olah - superacid chemistry 2e (wiley, 2009)

The chemistry of superacids, that is, of acid systems stronger than conventionalstrong mineral Brønsted acids such as sulfuric acid or Lewis acids like aluminumtrichloride, has developed

SUPERACID CHEMISTRY

SUPERACID CHEMISTRY SECOND EDITION

George A Olah

G K Surya Prakash

Arp
ad Moln
ar

Jean Sommer

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Superacid chemistry / by George A Olah [et al.] – 2nd ed.

1 Superacids I Olah, George A (George Andrew), 1927- II Olah,

George A (George Andrew), 1927- Superacids.

Preface to the Second Edition xvii

1.4.2.2 Exchange Rate Measurements Based

on Line-Shape Analysis (DNMR:

1.4.6 Theoretical Calculations and Superacidity

vii

2.2.2.10 Hydrogen Fluoride–Boron Trifluoride

2.4.3.1 Brønsted Acid-Modified Metal Oxides:

2.4.3.2 Lewis Acid-Modified Metal Oxides

3.1.1 Development of the Carbocation Concept: Early

3.3.6 Tool of Increasing Electron Demand 91

in Antimony Pentafluoride Solution

3.4.9 Propargyl and Allenylmethyl Cations

3.5.1.2 Multiply Protonated Methane Ions

3.5.2.5 Degenerate Cyclopropylmethyl and

6-dien-9-yl) Cations and Bicyclo[3.2.2]

4.2.1.6 Hydrogen Peroxonium Ion (H3O2þ)

4.3.4 Enium Ions of Group 16 Elements 424

4.4.4.2 Polyheteroatom Cations of Halogens

4.4.4.3 Polyheteroatom Cations of Chalcogens

5.1.1 Sigma-Basicity: Reversible Protonation or Protolysis

5.1.1.1 Deuterium–Hydrogen Exchange Studies 505

5.14.1.2 Synthesis of Nitrogen Heterocycles 685

AND UPDATED EDITION

More than 20 years passed since the publication of our book on Superacids The bookbecame out of print and much progress since was made in the field, which is gainingincreasing interest and significance Hence, it seems warranted to provide theinterested reader with a comprehensively updated review and discussion of the fieldwith literature coverage until early 2008 The title has been changed to ‘‘SuperacidChemistry” to reflect enormous progress in the field Some aspects of superelec-trophilic activation are also discussed (for more elaborate coverage, readers arereferred to G A Olah and D A Klump, ‘‘Superelectrophiles and Their Chemistry”Wiley-Interscience, 2008) Our friend and colleague, Arpad Molnar joined us as acoauthor and made an outstanding contribution to the revised new edition of our book,which we hope will be of interest and use to the chemical community Our publisher isthanked for arranging the new revised edition

xvii

The chemistry of superacids, that is, of acid systems stronger than conventionalstrong mineral Brønsted acids such as sulfuric acid or Lewis acids like aluminumtrichloride, has developed in the last two decades into a field of growing interest andimportance It was J B Conant who in 1927 gave the name “superacids” to acids thatwere capable of protonating certain weak bases such as carbonyl compounds andcalled attention to acid systems stronger than conventional mineral acids Therealization that Friedel–Crafts reactions are, in general, acid catalyzed with conjugateLewis–Brønsted acid systems frequently acting as the de facto catalysts extended thescope of acid catalyzed reactions Friedel–Crafts acid systems, however, are usually

interest in superacids and their chemistry The initial impetus was given by thediscovery that stable, long-lived, electron-deficient cations, such as carbocations,acidic oxonium ions, halonium ions, and halogen cations, can be obtained in thesehighly acidic systems Subsequent work opened up new vistas of chemistry and afascinating, broad field of chemistry is developing at superacidities Because acidity

is a term related to a reference base, superacidity allows extension of acid-catalyzedreactions to very weak bases and thus extends, for example, hydrocarbon chemistry tosaturated systems including methane

Some years ago in two review articles (Science 206, 13, 1979; La Recherche 10,

624, 1979), we briefly reviewed some of the emerging novel aspects of superacids.However, we soon realized that the field was growing so fast that to be able to provide amore detailed survey for the interested chemist a more comprehensive review wasrequired Hence, we welcomed the suggestion of our publisher and Dr Theodore P.Hoffman, chemistry editor of Wiley-Interscience, that we write a monograph onsuperacids

We are unable to thank all of our friends and colleagues who directly or indirectlycontributed to the development of the chemistry of superacids The main credit goes toall researchers in the field whose work created and continues to enrich this fascinatingarea of chemistry Professor R J Gillespie’s pioneering work on the inorganicchemistry of superacids was of immense value and inspiration to the development

of the whole field Our specific thanks are due to Drs David Meidar and KhosrowLaali, who helped with the review of solid superacid systems and their reactions.Professor E M Arnett is thanked for reading part of our manuscript and for histhoughtful comments

xix

Finally we would like to thank Mrs R Choy, who tirelessly and always cheerfullytyped the manuscript.

