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Characteristics of transfer reactions

Fixing the rate of heat transfer in a batch reactor is often not the best way to control the reaction. The heating or cooling characteristics can be varied with time to suit the characteristics of the reaction. Because of the complexity of hatch operation and the fact that operation is usually small scale, it is rare for any attempt to be made... [Pg.328]

Although widely used in the past and still used in special cases, the industrial sulfation with chlorosulfonic acid presents several problems which have caused the decline of this technique in favor of the more advantageous sulfation method with sulfur trioxide. These problems consist of evolution of the highly corrosive hydrogen chloride, heat transfer characteristics of the reaction, and the comparatively high level of chloride ion in the sulfated product compared with alcohol and alcohol ether sulfates obtained with sulfur trioxide. [Pg.228]

The temperatures and pressures developed are a function of the heat transfer characteristics of the reaction system. Hence, our observed pressures and temperatures relate only to this particular system. [Pg.355]

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... [Pg.192]

The polymerization reactor is of the heat-balance type because of the change in the heat transfer characteristics of the reaction mass during the polymerization. As the viscosity increases, the rate of heat dissipation by mixing will generally decline, which must be taken into consideration in setting up the equipment and in taking the appropriate measurements. [Pg.123]

Explanation The increase of both Y and DP with time implies a growing species of long life, which is characteristic of esters but not of carbenium ions, and an absence of transfer reactions. The suppression of all kinds of transfer reactions seems to require some very special features in the anionoid moiety of the ester which are not yet understood fully but which, by hypothesis, cannot affect the reaction pattern of an isolated, unpaired cation. The fact that living polymerisations can occur in toluene is a convincing demonstration that these reactions cannot involve carbenium ions, because growing cations alkylate toluene [33-37], a process which produces low DPs, independent of Y. [Pg.689]

The midpoint potential of a half-reaction E, is the value when the concentrations of oxidized and reduced species are equal, [Aox] = [Aredl- In biological systems the standard redox potential of a compound is the reduction/oxidation potential measured under standard conditions, defined at pH = 7.0 versus the hydrogen electrode. On this scale, the potential of 02/water is +815 mV, and the potential of water/H2 is 414 mV. A characteristic of redox reactions involving hydrogen transfer is that the redox potential changes with pH. The oxidation of hydrogen H2 = 2H + 2e is an m = 2 reaction, for which the potential is —414 mV at pH 7, changing by 59.2 mV per pH unit at 30°C. [Pg.253]

The stepwise formulation explains why boron becomes attached to the less-substituted carbon, but does not account for the fact that the reactions show no other characteristics of carbocation reactions. This could be because of an expected, extraordinarily fast rate of hydride-ion transfer to the carbocation. A more serious objection to the stepwise mechanism is that alkynes react more rapidly than alkenes, something which normally is not observed for stepwise electrophilic additions (cf. Section 10-5). [Pg.424]

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. [Pg.92]

Chemical reactions are studied in terms of elementary reactions involving only one step for bond breaking, bond formation, or electron transfer. A characteristic of elementary reactions is the molecularity. In other words, if... [Pg.107]

Complexity in multiphase processes arises predominantly from the coupling of chemical reaction rates to mass transfer rates. Only in special circumstances does the overall reaction rate bear a simple relationship to the limiting chemical reaction rate. Thus, for studies of the chemical reaction mechanism, for which true chemical rates are required allied to known reactant concentrations at the reaction site, the study technique must properly differentiate the mass transfer and chemical reaction components of the overall rate. The coupling can be influenced by several physical factors, and may differently affect the desired process and undesired competing processes. Process selectivities, which are determined by relative chemical reaction rates (see Chapter 2), can thenbe modulated by the physical characteristics of the reaction system. These physical characteristics can be equilibrium related, in particular to reactant and product solubilities or distribution coefficients, or maybe related to the mass transfer properties imposed on the reaction by the flow properties of the system. [Pg.104]

The kinetic characteristics of the reaction exert a considerable effect on the CSTR behaviour. If the kinetic curve is of the type represented in Fig. 32, in some parametric range self-oscillations of the reaction rate are possible in CSTR. Figure 33 illustrates the case in which the curve for the reaction rate (I) and the straight line for the substance transfer into the... [Pg.355]

The characteristics of liquid reaction with various values for M are listed in Table 7.1. It can be seen that, in the last two cases, i.e. Vm" <0.3, the processes are controlled by reaction kinetics and so enhancement of transfer becomes of no use in the medium case of 1, both diffusion and reaction kinetics affect the overall rate of the process, a measure of enhancing transfer may have a certain positive effect while in the former two cases, i.e. 4m >3, the reaction(s) in liquid proceed fast, the global processes are controlled by diffusion, and thus the measure of enhancing transfer will play a positive key role. [Pg.154]

The linearity of the plot of number-average molecular weight versus percentage conversion indicates that the amount of transfer reactions was low throughout the reaction. The increase in molecular weight was proportional to the degree of monomer conversion. The same characteristics have been observed for ROP of L-LA initiated by other cyclic tin alkoxides [83]. [Pg.51]

The experiments of Vos and eo-workers raise the question of whether the coherent nuclear motion associated with the P state that persists on the time-scale of eleetron transfer is coupled to the primary electron transfer reaction. In particular, do any of the nuclear vibrations coupled to the P state facilitate the transfer of electrons from P to Ba The observation of coherent nuclear motion that persists on the time-scale of primary electron transfer raises the possibility that this nuclear motion may be an important parameter that governs the characteristics of this reaction, which would place this process in a near-adiabatic regime. Of obvious importance is the question of whether it is possible to observe coherence in the formation of a produet state such as P Ba". A number of recent studies have addressed this difficult problem with conflicting conclusions (Sp"rlein et al., 1998 Streltsov et al., 1998 Vos et al., 1998 Streltsov et al., 1996) and, as discussed in reeent review (Vos and Martin, 1999) at present this question remains to be answered. [Pg.656]

This completes the first order description of transfer reactions within the NOx system. From this particular example we abstract the following characteristics which hold for all of the radical groups noted in Table 1. [Pg.348]

In spite of this success, dyads like 1 suffer from a major limitation as mimics of the natural electron transfer process. The very structural and electronic features which ensure rapid photoinitiated electron transfer, and consequently a high quantum yield, in these molecules also favor rapid charge recombination (step 3 in Figure 2). Thus, the P -Q " state lives at most a few hundred picoseconds in solution. The P-Q systems, and indeed other dyad-type artificial photosynthetic molecules, are unable to reproduce the long-lived charge separation characteristic of the reaction center. The stored energy is quickly lost as heat. [Pg.8]

Thus, the overpotential is logarithmically dependent on the current density. The parameter a and b are characteristics of electrochemical reactions and are readily obtained from a plot of t] vs. logf. The intercept of the straight line at t] = 0 gives the exchange current density and the slope can provide the transfer coefficient. Eq. (42) is commonly referred to as the Tafel equation. [Pg.2511]

From the characteristics of the reactions of type R1 effecting ionization in hydrogen and in oxygen, for which Exx+, the energy absorbed, is 15.6 and 12.2 e.v., respectively, estimates of the rate coefficients in electrolytic gas indicate that the predominant reaction for the range of X/n concerned is that generating the species 02 and the charge transfer reaction... [Pg.491]


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See also in sourсe #XX -- [ Pg.443 ]

See also in sourсe #XX -- [ Pg.443 ]




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