Polarity effects on solubilities

In addition to affecting boiling points and melting points, intermolecular forces determine the solubility properties of organic compounds. The general rule is that like dissolves like. Polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. We discuss the reasons for this rule now, then apply the rule in later chapters when we discuss the solubility properties of organic compounds. 

We should consider four different cases (1) a polar solute with a polar solvent, (2) a polar solute with a nonpolar solvent, (3) a nonpolar solute with a nonpolar solvent, and (4) a nonpolar solute with a polar solvent. We will use sodium chloride and water as examples of polar solutes and solvents, and paraffin wax and gasoline as examples of nonpolar solutes and solvents. 

Most vitamins contain charged groups, making them water-soluble. As a result, they are rapidly eliminated and generally nontoxic. Vitamins A and D, however, are nonpolar and are stored in the fattissue of the body, which is also nonpolar. Hence, these two vitamins are potentially toxic in large doses. 

Because water molecules are strongly polar, a large amount of energy is released when the sodium and chloride ions are hydrated. This energy is nearly sufficient to overcome the lattice energy of the crystal. The salt dissolves, partly because of strong solvation by water molecules and partly because of the increase in entropy (randomness or freedom of movement) when it dissolves. 

Polar solute in water (a polar solvent). The hydration of sodium and chloride ions by water molecules overcomes the lattice energy of sodium chloride. The salt dissolves. 

To predict relative boiling points, we should look for differences in (1) hydrogen bonding, (2) molecular weight and surface area, and (3) dipole moments. Except for neopentane, these compounds have similar molecular weights. Neopentane is the lightest, and it is a compact spherical structure that minimizes van der Waals attractions. Neopentane is the lowest-boiling compound. 

Neither n-hexane nor 2,3-dimethyl butane is hydrogen bonded, so they will be next higher in boiling points. Because 2,3-dimethylbutane is more highly branched (and has a smaller surface area) than -hexane, 2,3-dimethylhutane will have a lower boiling point than -hexane. So far, we have 

The two remaining compounds are both hydrogen-honded, and pentan-l-ol has more area for van der Waals forces. Therefore, pentan-l-ol should he the highest-hoiling compound. We predict the following order  

The actual boiling points are given here to show that our prediction is correct 

For each pair of compounds, circle the compound you expect to have the higher boiling point. Explain your reasoning. 

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Before leaving the subject of distribution of electrons within molecules, and its attribution to the origin of molecular polarity, with consequent effect on intermolec-ular forces (with further consequent effects on solubilities and melting points), it is pertinent to remind ourselves of two significant challenges faced by chemistiy instractors (i) to graphically represent forces of attraction between molecules and (ii) to develop the imagery that in the liquid state, orientation of molecules toward each other because of polarities is transitory, even if more probable, as they move past each other. 

What does the axiom "like dissolves like" mean There are four types of solute/sol-vent combinations polar solutes in polar solvents, nonpolar solutes in polar solvents, and so on. For each type of solution, discuss the magnitude of Structure, pressure, and temperature all have an effect on solubility. Discuss each of their effects. What is Henry s law Why does Henry s law not work for HCI(g) What do the terms hydrophobic and hydrophilic mean ... 

Fig. 14.3. Effects of temperature (A) and solvent polarity (B) on solubilities of three fi-CyDs in water. -CyD ...

Dual Polarity and Effects on Solubility To examine these ideas further, let s compare the solubilities of a series of alcohols in water and hexane, solvents with very different intermolecular forces. Alcohols are organic compounds that have a dual polarity, a polar hydroxyl (—OH) group bonded to a nonpolar hydrocarbon group ... 

In order to establish the mechanism of the reaction, the solvent effect on the reaction of pyridinyl radical with dibromomethane was investigated. As the results listed in Table 1 show, there is no solvent effect on the rate of the reaction. How could one reconcile the formation of a salt with the lack of solvent polarity effect on the rate Since the initial state (Py + RX) is not very polar (pyridinyl radical with a 1-ethyl group is soluble in n-hex ine, BrCH2Br has a dipole moment of ca. 1 Debye), the lack of... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

The Freundlich liquid phase isotherm can be used to determine the effect of solubility on the adsorptive capacity of activated carbon over a range of different concentrations. Phenol is highly soluble due to its polar nature whilst, in comparison, tetrachloroethylene (PCE) has a low solubility due to being non-polar. In the isotherms illustrated, the concentration of phenol is low relative to its solubility limit and consequently, the adsorptive capacity peaks at 18% maximum (see Figure 9). In comparison the concentration of tetrachloroethylene is relatively close to its solubility limit and, accordingly, the adsorptive capacity is exceptionally good. 

These general effects are summarized in Table 5.1. As can be seen in the table, unsymmetrical structure and flexible links adversely affect the glass transition. However, a combination of flexible and polar substituents can increase solubility without deleterious effect on Ts. 

As a result of its highly polar character, silica gel is particularly useful in the separation of polarizable materials such as the aromatic hydrocarbons and polynuclear aromatics. It is also useful in the separation of weakly polar solute mixtures such as ethers, esters and in some cases, ketones. The mobile phases that are commonly employed with silica gel are the n-paraffins and mixtures of the n-paraffins with methylene dichloride or chloroform. It should be borne in mind that chloroform is opaque to UV light at 254 nm and thus, if a fixed wavelength UV detector is being used, methylene dichloride might be a better choice. Furthermore, chloroform is considered toxic and requires special methods of waste disposal. Silica gel is strongly deactivated with water and thus, to ensure stable retentive characteristics, the solvent used for the mobile phase should either be completely dry or have a controlled amount of water present. The level of water in the solvent that will have significant effect on solute retention is extremely small. The solubility of water in n-heptane is... 

The toxicological or cumulative effect of illicit drugs on the ecosystems has not been studied yet. Moreover, their fate and transport in the environment is to a big extent still unknown. Due to their physical-chemical properties (octanol-water partition coefficient, solubility, etc.) some of them, such as cannabinoids, are likely to bioaccumulate in organisms or concentrate in sediments whereas the rest, much more polar compounds, will tend to stay in aqueous environmental matrices. However, continuous exposure of aquatic organisms to low aquatic concentrations of these substances, some of them still biologically active (e.g., cocaine (CO), morphine (MOR) and MDMA) may cause undesirable effects on the biota. 

Chlorofonn is too non-polar to dissolve the phenolic compounds under study, but it dissolves many of the monoterpenes, at least to some extent. Because the solubility of some monoterpenes into chloroform was low, different solvent/ solid ratios were tested. These were 50,20,10 and 5 1/kg of dry phloem. The extracts were bright yellow and the strongest colour was with the smallest solvent/solid ratio (51/kg). The colour of the solvent indicated that the solubility of the extractable compounds was not restricting the reaction even with the smallest solvent volume. The taste of the dry samples was evaluated by comparing them to the original phloem sample. The results showed that the mildest taste was in the phloem extracted with a solvent/solid ratio of 50 1/kg and 20 1/kg also had some effect on the taste. The taste of the chloroform-extracted phloem was stabile and it was the same after a week. 

A review on TLC and PLC of amino adds, peptides, and proteins is presented in the works by Bhushan [24,25]. Chromatographic behavior of 24 amino acids on silica gel layers impregnated tiraryl phosphate and tri-n-butylamine in a two-component mobile phase (propanol water) of varying ratios has been studied by Sharma and coworkers [26], The effect of impregnation, mobile phase composition, and the effect of solubility on hRf of amino acids were discussed. The mechanism of migration was explained in terms of adsorption on impregnated silica gel G and the polarity of the mobile phase used. 

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