Acid-base equilibria hydrolysis

Pre-equilibria. The base hydrolysis of trifluoroacetanilide is complicated by an acid-base equilibrium 22... 

Metal ion catalysis of salicyl phosphate hydrolysis is much more complicated than that of Sarin, since the former substrate can combine with metal ions to give stable complexes, and some of the complexes formed do not constitute pathways for the reaction. In addition the substrate undergoes intramolecular acid-base-catalyzed hydrolysis which is dependent on pH because of its conversion to a succession of ionic species having different reaction rates. Therefore a careful and detailed equilibrium study of proton and metal ion interactions of salicyl phosphate would be required before any mechanistic considerations of the kinetic behavior in the absence and presence of metal ions can be undertaken. 

FIGURE 7.4 Of the 16 chemistry topics examined (1-16) on the final exam, overall the POGIL students had more correct responses to the same topics than their L-I counterparts. Some topics did not appear on all the POGIL exams. Asterisks indicate topics that were asked every semester and compared to the L-I group. The topics included a solution problem (1), Lewis structures (2), chiral center identification (3), salt dissociation (4), neutralization (5), acid-base equilibrium (6), radioactive half-life (7), isomerism (8), ionic compounds (9), biological condensation/hydrolysis (10), intermolecular forces (11), functional group identification (12), salt formation (13), biomolecule identification (14), LeChatelier s principle (15), and physical/chemical property (16). 

The acid-base equilibrium constants of 3-oxy-7-nitrobenzofurazans were determined with the help of UV spectroscopy [1106, 1234], Kinetics of the reaction of methoxydegalogenization and hydrolysis of 4-chloro-7-nitro-2,l,3-benzoxadiazole has been investigated [1106], UV-spectra of some nitrobenzofurazan derivatives are described [1242-1244],... 

Y. Kurono, H. Tani, T. Kuwayama, K. Hatano, T. Yashiro, and K. Ikeda, Kinetics and mechanism of the acid-base equilibrium and the hydrolysis of benzodiazepinooxazines, Chem. Pharm. Bull. 39, 3323-3326 (1991). 

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

Metal ions of +3 oxidation state have a high surface charge density and consequently solvent molecules are tightly held. Resulting from this is the strong tendency for hydrolysis to take place according to an acid—base equilibrium of the type... 

Since NH4 is a weaker acid than H30, the reaction will go only a little to the right, but still far enough that a solution of ammonium chloride is acidic (pH < 7). Note that in the above reactions H2O can act as either an acid or a base. Note also that Na (aq) and Cl (aq) do not affect the above equilibria and are omitted from the net ionic equations for the hydrolysis reactions such ions are termed "spectator" ions since they do not change the acid-base equilibrium, even though they are present in solution. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Acetal formation is reversible (K for MeCHO/EtOH is 0-0125) but the position of equilibrium will be influenced by the relative proportions of R OH and H2O present. Preparative acetal formation is thus normally carried out in excess R OH with an anhydrous acid catalyst. The equilibrium may be shifted over to the right either by azeotropic distillation to remove H2O as it is formed, or by using excess acid catalyst (e.g. passing HCl gas continuously) to convert H2O into the non-nucleophilic H3O . Hydrolysis of an acetal back to the parent carbonyl compound may be effected readily with dilute acid. Acetals are, however, resistant to hydrolysis induced by bases—there is no proton that can be removed from an oxygen atom, cf. the base-induced hydrolysis of hydrates this results in acetals being very useful protecting groups for the C=0 function, which is itself very susceptible to the attack of bases (cf. p. 224). Such protection thus allows base-catalysed elimination of HBr from the acetal (27), followed by ready hydrolysis of the resultant unsatu-... 

The acid-catalysed hydrolysis of an ester is the reverse reaction of the Fischer esterification. Addition of excess water drives the equilibrium towards the acid and alcohol formation. The base-catalysed hydrolysis of esters is also known as saponification, and this does not involve the equilibrium process observed for the Fischer esterification. 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

The 1,8-naphthyridine di-iV-oxide (145), was prepared by oxidation of the corresponding base.154 Hydrolysis of 145 gave the 2-hydroxy derivative (146) in equilibrium with the cyclic hydroxamic acid (147).155... 

