Reactions, rapidity

Adding up the steps in a mechanism must yield the net chemical reaction rapid reactions may follow the rate-controlling step. 

This has been a brief overview of a rich field. Details of enzyme stracture and catalytic activity are studied in laboratories worldwide. Moreover, genetic engineering makes it possible to manufacture key enzymes in large quantities, so enzymes may become industrial catalysts that accomplish reactions rapidly and selectively. 

Borane dissolved in THF or dimethyl sulfide undergoes addition reactions rapidly with most alkenes. This reaction, which is known as hydroboration, has been extensively studied and a variety of useful synthetic processes have been developed, largely through the work of H. C. Brown and his associates. 

The combustion of H2 and 02 in the cavitation bubble [20] and the consumption of these gases, when in the ratio of 1 3 and 2 1, favoured the maximum formation of H2O2. Since the conditions of gases in a cavitating bubble were different from those under flame, the mechanism could not precisely be the same. The intermediates of chain reactions rapidly reached cooler interfacial region of the bubble and therefore did not allow the chain length to go beyond 10 steps. Fe2+ ions present in the... 

Figure 12.2 is a plot of the effectiveness factor r] versus the Thiele modulus hT. For low values of hT (slow reaction, rapid diffusion), the effectiveness factor approaches unity. For values of the Thiele modulus above 2.0, tanh hT 1 and the effectiveness factor may be approximated by... 

Because of these requirements, and in particular, the need to perform the reactions rapidly, it is not surprising that microwaves were used in the PET synthesis area at a very early stage [92], However, the subsequent increase has not been as dramatic as one might have anticipated. Two recent reviews [8, 9] give an up-to-date picture whilst here we present some noteworthy examples. 

It is obvious, then, that only the H2—Cl2 reaction can be exploded photo-chemically, that is, at low temperatures. The H2—Br2 and H2—12 systems can support only thermal (high-temperature) explosions. A thermal explosion occurs when a chemical system undergoes an exothermic reaction during which insufficient heat is removed from the system so that the reaction process becomes selfheating. Since the rate of reaction, and hence the rate of heat release, increases exponentially with temperature, the reaction rapidly runs away that is, the system explodes. This phenomenon is the same as that involved in ignition processes and is treated in detail in the chapter on thermal ignition (Chapter 7). 

While the rates of alkaline and pH-independent hydrolysis of [93] and [94] are similar, hydronium ion catalysis is 2500 times less favourable in the case of the cyclic acylal. The large rate difference in comparison with the open chain analogue may reflect the fact that, if a carbonium ion intermediate with a carboxyl group held rigidly adjacent to the carbonium carbon atom were formed in an A-1 reaction, rapid reclosure of the ring would result from recapture of the carbonium carbon by the carboxyl group [equation (55)]. A greatly diminished rate of hydrolysis would result. Perhaps there is a... 

In addition to the activity, other important requirements for the catalyst are the capability to start the reaction rapidly without the necessity for previous reduction with hydrogen and to perform effectively with intermittent operation these are essential properties for the catalyst in reformers, especially for portable and small-scale stationary fuel cell applications. In this respect, Dias and Assaf [61] focused on the potential of Pd, Pt and Ir to promote fast and intermittent ignition of methane ATR in Ni/y-Al203. They concluded that the three metals are very good promoters of the reduction of the nickel catalyst with methane, but the lower cost of palladium makes this metal more suitable than Pt and Ir for small fuel cells. 

The reaction cannot begin without initiation of See radicals, and H2 dissociation is very slow. Therefore, the few H atoms produced by the initiation steps may diffuse to the walls of the vessel and recombine before they can begin chain reactions. However, the presence of H2O accelerates the rate of the reaction because it causes formation of traces of H2O2, which easily dissociates and forms more H and -OH, which initiate the reaction by attack of H2 and O2, and these reactions rapidly produce more radicals, which strongly accelerate the process. 

The above reaction is reversible above 1,850°C. The metal produced as vapor must be cooled rapidly to prevent any reversible reactions. Rapid cooling (shock cooling) can quench the reaction giving finely divided pyrophoric dust... 

Encyclopedia 3(PATR 2700) (1966), pp C275 to C288 (Chromates and Dichromates, Including Their Analytical Procedures) C411 to C417 (Analytical Procedures Based on Color Reactions. Rapid Method... 

However, these cations are stabilized by an arenechromium tricarbonyl group in the a-position. Thus benzyl alcohol complexed with chromium tricarbonyl undergoes Ritter reactions rapidly and in high yield (equation I). On the other hand, the... 

Most enzymes have certain kinetic properties in common. When substrate is added to an enzyme, the reaction rapidly achieves a steady state in which the rate at which the ES... 

Folic acid and its polyglutamyl derivatives can be reduced to the THF coenzymes in two stages the first step is a slow reduction with NADPH to 7,8-dihydro-folate (step a, Fig. 15-18). The same enzyme that catalyzes this reaction rapidly reduces the dihydrofolates... 

These reactions rapidly found wide use and success, and many other sulfur ylides have been prepared and exploited [194, 195, 203, 204]. Various experimental procedures are to be found in the detailed monograph by Trost and Melvin [204] for sulfonium salts, ylides, epoxidations and cyclopropanations. 

Analytical Procedures Based on Color Reactions Rapid Method of Identification af Common Explosive Compounds and of Explosive Mixtures... 

Addition of a cyanohydrin acetal anion to [(benzene)Cr(CO)3] followed by reaction with allyl bromide produces the cyclohexadiene derivative (73) in 94% yield, which undergoes a Diels-Alder reaction rapidly to give a tricyclic framework (74). After quenching with methyl iodide and disassembling of the cyanohydrin group, the diketone (75) is obtained in 50% yield overall (equation 51).125 These products are obviously interesting as potential intermediates for synthesis. 

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