbon and two ligands. In CH₅⁺, multiple such 3c-2e interactions distribute the positive charge and facilitate the pentacoordinate arrangement, contrasting with the localized bonding in trivalent species. These ions were first characterized in highly acidic environments, such as superacid solutions. Carbonium ions exhibit fluxional dynamics, characterized by rapid intramolecular migrations of protons or substituents that interconvert equivalent configurations on a picosecond timescale. For instance, in CH₅⁺, the hydrogens undergo degenerate exchanges, resulting in time-averaged symmetry observable in spectroscopic measurements.^[19] Non-classical carbenium ions differ from their classical counterparts by incorporating partial σ-bond bridging, leading to charge delocalization across more than three atoms via 3c-2e interactions. The 2-norbornyl cation exemplifies this, with the positive charge shared between the C2 position and the C1-C6 bond through a bridged C-C σ-interaction, forming a symmetric, non-planar structure.^[20] The structural assignment of the 2-norbornyl cation fueled a prominent controversy during the 1960s and 1970s, pitting classical localized models against bridged non-classical proposals; resolution came through nuclear magnetic resonance evidence showing magnetically equivalent carbons at the bridgeheads, supporting the delocalized form.^[21] Subsequent X-ray crystallographic analysis of the isolated cation in 2013 provided definitive structural confirmation of this non-classical geometry. Additional illustrations of non-classical carbenium ions include the 7-norbornenyl cation, where delocalization involves the endo-methylene bridge and the C2-C3 double bond in a 3c-2e framework.

Properties and Stability

Geometric and Electronic Properties

Carbocations, particularly classical carbenium ions, exhibit a trigonal planar geometry at the positively charged carbon atom, arising from sp² hybridization that positions the empty p-orbital perpendicular to the plane of the three attached substituents.^[22] The electronic structure of carbenium ions features a vacant p-orbital on the central carbon, rendering the species highly electrophilic and prone to interaction with nucleophiles. This electron deficiency results in the positive charge being partially delocalized onto adjacent atoms through inductive effects, where electron density from neighboring bonds shifts toward the charged center, rather than residing fully on the carbon. Carbocations thus function as strong Lewis acids, capable of accepting electron pairs into the empty orbital to form stable adducts.^[22]^[23]^[22] Vibrational properties of carbocations can be probed via infrared (IR) spectroscopy, particularly in allylic systems where charge delocalization affects bond orders. For instance, in the allyl cation (C₃H₅⁺), IR spectra reveal C=C stretching vibrations in the range of 1564–1588 cm⁻¹, shifted to lower wavenumbers compared to neutral alkenes due to the partial single-bond character induced by resonance.^[24] Carbocations lack paramagnetism owing to their even-electron configuration, in which all electrons are paired, resulting in diamagnetic behavior. Solvation effects differ markedly between gas and solution phases; in the gas phase, carbocations display intrinsic electronic properties without solvent interactions, whereas in solution, polar solvents form tighter solvation shells around the charged carbon, enhancing stability through electrostatic interactions but also influencing reactivity rates.^[25]

Factors Influencing Stability

The stability of carbocations is profoundly influenced by the number and nature of substituents attached to the positively charged carbon atom. Alkyl groups exert a positive inductive effect (+I), donating electron density through sigma bonds to alleviate the electron deficiency, resulting in the stability order tertiary > secondary > primary > methyl.^[13] This effect is evident in the relative ease of formation and persistence of tertiary carbocations compared to less substituted analogs.^[16] Hyperconjugation provides an additional stabilization mechanism, wherein electrons from adjacent σ C-H bonds delocalize into the empty p-orbital of the carbocation, effectively spreading the positive charge. The extent of this interaction scales with the number of available α-hydrogens; for instance, the tert-butyl cation benefits from nine such hydrogens, markedly enhancing its stability relative to the methyl cation, which lacks any.^[26] This σ-π overlap lowers the energy of the system and is a key contributor to the inductive stabilization observed in alkyl-substituted ions.^[13] Resonance delocalization offers even greater stabilization when π-systems are adjacent to the carbocation center. In allylic cations, such as the allyl ion, the positive charge is distributed across two carbons through resonance:

