A very special class of esters are the esters which are formed by the reaction of fatty acids with glycerol, which has three alcohol groups. Glycerol is also called 1,2,3-propantriol. These are called triesters or triglycerides.
Fats and Oils
These esters can be called fats or oils, depending upon the degree of unsaturation in the compound. There are several ways of distinguishing between fats and oils: physical properties and structural formulas are two of them.
Below (and in Example 16-b in your workbook) you can see a reaction equation which indicates how such a fat might be made from three different fatty acids reacting with the three hydroxyl groups on the glycerol to form the fat molecule and splitting out three molecules of water. A fat molecule is called a triester because there are really three ester groups in the same molecule. Triesters are also commonly called triglycerides.
The reverse process of the formation of esters can also occur. That process involves the addition of water to an ester to reform the acids and the alcohols that went into making the ester. Such a reaction is referred to as hydrolysis, and it is exactly the opposite of dehydration.
If we were to take the reaction shown above and reverse the direction of the arrow in the equation, this would be an example of a hydrolysis reaction. That has been done here (and in Example 17-a in your workbook), with some simplification of the formulas.
Here we start with a triester (or triglyceride as it can also be called) and three molecules of water with whatever chemicals are needed to cause the reaction to occur (such as hydrogen ion or enzymes). The reaction results in the formation of the three fatty acids and glycerol.
This kind of hydrolysis can also be carried out in the presence of a strong base like NaOH. When this is done, the acids formed by the hydrolysis are immediately neutralized and converted into fatty acid salts, as shown in the equation below (and in Example 17-b). More likely the acids were never formed because the hydrogen never got a chance to attach to the oxygen. The result is soap and glycerol. The process is called saponification instead of hydrolysis, because soap is formed.
A soap essentially is a fatty acid salt. Usually it is the sodium salt of a fatty acid; sometimes the potassium salt. In either case, they are usually made from the hydrolysis of a fat of some kind to form glycerol and the fatty acid. The addition of the base causes the fatty acid to be converted to the salt of the fatty acid. Then after that has happened, more sodium ion is added (usually in the form of NaCl) to precipitate out the fatty acid salt. Remember that sodium salts are generally fairly soluble, so you need to add lots of sodium in order to shift the solubility equilibrium position to get the salt to precipitate. Since this is usually done by the addition of sodium chloride, that portion of the reaction is called salting out the soap.
E-mail instructor: Sue Eggling
Clackamas Community College