General Aspects

1.1 DEFINING ACIDITY

1.1.1 Acids and Bases

The concept of acidity was born in ancient times to describe the physiological propertysuch as taste offood or beverage (in Latin: acidus, sour; acetum, vinegar) Later duringthe development of experimental chemistry, it was soon realized that mineral acidssuch as sulfuric, nitric, and hydrochloric acids played a key role in chemicaltransformations Our present understanding of acid-induced or -catalyzed reactionscovers an extremely broad field ranging from large-scale industrial processes inhydrocarbon chemistry to enzyme-controlled reactions in the living cell

The chemical species that plays a unique and privileged role in acidity is the

prone to electronic repulsion and by itself has a powerful polarizing effect Due to itsvery strong electron affinity, it cannot be found as a free “naked” species in thecondensed state but is always associated with one or more molecules of the acid itself

or of the solvent Free protons exist only in the gas phase (such as in mass spectrometricstudies) Regardless, as a shorthand notation, one generally depicts the proton in

cation) and the fact that only the 1s orbital is used in bonding by hydrogen, protontransfer is a very facile chemical reaction and does not necessitate importantreorganization of the electronic valence shells Understanding the nature of the proton

The first clear definition of acidity can be attributed to Arrhenius, who between

1880 and 1890 elaborated the theory of ionic dissociation in water to explain the

conductance measurements, he defined acids as substances that dissociate in water andyield the hydrogen ion whereas bases dissociate to yield hydroxide ions In 1923, J N

that can donate a proton and defined a base as a species that can accept it This

Superacid Chemistry, Second Edition, George A Olah, G K Surya Prakash,
Arp
ad Moln
ar, and Jean Sommer Copyright
2009 John Wiley & Sons, Inc.

1

definition is generally known as the Brønsted–Lowry concept The dissociation of anacid HA in a solvent S can be written as an acid–base equilibrium [Eq (1.1)].

HA + S A − + SH+ ð1:1Þ

Equation (1.1) has a very wide scope and can be very well applied to neutral andpositively and negatively charged acid systems The acid–base pair that differs only by

acid ionizes depends on the basicity of the solvent in which the ionization takes place.This shows the difficulty in establishing an absolute acidity scale Acidity scales areenergy scales, and thus they are arbitrary with respect to both the reference point andthe magnitude of units chosen

Fortunately, many of the common solvents by themselves are capable of acting

as acids and bases These amphoteric or amphiprotic solvents undergo self-ionization[e.g., Eqs (1.2) and (1.3)], which can be formulated in a general way as in Eq (1.4)

2 H 2 O H 3 O + + OH − ð1:2Þ

2 HF H 2 F+ + F − ð1:3Þ

2 HA H 2 A+ + A − ð1:4Þ

the usual high dilution conditions can be written as in Eq (1.5)

later

G.N Lewis extended and generalized the acid–base concept to nonprotonic

base as a substance that can donate electrons Lewis acids are electron-deficient

that contain readily available nonbonded electron pairs (as in ethers, amines, etc.)[Eq (1.6)]

BF 3 + :O(CH3 ) 2 BF 3:O(CH3 ) 2 ð1:6Þ

acids and bases also fall into the Lewis categories

Considering the general equation (1.4) for the auto-ionization of solvent HA, one

includes both Lewis’ and Brønsted’s concepts, is used in practice while measuring theacidity of a solution by pH

A number of strategies have been developed for acidity measurements of bothaqueous and nonaqueous solutions Wewill briefly review the mostimportant ones anddiscuss their use in establishing acidity scales.

