Solvents, initiation rates

Modifier Solvent Initial rate (mmol h g ee (%) Major enantiomer... 

Solvents Initial rate x 10 Conversion Selectivity Dielectric... 

Table 2.7 Solvent effects on the Entry Solvent initiation rate of GH2 k, (L mor S- ) ki (relative to DCM) T...

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

Unprotected racemic amines can be resolved by enantioselective acylations with activated esters (110,111). This approach is based on the discovery that enantioselectivity of some enzymes strongly depends on the nature of the reaction medium. For example, the enantioselectivity factor (defined as the ratio of the initial rates for (3)- and (R)-isomers) of subtiHsin in the acylation of CX-methyl-ben zyl amine with tritiuoroethyl butyrate varies from 0.95 in toluene to 7.7 in 3-methyl-3-pentanol (110). The latter solvent has been used for enantioselective resolutions of a number of racemic amines (110). 

A second order reaction is conducted in two equal CSTR stages. The residence time per stage is T = 1 and the specific rate is /cCq = 0.5. Feed concentration is Cq. Two cases are to be examined (1) with pure solvent initially in the tanks and (2) with concentrations Cq initially in both tanks, that is, with Cio = Coq = Cq. 

For less polar monomers, the most extensively studied homopolymerizations are vinyl esters (e.g. VAc), acrylate and methacrylate esters and S. Most of these studies have focused wholly on the polymerization kinetics and only a few have examined the mierostructures of the polymers formed. Most of the early rate data in this area should be treated with caution because of the difficulties associated in separating effects of solvent on p, k and initiation rate and efficiency. 

The effect of f-BuX initiator and MeX solvent on PIB yield was studied in the temperature range from —20° to —60 °C. Results are shown in Table 3. Initial rates of polymerization at —40 °C were determined from time-conversion plots (Fig. 1). 

Based on initiator efficiencies, polymerization rates and floor temperatures, relative initiator reactivity in conjunction with Et2 A1C1 was r-BuCl > f-BuBr > f-BuI = 0. Further, depending on the nature of solvent, initiator reactivity decreased as MeCl > MeBr > Mel = 0,... 

Table 1.3 Influence ofthe organic solvent on the enantioselectivity of the protease subtilisin in the kinetic resolution ofthe racemic amine (9) (expressed as the ratio ofthe initial rate of acylation of the pure enatiomers, Vs/vr).

Later on the crucial role played by the solvent was enlightened in the protease-catalyzed resolution of racemic amines [26]. As shown in Table 1.3, the ratio of the initial rates of acylation of the (S)- and the (Ji)-enantiomers or racemic a-methyl-benzylamine (9) varied from nearly 1 in toluene to 7.7 in 3-methyl-3-pentanol. Similarly, the same authors found a significant solvent effect for the subtilisin-catalyzed transesterification of racemic 1-phenylethanol (10) using vinyl butyrate as acyl donor (Table 1.4 [27]). 

As shown in Fig. 6, when the salt [28 2 ] is dissolved in a variety of solvents, the anion [2 ] is rapidly consumed by reaction with the cation [28 ] and reaches an apparent equilibrium, the position of which depends on the solvent polarity. Clearly, the initial rate of the cation-anion reaction increases as the solvent polarity decreases. 

Data illustrating the relationship of the initial rate to the concentration of monomer at fixed initiator concentration are given in Table X for styrene in benzene and for methyl methacrylate polymerized at various concentrations in the same solvent. If the efficiency / of utilization of primary radicals is independent of the monomer concentration, the quantity given in the last column should be... 

When a heptane solution of 5-Br-PADAP and an aqueous solution of Ni " " were stirred, the ligand in the organic phase was continuously consumed according to the complexation, but there was no extraction of the complex. The complex formed was completely adsorbed at the interface. On the other hand, in a toluene system the complex was extracted very slowly (Fig. 6). The complexation mechanism in the two solvent systems could be analyzed by taking into account the interfacial adsorption of the ligand. The next equation was derived for the initial rate of the consumption of HLq in the heptane system ... 

In the ion-association extraction systems, hydrophobic and interfacially adsorbable ions are encountered very often. Complexes of Fe(II), Cu(II), and Zn(II) with 1,10-phenanthro-line (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivity, although the ligands themselves can hardly adsorb at the interface, except for protonated species [19-21]. Solvent extraction photometry of Fe(II) with phen is widely used for the determination of trace amounts of Fe(II). The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform are shown in Fig.9. In the presence of 0.1 M NaC104, the interfacial adsorption of phen complex is most remarkable. The adsorption of the extractable complex must be considered in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r° shows the relation... 

Using l,8-diphenyloctatetra-l,3,5,7-ene, (DOT), as a model compound either in dilute, ( 10-5m), hexane or ethanol solutions or incorporated into a film of undegraded PVC confirmed that in the presence of HC1 it underwent a photochemical reaction which resembled that of the polyenes in thermally degraded PVC. The results indicated that the initial rates of reactions proceeding in either solvent showed a second order dependence on HC1 pressure and that the reaction was considerably slower in ethanol than in hexane. Further, when cast in PVC films, the characteristic absorption maxima of DOT were shifted about 16nm to longer wavelengths compared with their absorption in hexane and there... 

Diphenylmethane reacts with dioxygen in the presence of potassium 1,1-dimethylethoxide in various solvents (dimethylformamide [DMF], hexamethylphosphoramide [HMPA], pyridine) to produce nearly 100% yields of benzophenone [284]. The adduct of benzophenone with dimethylsulfoxide (DMSO) [l,l-diphenyl-2-(methylsulfinyl)ethanol] is formed as the final product of the reaction. The stoichiometry of the reaction and the initial rate depends on the solvent (conditions 300 K, [Ph2CH2] = 0.1mol L [Me3COK] = 0.2mol L 1,p02 = 97kPa). 

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

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