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Acids and bases
Diagrammatic representation of the dissociation of acetic acid in aqueous solution to acetate and hydronium ions.
Acid types
Base types

In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (H2SO4),[1] which has a Hammett acidity function (H0) of −12. According to the modern definition, a superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid.[2] Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfuric acid (HSO3F), both of which are about a thousand times stronger (i.e. have more negative H0 values) than sulfuric acid. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. A strong superacid of this kind is fluoroantimonic acid. Another group of superacids, the carborane acid group, contains some of the strongest known acids. Finally, when treated with anhydrous acid, zeolites (microporous aluminosilicate minerals) will contain superacidic sites within their pores.[3] These materials are used on massive scale by the petrochemical industry in the upgrading of hydrocarbons to make fuels.[4]

History

[edit]

The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids.[1] This definition was refined by Ronald Gillespie in 1971, as any acid with an H0 value lower than that of 100% sulfuric acid (−11.93).[5] George A. Olah prepared the so-called "magic acid", so named for its ability to attack hydrocarbons, by mixing antimony pentafluoride (SbF5) and fluorosulfonic acid (FSO3H).[6] The name was coined after a candle was placed in a sample of magic acid after a Christmas party. The candle dissolved, showing the ability of the acid to protonate alkanes, which under normal acidic conditions do not protonate to any extent.

At 140 °C (284 °F), FSO3H–SbF5 protonates methane to give the tertiary-butyl carbocation, a reaction that begins with the protonation of methane:[6]

CH4 + H+ → CH+
5
CH+
5 → CH+
3 + H2
CH+
3 + 3 CH4 → (CH3)3C+ + 3H2

Common uses of superacids include providing an environment to create, maintain, and characterize carbocations. Carbocations are intermediates in numerous useful reactions such as those forming plastics and in the production of high-octane gasoline.

Origin of extreme acid strength

[edit]

Traditionally, superacids are made from mixing a Brønsted acid with a Lewis acid. The function of the Lewis acid is to bind to and stabilize the anion that is formed upon dissociation of the Brønsted acid, thereby removing a proton acceptor from the solution and strengthening the proton donating ability of the solution. For example, fluoroantimonic acid, nominally (H
2FSbF
6), can produce solutions with a H0 lower than −28, giving it a protonating ability over a billion times greater than 100% sulfuric acid.[7][8] Fluoroantimonic acid is made by dissolving antimony pentafluoride (SbF5) in anhydrous hydrogen fluoride (HF). In this mixture, HF releases its proton (H+) concomitant with the binding of F− by the antimony pentafluoride. The resulting anion (SbF−
6) delocalizes charge effectively and holds onto its electron pairs tightly, making it an extremely poor nucleophile and base. The mixture owes its extraordinary acidity to the weakness of proton acceptors (and electron pair donors) (Brønsted or Lewis bases) in solution. Because of this, the protons in fluoroantimonic acid and other superacids are popularly described as "naked", being readily donated to substances not normally regarded as proton acceptors, like the C–H bonds of hydrocarbons. However, even for superacidic solutions, protons in the condensed phase are far from being unbound. For instance, in fluoroantimonic acid, they are bound to one or more molecules of hydrogen fluoride. Though hydrogen fluoride is normally regarded as an exceptionally weak proton acceptor (though a somewhat better one than the SbF−
6 anion), dissociation of its protonated form, the fluoronium ion H2F+ to HF and the truly naked H+ is still a highly endothermic process (ΔG° = +113 kcal/mol), and imagining the proton in the condensed phase as being "naked" or "unbound", like charged particles in a plasma, is highly inaccurate and misleading.[9]

More recently, carborane acids have been prepared as single component superacids that owe their strength to the extraordinary stability of the carboranate anion, a family of anions stabilized by three-dimensional aromaticity, as well as by electron-withdrawing group typically attached thereto.

