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ChemWiki: The Dynamic Chemistry E-textbook > Biological Chemistry > Lipids > Fatty Acids

Fatty Acids

Fatty acids are merely carboxylic acids with long hydrocarbon chains. The hydrocarbon chain length may vary from 10-30 carbons (most usual is 12-18). The non-polar hydrocarbon alkane chain is an important counter balance to the polar acid functional group. In acids with only a few carbons, the acid functional group dominates and gives the whole molecule a polar character. However, in fatty acids, the non-polar hydrocarbon chain gives the molecule a non-polar character.

Introduction

The most common fatty acids are listed. Note that there are two groups of fatty acids--saturated and unsaturated. Recall that the term unsaturated refers to the presence of one or more double bonds between carbons as in alkenes. A saturated fatty acid has all bonding positions between carbons occupied by hydrogens.

The melting points for the saturated fatty acids follow the boiling point principle observed previously. Melting point principle: as the molecular weight increases, the melting point increases. This observed in the series lauric (C12), palmitic (C16), stearic (C18). Room temperature is 25oC, Lauric acid which melts at 44o is still a solid, while arachidonic acid has long since melted at -50o, so it is a liquid at room temperature.

Table 1: Common Fatty Acids

Fatty Acids
Acid
Name
 Structure
Melting
Point
Graphic
SATURATED
Lauric CH3(CH2)10COOH
 +44
 
Palmitic CH3(CH2)14COOH
 +63
 
 Stearic CH3(CH2)16COOH
 +70
 
UNSATURATED
Oleic  CH3(CH2)7CH=CH(CH2)7COOH
 +16
 
Linoleic CH3(CH2)4(CH=CHCH2)2(CH2)6COOH
 -5
 
Linolenic  CH3CH2(CH=CHCH2)3(CH2)6COOH
 -11
 
Arachidonic  CH3(CH2)4(CH=CHCH2)4(CH2)2COOH
 -50
 

Melting Points of Saturated vs. Unsaturated Fatty Acids

Note that as a group, the unsaturated fatty acids have lower melting points than the saturated fatty acids. The reason for this phenomenon can be found by a careful consideration of molecular geometries. The tetrahedral bond angles on carbon results in a molecular geometry for saturated fatty acids that is relatively linear although with zigzags. See graphic on the left.

This molecular structure allows many fatty acid molecules to be rather closely "stacked" together. As a result, close intermolecular interactions result in relatively high melting points.

On the other hand, the introduction of one or more double bonds in the hydrocarbon chain in unsaturated fatty acids results in one or more "bends" in the molecule. The geometry of the double bond is almost always a cis configuration in natural fatty acids. and these molecules do not "stack" very well. The intermolecular interactions are much weaker than saturated molecules. As a result, the melting points are much lower for unsaturated fatty acids.

 

Percent Fatty Acid Present in Triglycerides
 Fat or Oil
Saturated
Unsaturated
 
   Palmitic Stearic  Oleic Linoleic  Other
Animal Origin
 Butter 29 9 27 4 31
 Lard 30 18 41 6 5
 Beef 32 25 38 3 2
 Vegatable Origin
 Corn oil  10 4 34 48 4
 Soybean 7 3 25 56 9
 Peanut 7 5 60 21 7
 Olive 6 4 83 7 -

Saturated vs. Unsaturated Fatty Acids in Fats and Oils

Examine the table on the left, if you want the most unsaturated fatty acids in your diet, which is the most healthy, which fat or oil should you use the most? Answer = olive oil.

Which fat or oil contains the most saturated fatty acids? Answer = beef fat.

General Principle: Vegetable oils contain more unsaturated fatty acids and Animal fats contain more saturated fats.

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Last Modified
12:51, 21 Jan 2014

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