Nonpolar compounds, boiling point

On nonpolar columns, the compounds of a homologous series separate as a function of their boiling points, and linear relationships have been established between the logarithms of the retention volumes and the number of carbon atoms in the 2-, 4-, and 5-positions (see Fig. III-l). 

Both polar compounds ethanol and fluoroethane have higher boiling points than the nonpolar propane We attribute this to a combination of dipole/mduced dipole and dipole-dipole attractive forces that are present m the liquid states of ethanol and fluo roethane but absent m propane... 

Boiling-point separation of broad molecular weight range of compounds nonpolar phases ... 

Alkenes — Also known as olefins, and denoted as C H2 the compounds are unsaturated hydrocarbons with a single carbon-to-carbon double bond per molecule. The alkenes are very similar to the alkanes in boiling point, specific gravity, and other physical characteristics. Like alkanes, alkenes are at most only weakly polar. Alkenes are insoluble in water but quite soluble in nonpolar solvents like benzene. Because alkenes are mostly insoluble liquids that are lighter than water and flammable as well, water is not used to suppress fires involving these materials. Because of the double bond, alkenes are more reactive than alkanes. 

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

The compound with the lowest boiling point is the one with the weakest intermolecular forces of attraction. Compound (2) which is nonpolar and the same approximate size as the others should therefore have the lowest boiling point. 

Melting points and boiling points are related to the strength of the intermolecular forces between solvent molecules, and to the molecular weight of the solvent. Dispersive forces, hydrogen bonding and permanent dipole moments all contribute. Typically, for molecules of similar mass, nonpolar compounds which... 

Alkanes and alkenes are nonpolar compounds whose corresponding structures have almost identical molecular weights. Boiling points of alkenes are close to those of alkanes and similarly have 20° increments per... 

Polarity is a physical property of a compound, which relates other physical properties, e.g. melting and boiling points, solubility and intermolecular interactions between molecules. Generally, there is a direct correlation between the polarity of a molecule and the number and types of polar or nonpolar covalent bond that are present. In a few cases, a molecule having polar bonds, but in a symmetrical arrangement, may give rise to a nonpolar molecule, e.g. carbon dioxide (CO2). 

Reversed-phase chromatography is the term commonly applied to a system where a nonpolar liquid phase is coated on the solid support and elution carried out with an immiscible polar phase. Such systems are often necessary for separations which cannot be carried out by normal partition or adsorption chromatography. For TLC, the stationary phase is normally a liquid of high boiling point which does not readily evaporate from the adsorbent. Paraffin oil, silicone oil or n-tetradecane coated on silica gel or Kieselguhr are frequently used with water-based mobile phases such as acetone—water (3 2) or acetic acid-water (3 1). Reversed-phase chromatography is very useful for the TLC analysis of lipids and related compounds. 

Aldehydes and ketones are polar compounds that can be separated on a polar or an intermediate polar column. Polyethylene glycol (PEG)-type phase, such as Carbowax 20 M, Supelcowax 10, VOCOL, DBWax, or equivalent are suitable for the purpose. Compounds may also be separated according to their boiling points on a nonpolar column. A 60 m long, 0.53 mm ID and 1 pm film or other appropriate dimension methyl silicone capillary columns, such as SPB-1, DB-1, or DB-5. 

CH3CH2CH3 -42° Small nonpolar compounds have low boiling points. 

Let us look at the benzene-cyclohexane separation more closely as we summarize how GC works. The boiling points of benzene and cyclohexane are nearly the same, 80.1 and 81.4°C respectively. Any GC separation will have to depend on differences in the intermolecular interactions between the stationary phase and these two analytes, both of which are nonpolar hydrocarbons. What differences could be exploited with GC Benzene has a -n-electron cloud, which should make it more susceptible to induction effects and perhaps dispersion attractions (Chapter 3). Therefore we should choose a stationary liquid phase that would accentuate this difference—a polar one also, using the like-dissolves-like rule we might choose an aromatic compound that would interact more with benzene than with cyclohexane. One possible liquid phase that meets these criteria is dinonylphthalate, and it has been used to separate benzene and cyclohexane. The relative retention has been found to be 1.6, which represents a very good separation.1... 

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. 

Identify polar and nonpolar molecules. Predict general trends in the boiling points and solubilities of compounds, based on their size, polarity, and ability to form hydrogen bonds. 

---