Accelerated reactions

Sulfonamide effect addition of MeS02NH2 enhances hydrolysis of Os (VI) glycolate (accelerates reaction)... 

Metabolic Functions. The functions of the thyroid hormones and thus of iodine are control of energy transductions (121). These hormones increase oxygen consumption and basal metaboHc rate by accelerating reactions in nearly all cells of the body. A part of this effect is attributed to increase in activity of many enzymes. Additionally, protein synthesis is affected by the thyroid hormones (121,122). 

The operating temperature also affects the fuel cell operating potential, A high operating temperature accelerates reaction rates but... 

High-temperature corrosion is induced by accelerated reaction rates inherent in any temperature reaction. One phenomenon that occurs frequently in heavy oil-firing boilers is layers of different types of corrosion on one metal surface. 

The sensitivity of cellular constituents to environmental extremes places another constraint on the reactions of metabolism. The rate at which cellular reactions proceed is a very important factor in maintenance of the living state. However, the common ways chemists accelerate reactions are not available to cells the temperature cannot be raised, acid or base cannot be added, the pressure cannot be elevated, and concentrations cannot be dramatically increased. Instead, biomolecular catalysts mediate cellular reactions. These catalysts, called enzymes, accelerate the reaction rates many orders of magnitude and, by selecting the substances undergoing reaction, determine the specific reaction taking place. Virtually every metabolic reaction is served by an enzyme whose sole biological purpose is to catalyze its specific reaction (Figure 1.19). 

Enzymes display enormous catalytic power, accelerating reaction rates as much as lO over uncatalyzed levels, which is far greater than any synthetic catalysts can achieve, and enzymes accomplish these astounding feats in dilute aqueous... 

Metals that catalyze reaction (2) generally catalyze reaction (1) as well, and the rate for (1) is usually somewhat higher. Given such a catalyst, what product(s) would be obtained were phenylacetylene to be combined with one equivalent of H2 What would happen if the catalyst accelerated reaction (2) more effectively than reaction (1) ... 

Microwave chemistry has been found to be a useful method for accelerating reactions or catalyzing reactions that are difficult to carry out by other methods. A modification of the Hantzsch method to directly obtain pyridines has been communicated. A dry medium using ammonium nitrate bentonitic clay system with microwave irradiation affords pyridines 96 in a single pot within 5 minutes. When the pyridine is not the major product (> 75% yield), the dealkylated pyridine 97 becomes an... 

When a positively charged substituent such as the trimethylam-monio group is anywhere on the ring, but most effectively when it is ortho to the leaving group, it can favorably affect the entropy of activation with anionic nucleophiles and accelerate reaction. A recent example of reagent-substituent interaction is the electrophilic substitution of 2-carboxybiphenyl, nitration (non-polar solvent) of which occurs only at the 2 -position and not the 4 -position and has been postulated to be due to the interaction of the nitronium ion with the carboxyl group. 

This form assumes that the effect of pressure on the molar volume of the solvent, which accelerates reactions of order > 1 by increasing the concentrations when they are expressed on the molar scale, has been allowed for. This effect is usually small, ignored but in the most precise work. Equation (7-41) shows that In k will vary linearly with pressure. We shall refer to this graph as the pressure profile. The value of A V is easily calculated from its slope. The values of A V may be nearly zero, positive, or negative. In the first case, the reaction rate shows little if any pressure dependence in the second and third, the applied hydrostatic pressure will cause k to decrease or increase, respectively. A positive value of the volume of activation means that the molar volume of the transition state is larger than the combined molar volume of the reactant(s), and vice versa. 

It has been frequently suggested that dynamical factors are important in enzyme catalysis (Ref. 9), implying that enzymes might accelerate reactions by utilizing special fluctuations which are not available for the corresponding reaction in solutions. This hypothesis, however, looks less appealing when one examines its feasibility by molecular simulations. That is, as demonstrated in Chapter 2, it is possible to express the rate constant as... 

This chapter begins by explaining how the rates of reactions are determined experimentally and showing how their dependence on concentration can be summarized by succinct expressions known as rate laws. We then see how this information gives us insight into how reactions take place at a molecular level. Finally, we see how substances called catalysts accelerate reactions and control biological processes. 