In superacids, the proton is shuttled rapidly from proton acceptor to proton acceptor by tunneling through a hydrogen bond via the Grotthuss mechanism, just as in other hydrogen-bonded networks, like water or ammonia.[10]

Applications

[edit]

In petrochemistry, superacidic media are used as catalysts, especially for alkylations. Typical catalysts are sulfated oxides of titanium and zirconium or specially treated alumina or zeolites. The solid acids are used for alkylating benzene with ethene and propene as well as difficult acylations, e.g. of chlorobenzene.[11] In organic chemistry, superacids are used as a means of protonating alkanes to promote the use of carbocations in situ during reactions. The resulting carbocations are of much use in organic synthesis of numerous organic compounds, the high acidity of the superacids helps to stabilize the highly reactive and unstable carbocations for future reactions.

Examples

[edit]

The following are examples of superacids. Each is listed with its Hammett acidity function,[12] where a smaller value of H0 (in these cases, more negative) indicates a stronger acid.

See also

[edit]

References

[edit]
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Superacid

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A superacid is defined as a Brønsted acid whose strength exceeds that of 100% , typically quantified by the (H0) value being more negative than -12. These acids achieve their exceptional proton-donating power through the formation of highly stable, delocalized conjugate base anions, often involving electronegative elements like and . The concept of superacids traces back to the 1920s when the term was first coined by to describe acids stronger than conventional mineral acids, but systematic development occurred in the mid-20th century through the work of chemists Ronald J. Gillespie and George A. Olah. Gillespie provided a quantitative thermodynamic definition in the 1960s, emphasizing Brønsted superacids with H0 values far below those of , while Olah advanced their application in by demonstrating their ability to stabilize elusive carbocations at low temperatures. Olah's pioneering studies, conducted in the 1950s and 1960s at Dow Chemical and later at , earned him the 1994 for revealing the structures and mechanisms of carbocations using superacids. Prominent examples include fluoroantimonic acid (HSbF6), a mixture of hydrogen fluoride (HF) and antimony pentafluoride (SbF5) with H0 ranging from -21 to -24, capable of protonating even weak bases like alkanes to form carbocations. Other notable superacids are magic acid (a 1:1 adduct of fluorosulfuric acid, HSO3F, and SbF5), triflic acid (CF3SO3H, H0 = -14.6), and fluorosulfuric acid itself (HSO3F), all of which exhibit acidities billions of times greater than sulfuric acid due to their low nucleophilicity and high proton mobility. These media are often handled at cryogenic temperatures to prevent decomposition and are characterized by their corrosiveness and reactivity with glass, requiring specialized fluoropolymer containers. Superacids have revolutionized fields like and by enabling the study and manipulation of reactive intermediates such as carbocations, which were previously only postulated in mechanisms. Key applications include the and functionalization of alkanes, such as converting to higher hydrocarbons via electrophilic activation, and the synthesis of pharmaceuticals like the anticancer vinflunine through superacid-mediated fluorination. Additionally, they facilitate spectroscopic characterization (e.g., NMR and ) of nonclassical carbocations, resolving long-standing debates in reaction mechanisms, and find use in industrial processes like in petroleum refining. Despite their potency, safety concerns arise from their extreme reactivity, necessitating stringent handling protocols in laboratory and industrial settings.

Definition and Fundamentals

Definition

A superacid is defined as an acid medium exhibiting a Hammett acidity function (H0H_0) value less than -12, surpassing the acidity of 100% sulfuric acid, which has an H0H_0 of approximately -12. This scale extends the traditional pH measurement beyond dilute aqueous solutions, allowing quantification of extreme proton-donating ability in highly concentrated or non-aqueous environments. In aqueous media, the limits the observable strength of strong acids, as acts as a base that protonates to form ion (H3O+H_3O^+), rendering all acids stronger than appear equally potent. Superacids, therefore, require non-aqueous solvents—such as or other aprotic media—to fully manifest their enhanced protonating power without this solvent-induced equalization. The term "superacid" was coined by James Bryant Conant in 1927 to denote acids exhibiting extraordinary strength in non-aqueous solvents, beyond conventional mineral acids like . Although initially applied more broadly, the concept gained widespread acceptance through subsequent refinements, particularly in the mid-20th century. Superacids serve as the acidic counterpart to superbases, which are defined analogously as bases stronger than the hydroxide ion (OH−OH^-) in aqueous solution, often measured by their ability to deprotonate weak acids in non-aqueous settings. This duality highlights the extremes of Brønsted-Lowry acid-base theory in solvent-free or low-basicity environments.

