Acid base catalysis ester 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. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or h3o+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions are examples of acid-base catalysis. An important example of this type of reaction is esterification and its reverse, the hydrolysis of an ester. 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

The values of a and /3 are always between 0 and 1 for acid-base catalysis (except with some peculiar carbon acids),11 because complete transfer of a proton gives a value of 1, and no transfer a value of 0. The usual values for ester hydrolysis are 0.3 to 0.5, and for acetal hydrolysis about 0.6. 

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. 

Mechanism of acid-base catalysis It is accepted that acid-base catalysis involves a reversible acid-base reaction between the substrate and catalyst. This is in agreement with the protonic concept of acids and bases, since acid catalysis depends on the tendency of the acid to lose a proton, while base catalysis depends upon the tendency of the base to gain a proton. The mechanism of reaction involving H and OH- ion catalysis may be expressed as follows, by taking the example of hydrolysis of esters. 

Many hydrolysis reactions are pH dependent with base catalysis predominant. The acid-base catalysis is especially significant with carboxylic acid esters. The pseudo-first-order hydrolysis rate constant it/,(time" ) can be expressed as... 

The reactivity, fate, and distribution of bound solutes are certainly changed by association with stream humic substances. The rate of photolysis of certain organic compounds (Zepp et al., 1981a,b), the rate of volatilization of polychlorinated biphenyls (Griffin and Chian, 1980), the bioaccumulation of polynuclear aromatic hydrocarbons in fish (Leversee, 1981), the rate of humic acid induced acid-base catalysis (Perdue, 1983), and the rate of microbiological decomposition are some specific examples. The octyl ester of 2,4-D (2,4 DOE) was predicted by theoretical and mathematical models and found by experimentation to be resistant to base hydrolysis when bound to humic substances (Perdue, 1983). The same model predicted the humic acid catalyzed hydrolysis of atrazine as demonstrated by Li and Felbeck 11972). 

A classical example of specific acid-base catalysis is the hydrolysis of esters. The hydrolysis is catalyzed by H30" and OH" but not by other acids or bases. The rate of hydrolysis in the absence of acid or base is extremely slow. 

Rates of chemical processes may be affected by general acid-base catalysis. For example, hydrolysis of aliphatic esters of halogen-containing carboxylic acids is catalysed by buffer anions, the rate increasing linearly with the buffer concentration. Usually, however, catalysis by buffer species is less important than catalysis by hydrogen or hydroxide ions. Amines can also catalyse ester hydrolysis at high pH values Tris reacts with p-nitrophenylacetate to form an ( -acetyl Tris derivative. 

The proximal hydroxyl group can cooperate in the hydrolysis by hydrogen bonding and the carbonyl function of the ester becomes a better electrophilic center for the solvent molecules. In this mechanism one can perceive a general acid-base catalysis of ester solvolysis (Chapter 4). 

Baltzer s group has recently described a fully-synthetic protein that is also capable of hydrolysing p-nitrophenyl esters the polypeptide, which contains 42 amino acids, was designed to fold into a hairpin helix-loop-helix motif that dimerises into a four-helix bundle. The dimer is predicted to present on its surface a shallow reactive site containing several histidine residues. The spectroscopic properties of the peptide are consistent with the predicted folded structure, and the molecule does indeed catalyse ester hydrolysis (and transesterification) more effectively than 4-methylimidazole does. However, there is little substrate selectivity, and not much turnover. The histidine array does not seem to act via general acid-base catalysis, but rather to bind and stabilise ester oxygens in the transition state. We return to this molecule below. 

RNase Tl cleaves P-05 ester bonds in ssRNA, specifically at the 3 -P of the guanylic acid residues. As in RNase A (see Fig. 3.3), the catalysis occurs by a two-step mechanism, i.e., the formation of a terminal guanosine 2, 3 -cyclic phosphate intermediate (transesterification step) and the hydrolysis of the cyclic ester to guanosine 3 -monophosphate (hydrolysis step). The transesterification step involves a general acid—base catalysis. 

The reaction of ester hydrolysis and backward reaction of esterification of carboxylic acids are studied in detail. Ester hydrolysis is catalyzed by both hydrogen ions and hydroxyl ions, that is, is an example of specific acid-base catalysis. Two different methods for the cleavage of ester bonds are possible and observed experimentally acyl-oxygen and alkyl-oxygen... 

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

The same framework of eight possible mechanisms that was discussed for ester hydrolysis can also be applied to amide hydrolysis. Both the acid- and base-catalyzed hydrolyses are essentially irreversible, since salts are formed in both cases. For basic catalysis the mechanism is Bac2-... 

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