Bransted equation

Through lack of an unambiguous method for direct determination, the acidity constants of carbon acids have, for many years, been estimated from the rate of proton abstraction by means of rate-equilibrium relationships. Thus, Bell (1943) (see also Hibbert, 1977) estimated the acetaldehyde, acetone and acetophenone acidity constants (19.7,20.0 and 19.2, respectively) by assuming that the rate constants for proton abstraction from several mono- and dicarbonyl compounds to a single base (A-) with pAHA = 4.0 obey a Bransted equation in its differential form (47). By taking curvature into account, the... 

Cations. The ammonium ion, NH,+, behaves as a weak acid in water a 0.10 M solution of NH4CI has a pH of about 5. The process by which the NH4+ ion lowers the pH of water can be represented by the (Bransted-Lowry) equation ... 

A very similar equation can be written to explain why solutions of zinc salts are acidic. Here it appears that the hydrated cation in water solution contains four H20 molecules bonded to a central Zn2+ ion. The Bransted-Lowry equation is... 

The problem is that a substance HA undergoing a reaction in which it behaves as a Bransted acid is not behaving as an acid in the Lewis sense it is, however, behaving as a Lewis acid-base adduct. The issue is further clouded by the fact that HA can enter into reactions in which it does behave as a Lewis acid, as for example in the formation of a hydrogen bond (Equation 3.61) in this case it is not, however, behaving as a Bransted acid, since the proton remains... 

Write a balanced net ionic equation for the reaction of each of the following ions with water. In each case, identify the Bransted-Lowry acids and bases and the conjugate acid-base pairs. 

Write balanced equations for the reactions of (a) H3PO4 and (b) B(OH)3 with water. Classify each acid as a Bransted-Lowry acid or a Lewis acid. 

Finally, in an interesting and unique transformation, the de-insertion of isonitrile from the nominally eight-coordinate molybdacyclopentenediimine complex 181 is induced upon protonation with strong Bransted acid (Equation 92) it is not obvious why the ring-contracted molybdacyclobutenimine product 182 should be thermodynamically more stable than the simple product of protonation <2001JCD1284>. 

Bransted LFER. Bronsted and Pederson (1923) were the first to describe a relationship between rates and equilibria for a series of compounds. They found that log/cB for base-catalyzed decomposition of nitramide, H2N202, varies linearly with log/CHB+, the acidity constant of the conjugate acid of the catalyst. Rate constants for many other acid- or base-catalyzed reactions (including the hydrolysis of amides, esters, carbamates, and organophosphates, and dissociation of acids and metal-ion complexes) are log-log related to the acid (or base) dissociation constant of the catalyst and follow either of the equations... 

Thus, when HCl dissolves in water (Equation 16.3), HCl acts as a Bronsted-Lowry acid (it donates a proton to H2O), and H2O acts as a Bransted-Lowry base (it accepts a proton from HCl). 

Acidity and basicity are fundamental properties of organic compounds, and acid-base reactions are essential steps in many organic transformations. Although there are several definitions of acidity and basicity, the Bransted theory and the Lewis theory are used most often in organic chemistryIn Lewis theory, an acid is an electron pair acceptor and a base is an electron pair donor, as in the reaction of a trialkylamine as Lewis base with boron trifluoride as Lewis acid (equation 7.1). 

The Bransted-Lowry Theory of Acids and Bases COAL 2 Given the equation for a Br0nsted-Lowry acid-base reaction, explain how or why it can be so classified. 

Yamamoto developed a conceptually new class of chiral boron-Lewis acids 174 (Equation 20) that can readily be assembled from a tetraphenol ligand and B(OMe)3 [93, 94]. These were shown to be especially effective with a wide range of a-substituted enals in enantioselective Diels-Alder reactions. One illustrative example is the cycloaddition between aldehyde 173 and cyclopentadiene to give 175 in 99% ee, > 99 1 exo/endo selectivity, and quantitative yield [93]. Yamamoto suggested that these catalysts 174 were Bransted-assisted chiral Lewis acids (termed BLAs), in which the phenol proton activates the enal substrates by hydrogen bonding, as depicted in the proposed transition state assembly 176. 

In equations (5.11) and (5.12), the acid molecules are acting as proton donors and the water molecules are acting as proton acceptors. Accoring to the Bransted-Lowry theory, an acid is a proton donor. 

---