Hydrogen Bronsted acids

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. 

This study was extended by Satchell517, who investigated the effect of stannic chloride with various Bronsted acids on the rate of tritiation of toluene (excess) at 25 °C. Using tritium-hydrogen chloride the data in Table 153 were obtained, and whereas the first-order rate coefficients were closely proportional to the stannic chloride concentration, they were relatively independent of the total concentration of hydrogen chloride at a constant tritium chloride concentration, the small rate... 

From this work the relative efficiencies of Bronsted acids, in the presence of stannic chloride (0.166 M) in promoting hydrogen exchange at 25 °C can be ascertained from the rate coefficients (106Art) as follows HC1 (44) H20 (27) AcOH (1.6) CF3COOH (very small). Thus the ability of the dual acid systems to transfer protons is not simply related to the conventional acid strength of the BrQnsted component. 

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. 

Bronsted acid A proton donor (a source of hydrogen ions, H+). Examples HC1 CH COOH HCO - NH4 ... 

Mond process The purification of nickel by the formation and decomposition of nickel carbonyl, monomer A small molecule from which a polymer is formed. Examples CH2=CH2 for polyethylene NH2(CH2)6NH2 for nylon, monoprotic acid A Bronsted acid with one acidic hydrogen atom. Example CH COOI I. monosaccharide An individual unit from which carbohydrates are considered to be composed. Example C6H(206, glucose, multiple bond A double or triple bond between two atoms. 

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...

In some cases the effect of the nature of aromatic hydrogen substituent has been also observed (Table 48.2). These results contrast with those obtained using typical homogeneous Bronsted acid catalysts (e.g., sulphuric) for the acylation of the same substrate with acetic anhydride, under the same experimental conditions, where the yields (98%, 95%, 91% for R=N02, H, OMe, respectively) do not significantly depend on the nucleophile s substituent nature [23]. These data imderline the contribution of the heterogeneous catalyst. 

Figure 4. Correlation between infrared data and model reaction, (a) Basicity Hydrogen transfer activity versus surface area of 1230 cm 1 band of adsorbed C02 species after RT evacuation, (b) Acidity Dehydration activity versus quantity of Bronsted acid sites able to retain DMP at 150°C.

It is of interest to consider the form of the Bronsted plot or Eigen plot to be expected for reaction of a series of related intramolecularly hydrogen-bonded acids with hydroxide ion by the mechanisms in Schemes 5 and 6. The effect of a substituent on the value of the dissociation constant of an intramolecularly hydrogen-bonded acid (8 log K) will be two-fold. The stability of the undissociated acid will be modified because of a substituent effect on the... 

The first hydration step was promoted by Bronsted acids containing weakly or noncoordinating anions. In the second step, an intramolecular hydrogen transfer in the secondary alcohol was catalyzed by ruthenium(III) salts with chelating bipyridyl-type ligands. The possible complexation of the latter with the diene did not inhibit its catalytic activity in the allylic rearrangements, under acid-catalyzed hydration conditions. 

Bond Thompson mechanistic proposal, 46 bridges, 6 bromide, 185 brookite, 56, 68 Bmst method, 93 Bronsted acidic site, 77 Brpnsted acid sites, 144 buta-1,3-diene hydrogenation, 283... 

In contrast, for a proton transfer from a hydrogen-bonded acid occurring by the mechanism in (24), the predicted dependence of kf and on the strength of the catalysing base is quite different. In this case, when the pA bh of the base and the p -value of the hydrogen-bonded acid are closely matched, that is at Ap 0, it would be predicted that the proton in the transition state will be roughly half-transferred and Bronsted exponents a and p of around 0.5 should be observed. 

Concerning the nature of the acid sites on dehydrated alumina, the chemical evidence is more in favor of Lewis-type than of Bronsted-type acids. Trambouze and Perrin (343) estimated the content of Lewis acid sites by thermometric titration of a slurry in benzene with dioxane. The authors stated that Lew is acidity was not observed in boehmite and its dehydration products, only in the products obtained from hydrargillite. As mentioned earlier, Webb (339) found no indication of Bronsted acidity ammonia did not form ammonium salts. The quantity of ammonia chemisoi-bed per unit area in the range from 175° to 500° was not a function of the hydrogen content. After exten.sive dehydration, more ammonia was chemisorbed at 100 mm pressure, even at 500°, than corresponded to the hydrogen present (341b). 

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