As BrOnsted

The species shown in red, HB and HA, act as Bronsted-Lowry acids in the forward and reverse reactions, respectively A and B (blue) act as Bronsted-Lowry bases. (We will use this color coding consistently throughout the chapter in writing Bronsted-Lowry acid-base equations.)... 

FIGURE 10.19 In water, Al3+ cations are hydrated by water molecules that can act as Bronsted acids. Although, for clarity, only four water molecules are shown here, a metal cation typically has six H20 molecules attached to it. 

Proton electroreduction catalyzed by metal complexes is different from reduction at a metal electrode. It definitely involves the formation of metal hydride species through protonation of electroreduced, low-oxidation-state metal complexes that function as Bronsted base (Equation (5)). From protonated... 

In the common parlance of physical organic chemists such phrases as product-like or reactant-like transition states are common. The degree of resemblance of transition states to either reactants or products is usually assessed for reaction series obeying the linear-free-energy principle on the basis of suitable reaction constants, such as Bronsted a- and (3-values, and Hammett reaction constants p. The question is inherently more complex for cyclisation reactions, since they are not expected to follow the linear-free-energy principle. 

Hydroxyl Groups as Bronsted Acids and Lewis Bases... 

B—All can behave as Bronsted bases. Only B cannot behave as an acid. 

Brensted-Lowry theory chem A theory that all acid-base reactions consist simply of the transfer of a proton from one base to another. Also known as Bronsted theory. ( jbran steth lau re, the-3-re ... 

Things are a little more complicated than implied by this statement. Molecules that donate protons and molecules that accept them are formally known as Bronsted acids and bases. There is a more general definition of acid and base— Lewis acids and bases—in which a Lewis acid is any molecule having the ability to accept a pair of electrons and a Lewis base is any molecule having the ability to donate a pair of electrons to a Lewis acid. The Bronsted concept will suffice for all our purposes. 

Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on ojqrgen, which makes them proton acceptors. 

Amines cire both Bronsted-Lowry bases (they accept hydrogen ions from acids) and Lewis bases (they furnish an electron pair to Lewis acids). As Bronsted-Lowry bases they have values. Aliphatic amines have values of approximately 1(H, and aromatic amines have values near 10 °. (These values compare to a value of =10" for ammonia.)... 

Primary, secondary, and tertiary amines behave as Bronsted-Lowry bases. These amines react like ammonia, adding H to produce an ammonium ion. 

Aminosulfides, important building blocks for the synthesis of various bioactive molecules, can be prepared by the thiolysis of aziridines in the presence of Bi(OTf)3 (5 mol%) [36]. Even though Lewis acids such as boron trifluoride etherate and zinc chloride, as well as Bronsted acids such as trifluoromethanesulfonic acid, have been employed as acid catalysts, these methods suffer from disadvantages like the use of stoichiometric amount of catalysts [37—40], harsh conditions, the use of large excess of thiols and necessitate anhydrous conditions to produce the desired products. Furthermore, these reagents cannot be recovered and recycled because they decompose or deactivate under quenching conditions. 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

To prove their hypothesis, Bamford and Block (51) applied the diagnostic test previously designed by Gold and Jefferson (58) in their studies of hydrolysis of carboxylic anhydrides catalysed by tertiary bases. The technique employed involves the use of a series of tertiary bases having different relative abilities to assodate with Lewis acids and to act as Bronsted bases. Pyridine, a-picoline and 2,6-lutidine form... 

Balance each equation and classify the reaction as Bronsted acid-base, Lewis acid-base, or redox. 

Step 1. List the species present before dissociation and identify them as Bronsted-Lowry... 

A remarkable acceleration of MBH reactions was observed in the presence of mild cooperative catalysts consisting of tributylphosphine and phenols or naph-thols as Bronsted acids (Table 5.10) [88-90]. The non-asymmetric addition of... 

This is the same as Bronsted s theory which was designed particularly for solutions. The concentration of the activated complex can be expressed in terms of the reactants and the equilibrium constant K. Also the heat of the reaction, AH, to give the activated complex, can be calculated approximately from the quantum theory or from the Arrhenius theory. Since AF= —RT In K and AF = AII — TAS, and since K can, in some cases, be calculated from known, fundamental constants, the entropy term remains the only unknown. Rodebush has long pointed out that the unknown quantity 5 in the formula k = se E/RT is related to an entropy term. As a first approximation it has been related to a collision frequency in bimolecular reactions and to a vibration frequency in unimolecu-lar reactions. Combining the two thermodynamic equations23... 

Aprotic protophillic solvents all nitrogen bases without protons, e.g., pyridine, and other conjugated amines, and fully alkylated amines or amides. The solvents function as Bronsted bases as well as Lewis bases and may have high permittivity. 

From this you can see that the cation from the salt comes from the base and the anion comes from the acid. Salts can act as Bronsted-Lowry acids or bases to produce solutions that are acidic or basic. The salts react with water in a reaction known as hydrolysis to yield either a conjugate acid and a hydroxide ion or a conjugate base and a hydrogen (hydronium) ion. If you know the origins of the components of a salt, you can make some predictions about the pH of the solution formed from a hydrolysis of a salt ion. 

Oxide surfaces contacted with water are generally covered with hydroxy groups which can be schematically represented as S-OH where S stands for Al, Si, Ti, Fe etc. Some of these groups may behave as Bronsted acids, whereas other hydroxy groups may behave as Brensted bases, giving rise to the following equations ... 

Decide whether each of the following may act as Bronsted acids, Bronsted bases, or both ... 

Polyoxometalates (POMs), also known as heteropolyacids (HPA),3 are a class of compounds formed from negatively charged inorganic metal-oxygen building blocks. When charge-balanced with cationic species, POMs self-assemble into unusual 3D structures with specific topological and electronic properties.214 POMs are commonly formed from polyanions of early transition metals such as W, Mo or V. These anions can be substituted with other transition metals. The diversities in POMs composition and structure make them attractive for many applications, particularly as Bronsted acid and redox catalysts. For example,... 

This representation is over-simplified, each of the ions being further solvated in each acid. Autoprotolysis constants have been reported as 3 x 10 13 mol2 kg-2 (0°C) for HF[6], 3.8 x 10 8 (25°C) for HS03F[7] and 7.9 x 1(T7 (25°C) for CF3S03H[8]. Protonic media are made more acidic by addition of an entity which increases the proton concentration. Superacids are themselves so very weakly basic that very few, if any, compounds can act as Bronsted acids to donate protons to the solvent directly. Lewis acids combine with X- to shift the autoprotolysis equilibria to increase the proton concentration. Superacids are rendered basic by direct addition of the X species, the base of the system, (e.g. from an alkali metal compound MX) or by addition of compounds which accept protons from the medium, increasing the concentration of the base X. ... 

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