Stearic Acid, a Fatty Acid
Stearic acid is an example of a fatty acid. Fatty acids are long molecules consisting
of a hydrocarbon chain with a carboxylic acid group (-COOH) at the end. The long tail of
the molecule, made up of carbon and hydrogen, is not attracted to water and is said
to by hydrophobic (literally, water-fearing). The carboxylic acid "head" can form hydrogen
bonds with water, and is therefore strongly attracted to water. It is said to be
hydrophilic (literally, water-loving). When a fatty acid is placed on a water surface,
the hydrophilic heads of the molecules are attracted to the water. As a result, the
molecules form a monolayer on the surface of the water with their heads sticking into
the water and their hydrophobic tails sticking up above the surface of the water.
stearic acid, C18H36O2
Writing out all of those C's and H's in the diagram of the molecule can get
tedious. They can also obscure the underlying structure of the molecule.
Therefore, a common shorthand is to leave them out. In the following diagram, at
the points where line segments join, it is understood that a carbon atom is present
with enough hydrogens to make sure all of the carbon bonds are filled.
stearic acid, alternative representation
To help you visualize the molecule, here is an interactive
space-filling model of a stearic acid molecule. Per convention,
carbon atoms are gray, hydrogen atoms are white, and oxygen atoms are red.
Back to the main molecular models page.