Acid dehydration-condensation reactions

Certain catalytic modes have been well exploited in flame retardant systems, namely the dehydrating action of compounds which yield strong acids under flaming or smoldering conditions. Friedel-Crafts and other acid catalyzed condensation reactions have been exploited to increase char. These mechanisms don t work very well for polymers of mainly hydrocarbon character. Are there other modes of catalysis which might work better ... 

With carboxylic acids, the condensation reaction is an unfavourable equilibrium, promoted by using non-aqueous solvent (if any) and a dehydrating agent such as sulphuric acid (which also catalyses the reaction). 

Strong heating of carboxylic acids forms an acid anhydride through a type of substitution called a dehydration-condensation reaction (Section 14.6), in which two molecules condense into one with loss of water ... 

Esters, like acid anhydrides, form through a dehydration-condensation reaction in this case, it is called an esterification ... 

The cellular precursors that form a nucleic acid are nucleoside triphosphates (Figure 15.23A). Dehydration-condensation reactions between them create a chain... 

Esters and acid anhydrides form through dehydration-condensation reactions, and water is the other product. 

The three types of natural polymers—polysaccharides, proteins, and nucleic acids— are formed by dehydration-condensation reactions. 

Condensa.tlon, A variety of condensation reactions involving the hydroxyl or the carboxyl or both groups occur with lactic acid. The important reactions where products can be obtained ia high yields are esterificatioa (both iatramolecular and with another alcohol or acid), dehydration, and aminolysis. 

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). 

As for the reaction path from pyruvic acid to citraconic anhydride, it is considered that a condensation reaction first takes place by a reaction between an oxygen atom of carbonyl group and two hydrogn atoms of methyl group in another molecule, followed by oxidative decarboxylation to form citraconic acid. The produced citraconic acid is dehydrated under the reaction conditions used. The proposed reaction path is shown in Figure 7. 

Iminium ions are intermediates in a group of reactions that form ,( -unsaturated compounds having structures corresponding to those formed by mixed aldol addition followed by dehydration. These reactions are catalyzed by amines or buffer systems containing an amine and an acid and are referred to as Knoevenagel condensations,2U The reactive electrophile is probably the protonated form of the imine, since it is a more reactive electrophile than the corresponding carbonyl compound.212... 

Classical Aldol. Aldol reaction is an important reaction for creating carbon-carbon bonds. The condensation reactions of active methylene compounds such as acetophenone or cyclohexanone with aryl aldehydes under basic or acidic conditions gave good yields of aldols along with the dehydration compounds in water.237 The presence of surfactants led mainly to the dehydration reactions. The most common solvents for aldol reactions are ethanol, aqueous ethanol, and water.238 The two-phase system, aqueous sodium hydroxide-ether, has been found to be excellent for the condensation reactions of reactive aliphatic aldehydes.239... 

The reaction mechanism is shown in Figure 4 and is adapted from work by Fiego et al. [9] on the acid catalysed condensation of acetone by basic molecular sieves. The scheme has been modified to include the hydrogenation of mesityl oxide to MIBK. The scheme begins with the self-condensation of acetone to form diacetone alcohol as the primary product. The dehydration of DAA forms mesityl oxide, which undergoes addition of an addition acetone to form phorone that then can cyclise, via a 1,6-Michael addition to produce isophorone. Alternatively, the mesityl oxide can hydrogenate to form MIBK. 

Many of the enone substrates used in polyamino acid-catalysed epoxidation reactions can be made via a simple aldol condensation, which leads directly to the desired enone after in situ dehydration. Enones that cannot be synthesised by the above route may often be synthesised using standard Wittig chemistry, (Scheme 6). The above methods of substrate synthesis provide compounds with a variety of groups R and enabling the incorporation of both aliphatic and aromatic moieties into the enone structure. 

Figure 11.4 Condensation, dehydration and reduction reactions in fatty add synthesis. These reactions constitute the major components of the pathway of fatty acid synthesis and are all catalysed by fatty acid synthase. The reduction reactions, indicated by addition of 2H in the diagram, involve the conversion of NADPH to NADP . (The re-conversion of NADP back to NADPH occurs in the pentose phosphate pathway.) The condensation reaction results in an increase in size of acyl-ACP by two carbon units in each step. The two carbons for each extension are each provided by malonyl-CoA. ACP - acyl carrier protein.

In another example, a catalytic amount of a Bronsted acid such as H3PO4 was dissolved in [NRR 3][NTf2] (R = hexyl, R = butyl) 119). The catalyst was applied for the condensation reaction of alcohols, which usually requires strongly acidic media and dehydrating conditions. The condensation of veratryl alcohol was facilitated because the water that formed was continuously removed as vapor, which assisted in driving the reaction to high yields. The product (cyclotriveratrylene) separation, however, required the addition of a co-solvent. 

Entries 4 and 5 in Scheme 2.1 depict acid-catalyzed aldol reactions. In entry 4, condensation is accompanied by dehydration. In entry 5, a /i-chloroketone is formed by addition of hydrogen chloride to the enone. 

Oxo-l,2-benzothiazine 1,1-dioxide 139 undergoes aldol condensation reactions upon deprotonation with NaOMe and treatment with an aldehyde 140 (Equation 19) <1992SC2621>. The intermediate aldol adducts are then dehydrated with acetic acid to afford condensation products 141 <2000JME2040>. 

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