Brdnsted acids protons

In this equation, r) the absolute hardness, is one-half the difference between /, the ionization potential, and A, the electron affinity. The softness, a, is the reciprocal of T]. Values of t) for some molecules and ions are given in Table 8.4. Note that the proton, which is involved in all Brdnsted acid-base reactions, is the hardest acid listed, with t — c (it has no ionization potential). The above equation cannot be applied to anions, because electron affinities cannot be measured for them. Instead, the assumption is made that t) for an anion X is the same as that for the radical Other methods are also needed to apply the treatment to polyatomic... 

Many of the boron hydrides have been shown to function as Brdnsted acids (See 34 for references). The bridging hydrogens are acidic. Pentaborane(9) is a monoprotic acid in the presence of a variety of bases (35,36,37). An example of such a reaction (38) is given in Scheme I, Reaction (4). By removing the bridging proton to give BsHs , a boron-boron bond is formed which is susceptible to insertion of electrophillic agents. 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

Inspired by the recent observation that imines are reduced with Hantzsch esters in the presence of achiral Lewis or Brpnsted acid catalysts (Itoh et al. 2004), we envisioned a catalytic cycle for the reductive amination of ketones which is initiated by protonation of the in situ generated ketimine 10 from a chiral Brdnsted acid catalyst (Scheme 13). The resulting iminium ion pair, which may be stabilized by hydrogen bonding, is chiral and its reaction with the Hantzsch dihydropyridine 11 could give an enantiomerically enriched amine 12 and pyridine 13. 

Acid strength determination.- Acid centres of both Brbnsted and Lewis types can exist on solid surfaces. The BreSnsted sites are groups able to lose protons, while Lewis centres are surface sites which can accept electron pairs from adsorbing molecules. Acidic properties of the centre are characterized by the value of acid strength. For the Brdnsted acid sites, acid strength is defined by acidity functions as for acid solutions. The most... 

The same holds for protons, the origin of the outstanding Brdnsted acidity in zeolites [69],... 

Aluminium ions dislodged by mild hydrothermal treatment could modify the acidity. This could lead to "superacidity". Recent proton NMR studies, however, show no enhanced Brdnsted acidity. On the other hand, IR shows new hydroxyl bands (which may be poisoned by Na) as clear evidence for very strong Brdnsted acidity. Whatever the interpretation, we know that mild hydrothermal treatment can increase acidity in several types of zeolites. 

Brdnsted acids donate protons, and Br0nsted bases accept protons. These are the definitions that normally underlie the use of the terms acid and base. ... 

Notably, optimization studies exposed the critical influence of phenol on the reactivity and enantioselectivity within this manifold, suggesting a two-step pathway as illustrated in Figure 9. Initially, enantioselective protonation takes place from the chiral Brdnsted acid 57 or oxonium ion pair 60, generated by rapid proton transfer between 57 and phenol, to silyl enol ether 61 to form chiral ion pair 62. This is followed by desilylation with phenol to form the corresponding ketone 63, silylated phenol, and catalyst 57 for further turnover. 

Electrically conductive PTs are prepared by polymerization in an electrolyte in the presence of polymeric compounds with sulfonic acid groups [692]. The electrochemical polymerization of thiophene and Nafion gives a self-supporting film showing a uniform distribution of sulfur across the film and a reversible voltammogram [693]. Alkoxy substituted thiophene derivatives are electro-chemically polymerized in a solvent in the presence of a proton source in the form of a Bronsted acid [694]. The electrochemical preparation of poly-(3-alkyloxythiophene) without a Brdnsted acid has also been studied [695]. 

The Brdnsted definition of acidity implies that the acidity of the hydroxyl group is proportional to the strength of the O-H bond which through the force constant is related to the stretching frequency, cafO-H). Hence, by measuring this stretching frequency of the acidic proton in various zeolites it is possible to... 

Sample Solution We are toid that a proton is transferred from HCi to NH3. Therefore, HCI is the Brdnsted acid and NHs is the Br0nsted base. 

In Chapter 4 we learned that a Brdnsted acid is a substance that can donate a proton and a Brpnsted base is a substance that can accept a proton [M Section 4.3]. In this chapter we extend our discussion of Brpnsted acid-base theory to include conjugate acids and conjugate bases. 

When a Brdnsted acid donates a proton, the anion that remains is a conjugate base. 

