14.8: Physical Properties of Alkanes
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Learning Objectives
- To identify the physical properties of alkanes and describe trends in these properties.
Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families. Let’s consider their physical properties first.
Table \(\PageIndex{1}\) describes some of the properties of some of the first 10 straight-chain alkanes. Because alkane molecules are nonpolar, they are insoluble in water, which is a polar solvent, but are soluble in nonpolar and slightly polar solvents. Consequently, alkanes themselves are commonly used as solvents for organic substances of low polarity, such as fats, oils, and waxes. Nearly all alkanes have densities less than 1.0 g/mL and are therefore less dense than water (the density of H 2 O is 1.00 g/mL at 20°C). These properties explain why oil and grease do not mix with water but rather float on its surface.
| Molecular Name | Formula | Melting Point (°C) | Boiling Point (°C) | Density (20°C)* | Physical State (at 20°C) |
|---|---|---|---|---|---|
| methane | CH 4 | –182 | –164 | 0.668 g/L | gas |
| ethane | C 2 H 6 | –183 | –89 | 1.265 g/L | gas |
| propane | C 3 H 8 | –190 | –42 | 1.867 g/L | gas |
| butane | C 4 H 10 | –138 | –1 | 2.493 g/L | gas |
| pentane | C 5 H 12 | –130 | 36 | 0.626 g/mL | liquid |
| hexane | C 6 H 14 | –95 | 69 | 0.659 g/mL | liquid |
| octane | C 8 H 18 | –57 | 125 | 0.703 g/mL | liquid |
| decane | C 10 H 22 | –30 | 174 | 0.730 g/mL | liquid |
| *Note the change in units going from gases (grams per liter) to liquids (grams per milliliter). Gas densities are at 1 atm pressure. |
Looking Closer: Gas Densities and Fire Hazards
Table \(\PageIndex{1}\) indicates that the first four members of the alkane series are gases at ordinary temperatures. Natural gas is composed chiefly of methane, which has a density of about 0.67 g/L. The density of air is about 1.29 g/L. Because natural gas is less dense than air, it rises. When a natural-gas leak is detected and shut off in a room, the gas can be removed by opening an upper window. On the other hand, bottled gas can be either propane (density 1.88 g/L) or butanes (a mixture of butane and isobutane; density about 2.5 g/L). Both are much heavier than air (density 1.2 g/L). If bottled gas escapes into a building, it collects near the floor. This presents a much more serious fire hazard than a natural-gas leak because it is more difficult to rid the room of the heavier gas.
Also shown in Table \(\PageIndex{1}\) are the boiling points of the straight-chain alkanes increase with increasing molar mass. This general rule holds true for the straight-chain homologs of all organic compound families. Larger molecules have greater surface areas and consequently interact more strongly; more energy is therefore required to separate them. For a given molar mass, the boiling points of alkanes are relatively low because these nonpolar molecules have only weak dispersion forces to hold them together in the liquid state.
Looking Closer: An Alkane Basis for Properties of Other Compounds
An understanding of the physical properties of the alkanes is important in that petroleum and natural gas and the many products derived from them—gasoline, bottled gas, solvents, plastics, and more—are composed primarily of alkanes. This understanding is also vital because it is the basis for describing the properties of other organic and biological compound families. For example, large portions of the structures of lipids consist of nonpolar alkyl groups. Lipids include the dietary fats and fatlike compounds called phospholipids and sphingolipids that serve as structural components of living tissues. These compounds have both polar and nonpolar groups, enabling them to bridge the gap between water-soluble and water-insoluble phases. This characteristic is essential for the selective permeability of cell membranes.
Key Takeaway
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Alkanes are nonpolar compounds that are low boiling and insoluble in water.