Kinetics of condensation reactions

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Sodium dodecyl sulfate and hydrogen dodecyl sulfate have been used as catalysts in the denitrosation iV-nitroso-iV-methyl-p-toluenesulfonamide [138]. The kinetics of condensation of benzidine and p-anisidine with p-dimethylamino-benzaldehyde was studied by spectrophotometry in the presence of micelles of sodium dodecyl sulfate, with the result that the surfactant increases the rate of reaction [188]. The kinetics of reversible complexation of Ni(II) and Fe(III) with oxalatopentaaminecobalt(III) has been investigated in aqueous micellar medium of sodium dodecyl sulfate. The reaction occurs exclusively on the micellar surface [189]. Vitamin E reacts rapidly with the peroxidized linoleic acid present in linoleic acid in micellar sodium dodecyl sulfate solutions, whereas no significant reaction occurs in ethanol solution [190]. 

Changing the equilibrium conditions by having condensate in the sample due to water in the carrier gas or the diffusion limitation of the condensate in larger particles changes the reaction speed. Although the kinetics of the reaction and the diffusion of the condensate are not the process Imitating steps they have an effect on the overall reaction rate as described above. 

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

This review focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. In the past two decades, progress in determining the absolute kinetics of radical reactions in general has been rapid. The quantitation of kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase has been particularly noteworthy, progressing from a few absolute rate constants available before 1980 to a considerable body of data we summarize here. 

The kinetics of polycondensation reactions might be expected to be similar to those found in condensation reactions of small molecules (evidence suggests that rate coefficients are independent of polymer size). Polyesterification reactions between dibasic carboxylic acids and glycols can be catalysed by strong acids. In the absence of added catalyst, it has been suggested that the acidic monomer should act as a catalyst, whereupon the rate of reaction should be given by... 

One approach to following reaction kinetics on a solid phase is as follows. A trace amount of resin beads is taken out of a reaction vessel, rinsed briefly with solvent, and subjected to single-bead FTIR analysis or analysis by FTIR with a beam condenser. As an example, the kinetics of the reaction shown in reaction 1 was studied,4 that is, a combination of Wang resin 1 with succinimidyl 6-(iV-(7-nitrobenz-2-oxa-l,3-diazo-4-yl)amino)hex-anoate 2 to produce compound 3. The IR spectra for this transformation are... 

The kinetics of the condensation of the Cr(H20)63+ ion and its corresponding deprotonated species have been studied in the pH region 3.5-5.0 [25°C, I = 1.0 M(NaC104)] (201). The study of this reaction is complicated by the formation of higher oligomers. Chromatographic analysis of the products as a function of time established the dinuclear species to be the main product for the first 5% of reaction, and the initial-rate kinetics of condensation were studied by a pH-stat technique. The observed pH dependence of the rate was interpreted in terms of the second-order rate constants defined by Eq. (44), and values for... 

Work by Sigwalt, Bischoff and Cypryk22 have used this inter-and intramolecular catalysis to explain the condensation kinetics and cyclic formation processes in siloxane condensations. The kinetics show a very complex dependence on siloxane chain length, complicated by equilibria involving acid, silanol and water. They do indicate that the dominating reaction in the process is condensation and that chain disproportionation and chain scrambling are negligible The kinetics of condensation are influenced by the involvement of triflic acid in several equilibria, i.e. the formation of triflate esters, shown in equation 3, the possible involvement of triflic acid in the reaction of these esters with silanol, shown... 

In a polymerization process the chain length distribution or molar mass distribution (MMD) is influenced by a large number of factors and conditions the kinetics of the reaction plays a very important role. The calculation of the resulting MMD is thus very complicated. For one of the simplest cases, a step reaction with polycondensation, a first-order approach is given here. As an example we take a hydroxy acid HO-R-COOH, which, upon condensation, forms the chain -[-O-R-CO-]n. 

Bruice. T. C. and Dombardo, A., Catalytic reactions involving azomethines. XI. The kinetics of condensation of histamine with 3-hydroxypyridine-4-aldehyde. An intramolecular. Mannich reaction, / Am. Chem. Soc., 91. 3009, 1969. 

Miscellaneous. The kinetics of the reaction of urea with the tetrakishydroxymethyl-phosphonium cation have been studied. The reaction involves the stepwise condensation of the hydroxymethyl groups with urea, and at least four different phos-phonium species are present in the reaction mixture. The proposed mechanism involves the dissociation of the initial cation as the rate-limiting step (Scheme 6). ... 

This reaction can easily be followed by observing the pressure increase in the region of initial velocities before water is condensed and anuuonia dissolved therein. It is readily catalyzed by metals and alloys in the form of heated wires. Up to the present, the kinetics of this reaction was studied on surfaces of 80% platinum-20 % iridium, and 95% platinum-5 % gold. On the former alloy, the velocity can be expressed by... 

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