Aprotic acids

The basicity of transition metal complexes in reactions with aprotic acids. B. V. Lokshin, A. G. Ginzburg and E. B. Nazarova, Russ. Chem. Rev. (Engl. Transl.), 1980, 49,115-130 (136). 

Aluminosilicate glasses are used in certain AB cement formulations, and the acid-base balance in them is important. The Bronsted-Lowry theory cannot be applied to these aluminosilicate glasses it does not recognize silica as an acid, because silica is an aprotic acid. However, for most purposes the Bronsted-Lowry theory is a suitable conceptual framework although not of universal application in AB cement theory. 

The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

Correlation Between Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites and Selectivity of Conversion of Alkylbenzenes... 

The catalytic activity Z -5 -Jype. zeolitesmodified by polyvalent cations (Ca, Mg, x, In, Dy, So, Ga, A1, Be ), were investigated in reactions of toluene al lation by ethylene and transalkylation of ethylbenzene. The presence in these samples of aprotic acid centres of different strength and absence of prot-ic centres were established by IR spectroscopy technique of adsorbed CO. The strength of aprotic centres was characterized by the heat of CO adsorption and was shown to be a main factor determining the selectivity of catalytic action of the systems studied. 

Hence bromine in dichloromethane converted 2-methylthiazole into 5-bromo-2-methylthiazole in 48% yield (76JAP76/48655). In this reaction HBr was present as a base acceptor. In its absence the yield was halved, and with an equimolar proportion of aluminium chloride the yield fell to 10%. It is evident that although 2-methylthiazole can be brominated under mild conditions, its activity is sharply reduced by protic or aprotic acids capable of complexing with the annular nitrogen (86CHE663). 

A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BFj. 

Using this theory, a number of organic reactions can be considered as acid-base reactions, because they do not have to occur in solution. Lewis acids are known as aprotic acids, compounds that react with bases by accepting pairs of electrons, not by donating protons. 

Specific adsorbents with positive surface charges. Acidic hydroxyl groups (hydroxylated acid oxides such as silica), aprotic acid centers, or small radius cations (zeolites) on the surface. Adsorbents of this type will interact with molecules which have locally concentrated electron densities, that is. Group B and Group D molecules. 

Lewis Acid Sites. Many other mechanisms (66, 85) are best described in terms of the more general concept of aprotic, or Lewis acidity which is defined in terms of the capacity to donate or share pairs of electrons. Aprotic acid sites are commonly derived from the coordinatively unsaturated cations at crystal edges or adsorbed on crystal faces, from deydration of hydroxylated surfaces, and from deamination or deamination and dehydration of silica-aluminum catalysts or similarly treated clays having extensive tetrahedral substitution (130, 132). Formation of Lewis acid sites by deamination or deamination-dehydration is dependent on inversion of the basal oxygens of the aluminum-substituted tetrahedron away from the surface, in order to expose the aluminum (131). 

The foregoing discussion has been couched in terms of the electronic structure of minerals and its consequent effects on absorbate reactions. Redox, aprotic acid, and covalent site types can all be considered to operate by electron/hole transfers, whether this be singly or in pairs, uni- or equi-lateral, partial or complete. Expressed in these terms, it is clear that these canonical site types, and reactions produced by them, represent limiting cases. The boundary lines between them must grow very fuzzy if additional electron delocalization is provided by excitation of either reactant or catalyst. Because of the electronic energy structure of minerals and their... 

Charge transfer comprises also particle transfer, especially when a proton is present. Therefore, even protonic acidity itself can not be completely distinguished from the capacity to transport electrons and holes, i.e. oxidation/reduction, if for instance, hole migration to the surface can provoke proton dissociation. Protonic and aprotic acidities have long been known to be interlinked. In some, but not all, cases, Lewis acids can be converted to Bronsted ones by addition of water (132). 

The thermal and chemical stability of C—M bonds (M = Si, Ge, Sn, Pb), and therefore of all organic compounds of the silicon subgroup elements, decreases, both in homolytic and in heterolytic processes, when the atomic number of the M element increases. For example, the thermal stability of tetraalkyl derivatives R4M diminishes essentially when M is changed consecutively from Si to Pb151 l52. The ease of oxidation of R4M compounds and the ease of cleavage of C—M bonds by halogens, protic and aprotic acids, etc.,... 

Aprotic acid-catalyzed reactions of ketones with nitriles.1463... 

Spectroscopic and calorimetric investigations of the adsorption of acetonitrile [122] have shown that the sorbent Si,Al-MCM-41 has strong Lewis (aprotic) acidity and poorly defined Br0nsted (protic) acidity [17]. The presence of strong Lewis centers on Si,Al-MCM-41 was confirmed by flow microcalorimetry for the adsorption of 1-butanol from a solution of n-hexane [123, 124]. Depending on the aluminum content of the structure, the number of strong Lewis centers on the surface of this sample was found to vary in the range of 43 to 70% of the total number of active centers. 

The conjugate base of a protic acid is often sufficiently nucleophilic to prevent rearrangements in epoxide-opening reactions. As a consequence, most studies aimed at examining rearrangements have utilized aprotic Lewis acids, especially BF3. Many such studies have compared protic and aprotic acids ... 

Earlier, the attempts to effect reactions of hydrocarbons and silicon compounds with Si-Cl bonds in the presence of AICI3 and other aprotic acids (Friedel-Crafts reactions) were successful only at high temperatures and pressures. Thus, the reaction with benzene according to scheme ... 

Opinions differ whether aprotic acids can initiate the polymerization of cyclic siloxanes. According to more recent careful work, the source of protons is indispensable. Earlier it was claimed that A1C13 23), FeCl3 24 SnCl4 25,26), TiCl427) and BF3 27) can induce polymerization in the absence of added cocatalyst. We think that by the use of proton traps, as it was shown by Kennedy and Chou28) and Gandini et al. for other processes 29), this question could be decided. If the polymerization of cyclic siloxanes is induced by aprotic initiators it should not be inhibited by a proton trap. 

In binary liquid mixtures of the so-called aprotic acids (Lewis acids or L-acids) formed by halogenides of metals of III-IV groups of the periodic system, there are often cases when one of the components is an anion-donor (base) and the second component is an anion-acceptor (acid) ... 

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