Solvents steric

The dimer-based mechanistic pathways in the deprotonation of ketones and imines have been carefully studied by theoretical (MNDO) methods, particularly for LDA and pinacolone, microsolvated by various solvents (Scheme 6)47. The results suggest that, even if no general rule can be drawn, the open dimer mechanism is expected to dominate under all circumstances, and particularly with increasing amide and solvent steric... 

Gr-FFF Particles >1 pm. Applicable to water and organic solvents Steric, adhesion... 

The anions of protected cyanohydrins derived from saturated aliphatic aldehydes undergo competitive 1,2- and 1,4-addition to unsaturated carbonyl electrophiles. The pnqmrtion of the adducts >pears to vary as a function of both structure and solvent Steric interactions that favor dissociation of the reversibly formed 1,2-addition product increase the proportion of the 1,4-addition product For example, increasing the size of the substituent R in a protect cyanohydrin (29) from methyl to n-pentyl increases the ratio of 1,4-addition product (30) to 1,2-addition inoduct (31) from 1.5 1 to 2.7 1 (equation 14). ... 

Jaffe, A. B., Skinner, K. J., McBride, J. M., Solvent Steric Effects. II. The Free radical Chemistry of Azobisisobutyronitrile and Azobis 3 cyano 3 pentane in Viscous and Crystalline Media, J. Am. Chem. Soc. 1972, 94, 8510 8515. 

In the good solvent, steric interactions are dominant, as mentioned above, and the polymer is swollen compared to the ideal chain. This swelling corresponds to a change in the fractal dimension of the chain, which now becomes smaller than 2. 

Drag reduction is shown by polysaccharides, which are linear in nature. Highly branched materials such as gum arabic and dextran do not seem to reduce drag (17). Polyelectrolytes, such as partially hydrolyzed polyacrylamide (64,106), polyacrylic acid (118), acrylamide sodium acrylate copolymer (119), and sodium alginate (120), show higher drag reduction when the chain is extended or there is increment in coil dimensions. The coil expansion takes place because of coulom-bian repulsion and solvent steric hindrance of side group and main chain (119). 

EDTA s. Ethylenediamine-tetraacetic acid Effect s. a. under Solvents steric s. Steric effect Einhom s. a. Tschemiac... 

Regioselective and stereospecific azidation of 1,2- and 1,3-diols has been accomplished by using azidotrimethylsilane via a Mitsunobu reaction. The regioselectivity is infiuenced by solvent, steric bulk at C(2) of the 1,2-diol or at C(3) of the 1,3-diol, and the presence of electron-donating or electron-withdrawing groups near the secondary carbinol carbon. 

The ORD curves of all PEEI (XLVIa) in various solvents obey the one-term Drude equation by contrast, the. AA-benzoylated polymer (XLVIb) shows anomalous curves depending on the nature of the solvent. Steric restriction between the side chain and the substituent may induce conformational preference of one helical screw over the other, according to the authors but a more accurate investigation, at shorter wavelength, of CD and ORD, seems necessary to verify this conclusion. 

Good solvent Poor solvent Steric stabilization Figure 12.6 Steric stabilization using polymers. For proper stabilization, the steric barrier needs to extent at least 10 nm from the particle surface and therefore polymers or oligomers are used (low molecular surfactants are too small). It works best with thick and dense layers, good solvents, strong adsorption and complete particle coverage... 

The use of alkyl halides has given rise to the question of control of the carbon stereochemistry configuration. The concerted mechanism (CM) would lead to retention of configuration whereas the dissociative pathway being assimilated to a Sn2 would lead to inversion. The experimental conditions (nature of the halide, solvent, steric congestion) have a major influence on the Sn2 mechanism [32]. The nature of the nucleophile, namely the donating character of the phosphines bound to the metal must also be considered. 

Other factors influencing kf and Kapp include the use of a hydrophobic solvent, steric effects, electrolytes in the aqueous phase and the use of surface active catalysts. These effects are illustrated in Table V. Thus, with the less hydrophobic dioxane instead of toluene as a solvent, both kf and are suppressed in runs conducted in... 

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