11.1: 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 and oils. They usually contain an even number of carbon atoms (typically 12–20), and can be classified by the presence and number of carbon-to-carbon double bonds. Note the presence of a nonpolar hydrocarbon chain (tail) and a polar carboxylic acid functional group (head) in the molecule. A fatty acid with one carbon-carbon double bond is a monounsaturated fatty acid. A fatty acid with two or more double bonds is a polyunsaturated fatty acid.

Figure \(\PageIndex{1}\): Saturated fatty acids have only single bonds while monounsaturated have one double bond and polyunsaturated have more than one double bond.

Saturated Fatty Acids

Saturated fatty acids contain all carbon-carbon single bonds. This causes the molecules to form straight chains, as shown in the figure 11.1.2. Palmitic acid is an example of a saturated fatty acid. The hydrocarbon tails of saturated fatty acids are able to be packed together very tightly with one another, interacting through London forces. London forces depends on the surface area of the molecule. The longer the hydrocarbon tail in a fatty acid, the stronger the interactions between molecules. The melting points of saturated fatty acids increases as the molecules get larger. This explains why saturated fatty acids are solids at room temperature. Animals use saturated fatty acids to store energy.

Unsaturated Fatty Acids

In an unsaturated fatty acids, the hydrocarbon chain contains at least one carbon-carbon double bond (alkene). The double bonds are usually cis. The presence of a cis double bond causes chains to bend or kink in the hydrocarbon tail (see figure 11.1.2). The bent tails in neighboring unsaturated fatty acids are farther apart from one another compared to saturated fatty acids. This reduction of contact between the tails weakens the london forces between them. So unsaturated fatty acids are liquids at room temperature. For fatty acids with the same number of carbon atoms, the presence of more double bonds lowers the melting point. Plants use unsaturated fatty acids to store energy.

Figure \(\PageIndex{2}\): Structures of saturated and unsaturated fatty acids.

Essential Fatty Acids

Two polyunsaturated fatty acids—linoleic and 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 average daily diet should contain about 4–6 g of the essential fatty acids.

Common Fatty Acids

Table \(\PageIndex{1}\) lists some common fatty acids. Chemists often use a shorthand notation for fatty acids along with the common name because they are all carboxylic acids with different numbers of carbons and number of double bonds. This notation gives the number of carbons followed by the number of double bonds present. For example: palmitic acid is a 16-carbon saturated fatty acid, would be represented by C16:0 and palmitoleic acid is a 16-carbon monounsaturated fatty acid, would be C16:1.

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

Name	Number of Carbons	Number of Double Bonds	Condensed Structural Formula	Melting Point (°C)	Shorthand Notation	Source
lauric acid	12	0	CH3(CH2)10COOH	44	C12:0	palm kernel oil
myristic acid	14	0	CH3(CH2)12COOH	58	C14:0	oil of nutmeg
palmitic acid	16	0	CH3(CH2)14COOH	63	C16:0	palm oil
palmitoleic acid	16	1	CH3(CH2)5CH=CH(CH2)7COOH	0.5	C16:1	macadamia oil
stearic acid	18	0	CH3(CH2)16COOH	70	C18:0	cocoa butter
oleic acid	18	1	CH3(CH2)7CH=CH(CH2)7COOH	16	C18:1	olive oil
linoleic acid	18	2	CH3(CH2)3(CH2CH=CH)2(CH2)7COOH	−5	C18:2	canola oil
linolenic acid	18	3	CH3(CH2CH=CH)3(CH2)7COOH	−11	C18:3	flaxseed
arachidonic acid	20	4	CH3(CH2)4(CH=CHCH2)4(CH2)2COOH	−50	C20:4	liver

ω-3 Fatty acids

Fats containing saturated fatty acids are considered not so healthy as they have a tendency to deposit in arteries forming plaque, i.e., atherosclerosis. It can lead to high blood pressure, rupture of arteries, or heart attack. Saturated fatty acids are more common in animal fats while unsaturated fatty acids which are considered more healthy are more common in vegetable oils.

In omega (ω) labeling the last C, i.e., the −CH₃ at the end opposite to carboxyl (−COOH) group is labeled ω-1, the C next to it as ω-2, then ω-3 and so on, as as shown in the figure 11.1.3. Omega (ω) is the last alphabet in Greek, so, ω-C is the last carbon. The (ω) label is used to refer to the position of the first C=C bond from the ω-end. For example, linolenic acid in figure below is an ω-3 fatty acid.

Figure \(\PageIndex{3}\): Labelling of C′s in fatty acids: systematic IUPAC numbers are in red, nonsystematic Greek letter labels are in black, and omega- (ω-#) are in blue. (Copyright; Public domain)

It has been observed that peoples from Alaska eat a higher amount of unsaturated fats and have a low occurrence of atherosclerosis and heart attack. Fats in Alaskan food is primarily from fish, such as salmon, tuna, and herring. Vegetable oils have higher contents of ω-6 fatty acids, such as linoleic acid and arachidonic acid. Fish oil mostly contains ω-3 fatty acids, such as linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid.

salmon

tuna

herring

ω-3 fatty acids in fish

Figure \(\PageIndex{4}\): Three common fish in the diet of people of Alaska and three common ω-3 fatty acids found in the fish: linolenic acid (top), eicosapentaenoic acid (middle), and docosahexaenoic acid (bottom). (Copyright: Public domain).

Fatty Acid Compositions of Common Fats and Oils

Table 11.1.2 shows the fatty acid compositions of some common fats and oils. The composition of any given fat or oil can vary depending on the plant or animal species it comes from as well as on dietetic and climatic factors. To cite just one example, lard from corn-fed hogs is more highly saturated than lard from peanut-fed hogs. Palmitic acid is the most abundant of the saturated fatty acids, while oleic acid is the most abundant unsaturated fatty acid. Terms such as saturated fat or unsaturated oil are often used to describe the fats or oils obtained from foods. Saturated fats contain a high proportion of saturated fatty acids, while unsaturated oils contain a high proportion of unsaturated fatty acids. The high consumption of saturated fats is a factor, along with the high consumption of cholesterol, in increased risks of heart disease.

Table \(\PageIndex{2}\): Average Fatty Acid Composition of Some Common Fats and Oils (%)*

Source	Lauric (C12:0)	Myristic (C14:0)	Palmitic (C16:0)	Stearic (C18:0)	Oleic (C18:1)	Linoleic (C18:2)	Linolenic (C18:3)
Fats
butter (cow)	3	11	27	12	29	2	1
tallow		3	24	19	43	3	1
lard		2	26	14	44	10
Oils
canola oil			4	2	62	22	10
coconut oil^†	47	18	9	3	6	2
corn oil			11	2	28	58	1
olive oil			13	3	71	10	1
peanut oil			11	2	48	32
soybean oil			11	4	24	54	7
*Totals less than 100% indicate the presence of fatty acids with fewer than 12 carbon atoms or more than 18 carbon atoms.
^†Coconut oil is highly saturated. It contains an unusually high percentage of the low-melting C8, C10, and C12 saturated fatty acids.