Reaction hydrolysis

During hydrolysis, water acts simultaneously as solvent, reactant and catalyst via self-dissociation. Often the addition of a further catalyst, usually acids or bases, is necessary to avoid undesirable side reactions. Hydrolysis reactions which have been investigated include the hydrolysis of amines [14-16], amides [17], nitriles 

The hydrolysis of acetamide, acetonitrile and benzonitrile was also investigated at higher temperatures (350-450 °C, 28-32 MPa) in a tubular reactor [17] without addition of catalysts. The measured activation energy of acetonitrile decreases with pressure, which was assumed to be a consequence of catalysis by H30 ions. At higher pressure the self-dissociation of water increases, leading to an increased concentration of OH and H3O+ ions. 

The hydrolysis of esters is of technical interest therefore many different esters such as acetates [18], phthalates [19], natural fats [20] and others were investigated. A detailed investigation of the hydrolysis of ethylacetate (tubular reactor, 23-30 MPa, 250-450 °C, 4-230 s) [7] without the addition of a catalyst shows a lower activation energy at subcritical conditions than at supercritical conditions, indicating two different reaction mechanisms. Under subcritical conditions nucleophilic attack on a protonated ester is assumed to be the rate-determining step of the hydrolysis process. The formation of a protonated ester is favored in the subcritical region because here the self-dissociation of water and the dissociation of the acid, formed via hydrolysis, increase. At 350 °C, 30 MPa, 170 s reaction time, and without additional acid, the conversion to acid and alcohol was 96 %, which is the equilibrium value. In other cases, mostly with unsaturated esters, the acids formed undergo decarboxylation, which leads to poorer yields [12]. 

Similar to the hydrolysis of esters, the hydrolysis of ethers occurs at high pressures without the addition of acid catalysts. As for other hydrolysis reactions, high density and the addition of NaCl improves the reaction rate and selectivity of hydrolysis relative to other degradation reactions. Under optimal conditions, the reaction leads only to the respective alcohols. Examples of ethers investigated are methoxynaphthalenes [21], dibenzylether [7, 22], anisols [23], and from cellulose to glucose, fructose and oligomers [24]. 

There are numerous important reactions in which a molecule of water reacts in such a way that part of the molecule (H) appears in one product and the remainder (OH) appears in another. In other words, a molecule of water is split or lysized, which leads to the name hydrolysis that describes these reactions. Typical processes include reactions such as those that follow, indicating that a base is produced  

However, there are also hydrolysis reactions that produce acidic solutions, as shown in the following equations. 

Characteristically, the covalent bonds between nonmetals and halogens are very susceptible to hydrolysis reactions. A few examples of this behavior are illustrated in the following equations, and numerous others will be seen in later chapters dealing with the chemistry of the nonmetallic elements  

In general, reactions such as these produce the hydrogen halide and an acid containing the nonmetal in the same oxidation state as the original halogen compound. One notable exception to this behavior is that of SF6 because it does not react with water (see Chapter 15). 

Analogous lysis reactions such as those shown previously also take place in other solvents. For example, in liquid NH3 the reaction 

Stirring an ester in an aqueous suspension of Dowex-50 is an efficient method for hydrolyzing the ester to the acid. Similar results have been obtained by refluxing an ester in an aqueous suspension of H-beta zeolite. Stirring acetals in an acetone suspension of wet silica gel or moistened Amberlyst-15 0 is a convenient method for converting an acetal to the aldehyde or ketone without having side reactions such as double bond isomerizations also take place (Eqn. 22.40). 9 

Oximes, hydrazones and semicarbazones have been hydrolyzed in very good yields by heating in an aqueous suspension of Dowex-50 or an aqueous acetone suspension of Amberlyst-15. 2 

Pedersen reported in 1967 that the dicyclohexyl-18-crown-6 complex of potassium hydroxide was soluble in toluene and in this medium could readily hydrolyze the very sterically hindered ester methyl mesitoate [33]. He later reported that this complex could also hydrolyze the tertiary-butyl ester [34]. Similarly, Lehn found that the [2.2.2]-cryptate complex of potassium hydroxide was even more effective in this saponification reaction under directly comparable conditions [35]. The hydrolysis reaction is formulated in equation 9.15. This method has recently been applied in the hydrolysis of C-labelled methyl tetradecanoate [36]. Starks found that tetra-decanoate anion acted as a catalyst poison and impeded further hydrolysis [7]. This is therefore one of the few examples where crown ether catalysis is clearly superior to quaternary ammonium ion catalysis. 

