Influences of Reaction Medium

Figu re 7.8 Evolution of polyhedral structures of gold nanocrystals with respect to the AgNOj amount added in the reaction mixture in which pentanediol was employed as a major reaction medium. (Adapted from Ref [25].) 

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Three fragments of the delicious peptide sequence lysine-glycine, serine-leucine-alanine and aspartic acid-glutamic acid-glutamic acid possess separately umami/salty, bitter and sour taste respectively but mixtures or combinations of them produce a similar taste of that corresponding to the complete octapeptide. The synthesis of CBZ-Lys-Gly-OMe and CBZ-Ser-Leu-OMe were carried out using immobilized trypsin and thermolysin. We have studied the influence of reaction medium (pH, temperature and substrate concentration) on the yield and initial reaction rate of synthesis. 

For polymerization systems considered by us the deviation from single reaction order by monomer for AG as we think connected with specific influence of reaction medium on reactivity of monomer and propagation radical in polymerization processes. 

Figure 3. Influence of reaction medium of amine-glycidyl ether reaction.

Wehrli, J.T., Baiker, A., Monti, D.M., Blaser, H.U., Jalett, H.P. (1989) Enantioselective hydroganation of alpha-ketoesters influence of reaction medium and conversion, J. Mol Catal. A. Chem. 57,245-257. 

K. Vandamme, V. Melkebeek, E. Cox, D. Deforce, J. Lenoir, E. Adriaens, C. Vervaet, and J.P Remon, Influence of reaction medium during synthesis of Gantrez AN 119 nanoparticles for oral vaccination, Eur. J. Pharm. Biopharm., 74,202-208, 2010. 

The study of lactonization via an intermediate phenonium ion has been further pursued for several methyl 4-aryl-5-tosylhexanoates (55) as substrates. The intermediate phenonium ion (56) has two possibilities for ring closure, yielding products (57) or (58). In all the substrates, Ar contained one or two methoxy groups and sometimes also a methyl group. The effects of reaction medium, temperature, and time on the product ratios were examined. It was concluded that substrates (55) give y-lactone (57) selectively under thermodynamic conditions, but 5-lactone (58) under kinetic conditions. Substituents in Ar influence the selectivity through their electronic effects. 

The kinetic theory of collisions, which has been so effective in developing the kinetics of vapor-phase reactions, has substantially influenced research on the processes of liquid-phase oxidation and in describing these processes. It has been thought that the lack of laws on which to base liquid-state theory (in contrast to the well-developed kinetic theory of gases) would in principle severely limit the development of a quantitative theory of liquid-phase reactions. At present the characteristics of the liquid state are carefully considered in discussing the mechanism of intermolecular reactions, influence of the medium on reactivity of compounds, etc. 

The eluent can strongly influence the reaction medium. In many cases this can lead to a serious limitation in the choice of mobile phase due, for example, to solubility problems with the reagents or reaction products or to possible side reactions with the solvent itself. 

Figure 4. Influence of reaction time on copolymerization of lignosulfonate and methacrylic acid in water medium at 30°C , total monomer conversion O, grafting efficiency A, degree of LS reacted.

An impressive illustration of the influence of the medium on the photo-Fries reaction product distribution can be found in Sch. 5 where the products of photolysis of 1-naphthyl 2-methyl 2-phenyl propanoate in hexane solution and within NaY are compared. Remarkably, whereas in solution eight products are formed, within NaY zeolite a single product dominates the product mixture. Cations present within zeolites help to anchor the reactants, intermediates and products to the surfaces of the zeolite and... 

In the absence of spectral information in the gas phase, it is common to compare calculated features of the vibrational spectrum to data measured in rare gas matrix, the premise being that the latter medium perturbs the H-bonded system as little as possible. The influence of the medium was considered via a self-consistent reaction-field formalism wherein inductive interactions between the polar system and the polarizable medium are incorporated into a model Hamiltonian ". The calculations made use of the 6-31G basis set at the SCF level. 

The results obtained with the K 1481 resin reported in Table 5 show that the acid conversion normally increases with the increase of the amount of catalyst in the medium. The variations of the activity and the selectivity observed in these experiments are similar to those presented in the previous paragraph concerning the influence of reaction time. It can thus be concluded that there is no significant modification caused by external diffusion phenomena. 

The synthesis of chiral poly(depsipeptides), polymers with alternating amide and ester bonds, by lipase-catalyzed ring opening of 3-isopropyl morpholino-2,5-dione (19) was shown by Hocker and coworkers (Scheme 11.5) [26], Various lipases were tested for the bulk polymerization of these heterocyclic monomers at temperatures of 100 °C or above. PPL and lipase type III from a pseudomonas species were shown to be effective catalysts. The isolated polymers showed Mn values of 3.5-17.5 kgmol-1. The influence of reaction temperature, the amount of enzyme and the presence of water in the reaction medium were shown to be important factors on the high molecular weight fraction and were investigated in detail [26b]. Comparison of optical rotation values for polymers prepared by... 

