Materials Included In Kit

Additional Materials Required

Safety Precautions

Concentrated sulfuric acid is severely corrosive to eyes, skin and other tissue; use extreme caution when handling. Ethyl alcohol is a flammable liquid and a dangerous fire risk; the addition of denaturants makes ethyl alcohol poisonous. Acetic acid is corrosive to skin and tissue; it is a moderate fire risk. The ester produced in this experiment, ethyl acetate, is a dangerous fire hazard; it is irritating to skin and eyes and slightly toxic by ingestion and skin absorption. Use extreme caution when distilling mixtures containing flammable liquids. Never smell chemicals directly by putting them under the nose. Instead, hold the compound at least eight inches from the face with one hand and use the other hand to gently waft the vapors toward the nose. This lab should be performed in a fume hood or well-ventilated area. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult Safety Data Sheets for additional safety information.

Disposal

Please consult the current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Acetic acid and the water layer remaining after extraction can be disposed of according to Flinn Suggested Disposal Method #24a. The ethyl alcohol may be disposed of according to Flinn Suggested Disposal Method #26b. The ethyl acetate can be saved and used as a solvent or can be evaporated in a fume hood according to Flinn Suggested Disposal Method #18a.

Lab Hints

• At least two 45-minute lab periods are needed to prepare, isolate and distill the ester. Carry out the preparation and isolation steps in one laboratory period, then store the mixture in a stoppered container. The distillation and identification steps may then be carried out during the next laboratory period.
• Because a majority of the solvents and products are flammable, it is preferable to use hot plates or heating mantles rather than Bunsen burners to heat the reaction mixtures.
• Set the reagents and dispensers in a fume hood. Label each to avoid contamination.
• Check student setups for both refluxing and distillation. Pay particular attention to the joint seals and alignment. All joints must be tight. Ensure there are no closed sysems before students heat the flasks. A low-cost, one-piece organic distillation set (Flinn Catalog No. AP6351) that eliminates many of these joint connections is available from Flinn Scientific.
• Have the students check for any cracks in the round bottom flasks. Don’t use any that have cracks.
• The boiling point is measured when the thermometer temperature has stabilized after droplets have condensed on the bulb. Check thermometer bulb placement.
• For better results, cool the receiving flask in an ice bath. Because the ethyl acetate has a high vapor pressure, cooling the distillate in an ice bath will reduce the amount of evaporation from the recovery flask. The ice bath also reduces organic vapors in the laboratory.

Teacher Tips

• While the lab includes enough supplies for 12 students working independently, the amount of distillation equipment or hood space may limit the number of actual setups. The technique of distillation is an important lab skill to master. Choose the number of setups that provides the students and teacher the best opportunities for this learning to safely take place.
• In the solvent extraction step, two to three small washings with sodium carbonate solution may be used instead of one washing. This will give a more complete extraction and allow more practice using the separatory equipment.
• Fewer separatory funnels may be necessary since the isolation procedure is fairly quick.
• In the isolation procedure, the natural tendency is to toss away the wrong layer. Have students retain both layers until they are sure of the identity of both.

Further Extensions

AP^® Standards
This lab fulfills the requirements for the College Board recommended AP Experiment #22—Synthesis, Purification and Analysis of an Organic Compound. In addition, this lab provides the recommended familiarity with the distillation process.

Answers to Prelab Questions

1. Explain what an ester is.

   An ester is the product of the reaction between an organic acid and an alcohol.

2. Draw the structural formula for acetic acid.
   {10538_Answers_Figure_3}
3. Draw the structural formula for ethyl alcohol.
   {10538_Answers_Figure_4}
4. Write an equation using structural formulas for the reaction of acetic acid with ethyl alcohol. Name the product.
   {10538_Answers_Figure_5}

   The product is named ethyl acetate (or ethyl ethanoate).

5. Why are the reflux and distillation setups always open to the atmosphere?

   If these systems are closed, the pressure inside would rapidly increase creating a dangerous buildup of pressure. This, in turn, could create an explosive hazard with certain distillates.

6. How is distillation used to purify an ester?

   A solution will boil when the combined vapor pressures of the components equal the atmospheric pressure. Since the vapor pressure of the ethyl acetate is greater than that of the other components, its vapor is present in the greatest abundance as the solution begins to boil. Condensing the vapor using a distillation column produces a liquid greatly enriched in ethyl acetate, the more volatile component.

7. The theoretical yield of a product is the maximum amount of product that can be formed from a given amount of reactants. In this esterification reaction, 12 mL of 17.4 M acetic acid is combined with 10 mL of ethyl alcohol. The density of ethyl alcohol is 0.79 g/mL.
   1. Calculate the number of moles of each reactant.