Starks found that tricaprylylmethylammonium chloride catalyzed the hydrolysis of A2-dodecanesulfonyl chloride by aqueous hydroxide. Quantitative yields of the sulfonic acid were obtained in the presence of the quaternary ammonium catalyst whereas little reaction was observed in its absence [6, 7]. The hydrolysis of trichloro-methylbenzene to benzoic acid (see Eq. 9.16) was likewise catalyzed by a quaternary ammonium salt [37]. 

In the presence of water, an Fe salt dissociates to form the purple, hexa-aquo ion, i. e. 

The electropositive cation induces the H2O ligands to act as adds and, except at very low pH, hydrolysis, i. e. deprotonation of these ligands, takes place. The process is stepwise with ultimately all six ligands being deprotonated. The rates of water exchange, k x ° for Fe(H20) Fe0H(H20)5 and Fe(0H)2(H20)J were estimated to be 1.6 10 1.4 10 and 10 s respectively (Grant Jordan, 1981 Schneider Schwyn, 1987). Complete hydrolysis corresponds to formation of an Fe oxide or oxide hydroxide, i.e. 

Hydrolysis is commonly induced by addition of a base , by heating (forced hydrolysis) or by dilution it can also be induced by solvent extraction or ion exchange (Segal, 1984). A1 in the system enhances hydrolysis (Shah Singh Kodama, 1994). The many investigations of this process have been reviewed by Sylva (1972), Flynn (1984), Schneider and Schwyn (1987), Cornell et al. (1989), Rose et al. (1997) and Schwertmann et al. (1999). 

1) A problem with this method of hydrolysis is that addition of a base (especially NaOH) leads to local pH gradients and, hence, variations in the hydrolysed species that form (Schneider, 1984), and so makes reproducible results difficult to obtain. Attemps to minimize this effect have included replacement of NaOH with 

These equilibria are established rapidly. The relevant equilibrium constants are listed in Table 9.2. Above a threshold OH/Fe (ca 1). The low molecular weight species interact to produce species with a higher nuclearity, e. g. the dimer. 

In contrast to D-a-amino acids, the analog access for L-a-amino acids remained a challenge for a long time. Recently, however, such an efficient process technology was established at Degussa AG (now Evonik Industries AG), 

23 ASYMMETRIC SYNTHESIS WITH RECOMBINANT WHOLE-CELL CATALYSTS 

Synthesis of W-carbamoyl o-p-hydroxyphenylglycine with an immobiiized wiid-type whoie-ceii cataiyst from Bacillus brevis as key step in a process for D-p-hydroxyphenyigiycine. 

Synthesis of t-a-amino acids via dynamic kinetic resolution of hydantoins with a recombinant whole-cell catalyst containing a racemase, i-hydantoinase, and L-carbamolyase. 

A further hydrolytic process in which whole-cell catalysis turned out to be very suitable is the transformation of a racemic nitrile into the corresponding acid exemplified for the dynamic kinetic resolution of mandelonitrile into (R)-mandelic acid, (R)-IO. This reaction is catalyzed by means of a nitrilase, which is known as highly enantioselective enzyme. As early as 1991, researchers from Asahi Chemical Industry Ltd. reported such a reaction utilizing wild-type whole cells from Alcaligenes faeccdis bearing a suitable nitrilase [32]. When starting from racemic mandelonitrile, rac-7. 

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All hydrolysis reactions are catalysed by acid, or base, or both. 

The trichloride is obtained as a liquid, boiling point 349 K, when a jet of chlorine burns in phosphorus vapour. Care must be taken to exclude both air and moisture from the apparatus since phosphorus trichloride reacts with oxygen and is vigorously hydrolysed by water, fuming strongly in moist air. The hydrolysis reaction is ... 

