Reaction acceleration

The rates of several chemical reactions accelerate by factors of 10 or more between 0.1 and 100 MPa at ambient temperature, so much interesting chemistry occurs at these lower pressures. At such Tow pressures, Bridgman [26] even showed how to cook eggs at room temperature. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Above about 250°C, the vapor-phase oxidation (VPO) of many organic substances becomes self-sustaining. Such oxidations are characterized by a lengthy induction period. During this period, peroxides accumulate until they can provide a source of new radicals to sustain a chain reaction. Once a critical threshold peroxide concentration is reached, the reaction accelerates very rapidly. 

Continuous mass polymerisation units are extensively used for making polystyrene. Great care is necessary to prevent the heat of reaction accelerating the polymerisation to such an extent that the reaction gets out of control. The problem is made particularly difficult by the fact that heat can only be taken away from the points of higher temperature by conduction because of the very high... 

Other ingredients may be added to prevent sticking to moulds (lubricants), to promote the curing reaction (accelerators), to improve the flow properties (plasticisers) and to colour the product (pigments). 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

As suggested by the name, the products of an autocatalytic reaction accelerate the rate of the reaction. For example, an acid-catalyzed reaction may produce... 

Finally, the oxidation reaction has to been run under strict conditions of temperature, which are impossible to be operated in a batch reactor. Indeed, utility stream in the Shimtec reactor was heated to 47 °C, which first initiates the reaction, accelerates its kinetics, and then controls the temperature when the heat of the reaction is too important. In a batch reactor, working with such UF temperature is impossible because of security constraints. It would certainly lead to a reaction runaway. We now consider this question in the next section. 

This reaction accelerates very much in presence of the potassium hydroxide that serves as the catalyst. 

Changing process or mechanical conditions to reduce the potential for runaway reactions, accelerated corrosion or erosion, or other possible causes of undesirable events... 

Roskamp reported29 a similar intramolecular Diels-Alder reaction accelerated by silica gel saturated with water. The reaction led to the ready construction of the 11-oxabicyclo [6,2,1] ring systems (Eq. 12.4). The intramolecular Diels-Alder reaction has also been investigated by Keay.30 The Diels-Alder reaction of 2,5-dimethylpyrrole derivatives with dimethyl acetylenedicarboxylate in water generated the corresponding cyclization products.31... 

It has to be assumed that these processes are occurring on the boundary between SN1(P) and SN2(P) mechanisms in whose transition states considerable P—0(—Ar) bond cleavage takes place. The lifetime of the resulting, more or less free metaphosphate anion 102 then depends upon the nucleophilicity of the surrounding solvent. With pyridine, for example, a very fast reaction occurs so that the overall process approaches an SN2 reaction. Acceleration of the reaction by amines such as 2,6-lutidine, which are disqualified from acting as nucleophiles by steric hindrance, or by solvents such as dioxane, whiche are presumably too... 

A mixture of the (highly activated) dichloro compound with potassium fluoride in the solvent was heated to reflux to effect replacement of chlorine by fluorine. The reaction accelerated out of control and exploded, leaving much carbonised residue. Analogous reactions had been effected uneventfully on many previous occasions. See Dimethyl sulfoxide Acyl halides, etc. 

Benzyl acetate was prepared by addition of benzyl chloride (containing 0.6% pyridine as stabiliser) to preformed sodium acetate at 70°, followed by heating at 115°, then finally up to 135°C to complete the reaction. On one occasion, gas began to be evolved at the end of the dehydration phase, and the reaction accelerated to a violent explosion, rupturing the 25 mm thick cast iron vessel. This was attributed to presence of insufficient pyridine to maintain basicity, dissolution of iron by the... 

Dropwise addition of the chloride caused a steady exotherm to 60-65°C when a runaway reaction accelerated to explosion. 

A mixture of the hydride and solvent was heated to, and held at 50°C. An exothermic reaction, which increased the temperature to 75°C, could not be controlled by external cooling, and the pilot-scale reactor contents erupted. It was later found that an exotherm began to develop at 26° (or at 40-50° with carefully dried solvent), and the subsequent reaction accelerated rapidly. Avoidance of holding mixtures hot is recommended, particularly when scaling up reactions. Similar behaviour was seen with dimethylacetamide [1], A further similar plant-scale incident was reported, with onset of the exotherm at 40°C, followed by self-heating to 100°C in 10 min, even with maximum cooling applied [2],... 

White phosphorus begins to reduce the acid at 25-30°C and the vigorous reaction accelerates to explosion. Red phosphorus is similar at a higher initial temperature. 

A mixture of the sulfide, ethylene glycol and hexane in a mantle-heated flask spontaneously overheated and exploded at an internal temperature of around 180°C. It had been intended to maintain the reaction temperature at 60°C, but since alcoholysis of the sulfide is exothermic, presence of the heating mantle prevented the dissipation of heat, and the reaction accelerated continuously until explosive decomposition occurred [1], An incident in similar circumstances involving interaction of the sulfide with 4-methyl-2-pentanol also led to violent eruption of the flask contents. 

In many cases, the comparison of a reaction accelerated by microwave irradiation has been made with the same reaction in an oil bath at the same bulk temperature. Unfortunately, there have been quite a few reports in the chemical literature that have not been conducted with such proper control of conditions and consequently a fair comparison is not possible. Nevertheless, using this MW approach, the problems associated with waste disposal of solvents that are used several fold in chemical reactions, and excess usage of chemicals are avoided or minimized. The discussion pertaining to the preparation of supported reagents or catalysts has not been included in this chapter because numerous review articles are available on this theme [14—22],... 

Reactions accelerated by microwaves 363 Reactions not accelerated by microwaves 363 Superheating of liquid reaction mixture 3 64 Localized superheating in the solid phase 365 Selective heating 365 Hot spots 366... 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

The investigation team found that the reaction accelerated beyond the heat-removal capacity of the reactor. The resulting high temperature led to a secondary runaway decomposition reaction, causing an explosion that blew the hatch off the reactor and allowed the release of the contents from the vessel. 

Runaway reactions can be triggered by a number of causes, but, in most cases., their resultant features after initiation are similar [31]. Whenever the heat production rate exceeds the heat removal rate in a reaction system, the temperature begins to rise and can get out of control. The runaway starts slowly but the rate of reaction accelerates, and the rate of heat release is very high at the end. Most runaways occur because of self-heating with the reaction rate (and reaction heat output) increasing exponentially with temperature, while the heat dissipation is increasing only as a linear function of the temperature. 

Scheme 8.39. Asymmetric methylalumination reaction accelerated by addition of water.

Methylene blue and other reducible dyes were shown to enhance the activity of NADPHa-linked MHbR (K8, K9). This is confirmed by the finding that intravenous injections of methylene blue in methemo-globinemic patients result in a striking decrease of MHb levels (e.g., B14, K9, K10). This seems to be paradoxical, since methylene blue is capable of reacting with Hb with formation of MHb, but the dye reacts much more effectively as an artificial electron carrier in the NADPH2-MHbR Systran (B14). It has been stated (K10) that methemoglobin reduction is associated with the formation of pyruvate in equivalent amounts, but that in reactions accelerated by reducible dyes no correlation between pyruvate formation and MHb reduction could be found. 

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