1.1.2 The pH Scale

The concentration of the acid itself is of little significance other than analytical, with

itself is not satisfactory either, because it is solvated diversely and the ability oftransferring a proton to another base depends on the nature of the medium The real

The experimental determination of the activity of the proton requires the measurement

of the potential of a hydrogen electrode or a glass electrode in equilibrium with thesolution to be tested The equation is of the following type [Eq (1.7)], wherein C is aconstant

to the modern definition of the pH scale of acidity for aqueous solutions The pH of

a dilute solution of acid is related to the concentration of the solvated proton from

Eq (1.8) Depending on the dilution, the proton can be further solvated by two ormore solvent molecules

When the acid solution is highly diluted in water, the pH measurement isconvenient, but it becomes critical when the acid concentration increases and, evenmore so, if nonaqueous media are employed Since a reference cell is used with a liquidjunction, the potential at the liquid junction also has to be known The hydrogen ionactivity cannot be measured independently, and for this reason the equality of Eq (1.9)cannot be definitely established for any solution

Under the best experimental conditions, the National Bureau of Standard has set up

a series of standard solutions of pH from which the pH of any other aqueous solutioncan be extrapolated as long as the ionic strength of the solution is not higher than 0.1 M.For more concentrated solutions, the pH scale will no longer have any real signifi-cance In extending the limit to 1 M solutions, it is apparent that the available range

of acidity is directly related to the autoprotolysis constant [Eq (1.5)], because the

limiting conditions are rather unfortunate because many chemical transformations areachieved beyond this range and under much less ideal conditions

protonation of weakly basic indicators in acid solution The proton transfer

equilibri-um in the acid solution between an electro-neutral weak base B and the solvated protoncan be written as in Eq (1.10)

B + H 2 A+ BH+ + AH ð1:10Þ

equilibrium can be written as in Eq (1.11)

B + H + BH + ð1:11Þ

expressed as in Eq (1.12), in which a is the activity, C the concentration, and f theactivity coefficient

for a particular series of solutions of changing acidity The first application was made

the two indicators in the same acid solution using the relation of Eq (1.16)

were proposed related to acid–base equilibria in which the indicator is negatively,positively, or even dipositively charged The validity of all of these functions is based

on the simple assumption that the activity coefficient ratio is independent of the nature

case forgroups ofindicators with different structures, and especially for different basicsites, which often show significant deviations For this reason, it is well recognizednow that the above assumption does not have a general validity The measurement of a

are called Hammett bases

Equilibria other than proton transfer have also been used to determine acidityfunctions One of these is based on the ionization of alcohols (mainly arylmethylalcohols) in acid solution following the equilibrium in Eq (1.17)

ROH + H+ R+ + H 2 O ð1:17Þ

of the sulfuric acid–water and perchloric acid–water systems It shows a large

However, all these and other acidity functions are based on Hammett’s principleand can be expressed by Eq (1.19), in which B and A are the basic and the conjugateacidic form of the indicator, respectively They become identical with the pH scale inhighly dilute acid solutions The relative and absolute validity of the different acidityfunctions have been the subject of much controversy and the subject has been

Hx¼ pKA
logAB ð1:19Þ

Whatever may be the limitations of the concept first proposed by Hammett and

assess quantitatively the acidity of concentrated and nonaqueous strongly acidic

functions are discussed in Section 1.4

1.2 DEFINITION OF SUPERACIDS

first time in the chemical literature In a study of the hydrogen ion activity in anonaqueous acid solution, these authors noticed that sulfuric acid and perchloric acid

in glacial acetic acid were able to form salts with a variety of weak bases such asketones and other carbonyl compounds These weak bases did not form salts with theaqueous solutions of the same acids The authors ascribed this high acidity to theionization of these acids in glacial acetic acid, increasing the concentration of

0 5 10

100 50

Figure 1.1 H0and J0functions for H2SO4–H2O and HClO4–H2O systems HClO4: *, ref 11;

*, ref 13; H2SO4: &, ref 12; &, ref 14.

to call these solutions “superacid solutions.” Their proposal was, however, not furtherfollowed up or used until the 1960s, when Olah’s studies of obtaining stable solutions

of highly electron-deficient ions, particularly carbocations, focused interest on very

and trifluoromethanesulfonic acid are examples of Brønsted acids that exceed the

To reach acidities beyond this limit, one has to start with an already strong acid

achieved either by adding a strong Brønsted acid (HB) capable of ionizing in themedium [Eq (1.20)] or by adding a strong Lewis acid (L) that, by forming a conjugateacid, will shift the autoprotonation equilibrium by forming a more delocalizedcounterion of the strong acid [Eq (1.21)]