Key Properties

Superacids are characterized by conjugate bases with exceptionally low , enabling them to protonate a wide range of weak bases that resist by conventional strong acids. For instance, they can protonate saturated hydrocarbons at C-H or C-C bonds, forming stable carbocations such as the pentacoordinated CH5+ from , as demonstrated in George Olah's pioneering studies using mixtures like HF-SbF5. This capability extends to ketones, where occurs at the carbonyl oxygen, facilitating the of protonated carbonyl at low temperatures. Certain advanced superacids, such as acids like H(CHB11F11), possess even greater strength and can protonate to form HCO2+, whereas traditional mixed superacids like HF-SbF5 do not achieve this due to competing Lewis acid interactions. A defining feature of superacids is the low nucleophilicity of their conjugate bases, such as SbF6- or FSO3H-SbF5 systems, which minimizes unwanted side reactions and allows the isolation of highly reactive electrophiles like carbocations. This property arises from the weakly coordinating nature of these anions, enhancing the effective acidity in solution. Many superacids also display high thermal stability in specific formulations; for example, certain Lewis superacids based on aluminum, such as Al(OC6F5)3 derivatives, remain stable up to 180°C under inert conditions. They are notoriously corrosive, capable of attacking glass, metals, and organic materials, necessitating specialized handling in fluorinated polymers or lined reactors. Additionally, their volatility varies, with exhibiting a of 14 mm Hg at 18°C, contributing to its hazardous fume profile. Common superacids like (HF-SbF5) are viscous, syrupy liquids that appear colorless to pale yellow, with a of approximately 2.885 g/mL at 25°C. These physical traits reflect their ionic composition and make them suitable for low-temperature studies, often in solvents like SO2ClF. Superacids are generally defined by values H0 below -12, far exceeding that of (H0 ≈ -12). Thermodynamically, they achieve extreme acidity through very low effective pKa values, estimated at -31 for equimolar HF-SbF5, reflecting the high proton-donating power. In non-polar solvents, ion pairing between protons and conjugate bases predominates, further elevating the proton's and acidity compared to polar media.

Historical Development

Early Concepts

The concept of superacids emerged in the early as chemists sought to understand and quantify acid strengths beyond the limitations of traditional mineral acids like . In 1927, Norris F. Hall and introduced the term "superacid" to describe solutions capable of protonating weak bases, such as carbonyl compounds, that ordinary strong acids could not effectively handle. Their pioneering work involved studying dissolved in glacial acetic acid as a , where they employed the chloranil to measure hydrogen-ion activity and demonstrated that exhibited significantly greater acidity than under these non-aqueous conditions. This approach highlighted the potential for enhanced acidity in organic solvents, laying foundational groundwork for recognizing superacidic behavior. Building on such efforts, the development of suitable acidity scales became crucial for evaluating stronger acid systems. In 1932, Louis P. Hammett and Alden J. Deyrup proposed an acidity function, now known as the (H₀), to extend measurements beyond the aqueous scale, which was inadequate for concentrated strong acids and solutions due to leveling effects and low . Their method used nitroaromatic indicators to assess equilibria in sulfuric-perchloric acid mixtures, enabling quantification of acidities in highly concentrated media, including fuming sulfuric acid (), where traditional metrics failed. This innovation addressed key conceptual challenges in pre-1960s acid chemistry, such as the inability of aqueous scales to differentiate among very strong acids and the difficulties in performing accurate non-aqueous titrations without suitable reference electrodes or indicators. The evolving understanding of acidity during this period was also profoundly influenced by Gilbert N. Lewis's 1923 acid-base theory, which redefined acids as electron-pair acceptors rather than solely proton donors, thereby encompassing a broader range of superacidic systems involving Lewis acidic components like metal halides. This theoretical shift facilitated early interpretations of superacid mechanisms in non-protic environments, emphasizing coordination chemistry over simple proton transfer and encouraging experiments with mixed acid systems.

