Hydrolysis-Dehydration Reactions

These reactions with phosphinites, phosphonites and phosphonates as well as phosphites have been utilized for the preparation of peptides and active esters of amino acids in good yields. 

Considering that the chemical reactivity of carboxylic acids is similar to that of carbonic acids, as is observed in amide and ester formation, we have attempted the substitution of carbon dioxide for carboxylic acids in the coupling reaction with amines by using phosphites in pyridine or imidazole, and found that ureas are in fact produced in good yields (Eq. (4))6. Similarly, carbon disulfide reacts with amines to yield the corresponding thioureas (Eq. (5)). 

In addition, we have applied successfully the concept of hydrolysis-dehydration reaction with phosphorus compounds to the reaction with sulfur compounds such as sulfites7- as shown below  

Recently, we have found other types of the hydrolysis-dehydration reactions between carboxylic acids and amines or alcohols promoted by triphenyl phosphite and alkyl halides (Eq. (6)) 8 and metal salts like LiCl (Eq. (7))9  

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In these reactions, hydrolysis of diphenyl and triaryl phosphites to monoaryl phosphites and phenol was coupled by dehydration between carboxylic acids and amines or alcohols to the corresponding amides and esters. Therefore, the reaction can be generalized as a hydrolysis-dehydration reaction (Scheme 2). The concept of the hydrolysis-dehydration reaction using phosphites has been drown to be applicable also to reactions with other phosphorus compounds such as phosphinites, phos-phonites and phosphorates s Aryl esters of these phosphorus compounds are effective as condensing agents in the production of carboxylic amides and esters (from carboxylic acids and amines or alcohols, respectively) whereas alkyl esters are ineffective (Eqs. (1-3)) ... 

Theoretical Studies. Theoretical models for the Si(OR)4 hydrolysis, polycondensation, and dehydration reactions involved in sol—gel processes have been developed using semiempirical molecular orbital models. These have been reviewed (3,5). 

In the reaction of ethylene with sulfuric acid, several side reactions can lead to yield losses. These involve oxidation, hydrolysis—dehydration, and polymerization, especially at sulfuric acid concentrations >98 wt % the sulfur thoxide can oxidize by cycHc addition processes (99). 

The reactions of amides fall into hydrolysis, dehydration and degradation (to amines) acid hydrazides and acid azides undergo additional reactions. 

Harris, J.F. "Acid Hydrolysis and Dehydration Reactions for Utilizing Plant Carbohydrates," Applied Polymer Symposium. 1975, 28, 131-144. 

Aldehydes and ketones can react with primary and secondary amines to form Schiff bases, a dehydration reaction yielding an imine (Reaction 45). However, Schiff base formation is a relatively labile, reversible interaction that is readily cleaved in aqueous solution by hydrolysis. The formation of Schiff bases is enhanced at alkaline pH values, but they are still not stable enough to use for crosslinking applications unless they are reduced by reductive amination (see below). 

The powerful dehydrating property of hydrogen fluoride would cause it to be expected to assist in reactions in which water is a product. Such dehydration reactions would not in the true sense be catalytic. However, as the addition of water in the case of the hydrolysis of esters (Simons and Meunier, 66) has been shown to be catalyzed by hydrogen fluoride, the catalytic powers of hydrogen fluoride are probably involved in the reverse reaction, as a catalyst must necessarily accelerate the reverse reaction if it does so for the forward one. For this reason some of these reactions are included here. 

Abstract Polyfunctionality of carbohydrates and their low solubility in conventional organic solvents make rather complex their conversion to higher value added chemicals. Therefore, innovative processes are now strongly needed in order to increase the selectivity of these reactions. Here, we report an overview of the different heterogeneously-catalyzed processes described in the literature. In particular, hydrolysis, dehydration, oxidation, esterification, and etherification of carbohydrates are presented. We shall discuss the main structural parameters that need to be controlled and that permit the conversion of carbohydrates to bioproducts with good selectivity. The conversion of monosaccharides and disaccharides over solid catalysts, as well as recent advances in the heterogeneously-catalyzed conversion of cellulose, will be presented. 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

Silica gel is produced by the hydrolysis, dehydration, and polymerization (gelation) of TEOS in solution according to the following reactions ... 

Fig. 23a. Hydration and dehydration reactions of the silica surface and the IR absorption frequencies of the surface species. Chemisorption of water produces surface silanols, which serve as adsorption sites for water, b Reaction of octadecyltrichlorosilane (OTS). Hydrolysis of the chloride group by trace amounts of water in solution to silanol is followed by condensation with surface silanols, resulting in covalent bond formation between the monolayer and the substrate. OTS molecules can also cross-link to form polymeric species during film curing [201]...

The transition from left to right involves dehydration reactions the transition from right to left involves hydrolysis reactions. 

Since a molecule of water is eliminated for each linkage formed, the anabolic reactions leading to macromolecule formation are dehydration reactions. Recall from Section 4.2 that when macro-molecular nutrient molecules are digested prior to their entering the body s system, a molecule of water is added for each linkage broken — a hydrolysis reaction. The energy required for the anabolic synthesis of macromolecules is provided by catabolic processes of glycolysis, the citric acid cycle, and electron transport. 

The meehanism of the conjugate addition is the same as that in the previous example and the meehanism for ester hydrolysis was covered in Chapter 12, The key step in the dehydration reaction is the formation and eyclization of the mixed anhydride formed from the diacid and acetic anhydride, Both steps have the same mechanism, attack of an acid on an anhydride, but the second step is intramolecular. Like most eyelizations the reaction is entropically favoured as two molecules react to give three—the cyclic anhydride and two molecules of acetic acid... 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

As pointed out above, many studies on TOP "decomposition" emphasize the influence of water formed in a dehydration reaction of alcohol present either as solvent or as traces. Such a chain mechanism involving a first alcohol molecule producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules has been shown by Bradley and al. studying zirconium alkoxides [11]. Curiously, such a "hydrolytic" decomposition of titanium derivatives has not been taken into account in the most recently reported studies on CVD experiments from TOP [8-10]. 

Reaction studies include cyanations using hydroxy nitrile lyases, hydrolysis of amides using acylases, amidases or lipases, and even dehydration reactions of aldoximes to nitriles using aldoxime dehydratase. This reaction is quite... 

Methenyl-H4MPT cyclohydrolase is usually assayed in the direction of hydrolysis (Reaction 14), although the dehydration reaction has been demonstrated [100,113, 122,186,356]. While assaying cyclohydrolase in the direction of hydrolysis, the rates... 

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