Br nsted acidity

Classification according to Br nsted acid-base properties is useful. 

Because the breadth of chemical behavior can be bewildering in its complexity, chemists search for general ways to organize chemical reactivity patterns. Two familiar patterns are Br< )nsted acid-base (proton transfer) and oxidation-reduction (electron transfer) reactions. A related pattern of reactivity can be viewed as the donation of a pair of electrons to form a new bond. One example is the reaction between gaseous ammonia and trimethyl boron, in which the ammonia molecule uses its nonbonding pair of electrons to form a bond between nitrogen and boron ... 

C21-0022. Draw molecular pictures showing a t q)ical Lewis acid-base reaction and a typical Br< )nsted acid-base reaction. Describe in words the differences and similarities of these two reactions. 

A number of carboxylic acids other than acetic were investigated as solvents or promoters. All of these acids which were stable to reaction conditions were found to be effective in promoting glycol ester production (e.g., propionic, pivalic, benzoic, etc.). However, other Br nsted acids of non-carboxylic nature were not found to be effective promoters. Thus penta-chlorophenol, although it has a pKa value (4.82) very close to that of acetic acid (4.76), is not a comparable promoter (Table I, reaction 13). Likewise, phosphoric acid (pK 2.15) is not an effective solvent or co-solvent with acetic acid (Table I, reaction 8). Experiments with lower concentrations of these acids in sulfolane solvent also showed that carboxylic acids are unique in promoting glycol formation. The promoter function of carboxylic acids thus appears not to be dependent (only) upon their acidity, but on some other chemical or structural property. 

Lewis acidity. Br nsted acidity refers to the ability of the mineral surface to donate a proton and Lewis acidity to the ability to accept electrons, i.e., to act as an oxidizing agent (39) We will discuss examples of heterogeneous processes involving mineral surfaces and organic or inorganic species. The review has been restricted to systems which can be related to sedimentary conditions or which have mechanistic significance. 

The source of H+ is the dissociation of coordinated water (see section on Br nsted acid). The protonation of benzidine (Equation 8) shifts the oxidation reaction (Equation 7) to the left. 

Redox disproportionation increased with the acidity of the clay and may be Br nsted acid-catalyzed. The mechanism was not established, however. 

Br nsted Acidity of Clay Minerals. The Br nsted acidity of clays primarily arises from the dissociation of water coordinated to exchangeable cations (6, 36, 65) ... 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

Both acid- and base-promoted reactions may be affected by acidic surfaces and, hence, by the factors which influence the surface acidity. Kinetic evidence for increased Br nsted acidity at clay surfaces has been presented by McAuliffe and Coleman (80) who studied the hydrolysis of ethylacetate and the inversion of sucrose. They noted that potentionmetrie pH measurements did not explain the catalytically effective H+-concentration at the clay surface. 

Addition to Double Bonds and Elimination Reactions. Both Lewis and Br nsted acidity of mineral surfaces can promote addition and elimination reactions (37). Equation 14 shows an example for an addition/elimination equilibrium catalyzed by Br nsted acidity ... 

If an a-particle (4He nucleus) adds a d-quark, the energy difference should be almost 4 times the case of a proton. The first electron is bound (5/3)2 rydberg or 38 eV. The binding of the second electron can be extrapolated from the parabolic variation (20) in the isoelectronic series He, Li+, Be+2,... to be 13 eV, comparable with oxygen and chlorine atoms. Hence, the species He(d)-1/3 is not particularly reactive, though its proton adduct He(d)H+2/3 should be far less acidic than HeH+ (which is already stable toward dissociation in the gaseous state, but too strong a Br nsted acid to persist in any known solvent). 

The amount of coke decreased drastically above 0.25M. Since U.D. is closely related to the amount of framework aluminum, the good correlation between the amount of coke and U.D. suggests that coke forms on Br nsted acid sites in zeolite. The removal of framework aluminum corresponding to BrjJnsted acid may be effective for decreasing coke formation. Furthermore, this result also indicates that the active iron cluster is inactive for coke formation in spite of high activity for toluene disproportionation. 

Figure 8. Correlation between catalyst activity for gas-oil cracking and Br nsted acidity of Ca, Mn, and REX zeolites...

Krause and Belting studied a tandem catalyzed reaction, in this case intramolecular cyclization and intermolecular hydroalkoxylation. The substrates were various homo-propargylic alcohols in the presence of non-tertiary alcohols and a dual catalyst system consisting of Br( >nsted acids and a gold precatalyst (Equation 8.43). 

It is often useful to consider that sites for chemisorption result from surface coordinative unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br nsted acid sites. 

Infrared Spectra. Figure 2 shows the spectra of pyridine adsorbed on / -alumina. Two types of Lewis acid sites are present strong Lewis acid sites, which still bind pyridine on evacuation at 350°C and characterized by the 1622 and 1454 cm-1 bands and weak Lewis acid sites, characterized by the 1614 and 1450 cm- bands. Br nsted acid sites, which have characteristic bands around... 

An important features of the Brv /nsted theory is the relationship it creates between acids and bases. Every Br /nsted acid has a conjugate base, and vice versa. 

Table Typical Br /nsted Acids and Their Conjugate Bases...

A subtle, but important, point must be made before we can extend our understanding of acid-base chemistry to the reaction between a Grignard or alkyllithium reagent and a carbonyl group. The data in the table of Br /nsted acids and their conjugate bases reflect the strengths of common acids and bases when they act as Brif/nstedacids or bases. These data predict that methyllithium should react with acetylene to form methane and an acetylide ion, for example. 

In i.r. studies of the adsorption of CO Gerasimov et al.25 and Davydov et al44 observe adsorption both on V4+ and on V3+ ions in reduced catalysts, but hardly any adsorption in the oxidized state. Davydov et al45 concluded that propene is adsorbed on oxidized V205/A1203, mainly on Br nsted acid sites forming an alcoholate type complex. On reduced catalysts propene is adsorbed as a 7r-complex of V3+ and V4+ ions. 

As mentioned on p. 108, Spiridinova et a/.40,41 only find evidence for Lewis acid adsorption sites, whereas Goldberg et al 2 and Nowinska43 also find evidence for Br nsted acid sites. Similarly Belokopytov et al. 8 observe the adsorption of NH3 on both types of acid centers, Lewis sites showing a greater acid strength. On the other hand Inomata et al28,81,82 report that on V205 itself adsorption of NH3 only occurs as NH4 and that adsorption as... 

A MO investigation of Miyamoto et al.113 also supports the proposed mechanism. The calculations show NH3 to be stably adsorbed on a Br nsted acid site whereas NO is hardly adsorbed at all. The calculations show that electrons of the adsorbed NH3 are transferred to the antibonding orbitals of NO leading to its dissociation. 

Alcohols are amphoteric and thus can function both as weak Br nsted acids and as bases ... 

This equation, in conjunction with voltammetric measurements of half-wave potentials (Em) for the reduction of Br nsted acids at a platinum electrode in any solvent, permits the evaluation of C a)soi [pATa(sol)] 5... 

---