Specific acid and base

Specific acid and base Hydrolysis of esters Ri COOR. + H.O = Ri COOH + R.OH... 

Hydrolysis. Hydrolysis of synthetic organics occurs via several pathways. Specific-acid and -base catalyzed processes can be... 

See also specific acids and bases, such as sulfuric acid and sodium hydroxide, in alphabetically arranged entires throughout this Encyclopedia. 

In this equation, is the experimentally determined hydrolytic rate constant, /Cq h the uncatalysed or solvent catalysed rate constant, and /CgH- te the specific acid- and base-catalysis rate constants respectively, ttd ky - are the general acid- and base-catalysis rate constants respectively, and [HX] and [X ] denote the concentrations of protonated and unprotonated forms of the buffer. 

For a complete evaluation of the stability of the dmg, we need to evaluate the catalytic coefficients for specific acid and base catalysis and also to determine the catalytic coefficients of possible buffers which we might wish to use in the formulation. 

As we can see from Fig. 4.9, this dmg is very stable in unbuffered solution over a wide pH range but degrades relatively rapidly in the presence of strong acids or bases. Since the influence of buffer components has been removed, this plot allows us to calculate the rate constants for specific acid and base catalysis. Removing the terms for the effect of buffer from equation (4.44), we have... 

Alcohol- or water-producing condensation generates a three-dimensional network. Pohl and Osterholtz (6) showed that condensation of alkylsilane-triols is specific acid and base catalyzed. Above the isoelectric point of silica (about pH 2.5) condensation proceeds by nucleophilic attack of deprotonated silanols on neutral silicates (6, 23). Below the isoelectric point the reaction proceeds by protonation of silanols followed by electrophilic attack (2, 6). These reactions favor less highly condensed sites, because these are the most electron rich, and lead to more extended, ramified structures. Above pH 2.5, reactions favor more highly condensed sites. 

Observed pH rate profiles typically consist of several regions including segments with linear dependence on [H+] or [ OH], pH-independent, and curved transitions between linear areas. The occurrence of [H+] (or [ OH]) in the rate expression indicates either that a protonated (or deprotonated) form of the reactant is involved (preequilibrium) or that H+ (or OH) is involved in the rate-determining step. Figure 3.28 shows some pH dependencies that may be components of a specific profile. Curves (a) and (b) show linear dependence on [H+] and [ OH] that is due to specific acid and base catalysis, respectively. The horizontal portion of the profile corresponds to a reaction that does not involve acid or base catalysis. Usually the slope of the linear part of the curve is -1(H+) or - -l( OH) because there is only one protonation (or deprotonation) step. 

Taking into consideration specific acid and base catalysis (the concentration of a catalyst is reflected in the rate law but is not reflected in the equilibrium constant), the hydrolysis rate term for hydrolyzable chemicals can be described by ... 

This is typical acid-base catalysis, known as specific acid-base catalysis because the specific acid and base involved are (or H30 ) and OH". The form of the pH dependence of the rate tells... 

Specific acid and base Hydrolysis of y-lactones Hydrolysis of amides Hydrolysis of esters 1 0 1 R CO-NH2 + H2O = R COONH4 R,C00R2 -I- H2O = Ri-COOH -1- R2OH... 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

The kinetics for ligand exchange of the water-soluble (11 R = CH2CH2C02 ) as a function of pH and thiol concentration are in accord with four reversible mer-captan-lyate species exchange reactions followed by product formation through specific acid and base catalysis. It has also been found that (11) (R = Me, Bu , or Bu ) represent the basic forms of weak acids, with pKa, comparable to those of carboxylic acids. 

Pohl and Osterholtz [50] used C and Si NMR to investigate the condensation of alkylsilanetriol to bis-alkyltetrahydroxydisiloxane in buffered aqueous solutions as a function of pD (-loglDjO" ]). The second-order rate constants for the disappearance of the triol are plotted in Fig. 18a as a pD rate profile. The slopes of the plot above and below the rate minimum at pD 4.5 are -t-1 and -1, respectively, indicating that the condensation is specific acid- and base-catalyzed. Assink [30] investigated the acid-catalyzed condensation of TMOS over a narrow range of pH. He observed that the condensation rate was proportional to [H30 ], which is also consistent with specific acid catalysis. 

Hydrolytic decomposition represents an important transformation process for organophosphorus insecticides in soil and runoff water. Hydrolysis in homogeneous aqueous solutions has been extensively studied. Nucleophilic displacement (Sn2) occurs at either the phosphorus or carbon atom with an alcohol or diester being the leaving group (21). Hydrolysis of an organophosphorus insecticide (Q depends on the specific acid and base-catalyzed and neutral reactions - d[C]/dt = ki [C] = a[H ][C] + b[OH ][C] + n[C], where ki, kp, k and are the overall and specific acid, base and neutral... 

---