Bronsted acid water

Oxygen liabilities correlate with (1) structural positions in the complex, (2) bond lengths, and (3) Bronsted acidities. Waters trans to a p-hydroxo or p-oxo site are commonly more reactive and acidic than waters cis to the bridges. Rates of exchange of the water molecules cis or trans to the p-oxo bridges in Mo304(OH2)9 differ by a factor of 10 [17]. 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

It is obvious that the reaction is accelerated markedly by water. However, for the first time, the Diels-Alder reaction is not fastest in water, but in 2,2,2-trifiuoroethanol (TFE). This might well be a result of the high Bronsted acidity of this solvent. Indirect evidence comes from the pH-dependence of the rate of reaction in water (Figure 2.1). Protonation of the pyridyl nitrogen obviously accelerates the reaction. 

Table 2.9 shows the endo-exo selectivities for the Diels-Alder reaction between 2,4c and 2,5 catalysed by Bronsted-acid and four different metal ions in water. 

The catalyst acid sites are both Bronsted and Lewis type. The catalyst can have either strong or weak Bronsted sites or, strong i)i weak Lewis sites. A Bronsted-type acid is a substance capable of donating a proton. Hydrochloric and sulfuric acids are typical Bronsted acids. A Lewis-type acid is a substance that accepts a pair of electrons. Lewis acids may not have hydrogen in them but they are still acids. Aluminum chloride is the classic example of a Lewis acid. Dissolved in water, it will react with hydroxyl, causing a drop in solution pH. 

Ammonia is a base because, as we see from Eq. 1, it accepts protons from water and forms NH4+ ions. Notice that because water donates a hydrogen ion it is acting as a Bronsted acid. 

Self-Test J.lA Which of the following compounds are Bronsted acids or bases in water (a) HN03 (b) QH (c) KOH (d) C H5COOH. 

Many nonmetal oxides are Lewis acids that react with water to give Bronsted acids. An example is the reaction of C02 with water ... 

In this reaction, the C atom of C02, the Lewis acid, accepts an electron pair from the O atom of a water molecule, the Lewis base, and a proton migrates from an H20 oxygen atom to a C02 oxygen atom. The product, an H2C03 molecule, is a Bronsted acid. 

An acidic oxide is an oxide that reacts with water to form a Bronsted acid an example is C02, as we saw in Section 10.2. Acidic oxides are molecular... 

Because water is amphiprotic—because it is both a Bronsted acid and a Bronsted base—proton transfer between water molecules occurs even in pure water, with one molecule acting as a proton donor and a neighboring molecule acting as a base ... 

FIGURE 10.19 In water, Al3+ cations are hydrated by water molecules that can act as Bronsted acids. Although, for clarity, only four water molecules are shown here, a metal cation typically has six H20 molecules attached to it. 

Write the balanced chemical equation for (a) the thermal decomposition of potassium chlorate without a catalyst (b) the reaction of bromine with water (c) the reaction between sodium chloride and concentrated sulfuric acid, (d) Identify each reaction as a Bronsted acid—base, Lewis acid—base, or redox reaction. 

Scandium, Sc, which was first isolated in 1937, is a reactive metal it reacts with water about as vigorously as calcium does. It has few uses and is not thought to be essential to life. The small, highly charged Sc3+ ion is strongly hydrated in water (like Al3+), and the resulting Sc(H2())6]3+ complex is about as strong a Bronsted acid as acetic acid. 

B Boric acid acts as a Lewis acid. The boron atom in B(OH)3 has an incomplete octet and forms a bond by accepting a lone pair of electrons from a water molecule, which is acting as a Lewis base. The complex formed is a weak Bronsted acid in which an acidic proton can be lost from the H20 molecule in the complex. 

The modest endo/exo ratio observed when the reaction was carried out in basic chloroaluminate ionic liquids is ascribable to the polarity of the medium, while the high diastereoselectivity found in the acidic mixture is due to the increase of Lewis/Bronsted acidity of the medium. The rates of the reactions performed in basic and acidic chloroaluminates ([EMIMJCl AlCh, [BPJCl AlCh) are seven times slower and ten times faster, respectively, than those observed when the reactions were carried out in water [57]. 

