Nonpolar solvent, dissolving

Solubility. Cross-linking eliminates polymer solubiUty. Crystallinity sometimes acts like cross-linking because it ties individual chains together, at least well below T. Thus, there are no solvents for linear polyethylene at room temperature, but as it is heated toward its (135°C), it dissolves in a variety of aUphatic, aromatic, and chlorinated hydrocarbons. A rough guide to solubiUty is that like dissolves like, ie, polar solvents tend to dissolve polar polymers and nonpolar solvent dissolve nonpolar polymers. 

The general rule of solubility is like dissolves like. This means that polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes. 

Let s break it down into two parts polar and aprotic. Hopefully, you remember from general chemistry what the term polar means, and you should also remember that like dissolves like (so polar solvents dissolve polar compounds, and nonpolar solvents dissolve nonpolar compounds). Therefore, we really need a polar solvent to run substitution reactions. S 1 desperately needs the polar solvent to stabilize the carbocation, and Sn2 needs a polar solvent to dissolve the nucleophile. S il certainly needs the polar solvent more than Sn2 does, but you will rarely see a substitution reaction in a nonpolar solvent. So, let s focus on the term aprotic. 

Nonpolar Solute in a Nonpolar Solvent (Dissolves) Paraffin wax dissolves in gasoline. Both paraffin and gasoline are mixtures of nonpolar hydrocarbons (Figure 2-28). The molecules of a nonpolar substance (paraffin) are weakly attracted to each other, and these van der Waals attractions are easily overcome by van der Waals attractions with the solvent. Although there is little change in energy when the nonpolar substance dissolves in a nonpolar solvent, there is a large increase in entropy. 

Like dissolves like polar solvents dissolve polar solutes nonpolar solvents dissolve nonpolai solutes. 

Like Dissolves Like when trying to predict which solvents can dissolve which solutes, chemists use the rule of like dissolves like. This means that polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes. In the case of sugar and water, both are made up of polar molecules, so sugar is soluble in water. In the case of salt and water, the sodium and chloride ion pair is like the water molecule because it has a positive charge at one end and a negative charge at the other end. 

The statement like dissolves like means that polar solvents dissolve ionic and polar molecular solutes, and nonpolar solvents dissolve nonpolar molecular liquids. 

Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. 

These considerations account for the observed behavior that "like dissolves like." In other words, we observe that a given solvent usually dissolves solutes that have polarities similar to its own. For example, water dissolves most polar solutes, because the solute-solvent interactions formed in the solution are similar to the water-water interactions present in the pure solvent. Likewise, nonpolar solvents dissolve nonpolar solutes. For example, dry-cleaning solvents used for removing grease stains from clothes are nonpolar liquids. "Grease" is composed of nonpolar molecules, so a nonpolar solvent is needed to remove a grease stain. 

Solvents, like contaminants, may be polar or nonpolar. As a general rule, polar solvents dissolve polar residues while nonpolar solvents dissolve nonpolar residues. Thus, ionic residues such as chlorides, salts, acids, acid fluxes, and alkalis are best dissolved and removed with polar solvents such as water, isopropyl alcohol, ethanol, or methylethyl ketone. Greases, oils, silicones, rosin flux, and low-molecular-weight monomers are best dissolved and removed with solvents such as hydrocarbons, Freons , hydrochloro-fluorocarbons, xylene, terpenes, and naphtha. To remove both polar and nonpolar residues, a two-step process using both types of solvents may be used or, more conveniently, an azeotrope mixture of the two solvents can be used in a one-step process. Most of the chlorofluorocarbon solvents (Freons ) and their azeotropes with alcohols, methylene chloride, or ketones are being phased out due to their high ozone-depletion potentials. Solvent blends and azeotropes of hydro-fluoroethers and hydrochlorofluorocarbons (HCFC) are now replacing these solvents. 

In general, polar solvents dissolve polar or ionic solutes, and nonpolar solvents dissolve nonpolar solutes. This tendency is described by the rule like dissolves like. Thus, similar kinds of solvents dissolve similar kinds of solutes. Table 13.2 lists some common polar and nonpolar laboratory solvents. 

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