Acidity BrOnsted and

The NH4-Beta-300 (Zeolyst International, number denote Si02/Al203 molar ratio) was transformed to corresponding proton form using a step calcination procedure at 500 °C. H-Beta-300 was partially modified with Fe by repeated ion-exchange method (Fe(III)nitrate). The surface areas as well as acidities (Bronsted and Lewis acid sites) of Fe-Beta (iron content - 0.1 wt %) were determined by nitrogen adsorption and pyridine desorption at 250, 350 and 450 °C using FTIR spectroscopy [6]. 

There have been a number of studies of the reaction of diazoacetic ester in aprotic solvents, mainly with carboxylic acids (Bronsted and Bell, 1931 Hartman et al., 1946 and references cited). However, the information available hardly justifies conclusions about the mechanism. Addition of relatively basic phenols causes an acceleration in rate which can be interpreted in terms of nucleophilic catalysis of a rate-determining displacement of nitrogen, but the kinetic order in acid varies between one and two. Formally, a mixed order would result if proton loss from the diazonium ion was effected by carboxylate ions alone, while the less discriminating displacement of nitrogen involved competition between anions and unionized molecules. However, there are examples of high or mixed orders in other acid-catalysed reactions (Bronsted and Bell, 1931 Bell, 1941 1959) and in all probability large medium effects play a role. 

Besides triflic acid, other acids may also be considered initially. Table 2. shows performances obtained with other acids (Bronsted and Lewis as well as heterogeneous solid acids) in arbitrary and non-optimized conditions. 

An interesting case of a Lowry mechanism is the reaction of ethylene oxides with water, hydronium ion, chloride ion and hydrochloric acid. Bronsted and the Kilpatricks found that if the con-... 

Types of hydroxyl groups, acidity Bronsted and Lewis acid sites, adsorption behavior, and chemical reactivity... 

All types of acid (Bronsted and Lewis acids, complex acids, including Hustavson acids (HC1-2A1C13-3C6H5CH3) are anchored to a polymer matrix. Such... 

An acid was once defined simply as a substance which produces hydrogen ions, or protons. However, the simple proton, H , is never found under ordinary conditions, and this definition required amendment. Bronsted and, independently, Lowry, therefore redefined an acid as a susbstance able to donate protons to other molecules or ions, and a base as a substance capable of accepting such protons. If we consider hydrogen chloride, HCl, as an example, the HCl molecule is essentially covalent, and hydrogen chloride (gas or liquid) contains no protons. But anhydrous hydrogen chloride in benzene will react with anhydrous ammonia ... 

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. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

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. 

Both the Bronsted and Lewis acid sites on the catalyst generate carbenium ions. The Bronsted site donates a proton to an olefin molecule and the Lewis site removes electrons from a paraffin molecule. In commercial units, olefins come in with the feed or are produced through thermal cracking reactions. 

The equations can be readily expressed in a somewhat more general form when applied to a Bronsted-Lowry acid A and its conjugate base B ... 

Identify Bronsted and Lewis acids and bases in a chemical reaction (Self-Test 10.2). 

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. 

Fig. 8). The Os result was explained on the basis of an interaction between the Bronsted acid sites and Fe + species. The same type of Interaction is believed to indirectly expand the s- ave function and to decrease the s-electron density at the 57pe nucleus, thus giving an Increase in 6 at the same "Si02/Al203 ratio" of 17. 

We have shown that the Fe or Fe-Co/Zeollte systems with a vs H/T type results provide a unique probe for obtaining particle sizes (3) of the metal clusters, and can be extended for computing a particle-size distribution. In addition, electronic interactions between the Bronsted acid sites and the Fe species can be elucidated. 

The thermal decompositions are catalyzed by Bronsted and Lewis acids [68]. In general, when M is electron poor and Lewis acidic, the thermal decompositions occur efficiently and at low temperatures (typically between 100 and 200 °C, but sometimes at lower temperature). The addition of a catalytic amount of a Lewis or Bronsted acid (i.e., AICI3 or HCl) has been observed to accelerate the ehmination of isobutylene and the formation of three-dimensional network structures [64,124-126]. Pioneering studies on pyrolyses of various metal alkoxides by Bradley and others have also shown that alkene eliminations represent a primary decomposition pathway [104]. 

The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as a substance that gives up a proton differs little from that of Arrhenius. However, the same is not true of their definition of a base as a substance capable of accepting protons which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston Rychtman, 1982) ... 

The formation of telomers rests on Tjhallylic intermediates, and the ratio of formation of 1 1 vs. 1 2 telomers stems from the reaction of the amine on a C4-allylic complex vs. a Cg-allyhc complex, an excess of phosphine and the presence of an acidic (Bronsted or Lewis) co-catalyst favoring the qhcrotyl complex (Scheme 4-5) [178, 180, 189-196]. 

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. 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

If baking soda is added to hydrochloric acid, however, it does accept a hydrogen ion. So, according to Bronsted and Lowry s theories, baking soda is a base ... 

Scheme 10 Plausible catalytic mechanism for alkyne-carbonyl coupling as supported by the effect of chiral Bronsted acid catalyst and deuterium-labeling...

Table 1 Bronsted and Lewis acidity of fresh proton-form catalysts.

Identify each of the following terms (a) hydronium ion, (h) Bronsted theory, (c) proton (Bronsted sense), (d) acid (Bronsted sense), (e) base (Bronstcd sense), (/) conjugate, (g) strong, (h) acid dissociation constant, (/) ionization constant, (/) base dissociation constant, (k) autoionization, (/) pH, and f/w) K .. 

Bronsted and Guggenheim [J. /Am. Chem. Soc., 49 (2554), 1927] have studied the mutaro-tation of glucose as catalyzed by acids and bases. The reaction takes place slowly in pure water, is weakly catalyzed by hydrogen ions, and is strongly catalyzed by hydroxide ions. When strong acids and bases are employed as catalysts, the apparent first-order rate constants can be written as... 

The incorporation of Al in the framework of SBA-15 increased the acidity of the samples, depending on the amount of Al introduced. The acidic sites are heterogeneous and samples comprise both Bronsted and Lewis. These mesoporous solids demonstrated good adsorption capacities towards pollutant molecules. 

---