Superacid
Inchemistry,asuperacid(according to the original definition) is anacidwith an acidity greater than that of 100% puresulfuric acid(H2SO4),[1]which has aHammett acidity function(H0) of −12. According to the modern definition, a superacid is a medium in which thechemical potentialof theprotonis higher than in pure sulfuric acid.[2]Commercially available superacids includetrifluoromethanesulfonic acid(CF3SO3H), also known as triflic acid, andfluorosulfuric acid(HSO3F), both of which are about a thousand times stronger (i.e. have more negativeH0values) than sulfuric acid. Most strong superacids are prepared by the combination of a strongLewis acidand a strongBrønsted acid.A strong superacid of this kind isfluoroantimonic acid.Another group of superacids, thecarborane acidgroup, contains some of the strongest known acids. Finally, when treated withanhydrousacid,zeolites(microporous aluminosilicate minerals) will contain superacidic sites within their pores. These materials are used on massive scale by the petrochemical industry in the upgrading of hydrocarbons to make fuels.[citation needed]
History[edit]
The termsuperacidwas originally coined byJames Bryant Conantin 1927 to describe acids that were stronger than conventionalmineral acids.[1]This definition was refined byRonald Gillespiein 1971, as any acid with anH0value lower than that of 100% sulfuric acid (−11.93).[3]George A. Olahprepared the so-called "magic acid",so named for its ability to attackhydrocarbons,by mi xingantimony pentafluoride(SbF5) andfluorosulfonic acid(FSO3H).[4]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 toprotonatealkanes,which under normal acidic conditions do not protonate to any extent.
At 140 °C (284 °F), FSO3H–SbF5protonatesmethaneto give the tertiary-butylcarbocation,a reaction that begins with the protonation of methane:[4]
- 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 characterizecarbocations.Carbocations are intermediates in numerous useful reactions such as those forming plastics and in the production ofhigh-octanegasoline.
Origin of extreme acid strength[edit]
Traditionally, superacids are made from mi xing 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 aH0lower than –28, giving it a protonating ability over a billion times greater than 100% sulfuric acid.[5][6]Fluoroantimonic acid is made by dissolvingantimony pentafluoride(SbF5) in anhydroushydrogen fluoride(HF). In this mixture, HF releases its proton (H+) concomitant with the binding of F−by the antimony pentafluoride. The resultinganion(SbF−
6) delocalizes charge effectively and holds onto its electron pairs tightly, making it an extremely poornucleophileandbase.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, theprotonsin 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 SbF6–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.[7]
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 theGrotthuss mechanism,just as in other hydrogen-bonded networks, like water or ammonia.[8]
Applications[edit]
Inpetrochemistry,superacidic media are used as catalysts, especially foralkylations.Typical catalysts are sulfated oxides oftitaniumandzirconiumor specially treated alumina orzeolites.Thesolid acidsare used for alkylating benzene withetheneandpropeneas well as difficultacylations,e.g. ofchlorobenzene.[9]InOrganic Chemistry,superacids are used as a means of protonating alkanes to promote the use of carbocationsin situduring 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 itsHammett acidity function,[10]where a smaller value ofH0(in these cases, more negative) indicates a stronger acid.
- Helium hydride ion(HeH+,H0= -63)
- Fluoroantimonic acid(HF:SbF5,H0= -28)
- Magic acid(HSO3F:SbF5,H0= −23)
- Triflidic acid(CH(CF3SO2)3,H0= −18.6)
- Carborane acids(H(HCB11X11),H0≤ −18, indirectly determined and depends on substituents)
- Fluoroboric acid(HF:BF3,H0= −16.6)
- Bistriflimidic acid(NH(CF3SO2)2,H0= -15.8.[11]Estimated value calculated from pKa values in 1,2-dichloroethane in comparison to triflic acid)
- Fluorosulfuric acid(FSO3H,H0= −15.1)
- Hydrogen fluoride(HF,H0= −15.1)[12]
- Triflic acid(HOSO2CF3,H0= −14.9)
- Oleum(SO3:H2SO4,H0= −14.5)[13]
- Perchloric acid(HClO4,H0= −13)
- Sulfuric acid(H2SO4,H0= −11.9)
See also[edit]
References[edit]
- ^abHall NF, Conant JB (1927). "A Study of Superacid Solutions".Journal of the American Chemical Society.49(12): 3062–70.doi:10.1021/ja01411a010.
