Protonic Bronsted acidity

Proton -Bronsted acid Metal ion -Lewis acid Anion -Host -Electrophile -Hydrogen bond donor -Oxidant -Charge transfer donor -Ion -Radical -Solute -Adsorbent -Enzyme -Carrier (protein) -Receptor -Antibody -... 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

Protonic (Bronsted) acids initiate cationic polymerization by protonation of the olefin ... 

The question as to whether the acid properties of cracking catalysts are due to protons (Bronsted acids) or to electron-deficient atoms (Lewis acids) is somewhat more difficult to answer. Before considering this question, the origin of acid centers in silica-alumina compositions should be discussed. It is generally believed that acid centers, either the... 

A great number of theoretical works have been published on Diels-AIder reactions. Some of them deal with the interpretation of the increased reaction rate produced by Lewis acid catalysts (16-18). For the reaction reported in this paper, molecular orbital calculations were computed using the MNDO method (19) implemented in MOP AC (20) and performing full geometry optimization. The modelling of the catalytic activity was achieved by complexation of one reactant with either a proton (Bronsted acid) or with AICI3 (Lewis acid). The calculated reaction enthalpies showed that, for a Lewis acid catalyst, acrolein (-25.2 kcal/mol) is more complexed than dihydropyran (-8.2 kcal/mol). The same conclusion is obtained by comparison of protonation enthalpies of acrolein (-196.4 kcal/mol) and dihydropyran (-177.5 kcal/mol). 

In water, compounds having acidic protons (Bronsted acids) form hydronium HaO and higher hydrates, in which the proton is embedded in an unshared electron cloud on a water oxygen. Under... 

The most important idea in this chapter is the continuing expansion of the concept of Bronsted acidity and basicity to embrace the species known as Lewis acids and Lewis bases. The relatively narrow notion that acids and bases compete for a proton (Bronsted acids and bases) is extended to define all... 

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

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. 

Whereas the proton (H ) can be considered the ultimate Bronsted acid (having no electron), the helium dication (He ) is an even stronger, doubly electron-deficient eleetron aceeptor. In a theoretical, calculational study we found that the helionitronium trication (NOaHe" ) has a minimum structure isoelectronic and isostructural... 

We are told that a proton is transferred from HCI to NH3 Therefore HCI is the Bronsted acid and NH3 is the Bronsted base... 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites... 

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 review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

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. 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

A substance can act as an acid only if a base is present to accept its acidic protons. An acid does not simply release its acidic proton the proton is transferred to the base through direct contact. For example, HC1 is a Bronsted acid. In the gas... 

A Bronsted acid is a proton donor and a Bronsted base is a proton acceptor. 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

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. 

Self-Test 10.2A Identify (a) the Bronsted acids and bases in both reactants and products in the proton transfer equilibrium HN02(aq) + HP042 (aq) N02 (aq)... 

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 ... 

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

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. 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

Bronsted-acid-catalyzed Diels-Alder reactions are not frequent because of the proton sensitivity of many dienes and cycloadducts, especially when long reaction times and high temperatures are required. Examples in aqueous medium involving imines activated by protonation as dienophiles and a proton-promoted Diels-Alder reaction of glyoxylic acid with cyclopentadiene are considered in Section 6.1. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

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