      Moles of acetic acid

      {10538_Answers_Equation_3}

      Moles of ethyl alcohol

      {10538_Answers_Equation_4}
   2. Compare the ratio of reactant noles used in this experiment to the ratio of reactant moles in the balanced equation. Will both reactants be completely consumed? If not, which one will be consumed? How much of the excess reactant will be left over?

      No, both won’t be consumed. Only ethyl alcohol will be completely consumed.
      Moles of acetic acid left over = (0.21 – 0.17) moles = 0.04 moles

   3. The reactant that is completely consumed is called the limiting reactant, because its amount limits the amount of product that can be formed. Once the limiting reactant is consumed, the reaction stops. Determine the limiting reactant for this experiment and calculate the theoretical yield of ethyl acetate, in moles and grams.

      Ethyl alcohol is the limiting reactant.
      Theoretical yield of ethyl acetate:

      {10538_Answers_Equation_5}

Sample Data

Student data will vary.

Answers to Questions

1. In Prelab Question 6c, the theoretical yield of ethyl acetate, in grams, was calculated. Enter this value in the data table.

   In the Prelab Question 6, the theoretical yield of ethyl acetate was calculated to be 14.96 g.

2. Use the mass of the ethyl acetate collected from the distillation and the theoretical yield to calculate the percent yield of ethyl acetate. Record this value in the data table.

   The actual yield, in grams, divided by the theoretical yield and multiplied by 100 gives the % yield of the experiment.

   {10538_Answers_Equation_6}
3. Look up the literature value of the boiling point of ethyl acetate in a reference book, such as the Merck Index or the CRC Handbook of Chemistry and Physics. Compare the experimental and literature values of the boiling point.

   The value for the boiling point of ethyl acetate, from the Merck Index, is 77 °C.

References

Vonderbrink, Sally Ann; Laboratory Experiments for Advanced Placement Chemistry; Flinn Scientific, Batavia, IL, 1995, 157–163, 309–319.

Zubrick, Janes W.; The Organic Chem Lab Survival Manual; John Wiley & Sons, 1997, 194–201.

Introduction

An ester is a chemical compound that is formed when an organic acid reacts with an alcohol. Esters frequently have distinctive odors and are naturally occurring flavor and fragrance chemicals in many fruits and plants. In this experiment, the ester ethyl acetate (ethyl ethanoate) is prepared and purified by distillation.

Concepts

• Esters
• Distillation
• Reflux
• Theoretical yield
• Percent yield
• Solvent extraction

Background

The reaction between an organic acid and an alcohol in the presence of an acid (H⁺) catalyst is called esterification (Equation 1).

In the diagram, R and R′ represent organic groups such as hydrocarbons. The —OH group from the acid combines with the —H atom from the alcohol to form a water molecule. The R′—O— group from the alcohol attaches to the carbonyl carbon on the acid to produce the ester. The reaction is catalyzed by the addition of concentrated sulfuric acid, H₂SO₄, and the reaction is reversible. Adding concentrated sulfuric acid, a strong dehydrating agent, shifts the equilibrium to the products side by removing the water as it is formed.

Equation 2 illustrates a specific example of an esterification reaction, that of methyl alcohol and acetic acid, to form methyl acetate. The systematic name for acetic acid is ethanoic acid, and the systematic name for the ester product is methyl ethanoate.

Experiment Overview

In this experiment, a quantitative esterification reaction is performed. The process has three parts—reaction, isolation, and purification.

In the reaction step, known amounts of acetic acid, ethyl alcohol, and sulfuric acid are combined and heated under reflux. Refluxing a reaction mixture involves heating the mixture to its boiling point in a flask equipped with a reflux condenser that allows a continuous return of the volatile materials to the flask. Using a reflux condenser, the reaction is conducted at a higher temperature without losing reactants or products.

Once the reaction is complete and the flask is cool, the ester is isolated and collected by the technique of solvent extraction. A solution of saturated sodium carbonate is added to a separatory funnel along with the contents of the reaction flask. These contents include any unreacted acetic acid, ethyl alcohol, and sulfuric acid along with the ester and water. The saturated sodium carbonate solution is strongly basic (pH ≈ 10). This basic solution will convert any undissociated acetic acid to its salt (Equation 3)

By vigorously mixing this basic solution with the flask contents, the acetate ion along with the acidic and polar components are extracted from the flask mixture and two immisible layers result. The top, or less dense layer, contains the ester, while the bottom water layer contains sodium carbonate along with the water-soluble components acetate ion, ethyl alcohol, and sulfuric acid.

The ester layer is transferred back to the flask and purified by the technique called distillation. Distillation is the process of heating a compound to its boiling point and then removing the vapors by cooling them with a condenser. The purified ester product is identified by its boiling point and its odor. The condensed vapor, or distillate, is weighed, and the percent yield of ethyl acetate calculated.