HA + HB H 2 A+ + B − ð1:20Þ

2 HA + L H2A + + LA − ð1:21Þ

solution into the superacid region Therefore, it is clear that the proposed reference of

Gillespie’s definition of superacids relates to Brønsted acid systems BecauseLewis acids also cover a wide range of acidities extending beyond the strength ofconventionally used systems, Olah suggested the use of anhydrous aluminum chloride

as the arbitrary reference and we categorize Lewis superacids as those stronger than

measuring the strength of a Lewis acid)

It should be also noted that in biological chemistry, following a suggestion by

as “superacid catalysis.” Because the role of a metal ion is analogous to a proton, thisarbitrary suggestion reflects enhanced activity and is in line with previously discussedBrønsted and Lewis superacids

1.2.1 Range of Acidities

Despite the fact that superacids are stronger than 100% sulfuric acid, there may be asmuch or more difference in acidity between various superacid systems than betweenneat sulfuric acid and its 0.1 M solution in water

various measurements Meanwhile, however, Sommer and coworkers found that theweakest basic indicator of the para-methoxybenzhydryl cation family (4,4’-

stronger acidities are all based on indirect estimations rather than direct acid–base

present in the condensed phase because even compounds as weakly basic as methane

A quantitative determination of the strength of Lewis acids to establish similar

useful However, to establish such a scale is extremely difficult Whereas the Brønstedacid–base interaction invariably involves a proton transfer reaction that allowsmeaningful comparison, in the Lewis acid–base interaction, involving for exampleLewis acids with widely different electronic and steric donating substituents, there is

of Lewis acid” has no well-defined meaning

Regardless, it is important to keep in mind that superacidity encompasses bothBrønsted and Lewis acid systems and their conjugate acids The qualitative picture ofLewis acid strengths will be discussed in Section 1.4.7.

The acidity strength of solid acids is still not well known and is difficult tomeasure Claims of superacidity in solids are numerous and will be discussed later inChapter 2 Among the reviews related to acidity characterization of solids, those of

1.3 TYPES OF SUPERACIDS

As discussed, superacids, similar to conventional acid systems, include both Brønstedand Lewis acids and their conjugate systems Protic (Brønsted-type) superacidsinclude strong parent acids and the mixtures thereof, whose acidity can be furtherenhanced by various combinations with Lewis acids (conjugate acids) The followingare the most frequently used superacids

30 25

20 15

10

HBr−AlBr 3

(7) (0.6)

(10)

(50) (10)

(22)

(45) (6)

(50) (30)

HF HF HF

Magic Acid

BF 3 TaF 5 SbF 5 SbF5

B(SO 3 CF 3 ) 3 SbF 5 SbF 5

Figure 1.3 Acidity ranges for the most common superacids The solid and open bars aremeasured using indicators; the broken bar is estimated by kinetic measurements; numbers inparentheses indicate mol% Lewis acid

1.3.1 Primary Superacids

tris(pentafluoro-phenyl) borane, boron tris(trifluoromethanesulfonate), and aprotic organicsuperacids developed by Vol’pin and co-workers

1.3.2 Binary Superacids

2 Conjugate Brønsted–Lewis superacids:

b Hydrogen fluoride in combination with fluorinated Lewis acids such as

Solid superacids can be further divided into various groups depending on the nature

of the acid sites The acidity may be a property of the solid as part of its chemicalstructure (possessing Lewis or Brønsted sites; the acidity of the latter can be furtherenhanced by complexing with Lewis acids) Solid superacids can also be obtained bydeposition on or intercalation of strong acids into an otherwise inert or low-aciditysupport

1 Zeolitic acids

2 Polymeric resin sulfonic acids including sulfonic acid resins complexed withLewis acids and perfluorinated polymer resin acids (Nafion–H and Nafion–silica nanocomposites)

3 Enhanced acidity solids including Brønsted and Lewis acid-modified metaloxides and mixed oxides, as well as metal salts complexed with Lewis acids

4 Immobilized superacids and graphite-intercalated superacids

As with previous classifications, these are also arbitrary and are suggested forpractical discussion of an otherwise rather complex field The superacid character of

solids is discussed later in subsequent subchapters, and individual superacid systemsare discussed in Chapter 2.

