Molecular polarity relationship

The Relationship Between Molecular Shape and Molecular Polarity... 

Table 4.3 summarizes the relationship between molecular polarity and molecular shape. The letter A represents the central atom and the letter X represent a more electronegative atom. An atom represented by the letter Y is more electronegative than the atom represented by the letter X. After you examine this table, do the Practice Problems that follow. 

Wilbur Bergquist, "Do Likes Dissolve Likes An Illustration of Polar and Nonpolar Solvents," /. Chem. Educ., Vol. 69,1992,158. Iodine is shown to dissolve in ethyl alcohol and an alcohol/water mixture, but not in water alone. The relationship between solubility and the molecular polarities of solvent and solute are noted. 

Describe the relationships between molecular shapes and molecular polarities... 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

Nuclear dipole-dipole interaction is a veiy important relaxation mechanism, and this is reflected in the relationship between 7, and the number of protons bonded to a carbon. The motional effect is nicely shown by tbe 7 values for n-decanol, which suggest that the polar end of the molecule is less mobile than the hydrocarbon tail. Comparison of iso-octane with n-decanol shows that the entire iso-octane molecule is subject to more rapid molecular motion than is n-decanol—compare the methyl group T values in these molecules. 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

In 1868 two Scottish scientists, Crum Brown and Fraser [4] recognized that a relation exists between the physiological action of a substance and its chemical composition and constitution. That recognition was in effect the birth of the science that has come to be known as quantitative structure-activity relationship (QSAR) studies a QSAR is a mathematical equation that relates a biological or other property to structural and/or physicochemical properties of a series of (usually) related compounds. Shortly afterwards, Richardson [5] showed that the narcotic effect of primary aliphatic alcohols varied with their molecular weight, and in 1893 Richet [6] observed that the toxicities of a variety of simple polar chemicals such as alcohols, ethers, and ketones were inversely correlated with their aqueous solubilities. Probably the best known of the very early work in the field was that of Overton [7] and Meyer [8], who found that the narcotic effect of simple chemicals increased with their oil-water partition coefficient and postulated that this reflected the partitioning of a chemical between the aqueous exobiophase and a lipophilic receptor. This, as it turned out, was most prescient, for about 70% of published QSARs contain a term relating to partition coefficient [9]. 

A close relationship exists between physicochemical properties of pigment molecules and their ability to be absorbed and thus to exhibit biological functions. Carotenoids are hydrophobic molecules that require a lipophilic environment. In vivo, they are found in precise locations and orientations within biological membranes. For example, the dihydroxycarotenoids such as lutein and zeaxanthin orient themselves perpendicularly to the membrane surface as molecular rivets in order to expose their hydroxyl groups to a more polar environment. 

With the exception of rather small polar molecules, the majority of compounds, including drugs, appear to penetrate biological membranes via a lipid route. As a result, the membrane permeability of most compounds is dependent on K0/w. The physicochemical interpretation of this general relationship is based on the atomic and molecular forces to which the solute molecules are exposed in the aqueous and lipid phases. Thus, the ability of a compound to partition from an aqueous to a lipid phase of a membrane involves the balance between solute-water and solute-membrane intermolecular forces. If the attractive forces of the solute-water interaction are greater than those of the solute-membrane interaction, membrane permeability will be relatively poor and vice versa. In examining the permeability of a homologous series of compounds... 

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