Bronsted-type relation

In our earlier work [85] the literature data were analyzed under the assumption that the influence of solvents on the standard rate constant of the electrode reaction may be expressed by the Bronsted-type relation... 

Since acid-base catalysis involves proton-transfer between the catalyst and the substrate, Bronsted-type relations should also apply to the variation of reaction velocity with the acid-base strength of the substrate, and the same statistical corrections should apply, as may be seen from the following general considerations. 

The second hypothesis of Temkin. The second hypothesis of Temkin theory is that the relation between rate constants ki and equilibrium constants Ki = ki/k-i obeys a Bronsted-type relation ... 

To relate the rate constants in steps 8.1 and 8.2 to the affinity, which is the property characterizing the sites, Temkin next hypothesized that a Bronsted-type relation existed between the rate constant, ki, and the equilibrium constant, Ki = ki/k i, for each elementary step, i.e.. 

The close similarity of CTC of naphthalene adsorbed on the decationated zeolite, porous silica and alumina allows us to make an assumption that the acid sites of Bronsted and Lewis type related to the coordinatively unsaturated silicon and aluminium ions are the real electron-accepting sites. 

The Sn6P6 cages 19c and 19d are accessible by two different Bronsted acid-base reaction pathways Reaction of lc and Id, respectively, with two different stannanediyl derivatives furnished in 80-89% yield red-black crystals of the aggregates (Eq. 12) (39). The tin(II) phosphandiides are somewhat related to the previously described oligomeric bis (phosphaneyl) stannanediyls of the type PkSn, which easily form intermolecular aggregates (50, 51) or remain monomeric, if the phosphorus atoms bear very crowded organosilyl substituents (52). 

Streitwieser and co-workers have extended their measurements of equilibrium acidities in cyclohexylamine to determination of exchange rates.69 They have made quantitative correlations between exchange rate and the pKa s determined by equilibrium methods for various aromatic compounds and have thus been able to verify that the Bronsted relation holds for these substances and to find Bronsted coefficients a for various types of compounds. A third method for evaluating p/Ca of weak acids, which has been used by Applequist70 and by Dessy,71 involves the study of exchange reactions of organometallic compounds (Equations 3.54 and 3.55). 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

In addition to the Bronsted acidity in zeolites, in these materials the Lewis acidity is present as well. According to Lewis, an acid is an electron pair acceptor, a definition which is broader than that given by Bronsted, since a proton is a particular case of an electron pair acceptor. Then, the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only a particular type of process [128], The Lewis acidity is related to the existence of an extra-framework A1 (EFAL) species formed during the zeolite dealumination process [128],... 

Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the composition of the framework by increasing the silicon content increases the thermal stability of the samples. The catalytically active centres in zeolites are the acidic (Bronsted) hydroxyl groups associated with tetrahedrally coordinated framework aluminium atoms. Catalytic activity is thus strongly dependent on the concentration and location of aluminium in the framework. 

Since 1924, the Bronsted relation has been applied to many general acid and base catalysed reactions, such as those discussed in Sect. 2.2, as well as to proton transfer equilibria like (43)—(45). Over limited ranges of acid strength and for variation within a similar catalyst type, G and a or j3 are constant and the relation holds well. Different catalyst types in a reaction often do not fit on a single Bronsted plot, but give different Bronsted lines. This was observed for the decomposition of nitramide [68]. It has also been observed in proton transfer from l,4-dicyano-2-butene(51)... 

What are the molecular pathways for metal or ligand adsorption (4) How can we infer molecular information from dissolution experiments (5) Do the macroscopic properties of a surface in water, such as Bronsted acidity, change with size of the molecule (6) How do the microscopic properties of a molecule, such as the rates of bridge dissociation, relate to calculable bond properties, such as electronic charge densities (7) Over what time scales are different types of bond dissociations complete (8) Can geochemists predict rates of multi-step reactions ... 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

Three types of vanadium-containing species are present at the surface of the vanadia on titania catalysts. Monomeric vanadyl, polymeric vanadates, and crystalline vanadia depending on the vanadia loading (see Fig. 5.19). Moreover, Bronsted acid sites and Lewis sites are present at the surface of vanadia on titania catalysts. All species are needed to explain the mechanism of the SCR reaction over this type of catalyst. The active sites are related to previously mentioned vanadia/vanadium species. A variety of active sites were proposed such as two adjacent V =0 groups or co-ordinated vanadyl centres [81,84-86] V =0 and... 

In this chapter, we focus on recent achievements in the enantioselective synthesis of chiral amines using 1,1 bi 2 naphthol (BINOL) derived monophosphoric acid (1) or related phosphoric acids as chiral Bronsted acid catalysts 2, 3], The contents are arranged according to the type of bond forming reaction, including carbon carbon, carbon hydrogen, and carbon heteroatom bond forming reactions, followed by specific reaction types. 

Although the Bronsted relation was first discussed with respect to proton transfer reactions, it has been found to apply to other types of reactions including electron and atom transfer reactions. Equation (7.11.1) implies that there is also a linear relationship between the Gibbs activation energy for the reaction and the standard Gibbs energy change for the associated equilibrium. Thus, one may also write... 

Figure 2 shows for HYUS-5 that there are several types of differently coordinated aluminium. Indeed, there are bands at 60, 30 and 1.4 ppm, which are related to tetrahedral, pentahedral (5) or tetrahedrally distorted aluminium (6), and different types of octahedrally coordinated aluminium, respectively. The tetrahedr aluminium appearing at 60 ppm has always being attributed to FAL (7). However, the intensity of the 60 ppm lime (34%) becomes difficult to reconcile with the value of 140 for the zeolite framework Si/Al ratio obtained from the unit cell size (24.24 A). Indeed, from the X-ray unit cell contraction one should expect 4% of the total aluminium to be in framework tetrahedral position. Then we conclude that in steam dealuminated zeolites there is some aluminium which is not in framework positions but it is also tetrahedrally coordinated (EFAL ). The same is observe in the HYD samples. We have recently claimed (8) that the EFAL could be present as a special type of non-crystalline silica-alumina formed during dealuminatij. If this is so, there is not doubt that at least some of the EFAL could be associated with BronsJ d acid sites, which would not be taken into account if only the FAL is considered to be related to active Bronsted sites. 

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