Lipids

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Lipids are largely hydrocarbon like, and therefore do not dissolve in water but in nonpolar solvents like diethyl ether and benzene. The lipid family is very large and diverse. It includes cholesterol, hormones, and the edible fats and oils we eat.

Triacylglycerols

The edible fats and oils we eat are triacylglycerols (commonly called triglycerides). They are esters between glycerol (an alcohol with three OH groups) and any three of several long-chain carboxylic acids.
glycerol
triacylglycerol general structure

Fatty Acids

Fatty acids are the carboxylic acids used to make molecules of triacylgycerols. They generally have just one carboxyl group on a chain of even numbers of carbon atoms. They are unbranced in the most abundant ones. Some fatty acids have alkene groups. Long hydrocarbon chains are what make them nonpolar and like hydrocarbons. Lipids from vegetable sources are liquid at room temperature while lipids from animal fats are solid at room temperature. This is because vegetable oils have more alkene double bonds per molecule than animal fats, and are polyunsaturated materials.

Common Fatty Acids

Fatty acid

# of carbon
atoms

Structure

Melting
point (°C)
Myristic acid
 14 
CH3(CH2)12COOH
 54
Palmitic acid
 16 
CH3(CH2)14COOH
 63
Stearic acid
 18 
CH3(CH2)16COOH
 70
Oleic acid
 18 
CH3(CH2)7CH=CH(CH2)7COOH
 4
Linoleic acid
 18 
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
 -5
Linolenic acid
 18 
CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
 -11

Digestion of Triacylglycerols

We digest triacylglycerols by hydrolysis (using water). For example: 

triacylglycerol + 3 water ==> glycerol + 3 fatty acids

Hydrogenation of Vegetable Oils

Vegetable oils (liquids) are generally less expensive to produce than animal fats (solids). Using hydrogenation, chemists can break some of the carbon-carbon double bonds and replace them with hydrogen to make them chemically identical to the triacylglycerols in animal fats. Breaking one of the three double bonds in the molecule above can make the product melt above room temperature.

Saponification of Triacylglycerols

The hydrolysis of triacylglycerol in the presence of sufficient sodium hydroxide is called saponification. The fatty acids are released as sodium salts and the mixture of these salts of long-chain fatty acids is soap.
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