Substrate solubility, reaction rate

Solvation is an important parameter that influences macromolecular properties such as solubility, reaction rate equilibria, partition coefficients, enzyme-substrate, and ligand-receptor binding. In free energy calculations, solvent is represented either explicitly or implicitly. The decision of whether to use explicit or implicit solvent models in a given simulation typically depends on the available computer resources. Inclusion of explicit solvent molecules is more realistic than continuum models. 

Figure 4.3e considers a dense catalytic layer which is permeable to both the reactants present in the segregated phases. This is the case of several catalytic polymeric membranes, either unsupported or supported on porous substrates. The reaction rate is governed by the relative rate of diffusion and reaction in the thickness of the catalytic membrane. In this case the solubility of the components in the membrane layer should also be taken into account. 

The oxidation of alkenes and allylic alcohols with the urea-EL202 adduct (UELP) as oxidant and methyltrioxorhenium (MTO) dissolved in [EMIM][BF4] as catalyst was described by Abu-Omar et al. [61]. Both MTO and UHP dissolved completely in the ionic liquid. Conversions were found to depend on the reactivity of the olefin and the solubility of the olefinic substrate in the reactive layer. In general, the reaction rates of the epoxidation reaction were found to be comparable to those obtained in classical solvents. 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

In the case of aqueous multiphasic catalytic conversions, the reaction rate can be strongly affected by the ability of the substrate to move over into the catalyst phase. For biphasic hydroformylation, the velocity decreases with increasing chain length of the olefins due to their lower solubility in the aqueous phase [78]. 

Ru(II)-TPPTS to the corresponding unsaturated alcohols in biphasic mode. If one compares the reaction times until full conversion, it becomes clear that the reaction rate correlates with the solubility of the substrate in the aqueous phase, as expected. The latter decreases with increasing chain length or branching of the chain at the C3-atom. In contrast to heterogeneously catalysed hydrogenations of o , d-unsaturated aldehydes, the steric hindrance of substituents at the C3-atom only plays a minor role in the coordination mode of the substrate at the metal centre, since selectivity differences from croton-aldehyde to citral are marginal. 

Prior to the kinetic experiments, possible deactivation phenomena of the catalytic system were checked by recycling experiments with prenal and citral as substrates. These results provide not only important hints on the form of the rate equation, but also on which reaction is convenient for long-term investigations in the loop reactor. After the reaction, the aqueous and organic phases were separated and the catalyst phase was reused without further purification. Results on the hydrogenation of prenal are shown in Fig. 7. The reaction rate clearly decreases if the catalyst phase is reused. According to GC analysis and H-NMR studies, this can be attributed to the fact that the product of the reaction, prenol, is highly soluble in water. Consequently, a simple phase... 

The synthesis of aliphatic nitro compounds from the reaction of alkyl halides with alkali metal nitrites was discovered by Kornblum and co-workers and is known as the modified Victor Meyer reaction or the Kornblum modification. The choice of solvent in these reactions is crucial when sodium nitrite is used as the nitrite soiuce. Both alkyl halide and nitrite anion must be in solution to react, and the higher the concentration of nitrite anion, the faster the reaction. For this reason, both DMF and DMSO are widely used as solvents, with both able to dissolve appreciable amounts of sodium nitrite. Although sodium nitrite is more soluble in DMSO than DMF the former can react with some halide substrates.Urea is occasionally added to DMF solutions of sodium nitrite to increase the solubility of this salt and hence increase reaction rates. Other alkali metal nitrites can be used in these reactions, like lithium nitrite,which is more soluble in DMF than sodium nitrite but is also less widely available. 

A.9. Enhanced Reaction Rate Due to Increased Substrate Solubility... 

---