Skip to main content
Biology LibreTexts

2.3B: Hydrocarbons

  • Page ID
    12677
  • Hydrocarbons are important molecules that can form chains and rings due to the bonding patterns of carbon atoms.

    LEARNING OBJECTIVES

    Discuss the role of hydrocarbons in biomacromolecules

    KEY TAKEAWAYS

    Key Points

    • Hydrocarbons are molecules that contain only carbon and hydrogen.
    • Due to carbon’s unique bonding patterns, hydrocarbons can have single, double, or triple bonds between the carbon atoms.
    • The names of hydrocarbons with single bonds end in “-ane,” those with double bonds end in “-ene,” and those with triple bonds end in “-yne”.
    • The bonding of hydrocarbons allows them to form rings or chains.

    Key Terms

    • covalent bond: A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
    • aliphatic: Of a class of organic compounds in which the carbon atoms are arranged in an open chain.
    • aromatic: Having a closed ring of alternate single and double bonds with delocalized electrons.

    Hydrocarbons

    Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane (CH4). Hydrocarbons are often used as fuels: the propane in a gas grill or the butane in a lighter. The many covalent bonds between the atoms in hydrocarbons store a great amount of energy, which is released when these molecules are burned (oxidized). Methane, an excellent fuel, is the simplest hydrocarbon molecule, with a central carbon atom bonded to four different hydrogen atoms. The geometry of the methane molecule, where the atoms reside in three dimensions, is determined by the shape of its electron orbitals. The carbon and the four hydrogen atoms form a shape known as a tetrahedron, with four triangular faces; for this reason, methane is described as having tetrahedral geometry.

    image

    Methane: Methane has a tetrahedral geometry, with each of the four hydrogen atoms spaced 109.5° apart.

    As the backbone of the large molecules of living things, hydrocarbons may exist as linear carbon chains, carbon rings, or combinations of both. Furthermore, individual carbon-to-carbon bonds may be single, double, or triple covalent bonds; each type of bond affects the geometry of the molecule in a specific way. This three-dimensional shape or conformation of the large molecules of life (macromolecules) is critical to how they function.

    Hydrocarbon Chains

    Hydrocarbon chains are formed by successive bonds between carbon atoms and may be branched or unbranched. The overall geometry of the molecule is altered by the different geometries of single, double, and triple covalent bonds. The hydrocarbons ethane, ethene, and ethyne serve as examples of how different carbon-to-carbon bonds affect the geometry of the molecule. The names of all three molecules start with the prefix “eth-,” which is the prefix for two carbon hydrocarbons. The suffixes “-ane,” “-ene,” and “-yne” refer to the presence of single, double, or triple carbon-carbon bonds, respectively. Thus, propane, propene, and propyne follow the same pattern with three carbon molecules, butane, butene, and butyne for four carbon molecules, and so on. Double and triple bonds change the geometry of the molecule: single bonds allow rotation along the axis of the bond, whereas double bonds lead to a planar configuration and triple bonds to a linear one. These geometries have a significant impact on the shape a particular molecule can assume.

    image

    Hydrocarbon Chains: When carbon forms single bonds with other atoms, the shape is tetrahedral. When two carbon atoms form a double bond, the shape is planar, or flat. Single bonds, like those found in ethane, are able to rotate. Double bonds, like those found in ethene cannot rotate, so the atoms on either side are locked in place.

    Hydrocarbon Rings

    The hydrocarbons discussed so far have been aliphatic hydrocarbons, which consist of linear chains of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons, consists of closed rings of carbon atoms. Ring structures are found in hydrocarbons, sometimes with the presence of double bonds, which can be seen by comparing the structure of cyclohexane to benzene. The benzene ring is present in many biological molecules including some amino acids and most steroids, which includes cholesterol and the hormones estrogen and testosterone. The benzene ring is also found in the herbicide 2,4-D. Benzene is a natural component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and aromatic portions; beta-carotene is an example of such a hydrocarbon.

    image

    Hydrocarbon Rings: Carbon can form five-and six membered rings. Single or double bonds may connect the carbons in the ring, and nitrogen may be substituted for carbon.