Zigmond, 1988). The ATP-hydrolysis that accompanies actin polymerization, ATP —> ADP + Pj, and the subsequent release of the cleaved phosphate (Pj) are believed to act as a clock (Pollard et ah, 1992 Allen et ah, 1996), altering in a time-dependent manner the mechanical properties of the filament and its propensity to depolymerize. Molecular dynamics simulations suggested a so-called back door mechanism for the hydrolysis reaction ATP ADP - - Pj in which ATP enters the actin from one side, ADP leaves from the same side, but Pj leaves from the opposite side, the back door (Wriggers and Schulten, 1997b). This hypothesis can explain the effect of the toxin phalloidin which blocks the exit of the putative back door pathway and, thereby, delays Pi release as observed experimentally (Dancker and Hess, 1990). 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Only for hydrolysis reactions also monodentate substrates are encountered, but for these systems the extent of activation of these compounds by the metal ion is still under debate. 

There are a few documented examples of studies of ligand effects on hydrolysis reactions. Angelici et al." investigated the effect of a number of multidentate ligands on the copper(II) ion-catalysed hydrolysis of coordinated amino acid esters. The equilibrium constant for binding of the ester and the rate constant for the hydrolysis of the resulting complex both decrease in the presence of ligands. Similar conclusions have been reached by Hay and Morris, who studied the effect of ethylenediamine... 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

Pairwise hydrophobic interactions can be used to alter the reactivity of organic molecules in water. For instance, the rate of hydrolysis reactions may be influenced significantly by the presence of hydrophobic cosolutes. The effect on reactivity has been analysed by comparirg the interactions between initial state and cosolute with those between transition state and cosolute. ... 

The concentration of phenylacetate can be determined from the kinetics of its pseudo-first-order hydrolysis reaction in an ethylamine buffer. When a standard solution of 0.55 mM phenylacetate is analyzed, the concentration of phenylacetate after 60 s is found to be 0.17 mM. When an unknown is analyzed, the concentration of phenylacetate remaining after 60 s is found to be 0.23 mM. What is the initial concentration of phenylacetate in the unknown ... 

In aqueous solution, OF2 oxidizes HCl, HBr, and HI (and thek salts), Hberating the free halogens. Oxygen difluoride reacts slowly with water and a dilute aqueous base to form oxygen and fluorine. The rate of this hydrolysis reaction has been determined (23). 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

This hydrolysis reaction is accelerated by acids or heat and, in some instances, by catalysts. Because the flammable gas hydrogen is formed, a potential fire hazard may result unless adequate ventilation is provided. Ingestion of hydrides must be avoided because hydrolysis to form hydrogen could result in gas embolism. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

To produce the mtile titanium dioxide pigment, hydrolysis of the mother Hquor has to be carried out in the presence of a specially prepared hydrosol as a seeding agent. This hydrosol is made by the neutralization of a portion of the mother Hquor in the presence of hydrochloric or some other monohydric acid. Because of the large amount of the hydrosol that must be added to the mixture (about 6% concentration), the hydrolysis reaction takes only about 1 hr. 

Hydrolysis. Complexes formed by Pu ions with OH represent hydrolysis reactions. There is extensive interaction between Pu + and water. Pu(Ill) hydrolyzes at ca pH 7 (105) the first hydrolysis equiUbrium is as follows ... 

The enthalpies of these hydrolysis reactions have also been determined (112). Polynuclear complexes such as[(Pu02)2(OH2], [(Pu02)3 (OH) ), and [(Pu02)4(0H)y] have been inferred from potentiometric titrations (105). 

This is an example of an ammonolytic reaction ia which a chemical bond is broken by the addition of ammonia. It is analogous to the hydrolysis reactions of water. An impressive number of inorganic and organic compounds undergo ammonolysis. 

The principal reactions are reversible and a mixture of products and reactants is found in the cmde sulfate. High propylene pressure, high sulfuric acid concentration, and low temperature shift the reaction toward diisopropyl sulfate. However, the reaction rate slows as products are formed, and practical reactors operate by using excess sulfuric acid. As the water content in the sulfuric acid feed is increased, more of the hydrolysis reaction (Step 2) occurs in the main reactor. At water concentrations near 20%, diisopropyl sulfate is not found in the reaction mixture. However, efforts to separate the isopropyl alcohol from the sulfuric acid suggest that it may be partially present in an ionic form (56,57). 

