SuperacidsAcids up to billions of times stronger than sulfuric
acid have opened up fascinating new areas ofchemistry.
George A. Olah, G. K. Surya Prakash, Jean Sommer
Until recently, chemists generally con-sidered mineral acids, such as sulfu-ric, nitric, perchloric, and hydrofluoricacids, to be the strongest acid systemsattainable. That view has changed con-siderably as extremely strong acid sys-tems-hundreds of millions, even bil-
very closely to the degree of transforma-tion of a base with its conjugate acid,keeping in mind that this will depend onthe base itself and on medium effects.The advantage of this method was shownin the 1930's by Hammett and Deyrup(4), who investigated the proton donor
Summary. Superacids, although first referred to as early as 1927, were only exten-sively studied in the last decade. Acidities up to 1012 times that of sulfuric acid havenow been obtained. The extremely low nucleophilicity of the counterions in super-acidic systems is especially useful for the preparation of stable, electron-deficientcations, particularly carbocations. Many of these cations, which were formerly detect-able only in the gas phase, can now be studied in solution. Novel organic synthesesthat are not possible in ordinary acidic media can also be achieved in superacids,including syntheses of economically important hydrocarbons. The unique ability ofsuperacids to bnng about hydrocarbon transformations, even to activate methane toundergo electrophilic oligocondensation, can open up new fields in chemistry.
lions of times stronger than sulfuricacid-have been discovered. The objectof this article is to give a general over-view of the chemistry of these super-acids. Reviews have appeared describingindividual superacid systems and theirchemistry (1-3).A number of methods are available for
estimating acidity in solution. The bestknown one is the direct measurement ofhydrogen ion activity (aH+) used in defin-ing the pH.
pH = - log aH+ (I)
This can be achieved by measuring thepotential of a hydrogen electrode in equi-librium with a dilute acid solution. Inhighly concentrated acid solutions, how-ever, the pH concept is no longer appli-cable, and the acidity must be relatedSCIENCE, VOL. 206, 5 OCTOBER 1979
ability of the H20-H2S04 system overthe whole concentration range by mea-suring the extent to which a series of ni-troanilines were protonated. This wasthe first application of the very usefulHammett acidity function
Ho =pKBH+ - log B (2)
Here the pKBH+ is the dissociation con-stant of the conjugate acid (BN+), andBH+/B is the ionization ratio, which isgenerally measured by spectroscopicmeans [ultraviolet, nuclear magnetic res-onance (NMR), and more recently dy-namic NMR]. The Hammett acidityfunction is a logarithmic scale on which100 percent sulfuric acid has an Ho of-11.9 and anhydrous HF has an H. of-11.0.
The acidity of sulfuric acid solutioncan be increased by the addition of sol-utes that behave as acids in the system;that is
HA + H2S04 i± H3SO4+ + A-
These solutes increase the concentrationof the highly acidic H3SO4+ cation just asthe addition of an acid to water increasesthe concentration of the oxonium ion,H30+. Fuming sulfuric acid (oleum) con-tains a series of such acids, the poly-sulfuric acids, the simplest of which isdisulfuric acid, H2S207 (5), which ionizesas a moderately strong acid in sulfuricacid
H2S207 + H2S04 ;± H3SO4+ + HS207-
Higher polysulfuric acids, such asH2S3010 and H2S4013, also behave asacids and appear somewhat strongerthan H2S207. The value of the acidityfunction Ho increases continuously up toand beyond the composition of H2S207as S03 is added to H2S04.
In 1927, Hull and Conant (6) observedthat weak organic bases such as carbonylcompounds (ketones and aldehydes) arecapable of forming salts with perchloricacid in nonaqueous solvents. Because ofthe ability of perchloric acid in non-aqueous systems to protonate such weakbases, they called this acid system a su-peracid. According to Gillespie and co-workers (5, 7-9), who did much pioneer-ing work on inorganic aspects of acidsystems, all protic acids stronger than100 percent sulfuric acid should be clas-sified as superacids. Thus perchloric acid(HC104), fluorosulfuric acid (HS03F),and trifluoromethanesulfonic acid(CF3SO3H) are considered to be super-acids.
