Sulfuric acid freezing-point studies

Most carbocations are too reactive to be directly observable in ordinary solvents, and until relatively recently evidence has been obtained indirectly, primarily through the study of reaction kinetics and trapping processes, experiments discussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have long been known to produce observable concentrations of positive ions relatively easily. The triarylmethyl derivatives were the first of this type to be investigated the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,70 and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early observations by the freezing-point depression technique (see Section 3.2, p. 130) established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric acid, the reaction presumably being by way of Equation 5.14.71 Results in methane-sulfonic acid are similar.72... 

II. Unimolecular Acid-Catalyzed Reactions Involving Acyl-Oxygen Fission. At least one instance is known in which decomposition of the protonated complex does not seem to depend upon the attack of water or an alcohol molecule. It is the formation or hydrolysis of esters of 2,4,6-trimethylbenzoic acid in sulfuric acid solution. Since cryoscopic studies have shown that the acid gives a molar freezing point depression of four (p. 39), and the ester five (p. 225), we must conclude that decomposition of the protonated complex to the acyl carbonium ion 0... 

Let us now consider some of the evidence for this general mechanism. Such evidence has, of course, been gathered by study of specific reaction mechanisms. Only some of the most clear-cut cases are cited here. Additional evidence is mentioned when individual mechanisms are discussed in Section 9.4. A good example of studies that have focused on the identity and mode of generation of the electrophile is aromatic nitration. Primarily on the basis of kinetic studies, it has been shown that the active electrophile in nitration is often the nitronium ion, NO2+, which is formed by the reaction of nitric acid with concentrated sulfuric. Several other lines of evidence support the role of the nitronium ion. It can be detected spectroscopically and the freezing-point depression of the solution is consistent with the following equation ... 

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