- ^Himmel D, Goll SK, Leito I, Krossing I (2010). "A Unified pH Scale for All Phases".Angew. Chem. Int. Ed.49(38): 6885–6888.doi:10.1002/anie.201000252.PMID20715223.
- ^Gillespie, R. J.; Peel, T. E.; Robinson, E. A. (1971-10-01). "Hammett acidity function for some super acid systems. I. Systems H2SO4-SO3, H2SO4-HSO3F, H2SO4-HSO3Cl, and H2SO4-HB(HSO4)4".Journal of the American Chemical Society.93(20): 5083–5087.doi:10.1021/ja00749a021.ISSN0002-7863.
The work of Jorgenson and Hartter formed the basis for the present work, the object of which was to extend the range of acidity function measurements into the super acid region, i.e., into the region of acidities greater than that of 100% H2SO4.
- ^abGeorge A. Olah,Schlosberg RH (1968). "Chemistry in Super Acids. I. Hydrogen Exchange and Polycondensation of Methane and Alkanes in FSO3H–SbF5( "Magic Acid" ) Solution. Protonation of Alkanes and the Intermediacy of CH5+and Related Hydrocarbon Ions. The High Chemical Reactivity of "Paraffins" in Ionic Solution Reactions ".Journal of the American Chemical Society.90(10): 2726–7.doi:10.1021/ja01012a066.
- ^Olah, George A. (2005). "Crossing Conventional Boundaries in Half a Century of Research".Journal of Organic Chemistry.70(7): 2413–2429.doi:10.1021/jo040285o.PMID15787527.
- ^Herlem, Michel (1977)."Are reactions in superacid media due to protons or to powerful oxidising species such as SO3or SbF5?".Pure and Applied Chemistry.49:107–113.doi:10.1351/pac197749010107.S2CID98483167.
- ^Ruff, F. (Ferenc) (1994).Organic reactions: equilibria, kinetics, and mechanism.Csizmadia, I. G. Amsterdam: Elsevier.ISBN0444881743.OCLC29913262.
- ^Schneider, Michael (2000)."Getting the Jump on Superacids".Pittsburgh Supercomputing Center.Archived fromthe originalon 23 August 2018.Retrieved20 November2017.
- ^Michael Röper, Eugen Gehrer, Thomas Narbeshuber, Wolfgang Siegel "Acylation and Alkylation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000.doi:10.1002/14356007.a01_185
- ^Gillespie, R. J.; Peel, T. E. (1973-08-01). "Hammett acidity function for some superacid systems. II. Systems sulfuric acid-[fsa], potassium fluorosulfate-[fsa], [fsa]-sulfur trioxide, [fsa]-arsenic pentafluoride, [sfa]-antimony pentafluoride and [fsa]-antimony pentafluoride-sulfur trioxide".Journal of the American Chemical Society.95(16): 5173–5178.doi:10.1021/ja00797a013.ISSN0002-7863.
- ^Fuller, Maurice (2022).Coordination Chemistry and its Application(PDF).Bibliotex. pp. 45, 46.
- ^Liang, Joan-Nan Jack (1976).The Hammett Acidity Function for Hydrofluoric Acid and some related Superacid Systems (Ph.D. Thesis, advisor: R. J. Gillespie)(PDF).Hamilton, Ontario: McMaster University. p. 109.
- ^Olah, George (2009).SUPERACID CHEMISTRY(PDF).John Wiley & Sons, Inc. p. 47.