Reactions reactant-inhibited

Reaction of a i solid with a solid] As with solid and gas Normally the immobility of reactants inhibits reaction and barrier layer for-1 tion is more common... 

Negative orders with respect to products are fairly common. A negative order with respect to a reactant (reactant-inhibited reaction) is unusual. An example is olefin hydroformylation with a reaction order between zero and minus one in CO, a reactant (see eqn 6.12). This behavior results from a first step in which one CO ligand is displaced from the catalyst, and the fact that one of the two later steps in which CO re-enters the pathway occurs after an irreversible step. 

In some reactions, the rate increases rather than decreases as conversion progresses. This is loosely called autocatalysis, although no genuine catalysis may be involved. The acceleration may stem from promotion by a product or major early intermediate, or from consumption of a reactant that functions as inhibitor. In product-promoted reactions, the kinetics order with respect to a product (or early intermediate) is positive. This causes the rate to increase to a maximum and then to decline as the effect of consumption of the reactant or reactants begins to overcompensate that of promotion by the product. In reactant-inhibited reactions, the order with respect to a reactant is negative. The rate may increase until the respective reactant is used up and, in some cases, may theoretically approach infinity. The latter behavior is, of course, physically impossible, and another mechanism or event necessarily takes over. 

In a reactant-inhibited reaction, slow mass transfer of the inhibiting reactant from another phase accelerates the rate and may cause instability. 

In product-promoted reactions in which the accelerating species exits into another phase, or in reactant-inhibited reactions in which the respective reactant enters from another phase, slow mass transfer may boosts rather than depresses the reaction rate. In reactant-inhibited reactions, slow supply of the inhibiting reactant may cause the system to become unstable There may be a sharp stability limit beyond which catastrophic selfacceleration occurs until the phase has become depleted of the reactant or reactants or some other phenomenon has come into play. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

Rather than because of promotion by a product or intermediate, the rate may accelerate because a reactant that acts as inhibitor is consumed. For example, a small amount of inhibitor present initially may depress the rate of a chain reaction until used up (see Section 10.8). More interesting are reactions inhibited by one of the principal reactants (called substrate-inhibited in biochemistry parlance). An example is hydroformylation, in which CO is a reactant with negative reaction order (see Example 6.2 in Section 6.3). There is a subtle but important difference between product-promoted and reactant-inhibited reactions The rate of a product-promoted reaction builds up to a maximum and then declines as reactant depletion overpowers product promotion. In contrast, the rate of a reactant-inhibited reaction keeps escalating, possibly catastrophically, until the respective reactant is almost completely exhausted. Typically, some other mechanism then takes over. [The negative apparent reaction order of the respective reactant arises from an additive denominator term in a one-plus rate equation, but the other terms may be small or insignificant by comparison.] Possible mass-transfer implications of such behavior will be examined in Section 13.3. 

The acceleration and chance of instability in a reactant-inhibited reaction are less pronounced if mass transfer of the respective reactant is fast. Thus, the very high mobility of hydrogen makes the effects unlikely to occur in the most common situation of reactant inhibition, that is, in heterogenous hydrogenation (see Example 9.4 in Section 9.3.1). 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

Micelles have effects on spontaneous reactions which can be related to mechanism and the properties of the micellar surface. Inhibition of bimolecu-lar, micelle-inhibited reactions is also straightforward because micelles keep reactants apart. 

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

Even though the reaction is bimolecular, reactant inhibition does not occur for this type of reaction. 

During the slow oxidation of triethylamine, one of the products, diethylamine, appears to inhibit reactions leading to its own formation (9). Moreover, acetaldehyde is formed in the presence of oxygen and is stable at temperatures at which it is normally oxidized readily. This result was ascribed to inhibition of acetaldehyde by both the reactant (triethylamine) and the principal nitrogenous products (diethylamine and ethylamine). These findings preceded several detailed studies of the effect of aliphatic amines on the slow oxidation and the ignition of... 

Remarkably, DMF functions both as a solvent and reactant inhibition and trapping experiments support a SET mechanism producing difluorocarbene, which on further reaction with fluoride, generated by reaction of difluorocarbene with DMF, eventually leads to the trifluoromethylzinc reagent (Scheme 7). 

This rate expression was found to fit the results particularly well at low concentrations of ozone in mixtures with high concentrations of oxygen. The order of effectiveness of various gases for inhibition of the decomposition of ozone was found to be O2, 1 C02, 0.8 N2, 0.3 He, 0.13. Making allowance for the fact that 02 may be a reactant in reaction (27) as well... 

All of the previously mentioned nonlinearities are actually monotonic. Nonmonotonic functions are very common in gas-solid catalytic reactions due to competition between two reactants for the same active sites, and also in biological systems, such as in substrate inhibited reactions for enzyme catalyzed reactions and some reactions catalyzed by microorganisms. The microorganism problem is further complicated in a nonlinear manner due to the growth of the microorganisms themselves. 

There are a number of advantages of using membrane systems to conduct chemical reactions or syntheses. A single device could in principle integrate reaction, concentration, and separation functions. Segregating reactants from products would also enhance thermodynamically limited or product-inhibited reactions. Initially, the lack of membrane materials sufficiently resistant to high temperatures and chemical attack precluded the realization of mem-... 

The rate of reaction is approximately linear with cyclohexene partial pressure, however there are systematic deviations at higher partial pressures. The first order behavior with respect to hydrogen and the slight reactant inhibition of cyclohexene suggest the following kinetic correlation ... 

For the hydrodimerization of butadiene with water, attempts have been made to increase the reactivity by adding acidic solids [4], salts such as sodium phosphate [5], emulsifiers [6], carbon dioxide [7], or the like, with no satisfactory results. In particular, the reaction rate increases under a carbon dioxide pressure, but carbonate ions, not carbon dioxide itself, are considered to play an important role in this effect. It is known that the carbonate ion concentration in water is very low even under a carbon dioxide pressure. If the carbonate ion is the true reactant, the reaction rate should increase with the carbonate ion concentration. Since inorganic carbonates show almost no effect, the addition of various tertiary amines having no active hydrogen, under a carbon dioxide pressure was tested [8]. Diamines and bifunctional amines inhibited the reaction. The reaction rate increased only in the presence of a monoamine having a p/f of at least 7, almost linearly with its concentration (Figure 3). 

There are several other effects which result in deviations by real systems from the idealized models described above, (i) Subsidiary interfaces may develop resulting in a zone, rather than a surface, of reaction, (ii) The volume of product will generally be different from that of the reactant from which it was derived, and thus the effective reaction interface may not extend across the whole surface of the nucleus. This can result in particle disintegration, (iii) In reversible reactions, a volatile product may be adsorbed on the surface of the residual phase locally inhibiting reaction and hence the observed rate of product formation is less than that expected for the amount of reactant that has decomposed, (iv) Diffusion control may become significant in reversible reactions. 

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