Experiment #8: Lipids

Objective

Physical and chemical properties of lipids will be observed. Physical properties will be solubility and emulsification. The latter property will be explored in the preparation of a hand lotion. A chemical property will be observed with reaction of lipids with bromine.

Background

Lipids are a large and diverse group of naturally occurring organic compounds. Unlike carbohydrates and proteins, which are defined in terms of their structures, lipids are defined by the physical operation that is used to isolate them. In other words, lipids are related by their solubility in nonpolar organic solvents and general insolubility in water. There are many different classes of lipids – fatty acids, prostaglandins, triacylglycerols (fats), waxes, glycerophospholipids, sphingolipids, glycosphingolipids and steroids. Salts of fatty acids (soaps) as well as phospholipids can emulsify fats because of their dual nature, both nonpolar and ionic.

Fatty acids are classified as saturated or unsaturated, according to whether they have carbon-carbon double bonds or not. The carbon-carbon double bonds are sites where the molecules are not saturated with hydrogen and are susceptible to chemical attack. One substance that readily attacks these double bonds is the element bromine (Br₂). The product of this reaction is a brominated fatty acid.

Br

2

no reaction

Br

2

Phospholipids, such as lecithin, have a polar or charged portion and a nonpolar portion consisting of the long chain fatty acids within the same molecule. Consequently, the polar or charged portion of these molecules will mix with water and the nonpolar portion repels the water but mixes with lipids.

Nonpolar Tail

Phosphatidyl Choline (Lecithin)

Polar Head

These phospholipids will form small globules in water that make the water look opaque or milky. When lipids mix with water, this is known as emulsification and the mixture is known as an emulsion. This is what gives milk its opaque appearance.

SAFETY PRECAUTIONS

• Wear safety glasses at all times in the laboratory.
• Toluene is flammable. Do not have any open flames while working with toluene.
• Dispose of any leftover organic reactions as directed by instructor.
• Dispose of hand lotion as directed by instructor.
• Bromine in acetic acid is corrosive, handle with care.
• Use boiling chips for water bath. Dispose of chips as directed by instructor.
• Use beaker tongs or hot mitts to handle hot beakers.

EXPERIMENTAL PROCEDURE

In preparation for a later procedure, set up a hot water bath by placing a 400 mL beaker containing about 300 mL of DI water and 2-3 pieces of boiling chips on a hot plate. Keep an eye so that the water does not boil.

Solubility and Unsaturation Tests

1. Label 7 test tubes as cholesterol, Crisco, lanolin, lecithin, oleic acid, olive oil and stearic acid. To the respective labeled test tubes, add 10 drops of oleic acid, olive oil. To the remainder labeled test tubes, add a small amount (tip of a spatula, ~ 0.05 g) of cholesterol, Crisco, lanolin, lecithin, stearic acid. Note and record the physical state and any distinguishing characteristics, e.g.; color, viscosity, of each substance. To each test tube add 3 mL of toluene. Vortex and record your observations. [Note: If the substance doesn’t dissolve immediately, continue vortexing. Some substances dissolve slowly.]
2. To each test tube from above containing the mixtures, add 5 drops of 1% bromine in acetic acid solution. [Note: Before adding the bromine solution, record the initial color of the mixture.] Vortex. Record the color observed after addition of bromine. Dispose of all reactions as directed by your instructor. It is very important to wash your test tubes with soap and water to remove any organic residues.

60 Cuyamaca College

Experiment #8 Data and Observations

Solubility and Unsaturation Tests

Post-Laboratory Questions:

1. Draw the chemical formula and the structure for Toluene. In terms of polarity, what type of solvent is toluene?
2. Fill in a table below.

What is your general conclusion about the polarity of lipids?

1. a. Which fatty acids were solids?
   1. Which fatty acids were liquids?
   2. Explain why some fatty acids were solids and some were liquids?
   3. Which fatty acids reacted with bromine? Are these fatty acids saturated or unsaturated?

62 Cuyamaca College

1. a. Which triacylglycerols were solids?
   1. Which triacylglycerols were liquids?
   2. Which triacylglycerols reacted with bromine? Are these fatty acids saturated or unsaturated?
2. Did cholesterol react with bromine? Is it saturated or unsaturated?
3. Did lanolin react with bromine? Is it saturated or unsaturated?
4. Did lecithin react with bromine? Is it saturated or unsaturated