1.4 EXPERIMENTAL TECHNIQUES FOR ACIDITY MEASUREMENTS(PROTIC ACIDS)

From Eq (1.14) it is apparent that the main experimental difficulty in determiningacidities is the estimation of the ratio between the free base and its protonated ionicform of a series of indicators, their so-called ionization ratios

1.4.1 Spectrophotometric Method

based on the color change of the indicator The solutions containing the indicator were

when the indicator was colorless in its acid form, and 96% sulfuric acid (or 70%perchloric acid), when the indicator was colorless in the basic form

For example, when the indicator was colored in water the authors define a

stoichiometric concentrations of indicator in solution A and in water On the otherhand, the specific color intensity of the colored form relative to water is defined as

Sw¼ [B]w/[B]a, where [B]wis the concentration of the colored base in water and [B]aisconcentration in solution A Because the indicator exists only in its basic form in

form, and of the unknown solution, the ionization ratio is given by Eq (1.24)

For a greater precision in this determination, l should be chosen so as to have the

absorption line and the baseline of both acidic and basic forms of the indicator should

be measured and compared

Whereas the precision of the method is generally excellent, a number of drawbacksmay appear with some indicators First, Eq (1.24) is true only with the assumption that

separately in the same solution) The medium effect on the absorption spectrum

measurements However, large changes in the absorption spectrum during the increase

in ionization are difficult to correct Another difficulty that might appear is thestructural change of the indicator, during or after protonation The change in

dilute aqueous solution [Eq (1.25)]

It is to be noted that when the acid solution is very dilute, the presence of the

indicator has to be taken into account

As is apparent from Eq (1.14), an indicator is only useful over an acidity rangewhere its ionization ratio can be measured experimentally with sufficient precision.For spectrophotometric method, this means approximately 2 log units per indicator

concen-trated solution is not possible It is actually achieved by the method developed by

into account the overlapping of each indicator with the preceding and the followingone, each of which is useful for 2 log units, it appears that several indicators arenecessary (approximately as many indicators as the number of desired log units) This

is illustrated in Figure 1.4

establish Hammett acidity functions for aqueous acids between the years 1932 and

Since then, subsequent work seems to suggest that some of these values areincorrect This is particularly the case for some of the weaker bases whose quoted

that have since been proven to be unsatisfactory based on the strict definition of

covering the whole acidity range from dilute acid to the superacid media arecollected in Table 1.1

0

2

100 80

60 40

20 0

3 4 5

6

7 8

9

10 11 12

13

14 15

Figure 1.4 The ionization ratio as measured for a series of indicators in the 0–100% H2SO4–

H2O system.8

such as nitro compounds have to be used Although the acidity function scale basedupon nitro compounds as indicators may not be a satisfactory extension of the aniline

indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest cator in the aniline series, 2,4,6-trinitroaniline Thus, the acidity measurements usingthe nitro compounds may be considered to give the best semiquantitative picture of theacidity of the various superacid systems

indi-UV spectroscopy of adsorbed Hammett bases has also been used to estimate the

1.4.2 Nuclear Magnetic Resonance Methods

NMR spectroscopy, which was developed in the late 1950s as a most powerful tool forstructural analysis of organic compounds, has also proven to be useful for aciditydeterminations The measurement of the ionization ratio has been achieved by avariety of methods demonstrating the versatility of this technique If we consider thegeneral acid–base equilibrium Eq (1.26) obtained when the indicator B is dissolved

In NMR spectroscopy, when a species (for example, here [BHþ]) is participating in

of the mean lifetime is a first-order rate constant, called the rate of exchange (k ¼ 1/t),

no exchange were taking place

can be calculated from the line-shape analysis of the NMR bands of theexchanging species

Table 1.1 Selected pKBHþ Values for Extended Hammett Bases

3 “Fast Exchange” Conditions: k > 104s
1 The observed NMR bands appear asthe weighted average of the species participating in the equilibrium.

