Two Nonpolar Substances

Most organic solvents are nonpolar, so they are immiscible with water. However, most organic solvents are miscible with each other. An example of a nonpolar organic solvent is carbon tetrachloride (CCy. We said that oil and water don t mix. Oil and carbon tetrachloride, however, do mix—they are completely miscible. London forces are at work here. 

The relative strength of the London forces between oil molecules, between CCl molecules, and between oil and CCl are all of comparable strength. Therefore, oil molecules are just as likely to be surrounded by CCl molecules as they are to be surrounded by other oil molecules, and vice versa. Oil and CCl form a true solution. 

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Attractive and Repulsive Forces. The force that causes small particles to stick together after colliding is van der Waals attraction. There are three van der Waals forces (/) Keesom-van der Waals, due to dipole—dipole interactions that have higher probabiUty of attractive orientations than nonattractive (2) Debye-van der Waals, due to dipole-induced dipole interactions (ie, uneven charge distribution is induced in a nonpolar material) and (J) London dispersion forces, which occur between two nonpolar substances. 

At the molecular level the formation of a solution from two nonpolar substances, such as hexane and benzene, can be visualized as a process of simple mixing. These nonpolar molecules, having little tendency to either attract or repel one another, easily intermingle to form a homogeneous solution. 

Two nonpolar substances with similar structures but different scents are mixed in a perfume vial. One of the substances has a higher molar mass than the other. Which one is smelled when the perfume vial is first... 

It might seem that two nearby molecules A and B of the same nonpolar substance would be unaffected by each other... 

This combination of monomers is unique in that the two are very different chemically, and in thek character in a polymer. Polybutadiene homopolymer has a low glass-transition temperature, remaining mbbery as low as —85° C, and is a very nonpolar substance with Htde resistance to hydrocarbon fluids such as oil or gasoline. Polyacrylonitrile, on the other hand, has a glass temperature of about 110°C, and is very polar and resistant to hydrocarbon fluids (see Acrylonitrile polymers). As a result, copolymerization of the two monomers at different ratios provides a wide choice of combinations of properties. In addition to providing the mbbery nature to the copolymer, butadiene also provides residual unsaturation, both in the main chain in the case of 1,4, or in a side chain in the case of 1,2 polymerization. This residual unsaturation is useful as a cure site for vulcanization by sulfur or by peroxides, but is also a weak point for chemical attack, such as oxidation, especially at elevated temperatures. As a result, all commercial NBR products contain small amounts ( 0.5-2.5%) of antioxidant to protect the polymer during its manufacture, storage, and use. 

C3Hg (A/ = 44.0 g/mol)(-42.rc). For two substances with nearly the same molar masses, the polar substance has a higher boiling point than the nonpolar substance. 

These types of attractions occur when the charge on an ion or a dipole distorts the electron cloud of a nonpolar molecule. This induces a temporary dipole in the nonpolar molecule. These are fairly weak interactions. Like an ion-dipole force, this type of force requires the presence of two different substances. 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

The permeability, P (P = Pc x D), of a nonpolar substance through a cell membrane is dependent on two physicochemical factors (1) solubility in the membrane (Pc), which can be expressed as a partition coefficient of the drug between the aqueous phase and membrane phase, and (2) diffusivity or diffusion coefficient (D), which is a measure of mobility of the drug molecules within the lipid. The latter may vary only slightly among toxicants, but the former is more important. Lipid solubility is therefore one of the most important determinants of the pharmacokinetic characteristics of a chemical, and it is important to determine whether a toxicants is readily ionized or not influenced by pH of the environment. If the toxicant is readily ionized, then one needs to understand its chemicals behavior in various environmental matrices in order to adequately assess its transport mechanism across membranes. 

Of the two approaches to decomposing the thermodynamics for dissolution of nonpolar solutes into water, the first, from a reference point at the maximum of the free eneigy of transfer, leads to the concept of the compact state of the nonpolar substance. The compact state can be de-... 

The fugacity coefficients are calculated using a simplified equation of state described earlier 4,6). For this treatment the constants b and Oi are calculated by knowing the critical temperature and pressure. The attraction coefficient ° is given by generalized coefficients for nonpolar substances and with two other individual constants for polar substances. If Yi refers tp the vapor composition and n refers to binary vapor interactions, the equation becomes... 

In azeotropic distillation, the entrainer and the two components being separated can produce under some conditions three-phase equilibria. Two liquid phases may be in equilibrium with a vapor phase. For the three-phase equilibria the solubility of the nonpolar substance in the polar phase is denoted by x, the solubility of the polar substance in the nonpolar phase by y, and the corresponding activity coefficients by y and r, respectively. The relative volatility for components i and / is related to the total composition X according to... 

Some corresponding-statescorrelations use the critical compressibility factor Z, ratlier tlian tile acentric factor m, as a third parameter. The two types of correlation (one based on Tc, Pc, and Zc, the other on Tc, Pc, and w) would be equivalent were tliere a one-to-one correspondence between and w. The data of App. B allow a test of tliis correspondence. Prepare a plot of Z vs. w to see how well Z correlates witli w. Develop a linear correlation (Zc = a for nonpolar substances. 

Oil, most petroleum products, and other symmetric covalent molecules are nonpolar, whereas water and other asymmetric molecules are usually polar. When polar and nonpolar substances are mixed, they separate into two layers, as seen when oil floats on water. 

Physical properties of alkanes What types of physical properties do nonpolar compounds have A comparison of two molecular substances—one... 

Five constants must be known for each substance V°°, a, u°/k, /k, and C. Of these five only three must be determined from fitting experimental data, and these are V°°, <, and u°/k. Chen and Kreglewski suggested assigning values to the other two constants C was given the same value, 0.12, for all nonpolar substances and /k = 0.6 o> Tc. Chen and Kreglewski reported values of the constants for 11 substances (I). 

What properties would you predict for alkanes All of the bonds in these hydrocarbons are between either a carbon atom and a hydrogen atom or between two carbon atoms. A bond between two identical atoms, such as carbon, can never be polar. Because all of the bonds in alkanes are nonpolar, alkane molecules are nonpolar, which makes them good solvents for other nonpolar substances, as shown in Figure 21.11. 

A full discussion of why nonpolar substances are insoluble in water requires the thermodynamic arguments that we shall develop in Chapters 4 and 15. However, the points made here about intermolecular interactions will be useful background information for that discussion. For the moment, it is enough to know that it is less favorable thermodynamically for water molecules to be associated with nonpolar molecules than with other water molecules. As a result, nonpolar molecules do not dissolve in water and are referred to as hydrophobic ( water-hating, from the Greek). Hydrocarbons in particular tend to sequester themselves from an aqueous environment. A nonpolar solid leaves undissolved material in water. A nonpolar hquid forms a two-layer system with water an example is an oil shck. The interactions between nonpolar molecules are called hydrophobic interactions or, in some cases, hydrophobic bonds. 

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