2.7: Carbon Skeletons
The backbone of all organic compounds is a carbon skeleton. Each carbon atom can make four bonds, and as the carbon skeleton increases in length, the possibility for structural changes arises, such as ring structures, double bonds, and branching side chains.
Carbon Is the Basis of Organic Molecules
Life on Earth is carbon-based because all of the macromolecules that make up living organisms depend upon carbon atoms. At the core of each organic molecule is a carbon skeleton to which other atoms bond. The variety of these other atoms give each molecule its unique properties. Carbon can form four bonds and only rarely becomes an ion, making it an extraordinarily flexible component of molecules. These properties make carbon an essential component of all life on Earth, and it is found in abundance found not only on this planet but throughout the Universe as well.
Carbon-carbon bonds form the basis of the carbon skeleton. Hydrogen atoms readily bond to the carbon atom. Molecules that contain only hydrogen and carbon are called hydrocarbons. Hydrocarbons usually form either long chains or will have branches protruding at various points. Changing the number of bonds changes the properties of the molecule: for example, a fatty acid with a long hydrocarbon tail with one or more double bonds will behave differently than a fatty acid with no double bonds.
Isomers Are Different Ways to Arrange the Same Number of Atoms
Molecules with the same chemical formula but with different structures are called isomers. One example of isomers can be seen in two different molecules that share the chemical formula C6H14. Hexane has a straight, single chain of carbon atoms, while isohexane has a branch point on the second carbon atom. Other isomers might have a different arrangement of chemical groups on either side of a carbon-carbon double bond, resulting in two possible structures. Others still might be mirror images of one another, also called enantiomers. Like the fingers and thumb of the left and right hand, the parts of enantiomers are all the same, but they do not line up when superimposed.
Functional Groups Build on Carbon Skeletons
The unique properties of biological molecules are conferred by functional groups—chemical groups bonded to the carbon skeleton, such as amino (–NH2) or methyl groups (–CH3). Functional groups can be made up of atoms other than carbon, altering the structural and chemical properties of the molecule. The interactions of functional groups are crucial for nearly everything that occurs in a biological system, and knowledge of the properties of functional groups influences many fields of study, such as synthetic drug design.