Highly concentrated perchloric acid(Ho -13.0) is also an extremely strongoxidizing agent, and its contact with or-ganic compounds must be considered ex-tremely dangerous. This is because ofthe ease of formation of covalent per-
George A. Olah is a professor of chemistry andscientific codirector of the Hydrocarbon ResearchInstitute, University of Southern California, LosAngeles 90007. Dr. G. K. Surya Prakash is a juniorresearch fellow at the Institute. Jean Sommer is aprofessor at the Louis Pasteur University, Stras-bourg, France.
chlorates of the type R3COC103 (whereR is an alkyl or aryl group), which arethermally unstable and can give rise toviolent explosions. We will not discussfurther the chemistry of perchloric acidin this article; however, it has been re-viewed (10). Fluorosulfuric acid and tri-fluoromethanesulfonic acid have acid-ities comparable to or higher than that ofperchloric acid without the associateddangers of the latter.
Fluorosulfuric acid. Fluorosulfuricacid (l) is one of the strongest protonicacids known (Ho= -15.6). Of all thesimple Bronsted acids studied so far, on-ly disulfuric acid (H2S207) appears to bemore acidic than fluorosulfuric acid. Itsother advantages include a conveniently
HO\ /F
wide liquid range (-89° to + 163°C) and aviscosity (1.56 centipoise at 25°C) lowerthan that of sulfuric acid systems. It isreadily available, relatively inexpensive,easily purified by distillation, and if pure(free from HF) it does not etch glass.The low freezing point (-89°C) has
proved to be advantageous in the studyof protonation (3), particularly of weakbases such as carbonyl compounds andaromatic hydrocarbons. At ambient tem-peratures proton exchange reactions inacids are generally much too fast to al-low the detection of protonated weakbases. Near the freezing point of fluoro-sulfuric acid, however, many proton ex-change reactions are slowed down suffi-ciently to be studied by NMR spectros-copy. Another advantage of fluorosulfur-ic acid is that it is not as good asulfonating agent toward organic com-pounds as H2S207, particularly at lowtemperatures. Most of the applicationsof fluorosulfuric acid are discussed in thesection on Bronsted-Lewis superacidmixtures.
Trifluoromethanesulfonic acid (triflicacid). This acid (2) was first prepared(1/) by the oxidation of bis(trifluoro-methylthio)mercury with aqueous hy-drogen peroxide. Commercially, it is pre-
HO\ yCF3
2
pared by electrochemical fluorination ofmethanesulfonic acid (12), and it can alsobe prepared from trichloromethanesul-fonyl chloride (Cl3SO2Cl) with HF fol-lowed by hydrolysis. It is a colorless liquid
14
that boils at 162°C, fumes in moist air, andis converted to a stable monohydrate,which is a solid at room temperature. Triflicacid and its conjugate bases have extremethermal stability and are resistant to bothoxidative and reductive cleavage. Strongnucleophiles are not known to displacefluoride from triflic acid or its deriva-tives. However, it is capable of formingcovalent derivatives. It has an Ho valueof -14.6 (2). Other properties of triflicacid, such as a relatively low freezingpoint (< -35°C) and viscosity (2.87 cen-tipoise) compared with sulfuric acid,have made it a highly useful solvent forthe generation of cation radicals (13) andcarbocations. It is also extensively usedas an acidic catalyst for chemical syn-theses (2).Higher homologous perfluoroalkane-
sulfonic acids. The higher homologs oftriflic acid, C6F2.+1S03H, are also super-acids, but their acidity decreases with in-creasing molecular weight (14). The acid-ities on the Ho scale of CF3SO3H,C2F5SO3H, C4H9FSO3H, and C5F,1SO3Hare -14.6, -14.5, -12.7, and -11.7, re-spectively. The C1 to C4 acids are liquidsat room temperature; the C6 to C8 acidsare solids with relatively low meltingpoints. Addition of metal fluorides ofhigher valence, such as antimony, tan-talum, or niobium pentafluoride, greatlyenhances the acidity of all these systems.Addition of 3 to 5 percent (by weight)antimony pentafluoride, for example,causes an increase of at least 105 in acid-ity.
Lewis Superacids
The study of Friedel-Crafts reactionsled to the realization (15) that only non-bonded electron pair donors such as alkyhalides can readily coordinate with Lew-is acid catalysts (AlC13 or BF3), whereaswith bonded electron pair donors such asolefins and aromatics, protic acid cataly-sis is needed. In a generalized sense,however, acids are electron acceptors,and thus no a priori differentiation be-tween Bronsted (protic) and Lewis acidsseems to be justified. In extending theconcept of superacidity to Lewis acidhalides, it is suggested that those strong-er than anhydrous aluminum chloride(the most commonly used Friedel-Craftsacid) should be categorized as super-acids. These superacidic Lewis acids in-clude such higher-valence halides asantimony, arsenic, tantalum, niobium,and bismuth pentafluorides. It should al-ways be kept in mind that superacidityencompasses both Bronsted and Lewisacid systems.