Major Advancements

The major advancements in superacid chemistry began in the late 1950s and through the independent and collaborative efforts of chemists Ronald J. Gillespie and George A. Olah. Gillespie, working at , developed early superacid systems such as mixtures of (HSO₃F) and (SbF₅) in the , conducting conductivity measurements to characterize their extreme acidity. In 1971, Gillespie provided a widely accepted quantitative definition of superacids as Brønsted acids with H₀ values lower than -12, the acidity of 100% . Concurrently, Olah demonstrated the isolation of stable carbocations using the superacid mixture fluorosulfuric acid (HSO₃F) and antimony pentafluoride (SbF₅), dubbed "magic acid" by a visiting researcher in 1966. His groundbreaking work began at in the late 1950s and early 1960s, including a pivotal 1963 publication detailing the of hydrocarbons, including alkanes, in superacid media, marking the first experimental of such transformations and laying the foundation for studying nonclassical s. This system, approximately 10¹⁴ times stronger than , enabled the direct observation and characterization of elusive carbocation intermediates via NMR , resolving long-standing debates on mechanisms. Olah continued this research after joining in 1965. In the 1970s and 1980s, Olah and collaborators refined superacid systems, notably developing (HF-SbF₅), which achieved Hammett acidity values (H₀) as low as -31, surpassing in strength and enabling protonation of even weaker bases like and . This period saw expanded applications in generating persistent alkyl cations from alkanes via hydride abstraction, with Olah's comprehensive studies culminating in his 1994 for contributions to carbocation chemistry. Concurrently, theoretical insights into effects in highly acidic media advanced understanding of ion pairing and stability, though practical superacid development remained dominated by Olah's and Gillespie's experimental innovations. From the 1990s onward, computational modeling transformed superacid research by simulating protonation equilibria and anion structures inaccessible experimentally, with (DFT) calculations revealing the role of weakly coordinating anions in enhancing acidity. High-level methods in the and predicted novel superacid motifs, such as those involving superhalogens, achieving strengths rivaling HSbF₆. Post-2010 developments emphasized environmentally benign solid superacids, exemplified by Nafion-based composites incorporating sulfated metal oxides like TiO₂-SO₄, which exhibit H₀ values below -12 while avoiding volatile liquid handling; these materials improved proton conductivity in membranes by up to 50% at low humidity, promoting greener catalytic processes.

Mechanisms of Superacidity

Acidity Measurement

The Hammett acidity function (H₀) serves as the primary measure for quantifying the strength of superacids, extending beyond the limitations of the pH scale in dilute aqueous solutions to highly concentrated or non-aqueous media. Developed by Louis Plack Hammett and Alden J. Deyrup in 1932, it was originally applied to mixtures of strong acids such as sulfuric and perchloric acid, and later extended to superacid systems through the use of increasingly weak base indicators. The function is defined by the equation H0=pK(BH+)+log⁡([B][BH+]),H_0 = \mathrm{p}K(\mathrm{BH}^+) + \log \left( \frac{[\mathrm{B}]}{[\mathrm{BH}^+]} \right), where B represents a neutral weak base indicator, BH⁺ is its protonated conjugate acid, and pK(BH⁺) is the negative logarithm of the acid dissociation constant of BH⁺ determined in a standard reference medium, typically aqueous sulfuric acid. This formulation parallels the Henderson-Hasselbalch equation but accounts for non-ideal behavior in strong acid environments by focusing on the protonation equilibrium of the indicator rather than hydrogen ion activity. Measurement of H₀ involves introducing the indicator into the superacid medium and determining the [B]/[BH⁺] through spectroscopic techniques, as the protonated and unprotonated forms exhibit distinct absorption spectra. Nitroaniline derivatives are commonly employed as indicators due to their tunable basicity via nitro group substitution, which allows overlap across acidity ranges; for instance, 2,4-dinitroaniline (pK(BH⁺) ≈ -4.5) is used for moderately strong acids, while more electron-withdrawing substitutions enable extension to superacid regimes. UV-visible is the standard method, relying on the color change or differential of the indicator species to quantify the extent. provides an alternative, particularly for opaque or viscous superacid media like ionic liquids, by detecting vibrational shifts in the indicator upon without requiring transparency. Although effective, the H₀ scale is inherently equilibrium-based, reflecting thermodynamic proton donation ability under static conditions and thus unsuitable for kinetic or non-equilibrium assessments. Its applicability is restricted to media where the indicator experiences comparable effects for both forms and does not undergo side reactions or leveling; heterogeneous or highly viscous systems can further complicate accurate ratio determination. For strong basic media, the analogous H₋ function measures basicity by inverting the approach to track equilibria of conjugate acids.

