A hydrocarbon chain is like a pearl necklace made of carbon atoms. However, each carbon pearl is adorned with one or up to three hydrogen atoms. Understanding the chemical properties of a carbon atom and the nature of covalent bonding will clarify why hydrocarbons take on the forms that they do. Covalent bonding is the sharing of electrons between two atoms to form a bridge that holds them together. Through covalent bonding, hydrocarbons become molecules that cells use to store energy in the form of fat. Knowing the difference between single and double covalent bonds helps clarify why olive oil is liquid and butter is solid at room temperature.
String of Pearls
A hydrocarbon chain is a chain of carbon atoms linked by covalent bonds, like a pearl necklace. A covalent bond unites two atoms when they share electrons. Individual carbon atoms have four valence electrons, which form covalent bonds and are located on the outermost energy level of the atom. Since carbon has four valence electrons and can make up to four single covalent bonds, each carbon in a hydrocarbon chain is bonded to up to two hydrogen atoms in addition to its neighboring carbons. Carbons at the head and tail are bonded to up to three hydrogen atoms and one carbon neighbor -- still fulfilling the four-bond rule. The presence of hydrogen and carbon is why the chain is called a hydrocarbon. The carbon atoms can also form double bonds with each other.
Saturated Versus Unsaturated
A hydrocarbon chain in which all carbon atoms are bound to as many hydrogen atoms as possible is said to be saturated. Saturation means that there is no more room on the carbons, each of which can only bond with four other atoms at most, to add another hydrogen atom or other atom. In contrast, an unsaturated hydrocarbon is one that has at least one double bond between two carbons somewhere in the chain. A double bond is two single bonds between the same two atoms. The carbons that are double-bonded with and facing each other are each bound to one other carbon behind them. These double-bonded carbons are only bound to one hydrogen atom each, so the total number of bonds for each of these carbons is still four. However, if the double bond is broken, each carbon sharing this double bond can take on another hydrogen atom, still maintaining the four-bond rule, but now using only single bonds. Thus, an unsaturated hydrocarbon can become saturated by breaking the double-bonds and adding hydrogen atoms.
Not as Many Fit in the Suitcase
Unsaturated hydrocarbons containing only one double bond are called monounsaturated hydrocarbons. Those that contain more than one double bond are called polyunsaturated hydrocarbons. The longer a saturated hydrocarbon is, the type without any double bonds, the higher its melting point -- meaning more heat is required to turn it into a liquid. Unsaturated hydrocarbons, however, have lower melting temperatures even if they have just one double bond. The double bond can cause the hydrocarbon chain to be bent like a boomerang. Bent hydrocarbons don't pack together as tightly, so melt more easily.
Cis Or Trans Makes A Difference
Unsaturated hydrocarbon chains have double bonds, but double bonds come in two forms, cis or trans. A cis double bond is what causes the hydrocarbon to bend like a boomerang. This is why olive oil, a monounsaturated fat, is liquid at room temperature. A trans double bond allows the hydrocarbon chain to remain straight, and increases the melting temperature compared to the same hydrocarbon that has a cis double bond in the same location in the chain.
- CK12: Covalent Bonding
- Clackamas Community College: Valence Electrons
- Tutorvista: Saturated and Unsaturated Hydrocarbons
- Everything Science: The Hydrocarbons
- Centers for Disease Control and Prevention: Polyunsaturated Fats and Monounsaturated Fats
- Elmhurst College, Virtual Chembook: Triglycerides
- Michigan State University, Dept. of Chemistry: Lipids
- BananaStock/BananaStock/Getty Images