Conjugate Bronsted-Lewis Superacids
In most Lewis acid-catalyzed sys-tems, as in the reactions of saturated orunsaturated hydrocarbons, the actualcatalysts are the strong conjugate acidsof Lewis acid halides with protonsources such as H20 or HCI, which arenearly always present in the system-that is, H+AlCl4-, H+BF4-, and so on (intheir solvated forms). These conjugateFriedel-Crafts acids have Ho values esti-mated at about -15 to -16. They arestronger acids than the usual mineralacids. For example, xylenes are notisomerized by mineral acids but are read-ily isomerized by Friedel-Crafts con-jugate acid systems. In 1952 McCauleyand Lien (16) demonstrated the useful-ness of acid systems such as HF-BF3 inhydrocarbon transformation reactions.Two superacid systems used very fre-
quently are HSO3F-SbF5 (Magic Acid)(17) and HF-SbF5 (fluoroantimonic acid).The acidity of anhydrous HF and HSO3Fincreases drastically on the addition ofLewis acid fluorides such as SbF5, whichform large complex fluoro anions that fa-cilitate dispersion of the negative charge(7, 9, 18).
2 HF + 2 SbF5 ;± H2F+Sb2F1l-2HSO3F + 2SbF5 T-±
H2SO3F+Sb2F1o (SO3F)-
The acidity function of HSO3F increasesfrom -15.6 to -21.0 on addition of 25mole percent SbF5 (19, 20), as shown inFig. 1. Extrapolation of the HSO3F-SbF5curve in Fig. I would lead to an Ho valueof about -25 for Magic Acid. Fluoroanti-monic acid is even stronger. As shown inFig. 1, with 4 mole percent SbF5 the Hovalue for HF-SbF5 is already -21.0, athousand times stronger than the valuefor fluorosulfuric acid with the sameSbF5 concentration. At present it is diffi-cult to estimate the acidity of 1: 1 HF-SbF5, but a value of -28 can be predict-ed (20) on the basis of isomerization ki-netics data. Thus, these superacidic sys-tems can be 1016 times stronger than 100percent sulfuric acid
Related superacid systems in whichSbF5 is replaced by AsF5, TaF5, NbF5,or BF3 are lower in acidity than MagicAcid and fluroantimonic acid. For ex-ample, BF3 forms no stable conjugateacid with HSO3F. However, HF-TaF5,HF-NbF5, and HF-BFi are very usefulsuperacids that are nonreducible and willnot cause oxidative side reactions.The conjugate superacids of triflic acid
with Lewis acid halides such as SbF5 al-so have greatly increased acidity, andthey have been used to prepare high-oc-
tane alkylates from reactions of alkanesor alkylaromatic hydrocarbons with ole-fins (21). The CF3SO3H-SbF5 system isalso useful for the isomerization ofstraight-chain (or slightly branched) al-kanes to their more highly branched iso-mers of higher octane numbers (22), a re-action important in the petroleum indus-try. With NbF5 and TaF5, triflic acidforms catalytic systems that are capableof converting benzene or toluene to eth-ylbenzene (22).
Solid Superacids
Acidic oxides such as silica and silica-alumina were used extensively as solidacid catalysts in the past and are not dis-cussed in this article (23). However, sol-id acid systems considerably strongerthan these have been developed recentlyand can be categorized as solid super-acids.As chemical applications of liquid su-
peracids became important, efforts weremade to attach superacids to solid sup-ports. There have been considerable dif-ficulties in achieving this goal. For ex-ample, BF3-based systems such as HF-BF3 cannot be well adsorbed onto solidsupports because the highly volatile BF3is easily desorbed. However, SbF5,TaF5, and NbF5 have much lower vaporpressures and are thus much more adapt-able to being attached to solids. Becauseof their extreme chemical reactivity,SbF5, HF-SbF5, and HSO3F-SbF5 can beattached preferentially only to fluori-dated alumina, fluorinated polyolefin res-in, graphite, or fluorinated graphite (23,24). On such carriers, at temperatures aslow as 70°C, HF-SbF5 and HSO3F-SbF5readily isomerize straight-chain alkanessuch as n-heptane or n-hexane. Similarsystems are also effective as alkylationcatalysts for alkanes with alkenes. Solidsuperacids based on TaF5 or NbF5 aremore stable than those based on SbF5 be-cause of their resistance to reduction.Solid perfluorinated resin sulfonic acidcatalysts, such as those based on theacid form of DuPont's Nafion ion mem-brane resin CF3(CF2),-O-CF2CF2SO3H,and higher perfluoroalkane sulfonicacids, such as perfluorodecanesulfonicacid, are also gaining interest as solid su-peracid catalysts (4).
