Reaction reversibility, effect

Here, the reaction proceeds effectively to completion (HCl) is very large relative to (Hj) and (Clj) and hence (and K ) are also large. In these circumstances the reverse arrow is usually omitted. 

A new class of synthetic aldehyde reactions involves effectively reversing the polarity of the carbonyl group by forming a dithiane intermediate. 

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

For a given reaction studied in a series of solvents, (8r- 8 f) is essentially constant, and most of the change in In k will come from the term — AV (8j — 8s)". If AV is positive, an increase in 8s (increase in solvent internal pressure) results in a rate decrease. If AV is negative, the reverse effect is predicted. Thus reactivity is predicted by regular solution theory to respond to internal pressure just as it does to externally applied pressure (Section 6.2). This connection between reactivity and internal pressure was noted long ago," and it has been systematized by Dack. -" ... 

The application of this principle to several different systems is shown in Table 12.7. In system 2, the number of moles of gas decreases from to 1 as the reaction goes to the right. Hence increasing the pressure causes the forward reaction to occur a decrease in pressure has the reverse effect. Notice that it is the change in the number of moles of gas that determines which way the equilibrium shifts (system 4). When there is no change in die number of moles of gas (system 5), a change in pressure has no effect on the position of the equilibrium. 

Preliminary experiments prove that the substitution pattern of the /V-aryl moiety of imine 1 is crucial for the stereoselectivity of this reaction. The 2-substituent on the aryl group is of special importance. Namely, introduction of a methoxy group leads to a considerable decrease of enantioselectivity compared to the corresponding 2-H derivative, probably due to disfavor-able coordination with the organolithium complex. In contrast, alkyl groups show the reverse effect along with increased bulkiness (e.g., Tabic 1, entries l-3a) but 2,6-dimethyl substitution provides lower ee values. Furthermore, the 4-substituent of the TV-aryl moiety is of minor importance for the stereoselectivity of the reaction [the Ar-phcnyl and the /V-(4-methoxyphenyl) derivatives give similar results], whereas a substituent in the 3-position results in lower stereoselectivities (e.g., Et, Cl, OCHj)41. 

In a catalysed system, the spontaneous reaction may also occur, but in many cases it is insignificant compared to the catalysed rate of reaction. The effect of a catalyst is to accelerate the rate of both the forward and reverse reaction, allowing equilibrium to be reached much more quickly. The concentration of the reactants is still important, as this affects the probability that the catalyst will interact with the reactants and trigger the reaction. 

Fig. 8 Typical cyclic voltammograms of pure electron transfer reactions (a) effect of quasi-reversibility ks decreases from solid to dashed line) (b) effect of relative values of...

A catalyst lowers the activation energy of a reaction by providing an alternative mechanism. A catalyst also increases the rate of the reverse reaction. What effect does a catalyst have on AH of a reaction ... 

The last reaction is effectively irreversible under the usual conditions employed to hydrogenate olefins however much information pertinent to this discussion has been obtained by studies of the exchange of saturated hydrocarbons with deuterium (7, 59), a reaction which is initiated through the reversal of reaction (4). 

If the addition of the second hydrogen atom is the rate-controlling surface reaction, then the preceding steps would tend to be reversed, the degree of reversibility being a function of the relative rates of the several reactions. Two effects are expected (1) the isomerization of the initial olefin is pronounced and (2) the proportion of saturated products should tend towards the equilibrium distribution. Indeed, such effects are commonly observed when palladium catalysts are employed (5, 65, 66) (Fig. 9). 

There were also studied processes that cause passivation of the zinc electrode in many cycling operations of Ni-Zn batteries [323]. Positive effect for increasing the reaction reversibility was found when zinc-ion additives were introduced to the positive electrode [324]. 

The H02-aldehyde reaction is in parentheses because, as we shall see later, it is a reversible reaction that is sufficiently fast in the reverse direction under typical tropospheric conditions that no overall reaction, in effect, occurs. 

Many thermal reactions are effectively irreversible under the conditions employed, but some are reversible and an equilibrium position is reached between substrates and products. The position of equilibrium depends on the standard free energy difference between the two (AC - = - RT In K and on reagent concentrations, and A varies with temperature. Such considerations rarely apply to photochemical reactions, the overwhelming maiority of which are effectively irreversible (1.3), and the products are not in thermodynamic... 

Since the triplet reactions occur at rates much faster than any measured radiationless decay, a likely remaining process which could lower the quantum yield is the reverse of the initial biradical forming reaction, in effect a disproportionation between the two radical sites. 

If the carbohydrate formed is cellulose, then the reaction in effect is the reverse of the burning of wood, and obviously requires considerable energy input. 

The ke[ values of photoinduced electron transfer reactions from [Ru(bpy)3]2 + to various nitrobenzene derivatives in the presence of 2.0 mol dm-3 HC104 are listed in Table 1, where the substituent effect is rather small irrespective of electron-withdrawing or donating substituents. A similar insensitivity to the substituent effect is also observed in the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2+ to acetophenone derivatives [46,47]. The stronger the electron acceptor ability is, the weaker is the protonation ability, and vice versa. Thus, the reactivity of substrates in the acid-catalyzed electron transfer may be determined by two reverse effects, i.e., the proton and electron acceptor abilities, and they are largely canceled out. Such an insensitive substituent effect shows a sharp contrast with the substituent effect on the acid-catalyzed hydride transfer reactions from Et3SiH to carbonyl compounds, in which the reactivity of substrates is determined mainly by the protonation ability rather than the electron acceptor ability. 

The intentional design of model systems can be envisioned, as for instance binary or multiple assemblies (clusters) of active components and poisons, for the examination of their activity in chemisorption, or specific reactions. The results can then be compared with respective clusters containing the active species only. Perhaps, such model systems will be amenable to computational methods capable of predicting their chemisorptive behavior and their surface reactivity. Such approaches are now employed for the design of improved multicomponent catalysts and can, obviously, be used to study the reverse effect, i.e., the mutual deactivation of the cluster components. 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

On the other hand, at high concentrations of acetaldehyde, when the intermediate enolate carbanion is rapidly captured by another molecule of aldehyde, reverse of the initial parallel proton-abstraction steps is prevented (k3[CH3CHO] k-1 and k 2[BH 1 ]), and the rate of the overall reaction is effectively limited by the initial proton abstractions these then constitute (parallel) rate-limiting steps. The overall process is now first order in acetaldehyde and shows general-base catalysis [5], i.e. the rate law is given by Equation 3.13 ... 

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