Superacidity acidity order

The polyaddition of alkylene oxides to hydroxyl groups is catalysed by the alkali hydroxides (KOH and NaOH) or low hindered tertiary amines or, to a much lesser extent, by acid catalysts [Lewis acids and Bronstedt superacids in order to generate short... 

The anions from conjugate Brpnsted-Lewis superacids represent the core lithium salts used for commercial lithium batteries (i.e., LiPFg and LiBp4). The acidity order determined from QC calculations is as follows HBF4 (287.7) [Pg.21]

In addition, a new physicochemical method to characterize the acid strength of acid and superacid solids also will be required in order to evaluate and compare acid strengths with those obtained with liquid acids. 

Extensive nmr studies of the protonation of trihydroxybenzenes and their methyl ethers (Olah and Mo, 1972) and mono- and dihydroxybenzenes and their methyl ethers (Olah and Mo, 1973) in four different superacid media have recently been published. In the order of decreasing acidity, the media used were ... 

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 low-temperature study in superacid media of mono-, di-, and tri-protonated thiourea has been earned out.302 The experimental results were confirmed by theoretical calculations. Monoprotonation occurs at sulfur and, whereas the mono- and di-protonated forms are thermodynamically stable, the triprotonated ion is only kinetically stable. The pyrolysis of A-acctylthiourea and A.A -diacetyIthiourea (335) are ultimolecular first-order eliminations.83 Acid-catalysed ethanolysis of N,N -di- and tri-substituted aryl- and alkylaryl-thioureas gives 0-ethyl AAaryl thiocarbamates and amines.303 The acid-catalysed hydrolysis of thiourea was first order in thiourea and acid.304... 

When the unsaturated acetal 26 was reacted with superacid at — 60°C and irradiated, a photostationary state is observed consisting of the two stereoisomers 26a and 26b (Scheme 4).21 Evaluation of the kinetics of stereomutation shows good first order kinetics, and the rate constant increases with acidity. Based on the kinetics results, it was proposed that protonation of the carboxonium group of 26b leads to the dication (27), and this facilitates isomerization through delocalization of the positive charge. 

We conclude that there is no evidence for WZ catalysts having superacidic properties or sites with the acidic character that would be necessary for initiation of catalysis by alkane protonation. In as much as WZ catalysts are some four orders of magnitude more active than zeolites for alkane isomerization,26 it is clear that there is no one-to-one correlation between acid strength of WZ and its catalytic activity. We therefore infer that although the acidity of WZ catalysts is important in alkane conversion catalysis, the reaction is most likely initiated by a reaction other than protonation of the alkane by the catalyst or a species formed from it. 

Some of these systems plus other equally acidic systems are described in this review. We may somewhat arbitrarily classify 100% H2SC>4 and other non-aqueous systems containing considerably more acidic species than the hydrated proton as superacid systems as their acidities are of a different order of magnitude from that encountered in the more familiar aqueous systems. 

In order to confirm the acidity results measured using the indicators shown in Table 17.1, we have investigated as many acid-catalyzed reactions as possible. The reactions are summarized in Table 17.4 [43, 48, 118, 119]. Among them, the skeletal isomerization of light paraffins, in particular butane and pentane, has been the most widely applied. The isomerization of butane at room temperature was a well known test reaction for superacidity at the beginning of this work [43, 48, 118]. The activity for many of the reactions tested correspond to the acidities as determined by use of the Hammett indicators. 

The addition of water causes the breakage of the coordination bonds to yield Bronsted acid sites strengthening Lewis acid sites, as shown in Scheme 17.4, for example. Many research groups report the simultaneous existence of Bronsted and Lewis acid sites or the reversible transformation between Bronsted and Lewis acidity upon hydration or dehydration [61, 106, 152]. Fraenkel suggests that in order to be an effective superacid, sulfated zirconia should contain a critical amount of moisture [155bj. Several workers propose that the strong acidity requires the presence of both Lewis and Br0nsted sites. 

In refining processes alkylation of isobutane with propene or butene is important in order to obtain all date which has a high octane number and a low vapour pressure. This process is not, however, directly relevant to the focus of attention of this paper and wifi therefore not be deah with in any detail It has been well reviewed recai% [36]. It is, however, worth noting that recent attempts to develop a zeolite as an ahemate to the currently used hydrofluoric or su huric acid do not appear to have been successful and it is now assumed that superacid catalysts are the most likely heterogeneous alternatives. For the petrochemical industry the alkylation of aromatics is an inportant route to the production of alkylaromatic such as ethylbenzene, xylenes, cume, alkylbenzoies, alky henol... 

In order to protonate a molecule the acidic OH band has to be heterolytically broken. Not withstanding its acidic character, the energy cost to deprotonate a zeolite side is of the order of 1250 kj/gat. This is one of the main reasons why energetically protonation reactions in zeolites are quite different from reactions in superacids. 

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