cations, which are too reactive to existas stable species in less acidic solvents.Stable superacidic solutions of a largevariety of carbocations, including alkylcations such as the tert-butyl cation (3)(trimethylcarbenium ion) or the isopro-pyl cation (4) (dimethylcarbenium ion),have been obtained (25). Some of the
CH31+CH3
/c\CH3 CH3
3
H
l+
CH3 CH34
carbocations, as well as related acyl cat-ions and acidic carboxonium ions, thathave been prepared in superacidic solu-tions or even isolated from them asstable salts are shown in Fig. 2. Spec-troscopic techniques such as 'H and 13CNMR and infrared, ultraviolet, and elec-tron spectroscopy have been employedto characterize carbocations (10). Typi-cal alkyl and cycloalkyl cations gener-
19
17
0I
15
13 H503 F-SbFsl5
HF-SbF5 lo
ated and studied in these superacidic me-dia are shown in Fig. 3, and the 'H NMRspectra of 3 and 4 as well as the tert-amylcation and the tricyclopropylmethyl cat-ion are shown in Fig. 4.Aromatic and homoaromatic cations
and carbodications. According to Huck-el's (4n+2) electron rule, if a carbocat-ion has aromatic character, it is stabi-lized by resonance. Some aromaticallystabilized Huckeloid systems generated insuperacid media along with some carbo-dications are shown in Fig. 5.
Static or equilibrating carbocations.Some carbocations tend to undergo fastdegenerate rearrangements through in-tramolecular hydrogen or alkyl shifts tothe related identical (degenerate) struc-tures (26). The question arises whetherthese processes involve equilibrationsbetween limiting "classical" ion inter-mediates, whose structures can be ade-quately described by Lewis-type two-electron, two-center bonds separated by
Fig. I (left). Relative acidities_ of HF and HSO3F on addition
of SbF5. Fig. 2 (below).Some carbocations generatedin superacids.
0.1 0.3
5 10 15 20
% SbF5
R +H2 R +HX RO +H2
ArH +
R2C =O H .
RCH=O H
.RS+ H2
-R2S+H
A _RCO-H20
Preparation of Carbocations in
Superacids
Superacids such as Magic Acid orfluoroantimonic acid have made it pos-sible to prepare stable, long-lived carbo-5 OCTOBER 1979
low-energy transition states, or whetherintermediate "nonclassical" hydrogen-or alkyl-bridged carbonium ions are in-volved, which also require the presenceof two-electron bonds between three ormore centers for their description (27-30). It is difficult to answer this questionby NMR spectroscopy because of itsslow time scale; however, NMR has
a b
,C)
a
b
H3C +C' C-CH3rC H3H-3C.
c
-CH-
been used to delineate structures wheredegenerate rearrangements lead to aver-aged shifts and coupling constants. Also,Saunders and Kates (30) recently in-troduced an ingenious technique inwhich deuterium labeling perturbs thedegenerate equilibria, allowing one todifferentiate between classical and non-classical equilibrating cations (the per-
Fig. 3 (left). Alkyl and cycloalkyl cations. (a)The tricyclopropylmethyl cation was the firstcyclopropylmethyl cation to be observed byNMR spectroscopy (52). (b) Bredt's rule (53)in its original form seems to exclude the possi-bility of formation of positive carbenium ioncenters at bridgehead positions. Indeed,bridgehead halides proved to be extremelyunreactive under solvolytic conditions. How-ever, 1-fluoroadamantane (as well as 1-halo-adamantanes) gives the stable I-adamantylcation in superacid solution (3). The stabilityof the ion is explained by the overlap of theempty p lobe with the back lobes of threebridgehead C-H bonds [ca-II hypercon-jugation]. (c) The tertiary l-methyl-l-cyclo-pentyl cation (54) is very stable in super-acids and can be generated from a variety ofstarting materials. Fig. 4 (below). ProtonNMR spectra of (a) tert-butyl cation (3), (b)tert-amyl cation, (c) isopropyl cation, and (d)tricyclopropylmethyl cation. The frequencywas 60 MHz in (a) to (c) and 300 MHz in (d).
