CHAPTER 11: Unit 5. Physical properties: Alkanes and Cycloalkanes

Learning ObjectiveIdentify 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 “Physical Properties of Some Alkanes” 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 H2O 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.Table: Physical Properties of Some AlkanesMolecular NameFormulaMelting Point (°C)Boiling Point (°C)Density (20°C)*Physical State (at 20°C)methaneCH4–182–1640.668 g/LgasethaneC2H6–183–891.265 g/LgaspropaneC3H8–190–421.867 g/LgasbutaneC4H10–138–12.493 g/LgaspentaneC5H12–130360.626 g/mLliquidhexaneC6H14–95690.659 g/mLliquidoctaneC8H18–571250.703 g/mLliquiddecaneC10H22–301740.730 g/mLliquid*Note the change in units going from gases (grams per liter) to liquids (grams per milliliter). Gas densities are at 1 atm pressure.Alkanes only contain nonpolar C-C and C-H bonds. Therefore, they exhibit very weak intermolecular forces. As a result, Alkanes are not soluble in water and have lower densities than water. Melting and boiling point increases with increasing carbon chain length or ring size. Increased surface area increases the force of attraction between molecules.

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. (For more information about lipids, see Chapter 7 “Lipids”.). 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.

Tripalmitin (a), a typical fat molecule, has long hydrocarbon chains typical of most lipids. Compare these chains to hexadecane (b), an alkane with 16 carbon atoms.

IN CLASS PRACTICE PROBLEM

1. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.a. pentane or butaneb. heptane or nonane
2. For which member of each pair is hexane a good solvent?Answer
   1. a. pentaneb. nonane

Homework Exercises

1. Without referring to a table, predict which has a higher boiling point—hexane or octane. Explain.
2. If 25 mL of hexane were added to 100 mL of water in a beaker, which of the following would you expect to happen? Explain.a. Hexane would dissolve in water.b. Hexane would not dissolve in water and would float on top.c. Hexane would not dissolve in water and would sink to the bottom of the container.

Answers

1. octane because of its greater molar mass
2. b; hexane is insoluble in water and less dense than water.