A rate law summarizes the dependence of the rate on concentrations. However, rates also depend on temperature. The qualitative observation is that most reactions go faster as the temperature is raised (Fig. 13.22). An increase of 10°C from room temperature typically doubles the rate of reaction of organic species in solution. That is one reason why we cook foods heating accelerates reactions that lead to the breakdown of cell walls and the decomposition of proteins. We refrigerate foods to slow down the natural chemical reactions that lead to their decomposition. 

The curved arrows show how one resonance structure relates to another. Notice that the formal negative charge is located on the ortho and para positions, exactly where reaction takes place most quickly. Other ortho- and para-directing groups include —NH2, —Cl, and —Br. All have an atom with a lone pair of electrons next to the ring, and all accelerate reaction. 

Incorrect vessel charge either due to automatic control failure or plant operator error could result in excess catalyst or reactant concentration, etc. This could cause a rapidly accelerating reaction rate or could initiate unexpected side reactions, which could be more severe than the normal reaction. 

There is sfill some dispufe about how microwave irradiation accelerates reactions. Besides the generally accepted thermal effects, one beheves that there are some specific (but also thermal) microwave effects, such as the formation of hot spots . There is still some controversy about the existence of non-thermal (athermal) microwave effects. At the present time, new techniques such as coohng while heating are being investigated and the problem of upscahng... 

It is possible to strongly accelerate reactions assumed to be slow by increasing the reaction temperature (for example, by acting on the pressure), increasing the amount of catalyst, reducing the amount of solvent, and therefore increasing the concentration of the reactants. 

Enzymes accelerate reaction rates by lowering the activation barrier AGp. While they may undergo transient modification during the process of catalysis, enzymes emerge unchanged at the completion of the reaction. The presence of an enzyme therefore has no effect on AG for the overall reaction, which is a function solely of the initial and final states of the reactants. Equation (25) shows the relationship between the equilibrium constant for a reaction and the standard free energy change for that reaction ... 

Catalysts accelerate reactions by orders of magnitude, enabling them to be carried out under the most favorable thermodynamic regime, and at much lower temperatures and pressures. In this way efficient catalysts, in combination with optimized reactor and total plant design, are the key factor in reducing both the investment and operation costs of a chemical processes. But that is not all. 

Catalysts accelerate reactions and thus enable industrially important reactions to be carried out efficiently under practically attainable conditions. Very often, catalytic routes can be designed such that raw materials are used efficiently and waste production is minimized. Consequently, the chemical industry is largely based upon catalysis Roughly 85-90 % of all products are made in catalytic processes, and the percentage is increasing steadily. 

The oxidation of a series of olefins reveals the reaction to be very insensitive to electronic effects . Phenyl and methyl substitution of the olefin mildly accelerate reaction. In all cases A , is pH-independent. Data are collected in Table 1. 

The Co(ni) perchlorate oxidation of substituted and unsubstituted benzal-dehydes has kinetics and a low isotope effect (2.3 at 10 °C) in complete analogy with cyclohexanol and formaldehyde . Ring-substitution by electronegative groups accelerates reaction. 

Murphy, E.R., MartineUi, LR-, Zaborenko, N., Buchwald, S.L., Jensen, K.F. (2007) Accelerating Reactions with Micro-Reactors at Elevated Temperatures and Pressures Profiling Aminocarbonylation Reactions. Angewandte Chemie, 119(10), 1764-1767. 

Phase-transfer (PT) catalysts accelerate reactions of two immiscible reactants. Without a PT catalyst reactions between substances located in different phases are often slow or do not occur at all. The PT catalyst usually has the function of transferring anions, in the form of an ion pair, from the aqueous phase to the organic phase, in which the reaction with the water-insoluble reactant takes place (see schematic representation in Fig. 3.56). 

In summary, in situ STM studies of CO titration on the oxygen precovered metal surfaces have demonstrated atomic details of CO oxidation on metal surfaces and have shown excellent agreement with macroscopic kinetic measurements. Moreover, in situ studies have revealed an interesting but not well-understood, nonlinear behavior of reaction kinetics. The accelerated reaction rate observed takes place only when surface oxygen islands, either compressed oxygen islands or surface oxide islands, are reduced to the nanometer size. The nonlinear reactivity of these nanoislands is in stark contrast with the large adsorbate layer and requires further investigations. 

---