Practically irreversible reaction

Excited-state intramolecular proton transfer (ESIPT) exhibits different regularities [49, 50]. Commonly, this is a very fast and practically irreversible reaction proceeding along the H-bonds preexisting in the ground state. Therefore, only the reaction product band is seen in fluorescence spectra. Such cases are not interesting for designing the fluorescence reporters. The more attractive dual emission is... 

Three basic types of MCRs are now known. The MCRs of type I are collections of equilibrating subreachons. In type II the educts and intermediate products equilibrate, but their hnal products are irreversibly formed. The MCRs of type III correspond to sequences of practically irreversible reactions that proceed from the educts to the product. ... 

Finally, the polymerization of vinyl monomers usually proceeds as a practically irreversible reaction. Exceptions are heavily substituted monomers like a-methylstyrene for which propagation is clearly reversible. In the ring-opening polymerization, the driving force for polymerization comes from ring strain, thus it varies greatly for different monomers. 

In the cationic polymerization of heterocycles, a similar phenomenon was observed by Goethals in the polymerization of propylene sulfide and trans 2,3-dimethyl-thiirane. The latter monomer polymerizes rapidly and quantitatively to a linear polymer which is then relatively slowly converted into 3,4,6,7-tetramethyl-l, 2,5-tri-thiepane (J67a). In this particular process, the macroring formation is a practically irreversible reaction and differs in this sense from the equilibrium processes discussed so far. The irreversibility is due to the formation of one molecule of cis-butene per one molecule of a cyclic trithiepane ... 

The synthesis of sucrose by Leloir and collaborators has been discussed previously. These workers found that UDP-D-glucose is the D-glucose donor for formation of sucrose. The synthesis takes place by means of two separate enzymes, one utilizing D-fructose as the acceptor, and the other, D-fructose 6-phosphate. The sucrose phosphate formed in the second reaction is hydrolyzed by a phosphatase, resulting in the formation of free sucrose. As the equilibrium of the reaction for the formation of sucrose phosphate lies to the right, and inasmuch as the large accumulation of sucrose in some plants could be better accounted for by hydrolysis of the sucrose phosphate with phosphatase (which is a practically irreversible reaction), it was suggested that sucrose in plants is most likely synthesized by way of the sucrose phosphate intermediate. 

Both systems share the advantage of using practically irreversible reactions on cheap co-substrates, being thus suitable for strongly driving the reaction equilibrium of reversible reactions, for example, those catalyzed by ADHs, toward product formation under economically acceptable conditions. 

The physical situation for the absorption of gas A followed by a practically irreversible reaction with liquid phase reactant B can be represented by a pair of time dependent diffusion equationso... 

Figure 12.5 is the equivalent of Figure 12.1, but for a practically irreversible reaction (Example 12.6). In Figure 12.1 the Ag° of the reaction was small, and the minimum in the curve fell at x 0.82, well away from either axis. With the Ag° of this combustion reaction. Figure 12.5 shows that the equivalent of the curved g line is practically straight, except very close to the 1.00 axis, where there exists a minimum, corresponding to the calculated equilibrium state. 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

In deciding whether to write an elementary reaction as either a reversible or an irreversible reaction, we take the practical view that if the reverse reaction is negligibly slow on the exp>erimental time scale, the reaction is essentially irreversible. Consider the alkaline hydrolysis of an ester, for which the rate equation is... 

However, considering practical limitations, that is, the availability of optically pure enantiomers, E values are more commonly determined on racemates by evaluating the enantiomeric excess values as a function of the extent of conversion in batch reactions. For irreversible reactions, the E value can be calculated from Equation 1 (when the enantiomeric excess ofthe product is known) or from Equation 2 (when the enantiomeric excess ofthe substrate is knovm) [la]. For reversible reactions, which may be the case in enzymatic resolution carried out in organic solvents (especially at extents of conversion higher than 40%), Equations 3 or 4, in which the reaction equilibrium constant has been introduced, should be used [lb]. 

Reaction rates almost always increase with temperature. Thus, the best temperature for a single, irreversible reaction, whether elementary or complex, is the highest possible temperature. Practical reactor designs must consider limitations of materials of construction and economic tradeoffs between heating costs and yield, but there is no optimal temperature from a strictly kinetic viewpoint. Of course, at sufficiently high temperatures, a competitive reaction or reversibility will emerge. 

