Aluminum cation acidity

You may be surprised to learn that many metal cations act as weak acids in water solution. A 0.10 M solution of A12(S04)3 has a pH close to 3 you can change the color of hydrangeas from red to blue by adding aluminum salts to soil At first glance it is not at all obvious how a cation such as Al3+ can make a water solution addic. However, the aluminum cation in water solution is really a hydrated species, A1(H20)63+, in which six water molecules are bonded to the central Al3+ ion. This spedes can transfer a proton to a solvent water molecule to form an H30+ ion ... 

Next, the mechanism of the Type II reactions is discussed. To discriminate one of the enantiofaces of the acceptor it is desirable to place and to activate the electrophiles in a chiral environment. At the same time, effective activation of the Michael donor is required. In Shibasaki s ALB-catalyzed reaction (Scheme 3), it was proposed that the aluminum cation functioned as a Lewis acid to activate enones at the center of the catalyst, and that the Li-naphthoxide moiety deproton-ated the a-hydrogen of malonate to form the Li enolate (Scheme 9). Such simultaneous activation of both reactants at precisely defined positions became feasible by using multifunctional heterobimetallic complexes the mechanism is reminiscent of that which is operative in the active sites of enzymes. The observed absolute stereochemistry can be understood in terms of the proposed transition state model 19. Importantly, addition of a catalytic amount of KOt-Bu (0.9equiv. to ALB) was effective in acceleration of the reaction rate with no deterioration of the... 

The synthesis ofhaloaluminate-based ionic liquids from halide salts and aluminum Lewis acids (most commonly AIX3 X=C1, Br) can generally be split into two steps (i) fomation of the desired cation by the reaction of a trialkylamine, trialkylphosphine or dialkylsulfide with a haloalkane, and (ii) formation of the haloaluminate anion by addition of an appropriate aluminum halide to this salt (Scheme 2.1). 

Acidic compounds, 355, 358 Acidity, 154-164 See Aluminum cation Exchangeable, 160, 162 Nonexchangeable, 160 OH groups, 135, 169 Organic matter, 131 Acids, 23-42... 

Adsorption isotherms, 178-190 Freundlich, 179 Langmuir, 183 S-type, 178-179 L-type, 178-179 C-type, 178-179 H-type, 178-179 Aerobic decomposition, 323 Alkalinity, 82—91 Definition, 88 Types of alkalinity, 82 Aluminosilicate clays, 102 Aluminum cation, 103, 160 Acidity, 160 Complexation, 160 Polymeric aluminum, 160 Exchangeable, 160,162 Hydrolysis, 69, 75 Solubility, 71 Soluble complexes, 69 Aluminum hydroxide, 78-80 Solubility, 78 pH effect, 79... 

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. 

For example, five types of hydroxyl groups are predicted to occur on the (100), (111), and (110) surfaces of aluminas, as depicted in Figure 1. The relative concentration and distribution of these groups depends on the exposed faces. Dehydroxylation of the surface, according to equation (1), yields surface oxo groups and exposes aluminum cations. The surface hydroxyl groups have Bronsted acidity, while exposed metal cations behave as Tewis acids. Alumina surfaces with Tewis acidity can be prepared from aluminum alkoxides under... 

ACF is very effective catalyst for other transformations via fluorinated carboca-tions, e.g. alkenylations [12], rearrangements [13], and isomerizations [14]. The advantage of ACF in this type of chemistry could partially come from the exceedingly high affinity of aluminum for fluorine, which enables the facile generation of a carbo-cation by C-F bond cleavage in organofluorine compounds. The authors focus on this aspect of aluminum Lewis acids later in the chapter. 

Furda (28) illustrates this postulation in a simplified model (Figure 5) where several hypothetical situations are shown. In the first case Fe2+ nearly neutralizes the fiber leaving a net positive charge of +3, in the second and third cases, Fe3+, the trivalent ion produces a net positive charge of +6 and +15 respectively. He explains the greater affinity of fatty acid to the ferric-pectin complex rather than to aluminum-pectin complex by the possibility of different size cations or by the hydrophilicity of the aluminum cation, but cautions that these explanations require further research. 

R Al—X C Work as Strong Lewis Acids Movement of Al-C group Source of Aluminum Cation ... 

A similar aluminum cation was also available in the Mukaiyama-aldol reaction. It is worth noting that the t-butyldimethylsilyloxy (TBSO) group, which otherwise is unable to make chelation complex with neutral bidentate Lewis acids, is under chelation control with excess Me2AlCl or MeAlfJh. [12]. Aldehyde and ketone carbonyls are capable of participating in the chelation-controlled aldol reaction to give anti-6 with high diastereoselectivity (Scheme 6.4). 

As already mentioned, neutral aluminum has strong Lewis acidity and high oxo-philicity. It has long been a concern of chemists whether cationic aluminum species would have unprecedented reactivity and even more Lewis acidity. Several aluminum cations were isolated and characterized by X-ray single-crystal analysis, as described in Section 6.1.2.2 most were, however, hexacoordinated aluminum that catalyzed bimolecular reactions. They were also useful in the polymerization of a-olefins, the subject of this section. 

The acidity constants of electrically neutral acids (HA) and of acid anions (HA, HA , etc.) diminish in going from water to alcohol, whereas cation acids (ammonium ion, quinine ion, aluminum ion) show an increased acidity constant. These qualitative conclusions have been substantiated by experiment. 

Figure 3. Unmodified asphalt shows a slight increase in shear strength with time but the rate of thickening is much more pronounced when maleic anhydride is present This reaction is probably cationic (acid catalyzed) since a trace of aluminum chloride increases the rate.

Figure 7. Size of aggregates as a function of reaction time for silicate solutions containing different concentrations of aluminum cations +, K only, Ah Si = 0.0 , Al Si = 0.01 A, Ah Si = 0.04 and , Ah Si = 0.10. The pH was 4.0, and the temperature was 25 0 C. The total concentration of silica was 5 wt %. Silica gels were prepared at pH 4.0 at room temperature (25 °C) by adding the silicate solution under vigorous stirring to a solution of hydrochloric acid (2 N) containing different concentrations ofAU(S04)3.

Aluminum carbonate is more soluble in acidic solution, forming aluminum cations. Write a reaction (or series of reactions) that explains this observation. 

Klyueva et al. have investigated the acidic properties of erionite modified by isomorphous substitution of B , Ga , and Fe " " by Si and Al . The incorporation of these elements in the aluminosilicate framework led to the generation of new acid centers. These acid centers have a lower concentration of aluminum cations than aluminosilicates, leading to s unples with lower acidity. Consequently, the rate of reactions involving hydrogen transfer, like olefin conversion into paraffins, was lower on isomorphous-substituted erionite samples. Table 5 shows that this enhanced the selectivity toward light olefins. The production of aromatics may... 

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