11.8: Physical Properties of Organic Compounds

Skills to Develop

• To identify the physical properties of organic compounds and describe trends in these properties.

Hydrocarbons

Hydrocarbons like alkanes, cycloalkanes, and alkenes all share similar physical properties because they only contain non-polar bonds, and thus only have London-dispersion intermolecular forces of attraction. None of the hydrocarbons are soluble in water (very polar), but are soluble in nonpolar and some slightly polar solvents. 

In Table \(\PageIndex{1}\) several physical properties of some of the first 10 straight-chain alkanes are listed. Other hydrocarbons exhibit the same trends. 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 gasoline and oil do not mix with water but rather float on its surface.

London-dispersion interactions are typically quite weak; this is why the first few alkanes below are all gases. As the size of the molecule increases, however, the larger electron clouds become more polarizable, increasing the strength of the dispersion attractions; notice that the larger alkanes exist as liquids at room temperature (and can be solids, if even larger). The boiling points and melting point increase as size increases because more energy is required to separate them. This general trend for melting and boiling points holds true for the straight-chain homologs of all organic compound families.

Table \(\PageIndex{1}\): Physical Properties of Some Alkanes

Molecular Name	Formula	Melting Point (°C)	Boiling Point (°C)	Density (at 20°C)	Physical State (at 20°C)
methane	CH4	–182	–164	0.000668 g/mL*	gas
ethane	C2H6	–183	–89	0.001265 g/mL*	gas
propane	C3H8	–190	–42	0.001867 g/mL*	gas
butane	C4H10	–138	–1	0.002493 g/mL*	gas
pentane	C5H12	–130	36	0.626 g/mL	liquid
hexane	C6H14	–95	69	0.659 g/mL	liquid
octane	C8H18	–57	125	0.703 g/mL	liquid
decane	C10H22	–30	174	0.730 g mL	liquid
*Gas densities are at 1 atm pressure.

Figure \(\PageIndex{1}\): Oil Spills. Crude oil coats the water’s surface in the Gulf of Mexico after the Deepwater Horizon oil rig sank following an explosion. The leak was a mile below the surface, making it difficult to estimate the size of the spill. One liter of oil can create a slick 2.5 hectares (6.3 acres) in size. This and similar spills provide a reminder that hydrocarbons and water don’t mix. Source: Photo courtesy of NASA Goddard / MODIS Rapid Response Team, http://www.nasa.gov/topics/earth/features/oilspill/oil-20100519a.html.

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.

Other Functional Groups

Hydrocarbons, like alkanes, are non-polar. Many other organic compounds contain contain additional highly-electronegative atoms, like oxygen or nitrogen, in functional groups with polar bonds. Table \(\PageIndex{2}\) compares physical properties of three organic compounds: an alkane, an alkyl halide, and an alcohol. As indicated by the mass and total electrons, all three approximately the same size. Notice that, as the number of intermolecular interactions increases, the boiling points and melting points increases. More types of attractive forces require more energy to separate the molecules (melt solid or boil liquid).

Table \(\PageIndex{2}\): Physical Properties of Different Functional Groups

SUBSTANCE name (formula)	MOLECULAR MASS (amu)	TOTAL ELECTRONS	TYPES OF INTERMOLECULAR INTERACTIONS	MELTING POINT (^0C)	BOILING POINT (^0C)	PHYSICAL STATE (at 20^0C)
ethane (CH3CH3)	30	18	dispersion	–183	–89	gas
methyl fluoride (CH3F)	34	18	dispersion dipole-dipole	–138	–78	gas
ethanol (CH3OH)	32	18	dispersion dipole-dipole hydrogen-bonding	–98	65	liquid

The types of forces also affects solubililty. Ethanol (beverage alcohol) is completely miscible with water, as both are extremely polar and have the same types of interactions. Methylfluoride, also polar, is soluble in water, but not as completely as ethanol. Non-polar ethane does not dissolve appreciably in water.

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 fat-like 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.

Figure \(\PageIndex{2}\): 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.

Concept Review Exercises

1. Without referring to a table, predict which has a higher boiling point:

   a. hexane or octane (explain);  b. An amine or an alkane, both of similar size (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.

   1. Hexane would dissolve in water.
   2. Hexane would not dissolve in water and would float on top.
   3. Hexane would not dissolve in water and would sink to the bottom of the container.

Answers

1. a. octane because of its larger size; b. an amine, RNH₂, because it would have more types of forces (three compared to one for alkane)

2. b; hexane is insoluble in water and less dense than water.

Key Takeaway

• Alkanes are nonpolar compounds that are low boiling and insoluble in water.

Exercises

1. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.

   1. pentane or butane
   2. Acetic acid (CH₃COOH) or methyl chloride (CH₃Cl)

2. Which would be more soluble in water, methanol (CH₃OH) or 1-heptanol (CH₃CH₂CH₂CH₂CH₂CH₂CH₂OH)? Explain.

Answer

1. a. pentane; b. acetic acid has more types of forces (three compared to two for methyl chloride)
2. Methanol would be more soluble in water. Because both are alcohols, the polar OH group will allow both to participate in dipole-dipole and hydrogen-bonding interactions with water. Because 1-heptanol has a much larger non-polar hydrocarbon portion, it is less soluble (not as much like water).