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used in considerable amounts for personal hygiene and other applications.
Experiment 4: Preparation of Soap 4 – 2 Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids): H3CH2CCH2H2CCH2H2CCH2H2CCH2H2CCH2H2CCH2H2CCH2H2CCH2COONaHydrocarbon chainCarboxylate groupFigure 4.1: A sodium carboxylate. Each soap molecule consists of two parts: a hydrocarbon chain and a carboxylate group. The hydrocarbon chain is compatible with dirt because dirt contains oily and greasy materials composed of hydrocarbon-like molecules. When soap molecules encounter a dirt particle, the hydrocarbon chains of the soap molecules dissolve in the dirt, leaving the carboxylate groups to cover the surface of the dirt particle. Because carboxylate groups are compatible with water, the entire structure—dirt particle surrounded by soap molecules—can be washed away with water: COOH2ODirtCOOCOONa+Na+Na+SkinSkinDirtCOONa+COONa+COOCOOCOOHHOHHOHHOHHOHHNa+Na+Na+OFigure 4.2: The cleaning action of soap. So-called hard water limits the effectiveness of soaps. Hard water contains Ca2+, Mg2+, and similar metal-ion salts. These salts react with soap to form insoluble precipitates (e.g., bathtub rings): H3C (CH2)16COONa+Ca2+H3C(CH2)16COOCa2+22 Na+(aq)(aq)(aq)(s)Water softeners prevent the formation of these precipitates by removing Ca2+and Mg2+from domestic sources of water. 2
Experiment 4: Preparation of Soap 4 – 3 In this experiment, soap will be synthesized via a variant of the traditional process. Vegetable oil will be reacted with sodium hydroxide (an alkali). Animal fats and vegetable oils are rich in compounds called glycerol esters, such as glycerol tristearate. Most glycerol esters found in nature contain more than one type of carboxylate component (i.e., the 16 in the (CH2)16will vary). When a glycerol ester is heated with an alkali solution, such as aqueous NaOH, it is converted into glycerol and salts of fatty acids. This process is known as saponification. An example of saponification involving glycerol tristearate follows: HCOHCOHCOHHC(CH2)16CH3OC(CH2)16CH3OC(CH2)16CH3O+3 NaOHH3C (CH2)16COONa+HCOHHCOHHCOHHHGlycerol tristearateSodium stearate(soap)Glycerol3Glycerol is soluble in water. The long-chain sodium carboxylates (soap) are much less soluble in water than is glycerol and can be readily precipitated by adding a concentrated sodium chloride solution to the reaction mixture. The soap can then be separated from the solution by filtration and purified by washing with cold water. Safety Precautions: •Safety goggles must be worn at all times while you are in the laboratory. •95% ethanol is flammable. Make sure that there are no open flames in the laboratory. Work in the fume hood. In the event of a fire, notify your instructor. A fire in a small container can be smothered by covering the vessel with a watchglass. Use a fire extinguisher to eliminate a larger fire. If the fire is burning over too large an area to be extinguished easily, evacuate the area and activate the fire alarm. If your clothing