The most significant difference between the alkoxysilanes and siUcones is the susceptibiUty of the Si—OR bond to hydrolysis (see Silicon compounds, silicones). The simple alkoxysilanes are often operationally viewed as Hquid sources of siUcon dioxide (see Silica). The hydrolysis reaction, which yields polymers of siUcic acid that can be dehydrated to siUcon dioxide, is of considerable commercial importance. The stoichiometry for hydrolysis for tetraethoxysilane is... 

The kinetics of hydrolysis reactions maybe first-order or second-order, depending on the reaction mechanism. However, second-order reactions may appear to be first-order, ie, pseudo-first-order, if one of the reactants is not consumed in the reaction, eg, OH , or if the concentration of active catalyst, eg, reduced transition metal, is a small fraction of the total catalyst concentration. 

Aqueous sulfamic acid solutions are quite stable at room temperature. At higher temperatures, however, acidic solutions and the ammonium salt hydroly2e to sulfates. Rates increase rapidly with temperature elevation, lower pH, and increased concentrations. These hydrolysis reactions are exothermic. Concentrated solutions heated in closed containers or in vessels having adequate venting can generate sufficient internal pressure to cause container mpture. An ammonium sulfamate, 60 wt % aqueous solution exhibits mnaway hydrolysis when heated to 200°C at pH 5 or to 130°C at pH 2. The danger is minimised in a weU-vented container, however, because the 60 wt % solution boils at 107°C (8,10). Hydrolysis reactions are ... 

Polylactic acid sutures are slowly degraded by the foUowing hydrolysis reaction shown and can take years to be completely absorbed (16). These sutures were never commercialized. 

The presence of catalyst residues, such as alkali hydroxide or alkali acetate, a by-product of the hydrolysis reaction, is known to decrease the thermal stability of poly(vinyl alcohol). Transforming these compounds into mote inert compounds and removal through washing are both methods that have been pursued. The use of mineral acids such as sulfuric acid (258), phosphoric acid (259), and OfXv o-phosphotic acid (260) has been reported as means for achieving increased thermal stability of the resulting poly(vinyl alcohol). 

The desired extraction process is the exothermic proton-catalyzed hydrolysis of isobutylene to tert-huty alcohol. This alcohol is further dehydrated to yield pure isobutylene. At low concentrations the hydrolysis reaction is favored ... 

Saltlike Carbides. Almost all carbides of Groups 1—3 of the Periodic Table are saltlike. Beryllium carbide and Al C may be considered as derivatives of methane ion) and most carbides having C2 groups, ie, ions, as derivatives of acetylene. This is supported to some extent by hydrolysis reactions ... 

Carbon disulfide is essentially unreactive with water at room temperature, but above about 150°C in the vapor phase some reaction occurs forming carbonyl sulfide (carbon oxysulfide) [463-58-1] and hydrogen sulfide [7783-06-4]. Carbonyl sulfide is an intermediate in the hydrolysis reaction ... 

The equihbrium expressions for the hydrolysis reactions (eq. 1) foUow and are the ionisation constants of HOX and water, respectively. 

The mother Hquor from the cmde ferrous sulfate crystallisation contains neady all the chromium. It is clarified and aged with agitation at 30°C for a considerable period to reverse the reactions of the conditioning step. Hydrolysis reactions are being reversed therefore, the pH increases. Also, sulfate ions are released from complexes and the chromium is converted largely to the hexaaquo ion. Ammonium chrome alum then precipitates as a fine crystal slurry. It is filtered and washed and the filtrate sent to the leach circuit the chrome alum is dissolved in hot water, and the solution is used as cell feed. 

The main type of hydrolysis reaction is that of halogenoaryl compounds to hydroxyaryl compounds, eg, the aqueous caustic hydrolysis of 0- and /)-chloronitrobenzene derivatives to nitrophenols. Another important reaction is the hydrolysis of A/-acyl derivatives back to the parent arylamine, where the acyl group is frequently used to protect the amine. 

A number of specific Upases are used for ester synthesis (eq. 4) transesterification, eg, acidolysis with 1,3 specific Hpase (eq. 5) and hydrolysis reactions, eg, with nonspecific Hpase (eq. 6). 

Generally, these two methods complement each other. With some rare exceptions an enzyme that produces an S ester in the hydrolysis reaction produces an R isomer in acylation reaction and vice versa. 

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