Homogeneous acid-base catalysis hydrolysis

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Comprehensive discussions are to be found in (a) M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins, Wiley, New York, 1971 (b) W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New York, 1969 (c) M. L. Bender, Ckem. Rev., 60, 53 (1960). For more specialized treatments of particular aspects, see (d) W. P. Jencks, Chem. Rev., 72, 705 (1972), general acid-base catalysis (e) S. L. Johnson, Advan. Phys. Org. Chem., 5,237 (1967), ester hydrolysis (f) L. P. Hammett, Physical Organic Chemistry, 2nd ed., McGraw-Hill, New York, 1970, chap. 10, acid—base catalysis. 

The most common and most thoroughly studied type of homogeneous catalysis is acid-base catalysis. It includes hydrolysis, alcoholysis, esterification, and condensation reactions among many others. It is characterized by the fact that the equilibrium between base and conjugate acid, or between acid and conjugate base, is coupled with the actual catalytic cycle. 

At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions, " leading to small and homogeneous particles. Base catalysis of sol-gel hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of the hydrolyzed precursor monomers and to dense silica particles formed by the ripening of aggregates during the collision of droplets. As a result, the microparticles show essentially no porosity, with the particles being stabilized by a water/surfactant layer on the particle surface that prevents particle precipitation. ... 

A reaction exhibiting general acid catalysis, the ester hydrolysis (XXXVIII), has been discussed in Sect. 2.3. The present section deals with a classic reaction which is subject to both general acid and base catalysis in homogeneous media, the mutarotation of D-glucose. 

While there are abundant rate data available on ester hydrolysis in homogeneous aqueous solution (e.g., Mabey and Mill, 1978), quantitative data on the effect of surfaces on reaction rates are rather scarce. Hoffmann (Chapter 3, this volume) and Stone (1989) have investigated the catalytic effect of oxide surfaces on the hydrolysis of a few carboxylic acid esters, and have found a rate enhancement for compounds for which base catalysis is important at neutral pH. 

The concept that a catalyst provides an alternate mechanism for accomplishing a reaction, and that this alternate path is a more rapid one, has been developed in many individual cases. The basis of this idea is that the catalyst and one or more of the reactants form an intermediate complex, a loosely bound compound which is unstable, and that this complex then takes part in subsequent reactions which result in the final products and. the regenerated catalyst. Homogeneous catalysis can frequently be explained in terms of this concept. For example, consider catalysis by acids and bases.. In aqueous solutions acids and bases can increase the rate of hydrolysis of sugars, starches, and esters. The kinetics of the hydrolysis of ethyl acetate catalyzed by hydrochloric acid can be explained by the following mechanism ... 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

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