\ce{CH2=CH-CH2^+ <=> ^+CH2-CH=CH2}

This delocalization reduces the charge density on any single atom, conferring exceptional stability.^[27] Similarly, benzylic carbocations are stabilized by resonance with the aromatic π-electrons of the phenyl ring, allowing the charge to be shared with the ortho and para positions.^[13] Aromaticity in cyclic conjugated systems further exemplifies resonance-driven stability, as seen in the tropylium cation (\ce{C7H7^+}), a seven-membered ring with six π-electrons satisfying Hückel's rule for aromaticity. This fully delocalized structure imparts remarkable thermodynamic stability, enabling isolation under ambient conditions.^[28] Steric effects from bulky substituents can modulate stability by enforcing a planar geometry optimal for p-orbital overlap in tertiary carbocations, thereby enhancing hyperconjugation and inductive donation. However, excessive bulk may impede solvation in condensed phases, indirectly affecting observed stability.^[13] Quantitative assessments, such as gas-phase hydride ion affinities (the enthalpy change for \ce{R^+ + H^- -> RH}), underscore these trends; the tert-butyl carbocation exhibits an affinity of approximately 240 kcal/mol, far lower than the ~314 kcal/mol for the methyl cation, reflecting its superior stability.^[29]

Generation and Reactivity

Methods of Formation

Carbocations are commonly generated through heterolytic bond cleavage in unimolecular nucleophilic substitution (SN1) reactions, where a leaving group departs from an alkyl halide in a polar solvent, forming a carbocation intermediate. For example, the ionization of tert-butyl bromide proceeds as follows:

\text{(CH}_3\text{)}_3\text{CBr} \rightarrow \text{(CH}_3\text{)}_3\text{C}^+ + \text{Br}^-

This process is favored for tertiary halides due to the relative stability of the resulting carbocation, with activation energies typically ranging from 20 to 30 kcal/mol in polar protic solvents like water or ethanol. Similar cleavages occur with secondary halides, such as isopropyl bromide:

\text{(CH}_3\text{)}_2\text{CHBr} \rightarrow \text{(CH}_3\text{)}_2\text{CH}^+ + \text{Br}^-

while primary halides like ethyl bromide form primary carbocations less readily:

\text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{CH}_2^+ + \text{Br}^-

Protonation of alkenes represents another key method, where addition of a proton to the double bond yields a carbocation according to Markovnikov's rule. For ethylene, this occurs as:

\text{H}^+ + \text{CH}_2=\text{CH}_2 \rightarrow \text{CH}_3\text{CH}_2^+

In acidic media, alcohols can also be protonated to form carbocations via loss of water; for instance, protonation of tert-butanol leads to the tert-butyl cation:

\text{(CH}_3\text{)}_3\text{COH} + \text{H}^+ \rightarrow \text{(CH}_3\text{)}_3\text{COH}_2^+ \rightarrow \text{(CH}_3\text{)}_3\text{C}^+ + \text{H}_2\text{O}

In superacid media, such as magic acid (a mixture of fluorosulfuric acid and antimony pentafluoride, HSO₃F–SbF₅), even stable hydrocarbons like methane can be protonated to form elusive species like protonated methane, a nonclassical carbonium ion (CH₅⁺)^[30]:

\text{H}^+ + \text{CH}_4 \rightarrow \text{CH}_5^+

This approach, pioneered by George Olah, allows isolation and study of otherwise transient carbocations at low temperatures. Electron impact ionization in mass spectrometry generates carbocations by high-energy electron bombardment of molecules, often cleaving C-C bonds to produce fragment ions; for example, ionization of propane may yield the ethyl carbocation:

\text{CH}_3\text{CH}_2\text{CH}_3 + e^- \rightarrow \text{CH}_3\text{CH}_2^+ + \text{CH}_3^\bullet + 2e^-

Thermal or photochemical dissociation of diazonium salts provides a route to aryl carbocations, where loss of nitrogen gas occurs upon heating or irradiation; benzenediazonium chloride decomposes as:

\text{C}_6\text{H}_5\text{N}_2^+ \rightarrow \text{C}_6\text{H}_5^+ + \text{N}_2

This method is particularly useful for generating reactive phenyl or substituted aryl cations in solution.