The subscript refers to a spherical particle. One should also remember that we limited ourselves to a first-order irreversible reaction. Other expressions can be derived but are beyond the scope of this book. Nevertheless, Eq. (35) has important practical implications, since it is possible to discuss the effectiveness of the system by a single dimensionless parameter, (fig. Figure 5.35 shows the effectiveness factor as a function of O,. 

A homogeneous irreversible reaction is characterized by a large value of the equilibrium constant so that the reverse reaction can be ignored and the reaction can be considered to proceed only in the forward direction. There is practically no major problem in the measurement of the rate of reaction which progresses only in one direction. It is, however, known... 

This may be accomplished by recognizing that E(t) 0 for times much longer than the mean residence time. Practically speaking, this usually occurs whenever t is 5 to 10 times the mean residence time. Hence, for an irreversible reaction, cA(t = 10F) = 0, and for a reversible reaction, cA(t = 101) = cAeq. Either of these initial conditions consequently represents cA at the inlet of the reactor, and permits the solution of 20.4-6. 

CO rapidly occupies the free sites on the Pd atom, and the rate-limiting step is reaction 2. Reactions 3 and 5 are very fast, making steps 2 and 4 practically irreversible... 

Practically irreversible multicomponent reactions (MCRs), like the Ugi 4-component reaction (U-4CR), can usually fulfill aU essential aspects of green chemistry. Their products can be formed directly, requiring minimal work by just mixing three to nine educts. Often minimal amounts of solvents are needed, and almost quantitative yields of pure products are frequently formed. 

Part of the discrepancies can be removed by considering a reaction which becomes important only in water. It was found that in acidic aqueous solutions water soluble phosphines react with activated olefins yielding alkylphosphonium salts [83-85] (Scheme 3.5). The drive for this reaction is in the fast and practically irreversible protonation of the intermediate carbanion formed in the addition of TPPMS across the olefinic bond. Under... 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

We need reaction-rate expressions to insert into species mass-balance equations for a particular reactor. These are the equations from which we can obtain compositions and other quantities that we need to describe a chemical process. In introductory chemistry courses students are introduced to first-order irreversible reactions in the batch reactor, and the impression is sometimes left that this is the only mass balance that is important in chemical reactions. In practical situations the mass balance becomes more comphcated. 

The mathematical apparatus presented in 2.3.3 is restricted to irreversible reactions. Hydrolysis, due to the large excess of one reactant, water, is for practical purposes irreversible. In a transesterification the concentration of the leaving alcohol (from... 

The electroanalytical chemist tends to use terms such as reversible and irreversible in a subtly different way, although the difference is more one of semantics rather than final understanding. Thus, the phrase totally irreversible is used by electroanalytical practitioners for reactions in the Tafel region [lT)l (RT/F)]. The physical electrochemist most certainly views such reactions as being thermodynamically irreversible, and distinctly so. However, it seems that the use of the term 101311/ irreversible may lead to misunderstanding, for it does not seem consistent with the ease of reversal in direction, which in practice can be made to occur even with the most (thermodynamically) irreversible reactions. 

When treating the overall transformation kinetics of an organic compound as we have done for the hydrolysis of benzyl chloride (Eq. 12-11), we assume that the reverse reaction (i.e., the formation of benzyl chloride from benzyl alcohol) can be neglected. For many of the reactions discussed in the following chapters we will make this assumption either because the reverse reaction has an extremely small rate constant (i.e., the reaction is practically irreversible), or because the concentration ) of the reactant(s) are very large as compared to the concentration(s) of the product(s). There are, however, situations in which the reverse reaction has to be taken into account. We have already encountered such a reaction in Illustrative Example 12.1. To demonstrate how to handle the reaction kinetics in such a case, we use the hydration of an aldehyde to yield a diol (Fig. 12.3). This example will also illustrate how the equilibrium reaction constant, Kn is related to the kinetic rate constants, kY and k2, of the forward and reverse reaction. 

The pellets of the commercial catalyst were crushed to grain size from 0.5 to 1 mm. A calculation on the basis of the measurements of the effective diffusion coefficient showed that the reaction proceeded in the kinetic region. Bed density of the catalyst was 1.23 g/cm3, specific surface after kinetic experiments was 36 m2/g. In the temperature range of 150-225°C reaction (342) is practically irreversible. The experiments proved (348) to be valid thus, the kinetics on low- and high-temperature catalysts is the same. 