Depending on these conditions, various NMR methods have been proposed andused to calculate the ionization ratio of weak bases in a superacid medium

the ionization ratio cannot be measured In fact, one of the major advantages of thesuperacidic media is the ease with which weak bases can be fully protonated anddirectly observed by NMR Because it is known that the protonation rates are

spectrum and no variable is available to measure the ionization ratio

Under “fast-exchange” conditions, however, the NMR spectrum presents aweighed average of the bands of the exchanging species, and with the sensitivity

limits (5–95%) the ionization ratio can be measured taking the chemical shift as a

variable The calculation is simply based on the observed chemical shift of the average

decreasing the acidity of the medium

By plotting the chemical shift variation against the acidity, one observes a typical

determination of ketone basicity and evaluation of medium acidity

Compared with spectrophotometry, the NMR method has a number of advantages:(i) The procedure is very rapid, and it can be used by observing the variation of

colored impurities and slight decomposition of the indicator (iii) In principle, it can beused over the whole range of known acidity The medium effect, which may be

method can be used with a wide variety of weak bases having a lone-pair containingheteroatoms as well as simple aromatic hydrocarbons

However, because the measurement of the ionization ratio requires the presence

of a minimum of 5% of one of the forms of the indicator, it necessitates theavailability of a family of structurally similar compounds of varying basicity tocover a large domain of acidity This condition has been met by Sommer and co-

strongest superacids

2.4 2.5 2.6 2.7 2.8 2.9 3 3.1

14 13 12 11 10 9 8 7

C H

R = 1: 4'-MeO, 2: 4'-Me, 3: 4'-H, 4: 4'-Br,

The protonation curves of indicators 1–4 in the HSO3F
SbF5system (Figure 1.6)show how the decreasing basicity of the indicator necessitates an increasing amount of

Brønsted acid (as a consequence of oligomeric anion formation)

The same indicators havebeen used to compare the relativeacidity of the three mostused superacids As shown on Figure 1.7, the half-protonation of indicator 3

acid, and hydrogen fluoride systems These results show the supremacy of the

in acidity (see also Section 2.2.2.7)

1.4.2.2 Exchange Rate Measurements Based on Line-Shape Analysis

exchange rate conditions, two possibilities have been considered:

1 The change in line shape can be directly related to the proton exchange

2 The change in line shape is due to a separate exchange process related to theproton exchange

Both methods have been exploited to determine ionization ratios

[Eq.(1.31)](eachbasecoveringapproximately3logunitsforagivenconcentration),Gold

acid-base equilibrium Besides measuring intermolecular processes like the protonexchange rates, DNMR often has been used to measure intramolecular processeslike conformational changes that occur on the same time scale When the activationenergy of such a process is very different in the acidic and basic forms for an indicator,DNMR can be used to measure the ionization ratio

Due to a partial p-character, aromatic carbonyl compounds have an activationenergy barrier for rotation around the phenyl–carbonyl bond, the value of which is

proton-ation of the methoxy group will drastically decrease their barrier The dependent NMR spectrum will reflect both exchange processes, intra- and intermo-lecular, as shown in Scheme 1.1

of a large difference in activation energies between the rotation barriers in the and diprotonated aldehyde, the observed rates arevery sensitiveto the concentration of

By combining this method with the previously discussed chemical shift method,which is sensitive in the 0.05–20 range for the ionization ratio, the acidity could be

complementarity of both methods

These are only approximate estimations, but are in reasonable agreement with more

Despite the evident advantages of the NMR methods, two points must be ered concerning the results of the acidity measurements First, the concentration of the

11 12 13 14 15 16 17 18

C

O

H O

Me

+ H

indicator cannot be neglected as in the UV method, especially when the BH2 þ

concentration Second, aldehydes and ketones that have been generally used in theNMR methods are not true Hammett bases and the acidity that is derived should beconsidered only in a relative sense

1.4.3 Electrochemical Methods

Electrochemistry provides a number of techniques for acidity measurements Thehydrogen electrode is the most reliable method in nonreducible solvents It has beenshown, however, that its reliability is limited to relatively weak acid solutions A more

suggested a method to measure the potential variation of a pH-dependent systemwith respect to a reference system whose potential was solvent-independent The

(1:1)/Pt, containing sulfuric acid solution up to 100% Strehlow defined an acidity

at proton activities x and unity, respectively

strong acids, this function should be a logarithmic measure of the proton activity aslong as the normal potential of the redox system, ferrocene–ferricinium, is constant.This was, however, not the case in very strong acid solutions because ferroceneunderwent protonation Other electrochemical pH indicators have been proposed,such as quinine–hydroquinone or semiquinone–hydroquinone, the basicity of whichcan be modified by substitution on the aromatic ring These electrochemical indicators

in anhydrous HF and HF containing superacids

because the indicators are chemically very different from the organic substratesgenerally used On the other hand, as the measurements are based on pH determina-tion, the length of the acidity scale is limited by the pK value of the solvents However,very interesting electrochemical acidity studies have been performed in HF byTr
emillon and co-workers, such as the acidity measurement in anhydrous HF solventand the determination of the relative strength of various Lewis acids in the samesolvent By studying thevariation of the potential of alkane redox couples as a function

of acidity, the authors provide a rational explanation of hydrocarbon behavior in the

