Triesters

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.

The physical distinction is that fats are solid at room temperature and oils are generally liquid at room temperature. The structural difference is that fats are saturated (they have single bonds between all the carbon atoms) and oils are unsaturated (they contain some double bonds between carbon atoms). A third common distinction is that fats come from animals and oils come from vegetables. Keep in mind that, although useful, these distinctions are generalities and there are exceptions.

Fats	Oils
solids	liquids
saturated	unsaturated
animal	plant

Structure

This diagram (also shown in Example 16-a in your workbook) shows you two ways of writing the general structural formula for a fat or an oil. You can see that it is fairly complex, particularly when each alkyl group, the R or R'or R", can have 12 to 18 carbon atoms, more or less. (Please excuse the lines on the extended bonds for not lining up precisely.)

CH₂-O-C-R
|   ||
|     O
|
CH-O-C-R'
|    ||
|    O
|
CH₂-O-C-R"
   ||
   O

  CH₂-O-C-R
|    ||
R'-C-O-CH   O
||   |
O   CH₂-O-C-R"
      ||
      O

Formation

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.

glycerol

H-O-C-R
||
O

H-O-C-R'
||
O

H-O-C-R"
||
O

fatty acids

ООЎ

CH₂-O-C-R
|     ||
|     O
|
CH-O-C-R'
|    ||
|   O
|
CH₂-O-C-R"
   ||
   O

a triglyceride

+ 3 H₂O

Reaction

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.

CH₂-O-C-R
|   ||
|     O
|
CH-O-C-R'
|    ||
|    O
|
CH₂-O-C-R"
   ||
   O

a triglyceride

+ 3 H₂O

H⁺ or
ООООЎ
enzymes

RCOOH

R'COOH

R"COOH

CH₂OH
|
CH₂OH
|
CH₂OH

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.

Saponification

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.

CH₂-O-C-C₁₅H₃₁
|     ||
|     O
|
CH-O-C-C₁₅H₃₁
|    ||
|    O
|
CH₂-O-C-C₁₅H₃₁
||
O

+ 3 NaOH

ООЎ

3 C₁₅H₃₁COONa

sodium palmitate
(a soap)

CH₂OH
|
CH₂OH
|
CH₂OH

glycerol

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.

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