Separation of Organic Compounds by Acid-Base Extraction Techniques
A commonly used method of separating a mixture of organic compounds is known as liquid-liquid extraction. Most reactions of organic compounds require extraction at some stage of product purification. In this experiment you will use extraction techniques to separate a mixture of an organic acid, a base, and a neutral compound. Organic acids and bases can be separated from each other and from neutral compounds by extraction using aqueous solutions of different pH values. Most organic carboxylic acids are insoluble or slightly soluble in water, but these compounds are highly soluble in dilute aqueous sodium hydroxide because the acid is deprotonated by the base producing the sodium carboxylate salt. RCO2H(solv) + OH–(aq) → RCO2–(aq) + H2O(aq) The carboxylic acid can be selectively isolated by dissolving the mixture in an organic solvent that is immiscible with water, and then extracting the solution with sodium hydroxide. The basic aqueous solution containing the carboxylate salt is acidified, causing the sodium carboxylate salt to convert back to the carboxylic acid, which is not water soluble. The acid will precipitate from the solution, as shown here. RCO2–(aq) + H+(aq) → RCO2H(s) Organic bases (e.g., amines) that are insoluble in water can be separated by extraction with hydrochloric acid. Addition of HCl to the amine produces the corresponding ammonium salt, which is soluble in water but not in organic solvents. The amine can be recovered from the aqueous solution by treatment with a base, converting the ammonium salt back to the amine. The amine is not water-soluble and will precipitate, as shown here. Using your understanding of these properties, separation of a mixture containing a carboxylic acid, an amine, and a neutral compound can be carried out via sequential acid and base extractions. The precipitates will be collected and characterized by melting temperature analysis.
In this experiment, you will Separate a mixture containing benzoic acid, 3-nitroaniline, and naphthalene. Calculate the percent recovery of each component in the mixture. Measure the melting temperature of each isolated compound.
RNH2(solv) + H+(aq) → RNH3+(aq)
RNH3+(aq) + OH–(aq) → RNH2(s) + H2O(aq)
Organic Chemistry with Vernier
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Part I Extraction
60 mL separatory funnel four 50 mL Erlenmeyer flasks two 100 mL beakers gravity filtration apparatus vacuum filtration apparatus support ring spatula disposable Pasteur pipets and bulb 10 mL graduated cylinder two watch glasses pH paper balance Part II Melting Temperature
weighing paper sample mixture diethyl ether 6.0 M hydrochloric acid solution 1.0 M sodium hydroxide solution 6.0 M sodium hydroxide solution sodium sulfate, Na2SO4, anhydrous cold distilled water in a wash bottle saturated sodium chloride solution ice compressed air
LabQuest or computer interface LabQuest App or Logger Pro Vernier Melt Station glass capillary tubes, one end closed tissues (preferably lint-free)
isolated samples from Part I mortar and pestle benzoic acid (optional) 3-nitroaniline (optional) naphthalene (optional)
Part I Extraction
1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood. 2. Weigh out approximately 1.0 g of the sample mixture. Record the mass to the nearest 0.001 g. Transfer the mixture to a 100 mL beaker and dissolve it in 15 mL of diethyl ether. CAUTION: Diethyl ether is flammable. Be sure that there are no open flames in the room during the experiment. 3. Clamp the support ring onto a ring stand and place the separatory funnel into the ring. Pour the solution into the separatory funnel and add 5 mL of 6.0 M hydrochloric acid. CAUTION: Handle the hydrochloric...