Hydrates of strong acids

Crystal hydrates of strong acids and bases have attracted the attention of crystallographers and spectroscopists over several decades (46-48, 123-125). The interest in these crystal systems lies in their possible use as proton conductors (126). Moreover, as proton transfer events often occur in complexes such as H5O2+ and these... 

Cyclic hydrogen-bond patterns involving 4, 5, 6 and 7 bonds are commonly observed in the ices and the clathrate hydrates, described in Part IV. Similar cyclic systems are also observed in the hydrates of strong acids and salts which contain the so-called hydrated proton, for example, (H20)nH + in HBr -4 H20 [112, 113]. 

Mootz D, Oellers E-J, Wiebcke M (1987) First examples of type I clathrate hydrates of strong acids polyhydrates of hexafluorophosphoric, tetrafluoroboric, and perchloric arid. J Am Chem Soc 109 1200-1202... 

There is evidence for the existence, in crystalline hydrates of strong acids, not only of H30+ but also of H502 and more heavily hydrated ions. Some examples are given in Table 5-2. 

The number of water molecules that it is associated with in a particular solution is difficult to determine. That one molecule is enough for its formation, however, can be demonstrated by examining the hydrates of strong acids like hydrochloric, nitric, and sulfuric. The lowest hydrates of these are as follows ... 

The method of moments has further advantages. First, since the second moment is inversely proportional to the sixth power of the internuclear distance, it is a very sensitive means of determining interatomic distances. Second, it can provide insights into the structure. For example, it was used to demonstrate the presence of groups of three equivalent protons in solid hydrates of strong acids, thus proving the... 

Dilute aqueous solutions of strong acids (e.g. HCl or H2SO4) contain sufficient concentrations of hydrated protons to oxidize many metals, to produce their most stable states in solution. The only thermodynamic condition for metal oxidation is that the reduction potential of the metal ion produced should be negative. In general, for the metal ion M + undergoing reduction to the metal, if the standard reduction potential for the half-reaction ... 

Aqueous solutions of the +2 and +3 metal salts of strong acids are always slightly acid. This would probably seem reasonable to you if you reasoned from our general statement about the hydrolysis of salts on p 358, assuming that the metal ions come from weak bases (though they are actually insoluble ). A more satisfactory explanation is that hydrated aquo metal ions can act as weak acids. For example, hexaaquo iron(III) could dissociate to give... 

The hydration reaction of alkynes leading to carbonyl compounds is generally carried out in dilute acidic conditions with mercuric 1on salts (often the sulfate) as catalysts (ref. 5). Only very reactive alkynes (phenylacety-lene and derivatives) can be hydrated in strong acidic conditions (HgSO ) without mercury salts (ref. 6). Mercury exchanged or impregnated sulfonic resins have also been used in such reactions (ref. 7). Nevertheless, the loss of the catalyst during the reaction and environmental problems due to the use of mercury make this reaction method not as convenient as it should be for the preparation of carbonyl compounds. 

The equilibrium constant for ethene hydration is considerably greater than for methanal hydration, largely because the carbon-carbon double bond is weaker. Even so, methanal adds water rapidly and reversibly at room temperature without need for a catalyst. The corresponding addition of water to ethene occurs only in the presence of strongly acidic catalysts (Section 10-3E, Table 15-2). 

Exercise 18-9 Use bond energies and the stabilization energy of ethanoic acid (18 kcal mole-1, Section 18-2A) to calculate AH° for the addition of water to ethanoic acid to give 1,1,1-trihydroxyethane. Compare the value you obtain with a calculated AH° for the hydration of ethanal in the vapor phase. Would you expect the rate, the equilibrium constant, or both, for hydration of ethanoic acid in water solution to be increased in the presence of a strong acid such as sulfuric acid Explain. 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Concentrated sulfuric acid and/or concentrated phosphoric acid are often used as reagents for dehydration because these acids act both as acidic catalysts and as dehydrating agents. Hydration of these acids is strongly exothermic. The overall reaction (using sulfuric acid) is... 

Similarly, the nitride, carbide, cyanide, carboxylate, and carbonate salts of aluminum are unstable in aqueous solution. Aluminum salts of strong acids form solutions of the hydrated cation (see Hydrates). These solutions are acidic owing to the partial dissociation of one of the coordinated water molecules (equation 6), the p/fa of [A1(H20)6] + being 4.95 (see Acidity Constants). Note that this is quite similar to that of acetic acid. The second step in the hydrolysis reaction yields a dihydroxide species that undergoes condensation to form polynuclear cations (see Section 8). Antiperspirants often include an ingredient called aluminum chlorhydrate that is really a mixture of the chloride salts of the monohydroxide and dihydroxide aluminum cations. The aluminum in these compounds causes pores on the surface of the skin to contract leading to a reduction in perspiration. 

Although other clay minerals can also be acid leached and improved somewhat by this treatment, they do not match activity levels attained by calcium montmorillonite. It has been hypothesized that calcium montmorillonite is superior in this regard because it simultaneously develops a high concentration of strong acid sites, and extensive porosity in 50-200-A diameter pores after acid-activation (85). In any case, only those clays that achieve high hydrated silica values upon acid leaching are found to be good candidates for the acid-activation process. 

The addition of iodine azide to a number of substituted acetyl-lenes has been shown to take place in the opposite regio-chemical sense to their well known hydration in the presence of strong acid. Thus 1-phenylpropyne adds iodine azide to form cis- and /ranr-2-azido-l-iodo-l-phenylpropyne whereas the acid hydration gives propiophenone (equation 97). Addition of water or iodine azide to 2-bromo-l-phenylethyne on the other hand gives similarly oriented products (equation 98). 

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