Key Reactions and Rearrangements

Carbocations are highly reactive intermediates that typically undergo substitution, elimination, or rearrangement reactions. In nucleophilic substitution via the SN1 mechanism, the rate-determining step is the formation of the carbocation, followed by rapid attack by a nucleophile on the planar sp²-hybridized carbon center.^[31] This leads to racemization of the product when starting from an enantiopure secondary or tertiary alkyl halide, as the nucleophile can approach from either face of the carbocation with equal probability, though ion pairing may result in partial inversion.^[32] Typical rate constants for SN1 solvolysis of secondary alkyl sulfonates in water at 25°C are on the order of 10⁻⁵ s⁻¹, reflecting the energy barrier for carbocation formation.^[33] Elimination reactions proceed via the E1 pathway, where the carbocation loses a β-hydrogen to form an alkene, often competing with substitution under similar conditions. A representative example is the dehydration of tert-butanol, yielding the tert-butyl cation that eliminates to produce isobutene and a proton:

\ce{(CH3)3C^+ -> (CH3)2C=CH2 + H^+}

This process is favored at higher temperatures due to the entropic drive toward the gaseous alkene product.^[34] Skeletal rearrangements frequently occur to generate more stable carbocations, such as 1,2-hydride or alkyl shifts. For instance, the primary n-propyl carbocation rearranges to the more stable secondary isopropyl carbocation via a hydride shift:

\ce{CH3CH2CH2^+ ->[H^- shift] (CH3)2CH^+}

This rearrangement is common in solvolysis reactions and alters product distributions.^[35] The Wagner-Meerwein rearrangement exemplifies alkyl shifts in bridged systems, as seen in the acid-catalyzed conversion of pinacol derivatives or terpenoids like borneol to camphene, involving migration of an alkyl group across a carbocation center to relieve strain.^[36] In electrophilic aromatic substitution, carbocations serve as electrophiles in Friedel-Crafts alkylation, where an arene attacks the carbocation to form a new C-C bond, releasing a proton:

\ce{ArH + R^+ -> Ar-R + H^+}

However, the generated alkylarene is more reactive than the parent arene, leading to polyalkylation as a common side reaction, and primary alkyl halides often undergo hydride shifts to yield branched products.^[35]

Modern Characterization and Applications

Spectroscopic Detection

Nuclear magnetic resonance (NMR) spectroscopy, particularly ¹³C NMR, enables the direct detection of carbocations by revealing characteristic downfield chemical shifts for the cationic carbon due to its sp² hybridization and positive charge. For instance, the tert-butyl carbocation displays a ¹³C shift at 335.2 ppm in superacid media, a value over 300 ppm deshielded relative to the neutral precursor. This downfield shift confirms the ionic structure and rules out bridged or covalent alternatives. George Olah pioneered these observations in the 1960s using low-temperature superacid solutions like SbF₅ to stabilize elusive alkyl carbocations, marking a breakthrough in their characterization.^[13]^[13]^[13]^[13] Infrared (IR) and Raman spectroscopy complement NMR by probing vibrational modes, including characteristic C-C stretching bands around 700–800 cm⁻¹ for symmetric tertiary carbocations and the absence of C-H bending vibrations on the planar cationic carbon, which lacks attached hydrogens and features an empty p-orbital. These spectra highlight hyperconjugative effects, such as unusually low C-H stretching frequencies near 2830 cm⁻¹ in the tert-butyl cation due to σ-donation into the vacant orbital. Olah's group applied IR and Raman in the 1970s to confirm structures of stable carbenium and carbonium ions in superacids, correlating vibrational data with electronic delocalization.^[13] Ultraviolet-visible (UV-Vis) spectroscopy detects carbocations via intense absorptions from charge-transfer or π→π* transitions, often in the visible range that imparts color to solutions. The trityl (triphenylmethyl) cation, a benchmark stable species, exhibits λ_max at 435 nm and 410 nm in acidic media, reflecting its delocalized charge across phenyl rings.^[37]^[37] Mass spectrometry facilitates gas-phase analysis of carbocations, identifying intact ions by their m/z ratio and elucidating reactivity through collision-induced dissociation (CID) fragmentation patterns, such as hydride or methyl shifts. This approach isolates ions from solvation effects, revealing intrinsic stabilities and rearrangements.^[38]^[38] Cryogenic matrix isolation traps carbocations in inert noble gas matrices at 4–77 K, enabling high-resolution IR studies by minimizing diffusion and recombination. This technique has characterized elusive ions like vinyl carbocations, with IR bands confirming bond orders and symmetries.^[39]^[39] Recent advances employ pulsed-laser photoionization and flash photolysis (post-2010) to generate and time-resolve transient carbocations, combining with transient absorption for kinetic insights into short-lived species.