Origins of Extreme Strength

The extreme strength of superacids arises fundamentally from the stability and weakness of their conjugate bases, which minimize the tendency to recombine with protons and thereby sustain high acidity levels. A prime example is the hexafluoroantimonate anion (SbF₆⁻), whose low basicity stems from the high of the surrounding atoms; this electronegativity draws electron density away from the central , effectively delocalizing the negative charge and rendering the anion highly resistant to . Such weak conjugate bases enable superacids to achieve (H₀) values far below -12, surpassing conventional strong acids like . Synergistic interactions in binary systems further amplify this acidity by pairing a Brønsted acid with a strong Lewis acid, generating pairs that enhance proton activity. In the combination of (HF) and (SbF₅), for instance, the reaction produces the cation (H₂F⁺) as the proton donor and SbF₆⁻ as the , resulting in a medium with H₀ ≈ -24 due to the mutual reinforcement of their acidities. This mechanism allows superacids to protonate substrates that are typically inert, as illustrated by the general equation for : RH + H⁺ → RH₂⁺, where R represents an and the process yields carbocations stable enough for spectroscopic observation. Several interconnected factors underpin these capabilities: the protons in superacid media experience minimal solvation owing to the low nucleophilicity of the fluorinated environment, preserving their electrophilic potency; the anions exhibit extensive charge delocalization, which further diminishes their basicity and stabilizes the dissociated state; and the systems are engineered to avoid autoionization, with autoprotolysis constants as low as 10⁻¹¹ for HF, preventing self-neutralization and maintaining proton availability. These elements collectively enable superacids to operate in non-aqueous, low-dielectric solvents where ion pairing is tight yet dissociation is favored. Advances in have elucidated these principles through quantum mechanical modeling, particularly (DFT) calculations that quantify proton affinities and energies in superacid systems. Post-2010 studies, including DFT calculations using B3LYP with def2-TZVPP, have quantified proton affinities of substrates in superacidic environments like HF/AsF₅, confirming the energetic favorability of .