H3C,+,,CH3CCl.'
+,CH3O-C%C
CH3
+IC-C-C H3
+,CH3H"'-C H3
d
I
turbation is minimal in the case of non-classical equilibrating ions).
Faster methods, such as infrared andRaman spectroscopy and especiallyESCA (electron spectroscopy for chem-ical analysis), are particularly useful forinvestigating these systems (31). Sometypical examples are depicted in Fig. 6.Heteroatom-stabilized cations. Het-
eroatom-substituted carbocations arestrongly stabilized by electron donationfrom the nonbonded electron pairs of the'heteroatoms adjacent to the cationic cen-ter. For example
R2C+-X + R2C=X+
where X can be Br, OR, NR2, SR, F, orCl (where R is an alkyl or aryl group).The stabilizing effect is enhanced whentwo or three electron-donating hetero-atoms coordinate with the electron-deficient center. Some examples areshown in Fig. 7.
Hydrocarbon Transformation Reaction
The astonishing acidity of Magic Acidand related superacids allows pro-tonation of exceedingly weak bases. Notonly all conceivable ir-electron donors(such as olefins, acetylenes, and aromat-ics) but also weak v-electron donors(such as saturated hydrocarbons, includ-ing the parent alkane, methane) are pro-tonated. The ability of superacids to pro-tonate saturated hydrocarbons (alkanes)rests on the ability of the two-electron,two-center covalent bond to share itsbonded electron pair with empty orbital(p or s) of a strongly electron-deficientreagent such as a protic acid:
R-H+ H+ (R-
Superacids are suitable reagents forchemical transformation, particularly ofhydrocarbons.Isomerization. The isomerization of
hydrocarbons is of practical importance(15). Isomeric dialkylbenzenes, such asxylenes, are starting materials for plas-tics and other products. Generally, onlyone of the possible isomers is used asthe starting material, and thus there is aneed for intraconversion (isomerization).Straight-chain alkanes with five to eightcarbon atoms have considerably loweroctane numbers than their branched iso-
4 3A . . . .- -.
- -
5 4 3 2 1ppm
mers, and hence there is a need toconvert them into the higher-octanebranched isomers. Isomerizations aregenerally carried out under thermody-namically controlled conditions and lead
to equilibria. The equilibria favor in-creased amounts of the higher-octanebranched isomers at lower temperatures.
Conventional acid-catalyzed isomeri-zation of alkanes can be effected withvarious systems. Superacid-catalyzedreactions can be carried out at muchlower temperatures, even at or belowroom temperature, and thus providemore of the branched isomers. This is ofparticular importance in preparing lead-free gasoline. Increasing the octane num-ber by this means is preferable to doingso by the addition of higher-octane aro-matics or olefins, which may pose envi-ronmental or health hazard problems(32). Isomerization of alkylaromatics canalso be effectively carried out with su-peracids (15, 33).
Alkylation. Alkylation of aromatics iscarried out industrially on a large scale;an example is the reaction of ethylenewith benzene to produce ethylbenzene,which is then dehydrogenated to sty-rene, the monomer used in producingpolystyrene. Traditionally, these alkyla-
+CH2=CH2 A1C13-HCl CH3
tions have been carried out in solutionwith a Friedel-Crafts acid catalyst suchas AIC13. However, these processes arequite energy-consuming, form complexmixtures of products, and require largeamounts of catalyst, most of which istied up as complexes and can be difficultor impossible to recover. The use of asolid superacidic catalyst permits clean,efficient heterogenous alkylations withno complex formation.
Aliphatic alkylation is widely used toproduce high-octane gasolines and otherhydrocarbon products (34, 35). Conven-tional paraffin-olefin (alkane-alkene) al-kylation is an acid-catalyzed reactionwhich involves the addition of a tertiaryalkyl cation generated from an isoalkane(through hydride abstraction) to an ole-fin. An example of such a reaction is theisobutane-ethylene alkylation, yielding2,3-dimethylbutane. The mechanism ofsuch reactions has been elucidated bySchmerling (36).The recent revival of interest in strong
acid chemistry is further exemplified bythe discovery (37) that lower alkanessuch as methane and ethane can be poly-condensed in Magic Acid at 50°C, yield-ing mainly C4 to C1O hydrocarbons of thegasoline range. The proposed mecha-nism (Fig. 8) necessitates the inter-mediacy of protonated alkanes (pen-tacoordinated carbonium ions), at leastas transition states. Until now, theseions have been observed only by massspectrometry (37). Because of the highS OCTOBER 1979
R
R bRa
R R
R c R
aCH3
(iCH3i)O
d CH3 e
d
g h
b c
H
He
6 etc.