The experiments were done at 70, 100, and 130°C and at pressures somewhat lower than atmospheric. Under these conditions reaction (368) is practically irreversible. Activated charcoal of the trademark Bayer AKT-4 ground to grain size 0.25-0.5 mm served as a catalyst. Estimation of the efficiency factor on the basis of the determination of the effective difusion coefficient of hydrogen in nitrogen or helium has shown that for this grain size the results of reaction rate measurements refer to the kinetic region. Estimation of relaxation time of the reaction rate from (67) showed the reaction to be quasi-steady at the condition of our experiments in the closed system. 

If the reaction is conducted under such conditions that it is practically irreversible, but is not too far from equilibrium, then in accordance with considerations in Section XI, one of its stages must be irreversible namely the slowest one. Therefore, one of the rate constants of backward reactions, k — or k 2, should be taken as equal zero. Assuming k 2 =0, we obtain from (380)... 

The comparison of our measurements of the rate of reaction (377) with (383) showed that this equation is not obeyed quantitatively. The disagreement of (383) with experimentation already noted in preceding works can be explained by the nonuniformity of the surface not being taken into account. To describe the rate on a nonuniform surface under the conditions of practical irreversibility of the reaction, (214) can be applied. It should, however, be remembered that this equation corresponds to the mechanism with the irreversible first stage, and in our case it is the second stage that is irreversible. Therefore, in order to make use of (214), the numbering of... 

Subsequently, aliphatic 1-substituted, 1,1-, and 1,2-disubstituted ethylenes were investigated (3) using RhH(CO)(PPh3)3 in the presence of (-)DIOP (diisooctylphthalate) (4) as the chiral ligand, and asymmetric induction was shown to occur during or before the formation of a rhodium-alkyl intermediate (R-[Rh]). This formation appears to be practically irreversible under the reaction conditions used (5). As another consequence of this important feature, regioselection in hy-droformylation is likely to occur during or before the formation of the rhodium-alkyl intermediates. 

Similarly to the hydroformylation, under certain reaction conditions the formation of the intermediate palladium-alkyl complex can be practically irreversible as shown by the different prevailing chirality of the 2-methylbutanoic acid ester obtained from 1-butene and (Z)-2-butene, as well as from ( )- and (Z)-2-butene. Therefore, re-gioselection and enantioface selection must occur, as in hydroformylation, during or before the formation of the postulated palladium-alkyl intermediate (see Scheme IV). 

Thus the approach to equilibrium always follows a first-order rate law, Equation (14), with the pH-dependent rate constant kobs = kE + kK. Figure 1 shows the concentration changes in time starting from a 1m solution of pure enol (full line) and of pure ketone (dashed line). The individual, unidirectional rate constants kE and kK can be determined as follows For most ketones the equilibrium enol concentration is quite small, i.e., tE = cE(oo)/ ck(oo)<<1. Hence kE kK [Equation (1)], so that enol ketonization is practically irreversible and kE may be neglected, kobskK. The rate constant of enolization kE, on the other hand, is equal to the observed rate constant of reactions for which enolization is rate-determining, such as ketone bromination (Scheme 2). 

Most organic radicals react practically irreversibly with 02 at close to diffusion-controlled rates (typically at 2 x 109 dm3 mol1 s 1), and in air-saturated aqueous solutions the lifetime of these radicals will only be 2 ps [reaction (1) for a compilation of rate constants see Neta et al. 1990],... 

In this chapter some of the theoretical concepts used in these models will be outlined. In particular, emphasis will be given to the chemical thermodynamic principles that can be used to predict the stable forms of a given element. Such chemical principles provide the theoretical foundation of the commonly used chemical models. These models can be used to predict the final extent of reaction but not the rate. It is probably fair to say that these laws as basic principles are indisputable scientific fact however, problems arise when we try to apply them to ill-defined complex natural media such as soils and soil solutions where some reactions are kinetically slow and practically irreversible. However inadequate our chemical models are in relation to real-world situations they are the best we have and can be used to give valuable insight and meaning into the processes we observe. 

---