Examples of Superacids

Prominent Inorganic Superacids

Fluoroantimonic acid, denoted as HSbF₆, represents the strongest known superacid, achieving a Hammett acidity function (H₀) values ranging from -21 to -28 in its mixtures of hydrogen fluoride (HF) and antimony pentafluoride (SbF₅). This compound is prepared by combining anhydrous HF with SbF₅, which promotes ionization through the equilibrium 2 HF + 2 SbF₅ ⇌ H₂F⁺ + Sb₂F₁₁⁻, forming a highly viscous, colorless to yellow liquid capable of protonating even weak bases like alkanes. Its extreme acidity stems from the low nucleophilicity of the Sb₂F₁₁⁻ counterion, enabling the stabilization of elusive carbocations. However, fluoroantimonic acid exhibits limited thermal stability, remaining viable primarily at temperatures between -150°C and 0°C, beyond which it decomposes, releasing toxic HF gas. Antimony pentafluoride (SbF₅) serves as a pivotal Lewis superacid component in many inorganic systems, recognized as one of the strongest Lewis acids in the condensed phase due to its ability to accept ions and form stable polyfluorostibate anions like SbF₆⁻ or Sb₂F₁₁⁻. It is typically prepared by fluorinating metal or SbF₃ with HF, yielding a hygroscopic, viscous oil that enhances the protonating power of Brønsted acids when mixed, as seen in formulations. SbF₅'s polymeric structure in pure form contributes to its reactivity, but it requires anhydrous conditions to prevent . In superacid media, it facilitates generation at low temperatures, such as -78°C, while maintaining stability when moisture is excluded. Fluorosulfuric acid (HSO₃F), a prominent pure inorganic Brønsted superacid, possesses an H₀ value of approximately -15.1, rendering it about 1000 times stronger than concentrated . It is synthesized by reacting (SO₃) with HF, followed by distillation to obtain a colorless, fuming with a wide range from -89°C to 163°C. This acid's superacidity arises from the electronegative substituent, which weakens the O-H bond and stabilizes the conjugate base HSO₃F₂⁻. While more stable than , it still demands dry handling to avoid . All these inorganic superacids exhibit extreme corrosiveness, rapidly reacting with water to liberate HF and other hazardous , necessitating specialized fluoropolymer-lined containment and rigorous safety protocols during preparation and use.

Organic and Mixed Superacids

Organic and mixed superacids combine organic Brønsted acids with Lewis acids or incorporate organometallic anions to achieve exceptional acidity while offering tunable properties for specific applications, such as stabilizing reactive carbocations. A classic example is magic acid, formed by mixing fluorosulfuric acid (HSO₃F) and antimony pentafluoride (SbF₅) in a 1:1 molar ratio, which reaches a Hammett acidity function (H₀) of approximately -23. This mixture is prepared by dissolving SbF₅ in HSO₃F under an inert atmosphere to exclude moisture and prevent decomposition. Magic acid has been instrumental in carbocation studies, enabling the observation and characterization of elusive species like the tert-butyl cation through NMR spectroscopy. Another important system involves trifluoromethanesulfonic acid (CF₃SO₃H, known as ) combined with (BF₃), yielding a superacid with H₀ ≈ -16. itself is a strong organic superacid (H₀ ≈ -14.6), but the addition of BF₃ enhances its protonating power by forming a conjugate complex, facilitating reactions like electrophilic aromatic substitutions. Carborane-based superacids, such as those derived from the HCB₁₁H₁₂⁻ anion (e.g., H(CHB₁₁Cl₁₁)), represent a significant advancement, achieving H₀ values below -18 and emerging prominently since the . These acids are notable for their exceptional stability and low nucleophilicity of the conjugate base, allowing them to function in weakly basic organic solvents without oxidative side reactions. Post-2000 developments have emphasized their "strong yet gentle" profile, with applications in protonating inert molecules like alkanes and CO₂, due to the delocalized charge over the icosahedral carborane cage. Recent research as of 2025 has further advanced carborane-based systems for catalysis and materials science. Additionally, new halogen-substituted silicon cations have been developed as Lewis superacids for green chemistry applications. General preparation of these organic and mixed superacids involves equimolar mixing of components under an inert atmosphere, such as or , to maintain conditions and ensure homogeneity. To address safety concerns associated with their corrosiveness and volatility, recent innovations include solid-supported variants, like poly(4-vinylpyridine)- (PVP-TfOH), which immobilize the acid on a matrix for easier handling, reduced toxicity, and recyclability in catalytic processes.

