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Alkanes and Cycloalkanes

Properties of Alkanes and Cycloalkanes

The main source of alkanes is petroleum from decomposed organic material. Alkanes are used for a variety of purposes. Alkanes have very weak intermolecular forces and, as a result, have low melting and boiling points and poor water solubility.

Methane (CH4) is the smallest alkane (hydrocarbon with single bonds). Alkanes with four or fewer carbons exist as gases at room temperature and atmospheric pressure.

Alkanes have a variety of purposes, including fuel and industrial solvents.

Uses of Hydrocarbons

Name Uses
Methane Fuel in electrical generators; main component of natural gas
Ethane Used in production of ethylene
Propane Used in heating and cooking, such as propane grills
Butane Lighter fluid and in some aerosols
Pentane Solvent used in research and teaching labs and in production of polystyrene
Hexane Component of glue used in shoes, leather products, and roofing
Heptane A component of gasoline with a zero-octane rating
Octane Gasoline additive (especially in the form of isooctane, 2,2,4-trimethylpentane)
Nonane Component of diesel fuel
Decane Component of jet fuel and diesel

Hydrocarbons are compounds that contain hydrogen and carbon only. Hydrocarbons have many important roles in manufacturing and industry.

Alkanes owe their low melting point and boiling point and low solubility to their relatively weak intermolecular forces. Intermolecular forces are attractive or repulsive forces between a molecule and a nearby molecule, atom, or ion. Methane, propane, and butane are colorless gases at room temperature, though propane and butane are commonly pressurized and sold as liquids. Straight-chain alkanes with 5–15 carbons are liquids at room temperature. Straight-chain alkanes with 16–19 carbons are semisolid waxes at room temperature., Straight-chain alkanes with 20 or more carbons are waxy solids at room temperature. In general, branched (or not straight) chains of carbons will have lower intermolecular forces than their straight-chain counterpart and therefore have lower melting and boiling points.

Ball-and-Stick Representations of Alkanes

Three-dimensional ball-and-stick models of methane, ethane, and propane show the arrangement of the atoms in space.
The negligible difference in electronegativity between carbon and hydrogen atoms in alkanes and cycloalkanes cause these molecules to be relatively nonpolar (no net dipole moment). Therefore, the only attractions between the hydrocarbon molecules result from van der Waals forces, which increase in intensity as the size of the molecule increases. Van der Waals forces include dipole-dipole interaction, London dispersion force, and intermolecular force between permanent molecular dipoles.

As the number of carbons increases, so does the compound's boiling point. Furthermore, isomers with less branching have higher boiling points because these molecules are able to arrange themselves in closer proximity to neighboring molecules. The opposite is true for highly branched alkanes. Fractional distillation exploits the distinct boiling points of different alkanes by boiling a mixture of compounds and separating the components as they vaporize into gas. In crude oil, for example, the lighter, smaller hydrocarbons vaporize and are collected first, with the heavier components requiring additional heat to boil and vaporize.

Cycloalkanes, alkanes in cyclic form, generally have higher boiling and melting points than alkanes because their ring shape permits more surface interaction with surrounding molecules, resulting in stronger van der Waals forces.

Alkanes have low water solubility because the water molecules are polar and are attracted to each other (and not the nonpolar alkanes). The alkanes remain separate from the water molecules and are considered hydrophobic, or water repellant.

When an alkane is present with oxygen and high temperatures, the alkane will burn into carbon dioxide and water in a combustion reaction. Smaller hydrocarbons are more volatile and ignite more easily, usually resulting in complete combustion and a blue flame. Larger molecules are less reactive because the van der Waals forces are stronger, and they require more heat to vaporize and mix with oxygen. Combustion of larger alkanes can result in incomplete combustion, giving a yellow flame with black smoke, creating a mix of carbon dioxide and carbon monoxide.
CH4+2O2CO2+2H2OΔH°=890kJMethane+OxygenCarbonDioxide+Water\begin{gathered}{\rm{CH}}_4&+&2{\rm {O}}_2&\rightarrow&{\rm{CO}}_2&+&2{\rm {H}_2{O}}\hspace{25px}\Delta{\rm {H}}\degree=-890\;{\rm {kJ}}\\{\text{Methane}}\;&+&\;{\text{Oxygen}}\;&\rightarrow&\;{\text{Carbon}}\;{\text{Dioxide}}\;&+&\hspace{-100pt}{\text{Water}}\end{gathered}