3.2: Fatty Acids

Learning Objectives

• To recognize the structures of common fatty acids and classify them as saturated, monounsaturated, or polyunsaturated.

Fatty acids are carboxylic acids that are structural components of fats, oils, and all other categories of lipids, except steroids. More than 70 have been identified in nature. They usually contain an even number of carbon atoms (typically 12–20), are generally unbranched, and can be classified by the presence and number of carbon-to-carbon double bonds. Thus, saturated fatty acids contain no carbon-to-carbon double bonds, monounsaturated fatty acids contain one carbon-to-carbon double bond, and polyunsaturated fatty acids contain two or more carbon-to-carbon double bonds.

Table \(\PageIndex{1}\) lists some common fatty acids and one important source for each. The atoms or groups around the double bonds in unsaturated fatty acids can be arranged in either the cis or trans isomeric form. Naturally occurring fatty acids are generally in the cis configuration.

Table \(\PageIndex{1}\): Some Common Fatty Acids Found in Natural Fats

Name	Abbreviated Structural Formula	Condensed Structural Formula	Melting Point (°C)	Source
lauric acid	C11H23COOH	CH3(CH2)10COOH	44	palm kernel oil
myristic acid	C13H27COOH	CH3(CH2)12COOH	58	oil of nutmeg
palmitic acid	C15H31COOH	CH3(CH2)14COOH	63	palm oil
palmitoleic acid	C15H29COOH	CH3(CH2)5CH=CH(CH2)7COOH	0.5	macadamia oil
stearic acid	C17H35COOH	CH3(CH2)16COOH	70	cocoa butter
oleic acid	C17H33COOH	CH3(CH2)7CH=CH(CH2)7COOH	16	olive oil
linoleic acid	C17H31COOH	CH3(CH2)3(CH2CH=CH)2(CH2)7COOH	−5	canola oil
α-linolenic acid	C17H29COOH	CH3(CH2CH=CH)3(CH2)7COOH	−11	flaxseed
arachidonic acid	C19H31COOH	CH3(CH2)4(CH2CH=CH)4(CH2)2COOH	−50	liver

Two polyunsaturated fatty acids—linoleic and α-linolenic acids—are termed essential fatty acids because humans must obtain them from their diets. Both substances are required for normal growth and development, but the human body does not synthesize them. The body uses linoleic acid to synthesize many of the other unsaturated fatty acids, such as arachidonic acid, a precursor for the synthesis of prostaglandins. In addition, the essential fatty acids are necessary for the efficient transport and metabolism of cholesterol. The average daily diet should contain about 4–6 g of the essential fatty acids.

Although we often draw the carbon atoms in a straight line, they actually have more of a zigzag configuration (Figure \(\PageIndex{2a}\)). Viewed as a whole, however, the saturated fatty acid molecule is relatively straight (Figure \(\PageIndex{2b}\)). Such molecules pack closely together into a crystal lattice, maximizing the strength of dispersion forces and causing fatty acids and the fats derived from them to have relatively high melting points. In contrast, each cis carbon-to-carbon double bond in an unsaturated fatty acid produces a pronounced bend in the molecule, so that these molecules do not stack neatly. As a result, the intermolecular attractions of unsaturated fatty acids (and unsaturated fats) are weaker, causing these substances to have lower melting points. Most are liquids at room temperature.

Figure \(\PageIndex{2}\): The Structure of Saturated Fatty Acids. (a) There is a zigzag pattern formed by the carbon-to-carbon single bonds in the ball-and-stick model of a palmitic acid molecule. (b) A space-filling model of palmitic acid shows the overall straightness of a saturated fatty acid molecule.

 

Summary

Fatty acids are carboxylic acids that are the structural components of many lipids. They may be saturated or unsaturated. Most fatty acids are unbranched and contain an even number of carbon atoms. Unsaturated fatty acids have lower melting points than saturated fatty acids containing the same number of carbon atoms.

Concept Review Exercises

1. Give an example of each compound.

   1. saturated fatty acid
   2. polyunsaturated fatty acid
   3. monounsaturated fatty acid

2. Why do unsaturated fatty acids have lower melting points than saturated fatty acids?

Answers

1. 1. stearic acid (answers will vary)
   2. linoleic acid (answers will vary)
   3. palmitoleic acid (answers will vary)

2. Unsaturated fatty acids cannot pack as tightly together as saturated fatty acids due to the presence of the cis double bond that puts a “kink” or bend in the hydrocarbon chain.

Exercises

1. Classify each fatty acid as saturated or unsaturated and indicate the number of carbon atoms in each molecule.

   1. palmitoleic acid
   2. myristic acid
   3. linoleic acid

2. Classify each fatty acid as saturated or unsaturated and indicate the number of carbon atoms in each molecule.

   1. stearic acid
   2. oleic acid
   3. palmitic acid

3. Write the condensed structural formula for each fatty acid.

   1. lauric acid
   2. palmitoleic acid
   3. linoleic acid

4. Write the condensed structural formulas for each fatty acid.

   1. oleic acid
   2. α-linolenic acid
   3. palmitic acid

5. Arrange these fatty acids (all contain 18 carbon atoms) in order of increasing melting point. Justify your arrangement.

   1. 
   2. 
   3. 

6. Arrange these fatty acids (all contain 16 carbon atoms) in order of increasing melting point. Justify your arrangement.

   1. CH₃(CH₂)₁₄COOH
   2. 
   3. 

Answers

1. 1. unsaturated; 16 carbon atoms
   2. saturated; 14 carbon atoms
   3. unsaturated; 18 carbon atoms

 

3. 1. CH₃(CH₂)₁₀COOH
   2. CH₃(CH₂)₅CH=CH(CH₂)₇COOH
   3. CH₃(CH₂)₃(CH₂CH=CH)₂(CH₂)₇COOH

 

5. c < a < b; an increase in the number of double bonds will lower the melting point because it is more difficult to closely pack the fatty acids together.