Computational Studies and Synthetic Applications

Computational studies of carbocations have advanced significantly through density functional theory (DFT) methods, particularly using the B3LYP functional with the 6-31G* basis set for geometry optimization. These calculations provide reliable structural predictions, with bond lengths accurate to within 0.01 Å compared to experimental data for various carbocation species.^[40] For instance, B3LYP/6-31G* optimizations of the ethyl carbocation reveal the characteristic bridged structure stabilized by hyperconjugation. Ab initio methods complement DFT by quantifying hyperconjugative interactions, estimating stabilization energies of approximately 10-15 kcal/mol per C-H bond in simple alkyl carbocations like the ethyl cation, where the total hyperconjugation energy reaches about 36 kcal/mol across three hydrogens.^[41] In synthetic applications, carbocations serve as key intermediates in asymmetric catalysis, enabling enantioselective transformations. For example, the Sakurai allylation, involving Lewis acid activation of allylsilanes with carbonyls, can proceed through chiral carbocation-like transition states when mediated by silver complexes with chiral phosphine ligands, achieving high enantioselectivity in the addition to ketones.^[42] This approach highlights the utility of carbocation reactivity in constructing stereogenic centers, with yields often exceeding 90% ee for aliphatic substrates. Carbocations also play a central role in polymerization chemistry, particularly as initiators in living carbocationic polymerization of isobutylene to produce polyisobutylene. Seminal work using tert-chloride initiators with Lewis acids like TiCl4/BCl3 allows precise control over molecular weight and polydispersity (Đ < 1.5), yielding telechelic polymers for applications in adhesives and sealants.^[43] Beyond synthesis, carbocations contribute to environmental processes, such as in atmospheric chemistry where protonation of volatile organic compounds (VOCs) under acidic aerosol conditions generates carbocations that drive oligomerization and secondary organic aerosol formation. For instance, protonated aldehydes in cloud droplets form carbocations susceptible to nucleophilic attack by water or alcohols, influencing particle growth and climate effects.^[44] Recent post-2020 studies have employed QM/MM simulations to elucidate enzymatic carbocation formations, particularly in terpene synthases. These hybrid methods reveal how active-site residues guide carbocation rearrangements in bornyl diphosphate synthase, with MD trajectories showing cation-π interactions stabilizing intermediates during cyclization, achieving free energy barriers of 15-20 kcal/mol.^[45] Additionally, machine learning models have emerged for predicting carbocation stability, using SMILES-based features to estimate heats of formation with errors below 5 kcal/mol, aiding design of superacid media for stable carbocation isolation.^[46]