Applications

In Chemical Synthesis

Superacids play a pivotal role in by enabling the of otherwise unreactive alkanes, facilitating shifts and rearrangements that drive isomerizations and other transformations. In these media, alkanes form transient alkanium ions (e.g., R-H^+), which can eliminate H_2 to generate s or rearrange via 1,2-shifts to more stable isomers. A representative example is the of ((CH_3)_4C) in (FSO_3H-SbF_5), where initial formation of the neopentyl alkanium ion leads to a 1,2-methyl shift, yielding the tert-amyl cation ((CH_3)_2C^+CH_2CH_3) through subsequent abstraction. This process, observed at low temperatures around -30°C, demonstrates how superacids promote skeletal rearrangements inaccessible under conventional acidic conditions. Such generation in superacids has revolutionized Friedel-Crafts alkylations by eliminating the need for traditional Lewis acids like AlCl_3, as the superacid medium directly ionizes alkyl halides or alkanes to produce electrophilic s. For instance, Olah and coworkers utilized SbF_5 or to effect alkylations of with alkanes or alkyl fluorides, achieving clean incorporation of the without polyalkylation or rearrangement side products common in H_2SO_4/AlCl_3 systems. These reactions proceed under milder conditions, often at temperatures below 0°C, yielding up to 90% isolated products in cases like tert-butylation, compared to the 50-70% yields and higher temperatures (typically 40-60°C) required with , thereby minimizing and improving selectivity. Beyond alkylations, superacids enable the synthesis and isolation of stable carbocations as reaction intermediates, providing direct insight into mechanisms and allowing their use in subsequent synthetic steps. Olah's seminal preparation of the tert-butyl cation ( (CH_3)_3C^+ ) involved ionizing tert-butyl fluoride in SbF_5 at -60°C, yielding a persistent species characterized by ^1H NMR (δ 4.2 ppm for the methyl protons), which could then be trapped with nucleophiles like alkenes for addition reactions. This approach not only confirmed carbocation structures but also facilitated higher-yield syntheses of complex hydrocarbons, such as adamantane derivatives, under conditions where traditional acids fail due to insufficient protonating power. Overall, these capabilities highlight superacids' superiority over sulfuric acid in providing precise control, enhanced stability of intermediates, and reduced byproduct formation in carbocation-mediated syntheses.

In Spectroscopy and Analysis

Superacids facilitate the stabilization of highly reactive carbocations at low temperatures, enabling their characterization by (NMR) spectroscopy, which provides detailed insights into their structures and dynamics. In particular, ¹³C NMR spectroscopy has been instrumental in observing deshielded carbon atoms in carbocations, indicative of their electron-deficient nature. For instance, the tert-butyl cation exhibits a ¹³C NMR shift for the central carbon at 335 ppm in SbF₅ diluted with SO₂ClF at temperatures down to -78°C, confirming its planar sp²-hybridized structure. This technique has allowed the study of equilibrating systems, where methyl substituent effects remain constant across various carbocations, aiding in the differentiation of classical and bridged species. Infrared (IR) and Raman spectroscopy complement NMR by probing vibrational modes of protonated species in superacid media, revealing bond length changes and symmetry alterations upon protonation. Protonated carbonyl compounds, such as acetone in HF–SbF₅, display IR absorptions shifted due to O-protonation, with the C=O stretch appearing around 1678 cm⁻¹, reflecting weakened carbonyl bonds and oxonium ion formation. For protonated alkenes, which form alkenyl carbocations, IR spectra show characteristic vibrations like the asymmetric C=C=C stretch at 1578 cm⁻¹ in the allyl cation, while Raman analysis confirms molecular symmetry, as seen in the triptyl cation's bands supporting its trivalent carbenium structure. These methods, often conducted at low temperatures to prevent decomposition, provide evidence for electron-deficient bonding without interference from nucleophilic counterions. Mass spectrometry (MS) in superacid media has been employed to study the formation and fragmentation of ions, offering complementary data on their stability and reactivity under controlled conditions. Techniques such as electrospray ionization allow the direct ionization and analysis of carbocations generated in superacids, revealing molecular weight and isotopic patterns for elusive species like dications. For example, studies of alkyl carbocations in SbF₅-based systems confirm their persistence and aid in identifying rearrangement pathways, though less frequently used than NMR due to the need for volatile samples. The integration of these spectroscopic methods in superacid environments marked a pivotal historical advancement, enabling the direct observation of non-classical carbocations and resolving long-standing debates on their bridged versus classical structures. Pioneering low-temperature NMR studies of the down to -159°C in SbF₅/SO₂ClF confirmed its symmetrically bridged form, supported by IR evidence of equivalent bridgehead carbons, thus validating non-classical bonding theories. This breakthrough, central to George A. Olah's Nobel-recognized work, transformed mechanistic by providing empirical proof for reactive intermediates previously inferred only indirectly.

