Mechanism in solvents

Chemical kinetics and mechanisms in solvent extraction of copper chelates. D. S. Flett, Acc. Chem. Res., 1977,10, 99-104(21). 

Since benzoyl chloride reacts by a mechanism which is either a combination of Sn1/Sn2 (unimolecular/bimolecular) or some intermediate mechanism, a great deal of work has been directed towards elucidation of the factors which control its reactivity. In the context of the 3 and 4-substituted benzoyl halides, it is seen from the evidence of Hammett plots to react predominantly by a bimolecular mechanism in solvents of low polarity and by a unimolecular mechanism in solvents of high polarity. 

Table I. Experimental Results Testing Reverse Order Mechanism in Solvent Extraction of Coal8...

The foregoing discussion reveals that in order to understand better the influenee of solvents on the ion-transfer type of reactions, further progress is needed both in the theory of such processes and also in experimental studies of their mechanism in solvents with different donor properties. 

A wide-ranging study by French researchers (IFP and Elf) on oxidation mechanisms in solvent refined, isomerized wax, and poly-alpha-olefin (PAO) base stocks oxidation was published in 1995.33 Included was the finding that in modified TFOUT tests, insolubles formation was related to aromatics content, becoming significant at about 6% CA, but surprisingly not increasing further with CA (Figure 5.19). 

The decisive flow mechanism in solvent extractors is the motion of swarms of droplets that is in close relation to the motion of single droplets. In liquid-liquid systems, drops reach their terminal velocity after a very short distance of fiee motion since the density difference between the drop and the continuous phase is... 

As mentioned previously, the literature on nonaqueous cleaning process is inadequate. More research is required to better understand the cleaning mechanism in solvent systems, particularly if liquid COj systems become commercial. 

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

Thus, solvent effects are consistent with z 8 2 mechanism in which... 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Thiazolium salts can be obtained either directly by slight modifications of ring-closure methods, already described for the parent bases, or by classical quaternization of the bases, the detailed mechanism of which have been reported in Chapter III the quaternization is best represented by a classical SNj mechanism, the solvent playing an important part (14) unless the reaction is run without any solvent. 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Methacrylate monomers do not generally polymerize by a cationic mechanism. In fact, methacrylate functionaUty is often utilized as a passive pendent group for cationicaHy polymerizable monomers. Methacrylate monomers also have been used as solvents or cosolvents for cationic polymerizations (90,91). 

Pigment dispersions are stabilized by charge repulsion and entropic, ie, steric or osmotic, repulsion. Although both types of stabilization force may be present in most cases, for pigment dispersions in solvent-bome coatings entropic repulsion is usually the most important mechanism for stabilization. 

In this chapter we provide an introductory overview of the imphcit solvent models commonly used in biomolecular simulations. A number of questions concerning the formulation and development of imphcit solvent models are addressed. In Section II, we begin by providing a rigorous fonmilation of imphcit solvent from statistical mechanics. In addition, the fundamental concept of the potential of mean force (PMF) is introduced. In Section III, a decomposition of the PMF in terms of nonpolar and electrostatic contributions is elaborated. Owing to its importance in biophysics. Section IV is devoted entirely to classical continuum electrostatics. For the sake of completeness, other computational... 

Further details and specialized information on the mechanisms, product qualities, and proce.sses applied to the heating of coal in solvents can be found in the abundant literature [20]. What follows are some of the results of research conducted at West Virginia University, where investigations of the conversion of coal into pitches suitable for graphite production have been carried out... 

The importance of solvent participation in the borderline mechanisms should be noted. Nucleophilic participation is minimized by high electronegativity, which reduces the Lewis basicity and polarizability of the solvent molecules. Trifluoroacetic acid and perfiuoro alcohols are among the least nucleophilic of the solvents used in solvolysis studies. These solvents are used to define the characteristics of reactions proceeding without nucleophilic solvent participation. Solvent nucleophilicity increases with the electron-donating capacity of the molecule. The order trifluoroacetic acid < trifluoroetha-nol <acetic acid < water < ethanol gives a qualitative indication of the trend in solvent nucleophilicity. More will be said about solvent nucleophilicity in Section 5.5. 

Table S.16 presents data on some representative nucleophilic substitution processes. The first entry illustrates the use of 1-butyl-l-r/p-bromobenzenesulfonate to dononstrate at primary systems react with inversion, even under solvolysis conditkms in formic acid. The observation of inversion indicates a concerted mechanism in fids weakly nucleophilic solvent.

A macroporous polystyrene-divinylbenzene copolymer is produced by a suspension polymerization of a mixture of monomers in the presence of water as a precipitant. This is substantially immiscible with the monomer mixture but is solubilized with a monomer mixture by micelle-forming mechanisms in the presence of the surfactant sodium bis(2-ethylhexylsulfosuccinate) (22). The porosity of percentage void volume of macroporous resin particles is related to percentage weight of the composite (50% precipitant, 50% solvent) in the monomer mixture. 

There is increasing interest in copolymer systems, which, due to their chemical heterogeneity, may require very complex eluent systems in order to dissolve the sample and ensure that the separation ensues hy a pure size exclusion mechanism. In these examples, the PLgel is also compatible with eluent systems containing mixed solvents of different polarity (including water as a cosolvent up to 10% hy volume) and in organic solvents modified with acids or bases (e.g., acetic or formic acid, triethanolamine) as it is stable in the pH range of 1-14. 

Solvent Effects on the Rate of Substitution by the S/s/2 Mechanism. Polar- solvents are required in typical bimolecular- substitutions because ionic substances, such as the sodium and potassium salts cited earlier in Table 8.1, are not sufficiently soluble in nonpolar-solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate. Other than the requirement that the solvent be polar- enough to dissolve ionic compounds, however, the effect of solvent polarity on the rate of Sn2 reactions is small. What is most imporiant is whether or not the polar- solvent is protic or aprotic. 

Mechanism Initial state Transition state Relative charge in transition state Rate effect of increase in solvent polarity... 

Electrical properties of liquids and solids are sometimes crucially influenced by H bonding. The ionic mobility and conductance of H30 and OH in aqueous solutions are substantially greater than those of other univalent ions due to a proton-switch mechanism in the H-bonded associated solvent, water. For example, at 25°C the conductance of H3O+ and OH are 350 and 192ohm cm mol , whereas for other (viscosity-controlled) ions the values fall... 

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