References

[1]
Illustrated Glossary of Organic Chemistry - Carbocation; carbonium ion
Carbocation: A contraction of carbon cation. A structure with a positive formal charge on carbon due to an open octet. Formerly called carbonium ion, ...
[2]
CHEM 125b - Lecture 11 - Carbocations and the Mechanism of ...
Relative cation stability can be understood in terms of hyperconjugation, hybridization, and solvation or polarizability. Stabilization of a carbocation via ...
[3]
Reactivity: Understanding Mechanism - csbsju
A secondary carbocation, with the positive carbon attached to two other carbons and a hydrogen atom, is intermediate in stability.Missing: definition | Show results with:definition
[4]
[PDF] CARBOCATION REARRANGEMENTS - GMU
Classify each carbocation carbon as primary, secondary or tertiary. b. For each carbocation that could potentially rearrange to a more stable carbocation, use.
[5]
carbocation (C00817) - IUPAC Gold Book
A carbocation is a cation with an even number of electrons, where a significant positive charge is on one or more carbon atoms.
[6]
radical (R05066) - IUPAC Gold Book
Depending upon the core atom that possesses the unpaired electron, the radicals can be described as carbon-, oxygen-, nitrogen-, metal-centered radicals. If ...
[7]
carbanion (C00804) - IUPAC
Generic name for anions containing an even number of electrons and having an unshared pair of electrons on a tervalent carbon atom (e.g. Cl A 3 C A − or HC ...Missing: definition | Show results with:definition
[8]
Ca to Cl - IUPAC nomenclature
(2) A carbocation, real or hypothetical, that contains at least one five-coordinate carbon atom. (3) A carbocation, real or hypothetical, whose structure cannot ...<|control11|><|separator|>
[9]
carbenium ion (C00812) - IUPAC Gold Book
A generic name for carbocations, real or hypothetical, that have at least one important contributing structure containing a tervalent carbon atom with a vacant ...
[10]
carbonium ion (C00839) - IUPAC Gold Book
A carbonium ion is a carbocation with at least one five-coordinate carbon atom, or whose structure cannot be described by two-electron two-centre bonds only.
[11]
https://goldbook.iupac.org/terms/view/C00839
[12]
[PDF] George A. Olah - Nobel Lecture
One of the most original and significant ideas in organic chemistry was the suggestion that carbocations (as we now call all the positive ions of carbon ...
[13]
100 Years of Carbocations and Their Significance in Chemistry 1
Unraveling spatially dependent hydrophilicity and reactivity of confined carbocation intermediates during methanol conversion over ZSM-5 zeolite.
[14]
Press release: The 1994 Nobel Prize in Chemistry - NobelPrize.org
In the early 1960s Olah and co-workers discovered that stable carbocations could be prepared through the use of a new type of extremely acid compounds – far ...
[15]
Stable carbocations. CXVIII. General concept and structure of ...
Carbocation catalysis in confined space: activation of trityl chloride inside the hexameric resorcinarene capsule. Chemical Science 2022, 13 (29) , 8618 ...
[16]
Astronomical CH3+ rovibrational assignments
The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 μm (1400 cm−1) features toward the d203-506 ...
[17]
Ab Initio/DFT/GIAO−CCSD(T) Calculational Study of the t-Butyl Cation
MP2/cc-pVTZ calculated C+−C bond lengths of 1.453 to 1.455 Å for Cs structure 3 were found to be in good agreement with Yannoni's nutation NMR experimental ...
[18]
Confirmation of pentavalent carbon in protonated methane (CH5+ )
These structures − trigonal bipyramidal (TBP) and square pyramidal (SP) − are only 0.4 kJ/mol and 3.4 kJ/mol higher in energy, respectively. However ...4. Results And Discussion · 4.1. Investigation Of... · 4.2. Insights From...<|control11|><|separator|>
[19]
Stable Carbonium Ions. X. 1 Direct Nuclear Magnetic Resonance ...
The 2-Norbornyl Cation is not a Single Minimum Energy System. Progress in Reaction Kinetics and Mechanism 2004, 29 (4) , 243-288. https://doi.org/10.3184 ...
[20]
Stable carbonium ions. LXXXVI. Carbon-13 nuclear magnetic ...
Carbon-13 nuclear magnetic resonance spectrum of the stable nonclassical norbornyl cation. ... 1-Norbornyl cation may be in equilibrium with 2-norbornyl cation.
[21]
Theoretical approaches to the structure of carbocations
Geometrical preferences of the crotyl anion, radical and cation. Tetrahedron 1977, 33 (19) , 2497-2501. https://doi.org/10.1016/0040-4020(77)80071-9. Joseph ...
[22]
Dative stabilization of carbocations - ScienceDirect.com
