Solution, sulfuric acid freezing points

The cryoscopic method was used in Hantzsch s original work 53, 54) and was later developed by Hammett and Deyrup 56) and by Gillespie et al. 48). Sulfuric acid freezes at 10.371 C and has a relatively large molal freezing-point depression or cryoscopic constant fc = 6.12 32). It is thus a convenient solvent for cryoscopic measurements provided that adequate precautions are taken to prevent absorption of water from the atmosphere. From freezing-point measurements the number, v, of moles of particles (ions and molecules) produced by one mole of any solute may be obtained. Some examples are given in Table IV. 

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

In a comparable solution of benzoic acid the freezing point is depressed only twice the predicted amount, indicating only a normal acid-base reaction. Further, a sulfuric acid solution of methyl mesitoate when poured into water gave mesitoic... 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

Triphenylcarbinol in sulfuric acid solution has a spectrum indicating the presence of the same carbonium ion responsible for the conductivity of triphenylmethyl chloride in liquid sulfur dioxide.171 In confirmation of this the freezing point depression is four times that of substances dissolving to give only one mole of particles per mole of dissolved substance.171-173... 

In some systems the acylium ion is formed reverisbly and to a detectable extent while in others, to be discussed in the next chapter, it is a hypothetical intermediate. The first evidence for a stable acylium ion was the fourfold depression of the freezing point of sulfuric acid by mesitoic acid.176 The presence of the acylium ion in the sulfuric acid solutions of carboxylic acids with fourfold depressed freezing points is confirmed chemically by the high yield of ester obtained on pouring the... 

In a comparable solution of benzoic acid the freezing point is depressed only twice the predicted amount, indicating only a normal acid-base reaction. Further, a sulfuric acid solution of methyl mesitoate when poured into water gave mesitoic acid, while a similar solution of methyl benzoate similarly treated did not.534 The AacI mechanism is also found when acetates of phenols or of primary alcohols are hydrolyzed in concentrated (more than 90%) H2SO4 (the mechanism under the more usual dilute acid conditions is the normal Aac2).535... 

Another method that has proved extremely useful in obtaining information about the nature of solutes in sulfuric acid solution is the measurement of freezing point depressions. The freezing point constant (k) for sulfuric acid is 6.12 kg °C mol-1. For idea) solutions, the depression of the freezing point is given by... 

For example, the observation that the freezing point of a 1 molal solution of acetone in sulfuric acid is depressed by twice the molal freezing-point depression constant of sulfuric acid is interpreted in terms of the reaction... 

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... 

Nitric acid alone fails to nitrate benzene and sulfuric acid also does not readily react with it, yet the mixed acid is an efficient nitrating reagent. Solutions of nitric acid in sulfuric acid show an approximately four-fold molar freezing-point depression and this has been attributed to the generation of four ions, as shown in equation (1) ... 

A good deal of confusion and much controversy between early workers 54, 55, 77) resulted from a lack of understanding of the nature and extent of the self-dissociation of sulfuric acid, which is repressed by most solutes and thus affects the freezing-point depressions that they produce. Following Hammett and Deyrup 56) it became common practice to carry out cryoscopic measurements in sulfuric acid containing sufficient water to depress the freezing point to approximately 10.0° in order to repress largely the solvent self-dissociation. This is not, however, an entirely satisfactory procedure. The self-dissociation is not completely repressed (J), and allowance for it should still in principle be made. Moreover water has been found, in the case of nonelectrolytes and weak electrolytes at least, to have an effect on the depression... 

The freezing-point depression constant of pure H2SO4 is 6.12 K kg mol . When 2.3 g of ethanol (C2H5OH) is dissolved in 1.00 kg of pure sulfuric acid, the freezing point of the solution is 0.92 K lower than the freezing point of pure sulfuric acid. Determine how many particles are formed as 1 molecule of ethanol goes into solution in sulfuric acid. 

There is good evidence for equations 7 and 10. Both triphenyl-carbinol and 2,4,6-trimethylbenzoic acid show a fourfold freezing-point depression in sulfuric acid solution.3 Again the most plausible explanation is that carbonium ions are formed. 

That the amide might be dehydrated by the sulfuric acid reaction mixture seems to be ruled out by the fact that benzamide shows a molar freezing point depression of two in sulfuric acid solution. Dehydration would cause a molar freezing point depression of four. Accordingly, it seems more likely58 that dehydration of the intermediate XC occurs before rearrangement since a series of steps similar to those outlined in equations 25 to 29 would lead to the nitrile directly ... 

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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