Steric effects hydrates

Case Steric effect Hydration effect Alkali Properties of alkali halides ... 

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

Electronic and steric effects operate m the same direction Both cause the equilib rium constants for hydration of aldehydes to be greater than those of ketones... 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

Effects of Structure on Rate Electronic and steric effects influence the rate of hydration in the sfflne way that they affect equilibrium. Indeed, the rate and equilibrium data of Table 17.3 parallel each other almost exactly. 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

For borderline metals and/or borderline ligands, the order is determined by other factors such as dehydration (removal of hydrate water), steric effects, etc. 

The amount of adsorbed chemical is controlled by both properties of the chemical and of the clay material. The clay saturating cation is a major factor affecting the adsorption of the organophosphorus pesticide. The adsorption isotherm of parathion from an aqueous solution onto montmorillonite saturated with various cations (Fig. 8.32), shows that the sorption sequence (Al > Na > Ca ) is not in agreement with any of the ionic series based on ionic properties. This shows that, in parathion-montmoriUonite interactions in aqueous suspension, such factors as clay dispersion, steric effects, and hydration shells are dominant in the sorption process. In general, organophosphorus adsorption on clays is described by the Freundhch equation, and the values for parathion sorption are 3 for Ca +-kaoUnite, 125 for Ca -montmorillonite, and 145 for Ca -attapulgite. 

The special, and somewhat complex, role played by a substituent in the 4-position was first observed in the quinazoline series. The carbon atom that received the —OH group during hydration was located by preparing all six C-methylquinazolines. Only when inserted in the 4-position did a methyl group prevent the addition of water.41,23 This inhibition has both an inductive (+1) and a steric component. A kinetic study of quinazoline-4-carboxamide showed that the carbamoyl substituent, because of its steric effect, slowed the attainment of hydration from less than a second to 2-4 hours. However, this substituent has the... 

The ion content of the organic phase of ethylenoxid-products indicate that under saturation conditions there are some water molecules whose properties are not too different from normal water. Polypropyleneoxide products which contain much less water, release under similar conditions water with a reduced ion content147. From this experience one gets a working hypothesis for the mechanism of semipermeable membranes. The membranes should have some secondary hydrate shell with movable water but by reason of solubility or steric effects, not too much secondary hydrate water to avoid normal water with common solubility properties. 

Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like surface hydration and steric effects, have received considerable attention over the past several decades [194,278],... 

Small-angle X-ray diffraction was used to identify the time-averaged location of amiodarone in a synthetic lipid bilayer. The drug was located about 6 A from the center of the lipid bilayer (Figure 4.13) [125, 126]. A dielectric constant of k = 2, which is similar to that of the bilayer hydrocarbon region, was used to calculate the minimum energy conformation of amiodarone bound to the membrane. The studies were performed below the thermal phase transition and at relatively low hydration of lipid. The calculated conformation differed from that of the crystal structure of amiodarone. Even though the specific steric effects of the lipid acyl chains on the confor-... 

Table 3.1). After oxidative workup, one isolates the alcohol in the a-position almost exclusively. According to what has already been stated, as the more bulky reagent, 9-BBN reacts with more sensitivity to steric effects than BH3 and its secondary products. It therefore makes possible alkene hydrations with almost perfect regiocontrol. 

The position of the equilibrium is also influenced by steric effects. The product, which is sp hybridized and approximately tetrahedral, has the groups bonded to the hydrate carbon closer together (bond angles 109°) than they are in the reactant, which is sp2 hybridized with bond angles of 120°. There is more steric hindrance between the groups in the product than there is in the reactant. Therefore, larger substituents shift the equilibrium toward the less crowded carbonyl form and decrease the equilibrium constant. 

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