Chemical reactions dehydration-condensation reaction

The cmde phthaUc anhydride is subjected to a thermal pretreatment or heat soak at atmospheric pressure to complete dehydration of traces of phthahc acid and to convert color bodies to higher boiling compounds that can be removed by distillation. The addition of chemicals during the heat soak promotes condensation reactions and shortens the time required for them. Use of potassium hydroxide and sodium nitrate, carbonate, bicarbonate, sulfate, or borate has been patented (30). Purification is by continuous vacuum distillation, as shown by two columns in Figure 1. The most troublesome impurity is phthahde (l(3)-isobenzofuranone), which is stmcturaHy similar to phthahc anhydride. Reactor and recovery conditions must be carefully chosen to minimize phthahde contamination (31). Phthahde [87-41-2] is also reduced by adding potassium hydroxide during the heat soak (30). 

In recent years, the rate of information available on the use of ion-exchange resins as reaction catalysts has increased, and the practical application of ion-exchanger catalysis in the field of chemistry has been widely developed. Ion-exchangers are already used in more than twenty types of different chemical reactions. Some of the significant examples of the applications of ion-exchange catalysis are in hydration [1,2], dehydration [3,4], esterification [5,6], alkylation [7], condensation [8-11], and polymerization, and isomerization reactions [12-14]. Cationic resins in form, also used as catalysts in the hydrolysis reactions, and the literature on hydrolysis itself is quite extensive [15-28], Several types of ion exchange catalysts have been used in the hydrolysis of different compounds. Some of these are given in Table 1. 

Combined condensation of melamine and of the acid residue is also shown by the phosphate and borate. In these cases however, the polymeric dehydrated compound derived from the acid is thermally stable up to above 500 C allowing chemical reactions with melamine condensation products simultaneously formed. The resulting material is stable to 950 C (phosphate, ca 30% of original salt) or to above 1100 C (borate, ca. 20% of original salt). 

Confalone et al. (85) also made use of an intramolecular cycloaddition step in the construction of a range of tri- and tetracyclic products. Phenyl allyl ethers, of the type shown in Scheme 3.94, underwent dehydrative condensation with the requisite amine to furnish the intermediate ylides, which suffered cycloaddition resulting in 285 and 286 in essentially quantitative yield. The ratio of cis/trans fused products was in the range of 10 1. Such a process has been developed to construct the alkaloid (+ / ) sceletium A4 by reaction of the intermediate 287 with amine 288 via the cycloaddition protocol already developed, followed by further chemical manipulation, in an efficient five step synthesis (Scheme 3.94). 

MSA does not contain any chiral carbon centers. Before the aromatization of the six-membered ring occurs, two prochiral carbons (C-2 and C-4 in the six-carbon intermediate) evolve, each of which loses a hydrogen in the process of the dehydratization/aromatization steps. In addition, C-3 of the six-carbon intermediate forms a chiral center when the ketone is reduced to a hydroxyl by a ketoreductase activity (Fig. 5). The chirality of this hydroxyl carbon is unclear since the intermediate has not been isolated. It is also unknown if this carbon retains its chirality in an eight-carbon intermediate or whether the hydroxyl is eliminated by dehydration prior to the third condensation reaction. The stereospecificity at the prochiral C-2 and C-4 carbons in the reaction intermediates was addressed using chemically synthesized (] )- and (S)-[1- C, 2- H]malonate precursors which were enzymatically converted into CoA derivatives via succinyl CoA transferase [127,128]. Thus, the prochiral methylene in malonyl CoA was replaced by chiral, double-labeled (S)- or (J )-[1- C, 2- H]malonyl CoA substrates in the reaction mixture with 6-MSAS. The condensation is expected to occur with inversion of configuration and the intact methylene... 

Acids (hydrogen ion) and bases (hydroxide ion) act as homogeneous catalysts for many important organic chemical reactions in solution. These include esterification, ester hydrolysis (see Box 9.2), hydration of alkenes, dehydration of alcohols, and condensation reactions. 

It is believed that at high concentration of chemicals reactions such as phosphorylation, esterificarion and/or condensation occurs on the surface with some destruction of wood by heat-pressed treatment, then dehydration and carbonization occur acceleratively by combustion. As a result a carbonized layer is formed at the surface which prevents fire development. 

As the examples of non-crystalline condensation disperse structures, one can name silicates and aluminosilicates (silica and aluminosilica gels, both hydrated and dehydrated). Silica gels form in the course of chemical reaction between sodium silicate and acid, namely [16,33] ... 

USE As a dehydrating agent in certain chemical reactions as a polymerizing or condensing agent. 

Tables 1 and 2 show some examples of self- or cross-aldol condensation reactions followed by dehydration to obtain a,/3-unsaturated carbonyl compounds. Also in Table 1, a review of methyl isobutyl ketone (a saturated ketone) synthesis in the presence of hydrogen is included (10). In Table 2, some examples of fine chemicals obtained by aldol-type reactions are shown.

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