g
Fig. 5 (top left). Aromatically sta- R- X -R' /bilized cations and dications and R,R'= alkyl or aryl CH2some bridgehead dications; (a) to XBr, C CI H2(h) are discussed in (55-62), re- +spectively. Fig. 6 (top right). a XDegenerate classical and non- n- 0,2,3classical carbocations. (a) Exten- X C,Brsive kinetic, stereochemical, and bspectroscopic investigations Br + +0C= C=0clearly support the nonclassical \>rL"= FI-\ C -nature of the 2-norbornyl cation c d e f(31). (b) The trishomocyclopro-penyl cation (63). (c) The 7r-de- SRlocalized nonclassical 7-norborne- / R\ + /R R /RR-C' + C=N C-Nnyl cation (3). (d) A pyramidal di- R/ \R R/ \Rcation (29, 30). (e) The hydrogen- SRbridged cyclodecyl cation was g hshown to exist by rate enhance- -
ment in solvolysis and by direct spectroscopic observation (64). (f) Rapidly eqtuilibrating bicy-clobutonium cations (65). (g) Rapidly equilibrating classical cyclopentyl cation (3). Fig. 7(bottom right). Heteroatom-stabilized cations: (a) to (d) are described in (66), (e) and (f) in(67), (g) in (68), and (h) in (69).
reactivity of primary and secondary ionsunder these conditions, the alkylation re-action is complicated by hydride transferand related competing reactions. How-ever, in this mechanism it is implicit thatan energetic primary cation will react di-rectly with methane or ethane. Thisopens the door to new chemistry throughactivation of these traditionally passivetnolecules.A convenient way to prepare an ener-
getic primary cation is to react ethylenewith superacid. This has been used (38)with HF-TaF5 catalyst to achieve ethyl-ation of methane in a flow system at50°C. With a methane-ethylene mixture(85:15), propane is the major product.This reaction clearly is a direct alkane al-kylation through a pentacoordinated car-bocation (Fig. 9). Alkylation of ethyleneby an incipient methyl cation can beruled out because the yield of propyleneand polymeric material was only a fewpercent. Methane alone does not reactunder these conditions, and ethylenealone yields mainly a mixture of prod-ucts with higher molecular weights.When a 1:9 ethylene-ethane mixture is
treated with the same catalyst, normalbutane is formed with high selectivity.
This again shows that no primary butylcation is formed from ethylation of ethyl-ene (Fig. 10) and that the ethyl cation al-kylates ethane in a C-H bond through apentacoordinated carbonium ion. Theseresults clearly suggest that carboniumions or tight ion pairs of a related pen-tacoordinated nature (37) are the react-ing species and alkylations can proceedwithout the concurrent isomerizationcharacteristic of trivalent alkyl cations.Polymerization. The key initiation
step in cationic polymerization of al-kenes is the formation of a carbocationicintermediate (39), which can then inter-act with excess monomer to start propa-gation. The mechanism of the initiationof cationic polymerization and poly-condensation has been extensively stud-ied. Trivalent catbenium ions play thekey role, not only in the acid-catalyzedpolymerization of alkenes, but also in thepolycondensation of arenes (ir-bondedmonomers) as well as the cationic po-lymerization of ethers, sulfides, and ni-trogen compounds (nonbonded electron-pair donor monomers). On the otherhand, pentacoordinated carbonium ionsplay the key role in the electrophilic re-actions of v-bonds (single bonds), in-
CH3ZCCHCH3"11 (right). Conversion of isobutane into acetone and methyl alcohol.
cluding the oligocondensation of alkanesand the cocondensation of alkanes andalkenes (40, 41).
