Carbon chains form the skeletons of most organic molecules. The skeletons vary in length and may be straight, branched, or arranged in closed rings. Some carbon skeletons have double bonds, which vary in number and location. Such variation in carbon skeletons is one important source of the molecular complexity and diversity that characterize living matter. In addition, atoms of other elements can be bonded to the skeletons at available sites.
Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. Atoms of hydrogen are attached to the carbon skeleton wherever electrons are available for covalent bonding. Hydrocarbons are the major components of petroleum, which is called a fossil fuel because it consists of the partially decomposed remains of organisms that lived millions of years ago.
Although hydrocarbons are not prevalent in most living organisms, many of a cell’s organic molecules have regions consisting of only carbon and hydrogen. For example, the molecules known as fats have long hydrocarbon tails attached to a nonhydrocarbon component. Neither petroleum nor fat dissolves in water; both are hydrophobic compounds because the great majority of their bonds are relatively nonpolar carbon-to-hydrogen linkages. Another characteristic of hydrocarbons is that they can undergo reactions that release a relatively large amount of energy. The gasoline that fuels a car consists of hydrocarbons, and the hydrocarbon tails of fats serve as stored fuel for animals.
The hydrocarbons may exist as linear carbon chains, carbon rings, or combinations of both. It is the three-dimensional shape or conformation of the large molecules of life (macromolecules) that decides their function.
- Hydrocarbon Chains
Hydrocarbon chains are formed by successive bonds between carbon atoms and may be branched or unbranched. Those hydrocarbons which contain linear chains of carbon atoms are called aliphatic hydrocarbons. And 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.
- Hydrocarbon Rings
Another type of hydrocarbon called 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 also the 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.