Brdnsted acid. A substance that donates a proton (H+), (4.3. 16.1)... 

Xhe Brdnsted acid strengths decrease from top to bottom. P.A Proton affinity (kJ/mol) ... 

In cationic polymerization typical catalysts are represented by strong electron acceptors including Lewis acids, Friedel-Crafts halides, Brdnsted acids and stable carbenium-ion salts. However, many of them are not sufficient to initiate the polymerization and, therefore, require small amounts of a co-catalyst, usually a proton donor such as alcohols. 

Both Lewis and Brdnsted acidity are involved in the dehydration reactions over acid catalysts, and selectivity control to limit the dehydration of DME to olefins and aromatics requires that the surface acidity not be too hi and the reaction temperature be below 300°C [65]. The olefins are generally thou t to be produced by a consecutive reaction in which methanol is first converted to DME, which in turn is converted to olefins and aromatics. Reaction mechanisms for DME formation have been proposed by various investi tors. According to Kubelkova et aL [78], the mechanism over Si-Al zeolites involves protonation of the hydroxyl group of methanol on a Bronsted acid site to form a skeletal methoxyL This methoxyl group reacts with a -phase methanol molecule to form DME at 180300°C and C2C5 aliphatics and aromatics above 300°C. According to these authors, Lewis acid sites (Al -OH), associated with nonskeletal alumina, can also form methoxyls according to the reaction... 

Zeolites become solid Brdnsted acids if protons play the role of charge-compensating cations. Proton forms of zeolites are exceedingly important as industrial catalysts, e.g., for hydrocarbon conversion in the petroleum industry. The most active catalysts include relatively few H/Al sites per Si, typically the Si/Al ratio, x — y)/y, is 10 or larger. The proton forms of zeolites are obtained from the ammonium form by heating ... 

The proton is a Lewis acid because it can accept an electron pair into its empty 1 s atomic orbital. It follows that all Bronsted bases are Lewis bases. All Brdnsted acids are also Lewis acids because they are hydrogen-bond donors, that is, electron acceptors (see below). 

According to the Lowry-Brdnsted theory, a Brdnsted acid is a proton donor, while a Brdnsted base is a proton acceptor. In Lewis concept, acid acts as electron-pair acceptor, while base is electron donor (such as molecules possessing electton lone pairs). Hence, a Lewis base is in practice equivalent to a Brpnsted base. However, the concepts of acidity are markedly different [27]. 

In the case of solid catalysts, any atomic (ionic) group at the surface that can donate a proton is a Brdnsted acid while any place where one empty electron orbital exists is Lewis acid. For example, in the case of zeolites, Brdnsted acid site is a part of microporous aluminosilicate framework—a bridging [= Si (OH) A1 =] configuration which is able to donate a proton to an acceptor while Lewis acid site is either tri-coordinated A1 atom or charge-balancing cation Me " " which are able to accept the electron pair. Accordingly to the same theories, any place at the solid surface which can accept proton is a Brdnsted base while any place which can donate electron(s) is a Lewis basic site. For example, in the case of MeOjt (metallic oxides), the oxygen ions (0 ) behave as Brpnsted bases (because they are proton acceptors) while cations at the surface possess Lewis acidity (they are electron acceptors) [27, 28],... 

The protonic acid needed to initiate the chain reaction is usually produced by UV irradiation of diaryl-iodonium or triarylsulfonium salts. The photolysis reaction, which produces both Brdnsted acid and free radicals in the presence of a hydrogen donor molecule, can be formally written as follows for the iodonium salt ... 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

Table 3.4 Bjerrum s a parameter as calculated from c and x and corresponding properties of the proton. The ligands most often given away by both green plant roots and fungal mycelia produce a 0, making uptake of metal ions less sensitive towards pH changes even though certain of these ligands are fairly readily protonated, i.e. are considerable Brdnsted bases. This does not hold for those amino acids which are employed by grasses, mosses mainly, pointing to decreased selectivity...

The Brdnsted-Lowry definition describes acidity in terms of protons positively charged hydrogen ions,... 

However, for liquid phase reactions, it is very difficult to correlate the results wiHi BrOnsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboiQ lic acids (14), methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fhictose (15), and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Brdnsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by BrSnsted species, the maximum observed at Si/Al=15 for HY zeolites being a good balance between the niunber 6ind the strength of the protonic species. 

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