The positively charged carbon atom is sp2 hybridized and has one empty p orbital. These are typically good Lewis acids, and react well with nucleophiles.Missing: vacant | Show results with:vacant
[23]
Isomers of the Allyl Carbocation C3H5+ in Solid Salts: Infrared Spectra and Structures
### IR Frequencies for C=C Stretches in Allyl Carbocation (C3H5+)
[24]
Stabilities and Reactivities of Carbocations - ScienceDirect.com
Carbocation stability is measured by equilibrium constants, often using water as solvent, and oxygen substitution affects stability. Solvolysis rates also ...
[25]
Stabilization of carbocations CH 3 + , C 2 H 5 + , iC 3 H 7 + , tert-Bu ...
Feb 13, 2017 · The isolated carbocations are stabilized due to the intramolecular charge distribution under the influence of hyperconjugation.
[26]
Resonance Energies of the Allyl Cation and Allyl Anion: Contribution ...
The resonance energies of the allyl cation and anion were determined to be about 20−22 and 17−18 kcal/mol, respectively.
[27]
The Cycloheptatrienylium (Tropylium) Ion - ACS Publications
Revisiting aromaticity and stability in the diboron actinide compound Pa 2 B 2. Chemical Science 2025, 103 https://doi.org/10.1039/D5SC02743H. Alena D ...
[28]
252. Reaction kinetics and the Walden inversion. Part I. Homogeneous hydrolysis and alcoholysis of β-n-octyl halides
### Extracted Information
[29]
Inadequacies of the SN1 Mechanism | Journal of Chemical Education
A planar free carbocation intermediate is used as a simple model to explain racemization in solvolysis; but the problem is that racemization is never complete.
[30]
1978 557 Hydrolysis of Secondary Alkyl Sulphonates. sN2 ...
Rate constants for hydrolysis of various secondary methanesulphonates (mesylates, ROMs) in water have been determined, and rate constants for 2-adamantyl ...
[31]
Crystal structure of the tert-butyl cation - ACS Publications
tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding.
[32]
BJOC - A review of new developments in the Friedel–Crafts alkylation
This strongly indicates a carbocation as reaction intermediate and rules out an SN1-type reaction mechanism. Additionally, low temperature NMR studies in ...
[33]
The Story of the Wagner-Meerwein Rearrangement - ACS Publications
This paper tells the story of the long, arduous journey toward comprehending the mechanism of molecular rearrangements.
[34]
Optical nature of non‐substituted triphenylmethyl cation: Crystalline ...
Sep 24, 2021 · The excitation spectrum at room temperature contains a peak at 560 nm, whereas at 77 K the maximum occurs at a wavelength shorter than 560 nm.Missing: primary | Show results with:primary
[35]
Collision-Induced Dissociation Mass Spectrometry: A Powerful Tool ...
Jul 1, 2015 · Here, we summarize advances in natural product structure elucidation based upon the application of collision induced dissociation fragmentation mechanisms.
[36]
(PDF) Matrix Isolation and Vibrational Spectroscopy of Carbocations
Aug 10, 2025 · Results of systematic investigations along these lines include studies of the complexes of nitriles and ketones with SbF5, allyl and propargyl ...
[37]
Vibrational spectroscopic and quantum chemical studies of the ...
The bond lengths are computed reasonably well with all theoretical methods. B3LYP calculates the most accurate bond lengths compared to the crystal structure.
[38]
Hyperconjugation in Carbocations, a BLW Study with DFT ...
Here, in the carbocation case, the charge delocalization corresponds to the interaction between the empty π orbital of the carbocation center, and at least one ...
[39]
Silver-Catalyzed Asymmetric Sakurai−Hosomi Allylation of Ketones
The complex of AgF and (R)-DIFLUORPHOS has been shown to be an effective catalyst for the asymmetric Sakurai−Hosomi allylation of simple ketones.
[40]
Living carbocationic polymerization. 48. Poly(isobutylene-b-methyl ...
Kinetic Treatment of Slow Initiation in Living Carbocationic Polymerization and Investigation of Benzyl Halides as Initiators for the Polymerization of ...
[41]
Acidity and the multiphase chemistry of atmospheric aqueous ... - ACP
Sep 10, 2021 · ... protonated under acidic conditions to form a carbocation. The carbocation then is subject to a nucleophilic attack by a hydroxyl-group ...
[42]
Decoding Catalysis by Terpene Synthases - ACS Publications
Sep 15, 2023 · QM/MM MD simulations of BcBOT2 and the carbocationic intermediates (A–F, pink) are shown in the dashed boxes (cation D is a transition state ...
[43]
Machine learning combined with molecular SMILES-based features ...
Machine learning combined with molecular SMILES-based features to predict the heat of formation of carbocations in oil processing.