Ammonia-catalyzed hydrolysis

The synthesis of Si02 particles by ammonia-catalyzed hydrolysis of TEOS has been also carried out in Aerosol OT reversed micelle systems. The particles precipitated in this systems are spheres, but they have generally a broad size distribution comparing with that of nonionic reversed micelles compared to the normalized standard deviation of <10% in polyoxyethylene(5) nonylphenyl ether, that in Aero-... 

T. Matsoukas and E. Gulari Dynamics of growth of silica particles from ammonia-catalyzed hydrolysis of tetra-ethyl-orthosilicate, J. Colloid Interface Sci., 124 (1988) 252-261... 

Many enzymes have absolute specificity for a substrate and will not attack the molecules with common structural features. The enzyme aspartase, found in many plants and bacteria, is such an enzyme [57], It catalyzes the formation of L-aspartate by reversible addition of ammonia to the double bond of fumaric acid. Aspartase, however, does not take part in the addition of ammonia to any other unsaturated acid requiring specific optical and geometrical characteristics. At the other end of the spectrum are enzymes which do not have specificity for a given substrate and act on many molecules with similar structural characteristics. A good example is the enzyme chymotrypsin, which catalyzes hydrolysis of many different peptides or polypeptides as well as amides and esters. 

Phosphoric acid is a relatively strong acid and can cause extensive damage to polysaccharides. Ammonium phosphate salts are good fire retardants (1,7,13,18,22), are inexpensive, and should not cause acid-catalyzed hydrolysis or dehydration. However, if ammonium phosphate-treated wood is exposed to heat, ammonia will be given off and phosphoric acid will be left (7,39-43). The critical temperature at which monoammonium phosDhate thermally disassociates has been reported to be 166° (39), 170° (47), or 190°C (40). 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

Silica Nanoparticles. The base-catalyzed hydrolysis of silicon aikoxides in microemulsions produces nanoparticles (20-39). Aqueous ammonia has been used primarily as the base, with AOT and nonionic polyoxyethylene ethers as the main surfactants. Figure 2.2.6 presents a flow diagram for the synthesis of pure silica (23-32) the microemulsion is first prepared and then the alkoxide is added. As can be seen from Table 2.2.1, the microemulsions include the systems AOT/ isooctane/water/ammonia, AOT/toluene/water/ammonia, NP-5/cyclohexane/water/ ammonia, and NP-4/heptane/water/ammonia. Typical reaction times are l -5 days. Various modified silica nanoparticles have also been prepared, including hydropho-... 

Ammonia is highly toxic to animal tissues. In the urea cycle, ornithine combines with ammonia, in the form of carbamoyl phosphate, to form citrulline. A second amino group is transferred to citrulline from aspartate to form arginine—the immediate precursor of urea. Arginase catalyzes hydrolysis of arginine to urea and ornithine thus ornithine is regenerated in each turn of the cycle. 

Structures bearing carbonyl groups are generally more sensitive to base-catalyzed hydrolysis this is particularly true of the sulfones (73) which are readily ring opened to arene acids (74), aldehydes and ammonia. 2,4-Dioxo and thionyl derivatives behave similarly. Where both carbonyl and thiocarbonyl groups are present primary amines form imino derivatives by selective attack at the thiocarbonyl site in the dithioxo compound (75 X = S) reaction takes place at position 2 (Scheme 27) (79KGS291). 

Amides undergo an acid- or base-catalyzed hydrolysis reaction with water in the same way that esters do. Just as an ester yields a carboxylic acid and an alcohol, an amide yields a carboxylic acid and an amine (or ammonia). The net effect is a substitution of -N by -OH. This hydrolysis of amides is the key process that occurs in the stomach during digestion of proteins. 

In 1951, Whistler and Durso reported68 the isolation of epimelibiose by partial, acid hydrolysis of the polysaccharide guaran, followed by charcoal-column fractionation of the products. ThA material was obtained in anhydrous crystalline form from a mixture of methyl and butyl alcohols it had m.p. 201°, [a]26D + 120.9° — +124.6° in 36 hr. (c 2, in water). The yield was 2.2% of the weight of guaran used. Epimelibiose has also been produced72" in crystalline form following the ammonia-catalyzed alkaline isomerization of melibiose. 

Urease catalyzes hydrolysis of urea to ammonia and carbamate at a rate at least lO " times faster than the spontaneous hydrolysis rate of urea in aqueous solution between pH 2 to 12. > The stability of urea is attributed to its resonance energy, which has been estimated at 30-40 kcalmoU. " The zwitterionic resonance forms of urea donate electrons to the carbonyl carbon, causing it to be less reactive toward nucleophiles. 

Catalytic reduction was, however, occasionally observed. TiCU can catalyze the reduction of N2 by aluminum metal in the presence of AlBr3 at 130 °C and N2 pressure of 100 atm. Yields of ammonia (after hydrolysis) usually vary from 0.01 to 1 mol moL transition metal compound, indicating that two transition metal atoms participate in the reduction of one N2 molecule, although in some systems (particularly with very strong reductants) up to 2 mol NH3 are produced per mol tr msition metal. 

Urease catalyzes hydrolysis of urea to ammonia and carbamate at a rate at least 10 times faster than the spontaneous hydrolysis rate of urea in aqueous solution between pH 2 to The stability of urea is attributed... 

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