Superacidity measurements

BriunstedSuper Clds. In the 1960s a class of acids hundreds of millions times stronger than mineral acids was discovered acids stronger than 100% sulfuric acid are caHed superacids (211). The determination of acidity by pH measurement does not hold for very concentrated acid solution. 

There is one other substituent which is comparable with the diazonio group in the sense that it is cationic and that it has, in one of its mesomeric structures, a triple bond between the atom attached to the aromatic system and the second atom. It is the acylium group in 7.9. However, no substituent constants are known for this group, obviously because this cation is detectable in measurable concentrations only in superacidic media (see review by Olah et al., 1976). 

The HF-SbFs system is known to be a superacid H34). The possible relevance of this to the intercalation process was pointed out by Vogel V12), who first reported on the extremely high electrical conductivity of graphite-SbFj measured normal to the crystallographic c-axis. The measured conductivity was approximately 40 times that of pristine graphite, and 50% greater than that of pure copper. Other workers... 

The a-cyclopropylcyclopropylidenmethyl cation 28 is the only vinyl cation experimentally accessible50 in superacid solution which enjoys neither re-bond nor heteroatom stabilization of the positive charge. The 13C NMR spectrum measured at -138 °C is shown in Fig. 4. 

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

OXYGEN, OXIDES 0X0 ANIONS PHOTOOXYGENATION SUPEROXIDE DETECTION/MEASUREMENT SUPEROXIDE DISMUTASES SUPERACID... 

In Ref. 6, the Bronsted superacidity of HCl in liquid chloroaluminate IL ([C2Cilm]Cl/AlCl3) was studied by the protonahon of arens during which the degree of protonahon was measured by using absorption spectroscopy. The arens were stable in the liquid chloroaluminate for many hours and their protonated forms (arenium ions) were stable for 1 h or more. 

For a monograph, sec Olah Prakash Sommer Superacids Wiley New York, 1985. For a review, sec Gillespie Peel Adv. Phys. Org. Chem. 1971, 9, 1-24. For a review of solid superacids, see Arata Adv. fatal. 1990,37. 165-211. For a review of methods of measuring superacidity, see Jost Sommer Rev. Chem. Interned. 1988, 9, 171-199. 

Additional evidence for the SnI mechanism—in particular, for the intermediacy of carbocations—is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization measured in superacid solutions (p. 166).34... 

In electrophilic oxygenation with ozone in superacidic media or in dry ozonation over silica gel, protonated ozone, namely, +C>3H, was suggested to be the de facto electrophile. These reactions are fundamentally different from conventional ozonations with O3, which is a highly dipolar molecule. In 03H+ the dipole is removed by protonation. Cacace and Sporenza were able to directly identify +C>3H in the gas phase and also measure the proton affinity of ozone.107... 

Arnett and Hofelich measured heats of reaction of a variety of alcohols with SbF5/FS03H in sulfuryl chloride fluoride to form their respective carbocations at constant temperature (-40 °C). In this superacid medium there were no ion-pair complications126 and hence reliable calorimetric data were obtained for various cyclopropyl and phenyl substituted cations. The heats of reaction for the formation of tricyclopropylcarbinyl cation (-59.2 kcalmol ), trityl cation (-49.0 kcalmol1) and ferr-butyl cation (-35.5 kcalmol1) show that the relative order of the stabilization of the cationic center is cyclopropyl >... 

Table XIII shows the strengths measured by Hammett indicators with pK.d values ranging from —5.6 to — 14.5. As described above, dried H3PW 204o possesses superacidity (127). The order of the acid strengths agrees with that...

A novel method of measuring migratory aptitudes has been published by Shubin and co-workers.52 They studied the temperature at which the two methyls of 39 became equivalent in superacid solution in the nmr and found the following results for various substituents X H, 70°C CH3, — 100°C Cl, — 55°C F, — 70°C CF3, 0°C. 

Figure 1.3. Acidity ranges for the most common superacids. The solid and open bars are measured using indicators the broken bar is estimated by kinetic measurements numbers in parentheses indicate mol% Lewis acid.

The acidity strength of solid acids is still not well known and is difficult to measure. Claims of superacidity in solids are numerous and will be discussed later in Chapter 2. Among the reviews related to acidity characterization of solids, those of Corma, Farneth and Gorte, and Fripiat and Dumesic are quite significantly representative.30... 

Up to a // value of — 10, all indicators are primary amines and are therefore suitable for the measurement of the Hammett H() function. For stronger acids, new indicators such as nitro compounds have to be used. Although the acidity function scale based upon nitro compounds as indicators may not be a satisfactory extension of the aniline indicator scale, Gillespie and Peel18 have shown that the most basic nitro compound indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest indicator in the aniline series, 2,4,6-trinitroaniline. Thus, the acidity measurements using the nitro compounds may be considered to give the best semiquantitative picture of the acidity of the various superacid systems. 

Chemical Shift Measurements. Under slow-exchange conditions, the ionization ratio cannot be measured. In fact, one of the major advantages of the superacidic media is the ease with which weak bases can be fully protonated and directly observed by NMR. Because it is known that the protonation rates are practically diffusion-controlled ( 409 liter mol-1 s-1), under these conditions (< 10 2 s 1) the indicator is totally in the acidic form described by the NMR spectmm and no variable is available to measure the ionization ratio. 

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 the availability of a family of structurally similar compounds of varying basicity to cover a large domain of acidity. This condition has been met by Sommer and co-workers22,43 using the para-methoxybenzhydryl cations as useful indicators for the strongest superacids. 

Like all the other acidity functions, W0(H) equals pH in dilute aqueous solution. In strong acids, this function should be a logarithmic measure of the proton activity as long as the normal potential of the redox system, ferrocene-ferricinium, is constant. This was, however, not the case in very strong acid solutions because ferrocene underwent protonation. Other electrochemical pH indicators have been proposed, such as quinine-hydroquinone or semiquinone-hydroquinone, the basicity of which can be modified by substitution on the aromatic ring. These electrochemical indicators have been used with success by Tremillon and co-workers48 for acidity measurements in anhydrous HF and HF containing superacids. 

In principle, the A 0(H) function is of limited interest for kinetic applications because the indicators are chemically very different from the organic substrates generally used. On the other hand, as the measurements are based on pH determination, the length of the acidity scale is limited by the pA" value of the solvents. However, very interesting electrochemical acidity studies have been performed in HF by Tremillon and co-workers, such as the acidity measurement in anhydrous HF solvent and the determination of the relative strength of various Lewis acids in the same solvent. By studying the variation of the potential of alkane redox couples as a function of acidity, the authors provide a rational explanation of hydrocarbon behavior in the superacid media.48... 

Brouwer and van Doom51 used this approach in the early 1970s. In the NMR study 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 rearrangement in various superacid combinations of HF, HS03F, and SbF5. 

Mota and co-workers have investigated the nature of superacid electrophilic species in HF-SbF5 by density functional theory63 and measured the ability of the system to protonate light alkanes (methane, ethane, propane, and isobutane).64... 

Hydrogen Fluoride-Trifluoromethanesulfonic Acid. The acidity of this binary Brpnsted acid system has not been measured, but the superacidic properties are mentioned in numerous patents concerning fluorination, olefin alkylation, and hydrocarbon conversion. 

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