Alkylation and oligocondensation re-
actions of alkanes giving higher-molecu-lar-weight alkanes have been achieved(40-42)-for instance, the oligoconden-sation of methane, which was discussedin the preceding section. When higherproportions of alkanes are treated in re-
lation to the superacid used, they under-go oligocondensation with increasingease. The oligocondensation of lower al-kanes to give polyalkanes is a reaction ofsubstantial practical interest; by this re-
action, natural gas can be converted intobranched liquid hydrocarbons in the gas-oline range or into other hydrocarbonproducts. A study of alkane poly-condensation in Magic Acid (43) hasshown that highly branched polyalkaneswith molecular weights up to 700 can beobtained by reacting a gaseous alkane(C1 to C4) with the liquid acid, even atroom temperature.The first step of this polycondensation
reaction involves protonation of the al-kane, with subsequent formation of one
molecule of hydrogen, at the expense ofthe superacid used. This difficulty can
be overcome, as pointed out by Olah
et al. (37), by adding a small amount ofan olefin such as ethylene to the reactionmixture as initiator. Roberts and Calihan(41) also showed that methane undergoespolycondensation in the presence of al-kenes such as propene, 1-butene, or 2-methylpropene, giving polyalkanes con-
taining block methylene groups (at leastfour straight-chain CH2 units).
Superacids in Organic Syntheses
Since their discovery (3), stable carbo-cations were known to be readilyquenched by various nucleophiles.These reactions, which were first used toconfirm the structure of the ions, provedto be very useful in organic syntheses.The selectivity of the reactions is basedon the fact that generally only the ther-modynamically more stable ions are
formed under the reaction conditions, re-
sulting in high selectivity. The new func-tional group created in the superacid me-
dium will itself undergo protonation, andthus be protected against any furtherelectrophilic attack. In this way, a num-
ber of new selective reactions with highyields were achieved, as shown in thefollowing examples.
Dienone-phenol rearrangement. Thisisomerization is of AUbstantial impor-tance in natural product syntheses, usu-ally catalyzed by a strong base. The re-action occurs with good yields in polycy-clic systems under superacidic condi-tions, as shown by Gesson and Jacque-sy (44).
CH3 0 CH130
HF-SbF5
HO 0H6
Reduction. Hydride ion transfer tocarbocations is a Wvell-known reaction inhydrocarbon chemistry. This reactionhas been used successfully in superacidto reduce a,43-unsaturated ketones withmethylcyclopentane as the hydride do-nor (43). Superacid-catalyzed reductionof aromatics, as shown by
HHF-SbF5
Wristers (45), requires both a hydridedonor and hydrogen
Carbonylation. The reaction betweencarbocations and carbon monoxide af-fording oxocarbenium ions (acyl cations)is a key step in the well-known Koch-Haaf reaction for the preparation of car-boxylic acids from alkenes (46). It hasbeen thoroughly studied by Hogeween(46) under superacidic conditions. Thereaction has recently been applied tofunctjonalize nonactivated carbon atoms(47).
H
H
H O
H
Oxidation. Novel oxidations of hydro-carbons in superacids with ozone or hy-drogen peroxide have been investigated(48). Protonated ozone (03H)+ or hydro-gen peroxide (H302)+ attacks the singlea-bond, resulting in oxygen insertion.These reactions can be followed by a
protolytic transformation, such as theconversion of isobutane into acetone andmethyl alcohol (Fig. 11) (48). By a sim-ilar procedure, aromatics (ArH) are alsohydroxylated in high yields at low tem-perature
ArH H202 ArOHHS03F-S02CIF
Miscellaneous reactions. Many acid-catalyzed reactions can be advanta-geously carried out by using solid super-
acids instead of conventional acid sys-
tems. The reactions may be in either thegaseous or liquid phase. For example,several simple procedures were recentlyreported (49) in which Nafion-H solidacid was used in alkylation, trans-bromination, nitration, acetalization,hydration, and so on.
Superacids in Inorganic Chemistry
Halogen cations. It has often beenpostulated that the monoatomic ions I+,Br+, and Cl+ are the reactive inter-mediates in halogenation reactions ofaromatics and alkenes. The search forsuch species has led to the discovery of12+ and other related halogen cations,which are stable in superacids (50). TheI2+ cation may be generated by the oxi-dation of 12 with S206iF2 in HSO:F solu-tion
2 12 + S206F2 2 I2+ + 2 S0OjF-
and a stable blue solution of this cationcan also be obtained by oxidizing iodinewith 65 percent oleum. In a less acidic
medium, the I2+ cation disproportionatesto more stable oxidation states. The elec-trophilic Br2+ cation is obtainable only inthe very strong superacid Magic Acid orfluoroantimonic acid, and it dispro-portionates in HS03F. The Cl2+ cation,which is much more electrophilic, hasnot yet been observed in solution. Mono-atomic halogen cations seem to be toounstable to be directly observed.
