Acid-base concepts aprotic acids

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. 

The preconditions of another approach to the treatment of acid-base concepts can be found in the classic solvosystem concept described above. Careful reading of this concept shows that the acid-base definition connects the terms acid and base only with the process of autodissociation of a molecular solvent or of one capable of ionization. Nevertheless, it is obvious that acid-base interactions can occur in those solvents, which are not able to form acid and base owing to a dissociation process. Aprotic solvents may serve as a typical example of solvents of such a kind another case of solvents incapable of the acid-base autodissociation takes place if we consider the Lux acid-base equilibria in molten oxygen-free media. Therefore, in relation to any given acid or base there exist two kinds of solvents of differing auto-dissociation ability with the formation of the said acid or base [36, 37, 44, 45]. 

The above classification is generally in agreement with Pearson s acid-base concept. Protic solvents are hard in nature, and they solvate small anions with strong hydrogen bonds, whereas dipolar aprotic solvents have a soft character, and they interact more strongly with the large, polarizable anions. 

In aprotic solvents, in which pKSn values are not definitive, it is difficult to estimate the activity of the lyate ion from the pH of the solution. It is different from the case in water, in which a(OI I ) can be estimated from the pH value. Although the pH scale in aprotic solvents has such a disadvantage, it is still useful to quantitatively understand the acid-base aspects of chemical phenomena. Wider use of the pH concept in non-aqueous solutions is desirable. 

Therefore, the generalized solvosystem concept proposed above is applicable to the description of protic and aprotic solvents, although just for this case this does not allow us to obtain basically new results. As for the case of ionic melts, it is more important, since the theoretical basis of acid-base interactions in this kind of solvents is not developed so extensively. Nevertheless, there are many results on oxoacidity obtained in ionic solvents of the first and the second kinds, which have been treated in a proper way. 

A simpler nonphosgene process for the manufacture of isocyanates consists of the reaction of amines with carbon dioxide in the presence of an aprotic organic solvent and a nitrogeneous base. The corresponding ammonium carbamate is treated with a dehydrating agent. This concept has been apphed to the synthesis of aromatic and aUphatic isocyanates. The process rehes on the facile formation of amine—carbon dioxide salts using acid haUdes such as phosphoryl chloride [10025-87-3] and thionyl chloride [7719-09-7] (30). 

Lowry and co workers141,222 studied the mutarotation of tetra-O-methyl-a-D-glucopyranose, and found that the rate of reaction is low in dry pyridine or in dry cresol, but high in a mixture of the two solvents or in either solvent when moist. Lowry and Smith57 concluded that the mutarotation requires an acid catalyst and a base catalyst, and that amphoteric solvents are complete catalysts for the process, whereas aprotic solvents are not. They also showed that molecules of undissociated acids, cations of weak bases, and anions of weak acids have catalytic properties. Much the same concept was developed independently by Bronsted and Guggenheim,189,223 and came to be known as generalized acid and base catalysis. It was found that the rate of mutarotation of a sugar in the presence of a mixture of several catalysts may be represented by an equation of the type ... 

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