As mentioned before, the catalytic behaviour of the resin strongly depends on the nature of the medium. In order to take into account which is the influence of the medium in the reaction kinetics, experiments should be carried out in conditions wherein the medium polarity varies significantly. It can be done performing experiments in the absence of diluents. 

For the purpose of this review, an attempt has been made to minimise the influence of the medium effect (using only the data reported for [Ni ],oi = 0.2 and 0.4 M) and to take into account the presence of NiCr complex. For the reaction ... 

The effectiveness with which irradiation enhances the safety of foods and extends their storage lifetime relates directly to the chemistry that takes place in the contaminating microorganisms, in the food itself, and in any protective packaging. This chemistry can be understood by considering separately the nature of the reactive entities formed in each major food constituent and the influence of the medium and irradiation conditions on their subsequent reactions to form stable radiolysis products. Such an understanding can be exploited to optimize the irradiation treatment and to minimize any objectionable effects. 

Lipases catalyze the hydrolytic cleavage of triglycerides into fatty acids and glycerol, or into fatty acids and mono or di-glycerides, at oil interfaces in nature. However, this hydrolytic reaction can be reversed and transformed into reactions of esterification (inter or transesterification), alcoholysis or aminolysis by engineering the medium polarity or the water content of the medium. Therefore, substrates for lipases can be esters, like the natural triglyceride substrates in hydrolytic reactions or, if the reaction is reversed, carboxylic acids, alcohols, amines or esters. The reaction medium not only determines the direction of the reaction (hydrolytic or synthetic), but also determines the solubility and stability of lipase substrates. Therefore, lipase activity and selectivity are strongly influenced by reaction medium. 

Secondly, a great many chemical reactions are catalysed by substances which can either receive or donate (unhydrated) hydrogen ions, so that the influence of a medium upon changes occurring in it is often largely determined by the hydrion concentration. 

In spite of an enormous number of papers dealing with the influence of the medium on the rate of chemical reactions, no strict quantitative theory capable of universal application has been put forward up to now. Thus, it is not yet possible to describe the relationship between the reactions rate constants and the equilibrium constants with the nature of the medium in which the reactions take place by means of a single equation. 

The absence of a general theory on the influence of the medium on the kinetics of a chemical reaction can be explained by the feet that the ehange of solvent can not only influence on the rate of the process but also frequently results in a complication (or change) of the reaction mechanism. The calculation of the individual contributions made by each factor is thus, in most cases, rather complicated and requires a deep and comprehensive study of the properties of the medium and of the reacting particles. This is because a quantitative evaluation of all types of interactions between the reacting particles with medium can only arrive at the basis of fiill knowledge of these properties. 

The basis of such an approach to the description of the diffusion-controlled reactions of radical polymerization has been laid in the works of Nourish and Smith [67]. In 1942, they explained the phenomenon of autoacceleration by a decrease of the constant rate of chain termination with increasing system viscosity. After that a series of investigations has been published in which the influence of the medium viscosity on the constant rate of chain termination has been studied. A whole range of the empirical models [54-56, 62, 68] was... 

We start by distinguishing between two major effects. The first involves the influence of the medium n which the reaction is carried out, while the second refers to the influence of the solid-state matrix in which the active site is embedded. We first summarize cavity, ligand and solvent effects which comprise the catalytic medium. This is then followed by a brief discussion on the different matrix effects. 

Apart from chemical factors that influence the reaction medium, physical variables may also affect crystallization of supramolecular isomers. The most commonly varied physical factors are temperature and pressure, which influence not only the energy of the precursors but also the final isomeric products (Figure 2). 

The book by J. Van t Hoff Etudes de dynamique chimique published in 1884 was an important scientific event in chemistry. In this book, the author generalized data on kinetic studies and considered the kinetic laws of monomolecular and bimol-ecular transformations, the influence of the medium on the occurrence of reactions in solutions, and phenomena named by him perturbing factors. The large section of the outlines is devoted to the temperature influence. Van t Hoff had come right up against the law, which was several years later justified by S. Arrhenius. Using the... 

The compensation effect is absent from gas-phase reactions of atoms and radicals with molecules, it is not either observed for radical reactions in solutions when one of two reactants is a nonpolar particle. One of the sources of this effect is the influence of the medium on the elementary act of polar particles. The rate constant of bimolecular reaction in a solution depends on the association constant of particles Kj q, amplitude of vibrations of particles a and dielectric constant e. All these... 

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