Cations of other nonmetallic ele-ments. Elemental sulfur, selenium, andtellurium give colored solutions whendissolved in a number of strongly acidicmedia. It has been shown (51) that S162 ,S82+, S42+, Se82+, Te42+, and Te2+ arepresent in such solutions. These cationsare formed by oxidation of the elementsby H2S207 or S206F2; for example
4 S + 6 H2S207 -* S42+ + 2 HS3010-+ 5 H2S04 + S02
Like the halogen cations, the sulfur, se-lenium, and tellurium cations are highlyelectrophilic and undergo disproportion-ation in media with any appreciable basicproperties, although, as would be antici-pated, the ease of disproportionation in-creases in the order tellurium < selenium< sulfur.
References and Notes
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istics of the primitive angiosperms: in-sect pollination and the closed carpel.My hypothesis is that insect pollinationand the closed carpel, in combination,provide a mechanism that greatly in-
Summary. In the primitive angiosperms, closed carpels are believed to haveevolved as protection for ovules, which would otherwise be injured by animal pollina-tors. The hypothesis is presented that, whatever the origin and other functions ofangiosperms, insect pollination and closed carpels may, in combination, reduce theinfluence of random variation on pollen tube competition, thus enhancing the ability ofnatural selection to act on the gametophytic phase of the life cycle. The micro-gametophytic phase represented by vast numbers of haploid individuals can thenserve, by insect pollination and. closed carpels, as a screen against any genome notfunctioning with a high degree of metabolic vigor. Poorly balanced genomes couldthus be eliminated at relatively little cost. Insect-pollinated angiosperms would there-fore benefit from positive aspects of sexual recombination. Such a system may haveallowed the angiosperms to undergo their rise to dominance.
sive in, the evolutionary rise of thisgroup. These include insect-pollinatedflowers, closed carpels, double fertiliza-tion, true endosperm, vessel elements,broad leaves, defensive alkaloids, andbird-dispersed fruits (1-3).There can be no doubt that many of
these hypotheses contain more than a
modicum of truth but, in this article, Ipropose yet another one, a hypothesisthat depends on, and thereby adds signif-icance to, two, outstanding character-
tensified selection pressures among themicrogametophytes. Thus, the micro-gametic phase of the life cycle served as
a barrier against poorly functioning hap-loid genomes and hence benefited the re-
mainder of the life cycle.To understand the operation and the
consequences of this mechanism, it isnecessary to consider some character-istics of a wind-pollinated preangio-spermous species, similar perhaps tothose that presumably gave rise to the
early angiosperms. Typically vast num-bers of pollen grains would have beenproduced since only a miniscule fractionof these randomly moving grains couldever reach a receptive surface. Thosegrains that did reach receptive surfaceswould most likely have done so singlyand, because such pollinations mighthave occurred over a long period of time,the first grains to arrive could have en-joyed a head start, compared to later ar-rivals. Thus, the success or failure of aparticular microgametophyte was influ-enced by chance. Upon arrival, thewind-borne pollen was passively carriedto the megasporangium either by fallingupon or being drawn to it by contractionof a pollination droplet.
In each of these above-mentionedcharacteristics, an insect-pollinated angi-osperm is in direct contrast with a wind-pollinated preangiosperm. In the angio-sperm, for example, fairly large numbersof grains will have reached receptive sur-faces, not by deposition of single grains,but rather by masses of pollen depositedsimultaneously by insect visitors. Fur-thermore, upon reaching a receptive sur-face, pollen grains, because of the closedcarpels, of necessity produce pollentubes that grow through fairly long sec-tions of stylar tissues.
While the transition from wind-polli-nated preangiosperm to insect-pollinatedangiosperm may have had many con-sequences, my hypothesis suggests someof the most significant effects may havebeen upon the nature of interactions be-tween microgametophytes. For ex-ample, with insect pollinations, morepollen grains would reach receptive sur-faces (stigmas) so that competitionamong microgametophytes would be in-tensified. Furthermore, the simultaneousarrival on the stigma of many pollengrains would generate, in itself, an epi-sode of intense competition. Finally, thelong passage through the style, as is ex-plained below, would provide an ex-
The author is an associate professor of botany atthe University of Massachusetts, Amherst 01003.
SCIENCE, VOL. 206, 5 OCTOBER 1979
The Rise of the Angiosperms:A Genecological Factor
The combination of insect pollination and closedcarpels may provide a unique selective mechanism.
David L. Mulcahy
To what factors do the angiospermsowe their preeminent position in presentworld floras? Numerous angiospermouscharacteristics have been suggested asbeing contributory toward, or even deci-