Industrial and Emerging Uses

Superacids play a critical role in petroleum refining, particularly in the of with olefins to produce high-octane components. Anhydrous (HF), a strong Brønsted acid with a (H₀) of approximately -11, is widely employed in this process due to its ability to generate stable carbocations that facilitate the reaction under mild conditions. The Phillips Petroleum Company's HF-based alkylation process, commercialized in the and still operational in numerous refineries, exemplifies this application, where HF acts as both catalyst and solvent, enabling efficient conversion with minimal side reactions compared to alternatives. This process accounts for a significant portion of U.S. alkylate production, enhancing quality while operating at temperatures around 20–40°C to control exothermic reactions. In polymerization catalysis, solid superacids such as sulfated zirconia (SZrO₂) have been developed as heterogeneous alternatives to traditional homogeneous systems for the production of polyolefins. These materials exhibit superacidity (H₀ ≤ -12) due to sulfate groups creating strong Brønsted and Lewis acid sites on the zirconia surface, enabling of monomers like isobutene to form polyisobutylene, a key precursor for and adhesives. Phillips Petroleum pioneered sulfated zirconia catalysts in the 1990s for olefin oligomerization and , offering advantages in catalyst recovery and reduced corrosion over liquid superacids. Recent advancements have extended their use to metallocene-supported systems, achieving high molecular weight polyolefins with controlled for improved material properties in packaging and automotive applications. Emerging applications of superacids emphasize sustainability, particularly in for conversion and advanced technologies. Protic ionic liquids incorporating superacidic anions, such as those based on trifluoromethanesulfonic , have shown promise post-2015 for the pretreatment and of lignocellulosic into biofuels and platform chemicals like 5-hydroxymethylfurfural (HMF). These recyclable media dissolve efficiently at mild temperatures (80–120°C), outperforming conventional s by minimizing use and degradation products, enabling high digestibility, often exceeding 90%. In , superacid-functionalized aromatic polymers serve as proton exchange membranes (PEMs) in cells, where perfluorosulfonic additives enhance conductivity under low (up to 0.1 S/cm at 80°C) by stabilizing ions in sulfonated nanostructures. These membranes, often layered with composites, support durable operation in cells, addressing limitations in applications. Handling superacids requires stringent protocols due to their extreme corrosivity and reactivity with , moisture, and organics. Operations typically occur in inert-atmosphere gloveboxes to prevent and explosive (HF) release, with all manipulations conducted under dry or . Storage and containment utilize vessels, such as polytetrafluoroethylene (PTFE) or perfluoroalkoxy (PFA) containers, which resist chemical attack and maintain integrity up to 260°C, unlike or metals that degrade rapidly. Personal protective equipment includes chemical-resistant suits, face shields, and gels for HF exposure mitigation. Environmental concerns with superacid use center on fluoride-containing waste streams, which pose risks of and ecosystem toxicity. In refinery , HF processes generate fluoride-laden effluents that can exceed industrial discharge limits (e.g., 10 mg/L in some standards), leading to in aquatic organisms and in . strategies involve neutralization with lime to form insoluble , though this produces voluminous sludge requiring secure landfilling to prevent leaching. Recent research highlights sustainable alternatives, such as immobilized superacids on silica supports for production, reducing fluoride emissions by over 80% through catalyst recycling and minimizing liquid waste in upgrading processes.

References

  1. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Inorganic_Chemistry_%28LibreTexts%29/06%253A_Acid-Base_and_Donor-Acceptor_Chemistry/6.03%253A_Brnsted-Lowry_Concept/6.3.04%253A_Brnsted-Lowry_Superacids_and_the_Hammett_Acidity_Function
  2. https://www.sciencedirect.com/topics/chemistry/superacid
  3. https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201703322
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