LIPIDS I Lipids II Lipid MiniTopics Review  Elmhurst College Fatty Acids Triglycerides Bilayer Membrane  Chemistry Department Wax Phosphoglycerides Soap  Virtual ChemBook

Table of Fatty Acids Acid Name  Structure Melt Point Graphic Chime SATURATED Lauric CH3(CH2)10COOH  +44  Graphic Chime Palmitic CH3(CH2)14COOH  +63 . Chime  Stearic CH3(CH2)16COOH  +70  Graphic Chime UNSATURATED Oleic  CH3(CH2)7CH=CH(CH2)7COOH  +16  Graphic Chime Linoleic CH3(CH2)4(CH=CHCH2)2(CH2)6COOH  -5  Graphic Chime Linolenic  CH3CH2(CH=CHCH2)3(CH2)6COOH  -11   Chime Arachi- donic  CH3(CH2)4(CH=CHCH2)4(CH2)2COOH  -50  Graphic Chime	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.  Quiz: Which acid (short chain or fatty) would most likely be soluble in water?   Answer water is polar, in short chain, acid group is polar, answer = short chain ... in hexane?   Answer hexane is non-polar, in fatty acid, long HC chain is non-polar answer = fatty acid Table of Fatty Acids on the left: 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.
Click for larger image	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. 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.  Quiz: If room tempereature is 25o, which of the following fatty acids is a solid or liquid at room tmeperature.  Palmitic   Answer still solid melts at +63.  Oleic Answer still liquid melts at +16.
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. Animal fats contain more saturated fats. Quiz on Fatty Acids: Write down your answers. Then check the answers from the drop down menu. Which fat or oil contains the most double bonds? Answer Soybean oil contains most linoleic with 2 double bonds per molecule. Which fats or oils are likely to be liquids at room temperature?   Answer All vegetable oils contain most unsaturated fatty acids, with low melting points. Which fats or oils have the least amount of unsaturated fatty acids?   Answer Animal fats