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Triglycerides (page 2)

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Author: Janice VanCleave

Why?

Molecules are two or more atoms bonded (linked) together. A bond is a force holding atoms together. Fats and oils make up a group of naturally occurring organic chemical compounds called triglycerides. Triglycerides are organic chemical compounds made of one molecule of glycerol (organic alcohol) with three molecules of fatty acids (organic acids). Each glycerol molecule has a structure of three carbon atoms with one fatty acid molecule branching from each carbon atom. Fatty acids are so named because they are commonly found in fats and oils. Organic acids or organic compounds with one or more acid groups are called the carboxyl group (COOH). Fatty acids have carbon "backbones,"usually from 4 to 24 carbons or more, with one carboxyl group at the end. The three acid side chains can be alike or different from one another. Each carbon atom in an acid chain can be attached to another carbon by either a single or double bond. Organic compounds with single carbon-to-carbon bonds are called saturated, while unsaturated organic compounds have one or more multiple carbon-to-carbon bonds. Saturated fatty acid contains all the hydrogens they are capable of holding, thus the acid is said to be saturated with hydrogen. As the number of double bonds increases, the number of hydrogens attached to the carbons decreases. If the fatty acid has one double bond it is called monounsaturated. If there are two or more double bonds, it is called polyunsaturated. A saturated triglyceride has only saturated fatty acid chains; a monounsaturated triglyceride has one or more monounsaturated fatty acid chains; and a polyunsaturated triglyceride has one or more polyunsaturated fatty acid chains.

Tincture of iodine contains elemental (the natural, noncharged form of an element) iodine (I2) dissolved in alcohol. When mixed with an unsaturated molecule, such as unsaturated triglycerol, the iodine causes the mixture to look reddish. After heating, the double bonds break and iodine bonds to the carbons. The elemental iodine, along with its color, disappears. (The heat speeds up the process.) The combination of iodine with ethene (an unsaturated organic compound) provides a simple illustration of how iodine bonds with an unsaturated molecule. The product resulting from adding iodine to ethene is colorless 1,2-diiodoethane, as shown in Figure 29.2. However, any remaining amounts of uncombined iodine generally are still present, causing the contents of the test tube to be yellow instead of clear.

Try New Approaches

How does the amount of oil affect additions to carbon-to-carbon double bonds? Repeat the experiment twice, first using more oil. Keep the amount of iodine the same but use 3 tablespoons (45 ml) of oil. Then repeat using less oil, 1 tablespoon (15 ml).

Design Your Own Experiment

The degree of unsaturation of triglycerides is represented by a value known as the iodine number, which is the number of grams of iodine that will react with 100 grams of the triglyceride. Table 29.2 gives the iodine number of some common oils. Design an experiment to determine for yourself that oils with a low iodine number are less saturated and will react with a lesser amount of iodine than those with a higher iodine number. Try repeating the original experiment using different oils but start with less iodine, such as one drop. When the oil clears, add another drop. Note: Since tincture of iodine is flammable, hold tubes away from the heating source when adding the iodine. Continue to add measured amounts of iodine to each tube of oil until the reddish brown color no longer clears. Science Fair Hint: Construct and display a bar graph comparing the number of iodine drops added to each sample of oil.

Get the Facts

  1. The number of carbon-to-carbon double bonds in a triglyceride is known as its degree of unsaturation. The greater the number of double bonds the greater its degree of unsaturation and the lower the melting point of the triglyceride. How does the length or number of carbons in the fatty acids of a triglyceride affect melting point? For information about triglycerides, see Carl H. Snyder's The Extraordinary Chemistry of Ordinary Things (New York: Wiley, 1997), pp. 405–406.
  2. Hydrogenation is the addition of hydrogen to an unsaturated molecule, which causes a decrease in saturation. If a product is made from a highly unsaturated oil, how does partial hydrogenation of the oil affect its physical and chemical properties? For information, see Snyder's The Extraordinary Chemistry of Ordinary Things, pp. 409–410.
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