1.4.4 Chemical Kinetics

The idea that the acidity function may be useful in determining the rates of catalyzed reactions was the main reason for development of the method first proposed

acid-by Hammett and Deyrup.8The parallelism between reaction rate and H0was noticed

substrate parallels the protonation of the Hammett bases, the observed rate constant

number of acid-catalyzed reactions and its limitation due to deviations in the

applied to obtain a qualitative classification of the relative acidity of various superacidsolutions

of the interconversion rates of alkyl tetrahydrofuryl ions 7 and 8 (Scheme 1.2),proceeding via dicarbenium ion intermediates, they measured the overall rate of

By making the assumption that the rates were only proportional to the concentration

of the dication and taking into account the temperature dependence of the rate, theycould estimate the relative acidity of these systems By repeating these experimentswith closely related reactions and varying the acid composition, they were able to

approximate in comparison with the results obtained by other techniques Moreover,rates can be affected by other factors than acidity such as solvation effects and

medium

superacids of great practical significance, and various techniques have been used to

lower than those of the fluoroacids discussed

+

+ +

OH

+ +

1.4.5 Heats of Protonation of Weak Bases

variety of carbonyl compounds given in the literature vary over an unacceptably widerange The variations are due not only to the activity coefficient problems, but also topractical difficulties such as the effect of media on position of the UV absorption

alternative method was proposed by Arnett and co-workers measuring the heats of

known from other methods They found a good correlation of these heats of

the acidity function procedure that all measurements are made in the same solvent

extended to HF-based superacids

1.4.6 Theoretical Calculations and Superacidity in the Gas PhaseThe knowledge of acidities or basicities independent from solvation effects is ofgeneral interest to chemists because it gives important information on the solventeffects It also allows the study of the intrinsic ability of the chemical structure of theacid or base to stabilize the anion/cations involved in the acid–base reaction and aquantitative structure–property relationship In the last two decades, with the devel-

gas phase These techniques operating under very different conditions of pressure andtime domain gave good agreement for the relative basicity measurements via protontransfer equilibria determination

proposes a quantitative intrinsic superacidity scale for sulfonic acids based onmeasurement of the proton transfer equilibrium between the superacid and its

order of the Brønsted superacids measured in the gas phase mirrors the acidity orderfound in solution However, the method will be difficult to apply for measuring thelarge acidity domain of these acids when combined as usual with strong Lewis acids

gas-phase acidities of a large number of very strong CH, OH, and SH Brønstedacids by using the pulsed FT ion cyclotron resonance (ICR) equilibrium constant

gas-phase acidity of compounds which make up the scale exceeds the acidity of such

traditionally strong mineral acids as HCl, HBr, HI, or H2SO4by more than 30 powers

gas-phase superacid

Subsequently, the development of both theoretical DFT methods and moresophisticated ab initio high-level MP2-type calculations has also spurred investiga-tions in the superacid field

orG2(MP2)level/andalsowiththeDFTmethod(B3LYP//6-311þG** level)tocalculate

the intrinsic acidities and gas-phase deprotonation enthalpies for 39 neutral strong orsuperstrong Brønsted acids, Brønsted–Lewis conjugate superacids, and even acidic

the neat acid showed a fairly linear correlation Similar DFT studies were carried out in

Mota and co-workers have investigated the nature of superacid electrophilic

intrinsic gas-phase acidities of a series of 21 Brønsted acids The computed results are

in excellent agreement with experimental gas-phase acidities in the range 342–

compared to the G3(MP2) results

1.4.7 Estimating the Strength of Lewis Acids

A quantitative method to determine the strength of Lewis acids and to establish similarscales as discussed in the case of Brønsted acids would be very useful However,establishing such a scale is extremely difficult and challenging Whereas the Brønstedacid–base interaction always involves proton transfer, which allows a meaningfulquantitative comparison, no such common relationship exists in the Lewis acid–baseinteraction The result is that the definition of strength has no real meaning with Lewisacids

The “strength” or “coordinating power” of different Lewis acids can vary widelyagainst different Lewis bases Thus, for example, in the case of boron trihalides, borontrifluoride coordinates best with fluorides, but not with chlorides, bromides, or