Reaction rates acceleration

There are many reactions in which the products formed often act as catalysts for the reaction. The reaction rate accelerates as the reaction continues, and this process is referred to as autocatalysis. The reaction rate is proportional to a product concentration raised to a positive exponent for an autocatalytic reaction. Examples of this type of reaction are the hydrolysis of several esters. This is because the acids formed by the reaction give rise to hydrogen ions that act as catalysts for subsequent reactions. The fermentation reaction that involves the action of a micro-organism on an organic feedstock is a significant autocatalytic reaction. 

Diels-Alder reactions rate acceleration promoted by a bisphenylendiol [93c]... 

There are many reactions in which the products formed can themselves act as catalysts for the reaction. Thus there will be a range of compositions for which the reaction rate accelerates... 

As a result of the catalytic center-chiral entity interaction the reaction rate accelerates substantially. This phenomenon was described for the first time by Sharpless [6], who coined the term ligand accelerated catalysis. Unfortunately, the reasons for this phenomenon are still not well... 

Uncatalysed Diels-Alder reactions usually have to be carried out at relatively high temperatures (normally around 100 °C)73, often leading to undesired side reactions and retro-Diels-Alder reactions which are entropically favoured. The Diels-Alder reaction became applicable to sensitive substrates only after it was realized that Lewis acids (e.g. A Clg) are catalytically active56. As a consequence, Diels-Alder reactions can now be carried out at temperatures down to — 100°C85. The use of Lewis acid catalysts made the [4 + 2]-cycloaddition applicable to the enantioselective synthesis of many natural compounds51,86. Nowadays, Lewis acid catalysis is the most effective way to accelerate and to stereochemically control Diels-Alder reactions. Rate accelerations of ten-thousand to a million-fold were observed (Table 7, entries A and B). 

Most importantly, the presence of lead compounds results in a strong acceleration of the fizz zone reactions, i. e., those in the gas phase close to the burning surface. Acceleration of the reactions in the subsequent dark zone or in the luminous flame zone is not significant. The net result of the fizz zone reaction rate acceleration is an increased heat feedback to the surface (e. g., by as much as 100 %), which produces super-rate burning. 

Supramolecular systems can be considered as new tools of modern physical organic chemistry. The study of catalytic processes using supramolecular model systems aims to explain the observed rate enhancement in terms of structure and mechanism. In some cases, the model systems may even provide a simplified simulation of the action of an enzyme and lead to further understanding of the different mechanism by which enzymes are able to achieve impressive reaction rate accelerations and turnover numbers. 

The kinetics of high-pressure cycloaddition reactions of tropone with dienophiles and enophiles have been reviewed.254 The Diels-Alder reaction of 9,10-dimethylanthracene with acrylonitrile has been investigated at high pressure in acetonitrile and in ethereal solutions of lithium perchlorate.255 The combination of high pressure and a solution of lithium perchlorate in diethyl ether is an excellent reaction rate accelerator in 4 + 2-cycloaddition reactions.256... 

Grieco, P. A. Giamer, P. He, Z.-M. "Micellar" catalysis in the aqueous intermolecular Diels-Alder reaction Rate acceleration and enhanced selectivity, Tetrahedron Lett. 1983, 24,1897-1900. 

In the reaction of oxalic acid with iodate, the oxidising species are molecular iodine and hypoiodous acid (principally the latter). The induction period observed in the reaction is due to build-up of the oxidising species it is shortened by exposure to light or by the presence of metal ions. As soon as the iodine concentration is appreciable, the reaction rate accelerates rapidly on account of the sequence of reactions... 

Combustion is defined as a rapid, high-temperature oxidation reaction. What happens in the reactor just described after the reaction rate accelerates dramatically is combustion, whereas the initial slow oxidation reaction between methane and oxygen to form CO2 and H2O and other reactions between these species, such as the formation reaction of... 

V-PjtoL OSP] N-Vinyl-2-pyrrdidone with stabilizer reaction rate accelerator fix systems incl. adhesives, coatings, cosm ics, textiles, syn. fibm, textile sizes, i otective coUoids, lobe oil additives. 

Enzymes demonstrate both high specificities and significant reaction rate accelerations. The relative values of enzymic over non-enzymic reactions may be from 10 ° to 10 (orotidine decarboxylase) and the turnover numbers range from one catalytic event per minute to 10 per second (hydration of CO2 to HC03 by carbonic anhydrase). The molecular entities of enzymes cover proteins, ribozymes and catalytic antibodies. 

Kelly, T.R., Meghani, P. and Ekkundi, V.S. (1990) Diels-Alder reactions rate acceleration promoted by a biphenylenediol. Tetrahedron Letters, 31, 3381-3384. 

At low concentrations of phospholipid substrates in water, the reaction catalyzed by a phospholipase occurs at a rather low rate. The reaction rate accelerates when the concentrations of the phospholipid substrates increase to the point that micelles are formed. How is this property of phospholipases related to their activity in the cell ... 

Non-chloroaluminate ILs, which are in general poor nucleophiles, have proven to be attractive alternative media for Lewis acid catalyzed reactions. ILs may have a reaction rate accelerating effect, and they may improve selectivity and facilitate catalyst recovery. This is the case for scandium triflate catalyzed Diels-Alder cycloaddition [8,9], three-component (aldehyde, aniline, triethylphosphite) synthesis of a-aminophosphonates [10], Claisen rearrangement and cyclization reactions [11], or Friedel-Crafts reactions [12, 13]. 

In a Diels-Alder reaction an electron-rich diene reacts with an electron-poor dienophile. By coordination of a Lewis acid the frontier orbital energies of the dienophile are lowered. ITius, die energy difference between the dienophile-LUMO and die diene-HOMO is decreased and the reaction rate accelerated. Most reactions are catalyzed by Lewis acids like BFa OEta, EtAlCl2, Et AlCl, TiCU or for example SnCU, and thus can be carried out even at low temperature (-70°C). 

Generally, the rate of heat transfer is low near the burner wall because the temperature differences are very small. (Load movement is counterflow to flame movement thus, the flame reactants are coolest as they leave any one zone whereas the load pieces are hottest as they leave any one zone.) As the distance from the burner wall increases, the load surface is colder and the flame temperature is hotter because the combustion reaction rate accelerates. However, a control T-sensor 15 ft (4.6 m) from the burner wall limits the furnace temperature at that point. (This temperature is held to a setpoint determined by the operator or by a model.) With high-spin burners, as one follows the temperature profile away from its maximum and in the direction of flame reactant flow, the furnace temperature declines quickly to the setpoint, and thereafter drops rapidly to the exit. 

Uses Reaction rate accelerator copolymerizes readily with most other vinyl monomers modifies hydrophilic properties in systems incl. adhesives, coatings, cosmetics, textiles, syn. fibers, textile sizes, protective colloids, lube oil additives reactive diluent in UV- and electron beam-curable systems (coatings, inks, adhesives) aids pigment disp. (inks) intermediate for modified phenolic resins of interest as plasticizers, dye intermediates, textile assistants... 

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