Thin self-supporting clay films (appropriate for IR measurement) readily take up organic amines such as cyclohexylamine with displacement of the major fraction of the intercalated water. For the Ua -exchanged sample the majority of the amine is present in the unprotonated form - there being insufficient Bronsted acidity generated by the interlayer cation. When Al + is the exchangeable cation, however, a major fraction of the intercalated amine becomes protonated (see Figure 2). 

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

At an industrial scale, the esterification catalyst must fulfill several conditions that may not seem so important at lab-scale. This must be very active and selective as by-products are likely to render the process uneconomical, water-tolerant and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. In a previous study we investigated metal oxides with strong Bronsted acid sites and high thermal stability. Based on the literature reviews and our previous experimental screening, we focus here on application of metal oxide catalysts based on Zr, Ti, and Sn. 

Instead of Bronsted acids, lanthanide triflates can be used to catalyze the reaction of indole with benzaldehyde (Eq. 7.7). The use of an ethanol/water system was found to be the best in terms of both yield and product isolation. The use of organic solvent such as chloroform resulted in oxidized byproducts.17... 

Montmorillonite K10 was also used for aldol the reaction in water.280 Hydrates of aldehydes such as glyoxylic acid can be used directly. Thermal treatment of K10 increased the catalytic activity. The catalytic activity is attributed to the structural features of K10 and its inherent Bronsted acidity. The aldol reactions of more reactive ketene silyl acetals with reactive aldehydes proceed smoothly in water to afford the corresponding aldol products in good yields (Eq. 8.104).281... 

It has also been shown that dimethylsilyl enolates can be activated by diisopropylamine and water and exhibit a high reactivity toward iV-tosyl imines to give Mannich-type reaction products in the absence of a Fewis acid or a Bronsted acid.51 For example, the reaction of [(1-cyclohexen-l-yl)oxy]dimethylsilane with 4-methyl-A -(phenylmethylene)benzene sulfonamide gave re/-4-methyl-N- (f )-[(15)-(2-oxocyclohexyl)phenyl-methyl] benzenesulfonamide (anti-isomer) in 91% yield stereoselectively (99 1 anti syn) (Eq. 11.30). On the other hand, Fi and co-workers reported a ruthenium-catalyzed tandem olefin migration/aldol and Mannich-type reactions by reacting allyl alcohol and imine in protic solvents.52... 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Bronsted-acid-assistcd chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phcnylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5A(trifluoromethy I) - be nzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,(3-enals and various dienes.62 The interesting aspect is the role of water, THF, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

In the case of the rhenium aqua-ion [Re(OH2)3(CO)3]+ (33b) the question has been posed whether complex-anion can be considered to be a Bronsted acid. Titrations with hydroxide in water yielded a pKa value of 7.55 which is exceptionally low for a +1 cation. After the deprotonation of one coordinated water molecule, polymer formation over (/r-OH) bridges was initiated and the two compounds [Re3(/T3-OH)(/T-OH)3(CO)9r (35) and [Re2(/i-OH)3(CO)6] were (36) isolated and structurally characterized (Scheme 6). 

The formation of heavy carbonaceous compounds in 5A calcium exchanged zeolites depends on the calcium content. These zeolites are able to protonated ammonia molecules in ammonium ions. This Bronsted acidity results from the presence of CaOH+ species which are formed by water dissociation on Ca2+ ions and have an IR signature at 3515 cm"1. 

During our investigations of the reactions mediated by LASCs, we have found that addition of a small amount of a Bronsted acid dramatically increased the rate of the aldol reaction (Eq. 5).[191 This cooperative effect of a LASC and an added Bronsted acid was also observed in the allylation ofbenzalde-hyde with tetraallyltin in water.1201 Although, from a mechanistic point of view, little is known about the real catalytic function of scandium and proton, this cooperative effect of a Lewis acid and a Bronsted acid provides a new methodology for efficient catalytic systems in synthetic chemistry. 

K. Manabe, Y. Mori, S. Nagayama, K Odashima, S. Kobayashi, Synthetic Readions Using Organometal-lics in Water. Aldol and Allylation Reactions Catalyzed by Lewis Acid-Surfadant-Combined Catalysts/ Bronsted Acids Systems Inorg Chim. Acta in press. 

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