Although acid-base effects can be important in adhesion our studies indicate that bonding to metals through silane coupling agents is not by an acid-base mechanism, but probably through Si-O-M oxane bonds. As with glass, the hydrolysis and formation of oxane bonds are true equilibria, but the individual equilibrium constants are not known. 

Esters are subject to general acid or base-catalyzed hydrolysis to form a carboxylic acid and an alcohol. Both are equilibrium reactions and readily occur when equilibrium is shifted to the right. Base-catalyzed hydrolysis... 

As a chemical phenomenon, weathering can be viewed as the result of the tendency of the rock-water-mineral system to attain equilibrium. This occurs through the usual chemical mechanisms of dissolution and precipitation, acid-base reactions, complexation, hydrolysis, and oxidation-reduction. 

The me.vo-diphcnylphospholanomide was obtained via a cycloaddition reaction of N,N-d -methylaminophosphinodichloride and 1,4-diphenylbutadiene followed by hydrogenation. The mcw-compound was transformed into an equilibrium mixture containing predominately the rac-compound on treatment with base. After hydrolysis, the phospholanic acid was kinetically resolved using quinine. Reduction and protection of the phospholane as the borane adduct allowed the double displacement of 1,2-ethyleneglycol ditosylate. 

Base-mediated ester hydrolyses have a high driving force. This is because of the acid/base reaction between the carboxylic acid formed in the reaction, and the base used as the reagent. The resonance stabilization of the carboxylate is approximately 30 kcal/mol, which means a gain of about 16 kcal/mol compared to the starting material, the carboxylic ester (resonance stabilization 14 kcal/mol according to Table 6.1). Accordingly, the hydrolysis equilibrium lies completely on the side of the carboxylate. 

Methyl Ester-Based Processes. The fatty methyl esters are produced predominantly by the transesterification of fats and oils with methanol in the presence of an alkaline catalyst under very mild reaction conditions.l5a,b They are used in the production of lauric-type (Cl2) alcohols. The short-chain fatty methyl esters (C8-Cl0), produced as by-products via the fractional distillation of crude lauric-type (coconut, palm kernel) methyl esters, are converted to fatty acids via acidic or alkaline hydrolysis (Fig. 36.12). The hydrolysis of short-chain fatty methyl esters by stream splitting or Twitchell-type processes is not very efficient because of unfavorable equilibrium constants.16a,b... 

Hydrolysis is an equilibrium between two conjugate acid-base pairs, in which water can play the part of a weak acid or a weak base. In the hydrolysis of acetate ions water acts as an acid ... 

In acid-catalyzed reactions, the distinction between single-species and complex catalysis is not always clear-cut. The actual catalyst is the solvated proton, H30+ in aqueous solution, and H20 (or a molecule of the nonaqueous solvent) may thus appear as a co-product in the first step and as a co-reactant in the step reconstituting the original solvated proton, possibly also in other additional steps, e.g., if the overall reaction is hydrolysis or hydration. Moreover, the acid added as catalyst may not be completely dissociated, and its dissociation equilibrium then affects the concentration of the solvated proton. At high concentrations this is true even for fairly strong acids such as sulfuric, particularly in solvents less polar than water. Such cases are better described as acid-base catalysis (see Section 8.2.1). 

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. 

Very many problems in solution chemistry are solved with use of the acid and base equilibrium equations. The uses of these equations in discussing the titration of weak acids and bases, the hydrolysis of salts, and the properties of buffered solutions are illustrated in the following sections of this chapter. 

Hydrolysis equilibria can be interpreted in a meaningful way if the solutions are not oversaturated with respect to the solid hydroxide or oxide. Occasionally, it is desirable to extend equilibrium calculations into the region of oversaturation but quantitative interpretations for the species distribution must not be made unless metastable supersaturation can be demonstrated to exist. Most hydrolysis equilibrium constants have been determined in the presence of a swamping inert electrolyte of constant ionic strength (/ = 0.1, 1, or 3 M). As we have seen before, the formation of hydroxo species can be formulated in terms of acid-base equilibria. The formulation of equilibria of hydrolysis reactions is in agreement with that generally used for complex formation equilibria (see Table 6.2). 

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