Equation, Brpnsted catalysis

In general acid catalysis, the rate is increased not only by an increase in [SH ] but also by an increase in the concentration of other acids (e.g., in water by phenols or carboxylic acids). These other acids increase the rate even when [SH ] is held constant. In this type of catalysis the strongest acids catalyze best, so that, in the example given, an increase in the phenol concentration catalyzes the reaction much less than a similar increase in (HjO ). This relationship between acid strength of the catalyst and its catalytic ability can be expressed by the Brpnsted catalysis equation ... 

The Hammett equation is not the only LFER." ° Some, like the Hammett equation, correlate structural changes in reactants, but the Grunwald-Winstein relationship (see p. 452) correlates changes in solvent and the Brpnsted relation (see p. 337) relates acidity to catalysis. The Taft equation is a structure-reactivity equation that correlates only field effects. ... 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

Brpnsted equations are also common in general-acid catalysis, as, for example, in the hydrolysis of certain acetals.8-10 In acid catalysis, a rather than /3 is used ... 

The term nucleophilicity refers to the relative rate of reaction of an electron donor with a given electrophile, as distinct from basicity, which refers to its relative affinity for a proton in an acid-base equilibrium. A quantitative relationship between rate and equilibrium constants was discovered by Brpnsted and Pedersen (1) in 1924. These authors found that the rate constants for the catalytic decomposition of nitramide by a family of bases, such as carboxylate ions (GCH2C02 ), could be linearly correlated with the acidities of their conjugate acids, pKHB. This observation led to the discovery of general base catalysis and the first linear free-energy relationship, which later became known as the Brpnsted equation ... 

For other applications of the Brpnsted equation to SN2 reactions, see Smith, G. F. J. Chem. Soc. 1943, 521-523. Hudson, R. F. Klopman, G. J. Chem. Soc. 1962, 1062-1067. Hudson, R. F. Chemical Reactivity and Reaction Paths Klopman, G., Ed. Wiley-Interscience New York, 1974 Chapter 5. For applications of the Brpnsted equation to acyl transfer and other reactions, see Hammett (2) and Jencks, W. P. Catalysis in Chemistry and Enzymology McGraw-Hill New York, 1969. 

The following data give the dissociation constants and rate of acetaldehyde hydration catalysis by each acid. Treat the data according to the Brpnsted equation and discuss the mechanistic significance of the results. 

Dehydrogenation of AB can also be accomplished through acid catalysis using either Lewis or Brpnsted acids. Addition of strong acids can initiate hydrogen release from AB even at 25°C, according to Equation 16.44 [129],... 

Cycloisomerization of methyl-substituted 1,6-diene is also catalyzed by tin(rv) triflate (Equation (8.8)). DFT computations proposed that the mechanism does not involve the direct addition of the tin(IV) cation to a double bond because the catalyst regeneration step would be energetically unfeasible [24]. The active catalyst is a hydrated triflate salt where water molecule plays a decisive role to enable the smooth completion of the catalytic cycle. The diastereoselectivity observed in the cycloisomerization was associated with the transition-state geometries. DFT calculations also showed that protonation and deprotonation occur on a single face of the substrate. These considerations, correlated to the experiments, showed that Brpnsted superacids are not effective in Lewis superacid catalysis. 

---