Materials

Prelab Questions

1. Explain what an ester is.
2. Draw the structural formula for acetic acid.
3. Draw the structural formula for ethyl alcohol.
4. Write an equation using structural formulas for the esterification reaction of acetic acid with ethyl alcohol. Name the product.
5. Why are the reflux and distillation setups always open to the atmosphere?
6. How is distillation used to purify an ester?
7. The theoretical yield of a product is the maximum amount of product that can be formed from a given amount of reactants. In this esterification reaction, 12 mL of 17.4 M acetic acid are combined with 10 mL of absolute ethyl alcohol. The density of ethyl alcohol is 0.79 g/mL.
   1. Calculate the number of moles of each reactant.
   2. Compare the ratio of reactant moles used in this experiment to the ratio of reactant moles in the balanced equation. Are both reactants completely consumed? If not, which one is consumed? How much of the excess reactant is left over?
   3. The reactant that is completely consumed in the reaction is called the limiting reactant, because its amount limits the amount of product that can be formed. Once the limiting reactant is consumed, the reaction stops. Determine the limiting reactant for this experiment and calculate the theoretical yield of ethyl acetate, in moles and grams.

Safety Precautions

Concentrated sulfuric acid is severely corrosive to eyes, skin and other tissue; use extreme caution when handling. Ethyl alcohol is a flammable liquid and a dangerous fire risk; the addition of denaturants makes ethyl alcohol poisonous. Acetic acid is corrosive to skin and tissue; it is a moderate fire risk. The ester produced in this experiment, ethyl acetate, is a dangerous fire hazard; it is irritating to skin and eyes and slightly toxic by inhalation, ingestion and skin absorption. Use extreme caution when distilling mixtures containing flammable liquids. Never smell chemicals directly by putting them under the nose. Instead, hold the compound at least eight inches from the face with one hand, and use the other hand to gently waft the vapors toward the nose. This lab should be performed in a fume hood or well-ventilated area. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Preparation of Ethyl Acetate

1. Place 10 mL ethyl alcohol, 12 mL glacial acetic acid, 15 drops of concentrated sulfuric acid, and a boiling stone in a 125-mL round bottom flask.
2. Place a 600-mL beaker, filled with approximately 450 mL of water and a few boiling stones, on a hot plate.
3. Place the round bottom flask in the beaker of water so that the reaction mixture is below the water line. Clamp the flask to a ring stand (see Figure 1).
   {10538_Procedure_Figure_1}
4. Attach a length of tubing to the inlet (bottom) and another to the outlet (top) of the condenser.
5. Attach the condenser to the round bottom flask, making sure the fitting between the two is secure and the condenser is vertical. If the connection between the two is a ground glass tapered joint, lightly grease the inner (male) joint to create a good seal. Clamp the condenser to the ring stand (see Figure 2).
   {10538_Procedure_Figure_2}
6. Attach the condenser inlet tubing to the water source and place the outlet tubing in the drain. Slowly run cold water through the condenser.
7. Turn the hot plate on and heat the round-bottom flask in the hot water bath. Raise the temperature of the hot water until the mixture in the round-bottom flask is gently boiling.
8. Continue the gentle boil of the reaction mixture for about 15 minutes. Turn off the hot plate and cool the mixture by removing the hot water bath.

Isolation of Ethyl Acetate

1. Prepare a saturated solution of sodium carbonate by combining 4.5 g of sodium carbonate with 15 mL distilled water in a 15 x 150-mm test tube.
2. Stopper the test tube with a cork, shake well, and allow any undissolved solid to settle.
3. In the hood, pour the clear Na₂CO₃ solution into a separatory funnel (see Figure 3) or, if none is available, into a 18 x 125-mm test tube.
   {10538_Procedure_Figure_3}
4. Add the reaction mixture to the separatory funnel (or test tube), stopper and mix the solution. If using a separatory funnel, turn it upside down and open the stopcock occasionally to vent the system. If using a test tube, remove the stopper with caution—some pressure may have built up. Invert at least 15–20 times.
5. Allow the two layers to separate. Ethyl acetate (density 0.90 g/mL) is less dense than water, therefore the top layer is ethyl acetate.
6. If using the separatory funnel, remove the stopper, open the stopcock and slowly drain off the waste aqueous layer into a 50-mL waste beaker, then close the stopcock.
7. Transfer the remaining layer (ethyl acetate) to a clean, dry, round-bottom flask.
8. If using the test tube, remove the top layer (ethyl acetate) with a glass capillary dropper and transfer the ethyl acetate to a clean, dry, round bottom flask.

Purification of Ethyl Acetate

1. Add a boiling stone to the round-bottom flask.
2. Set up the distillation as shown in Figure 4. For all ground glass connections, lightly grease the inner (male) joint to create a good seal.
   {10538_Procedure_Figure_4}
3. Place the distilling flask in the 600-mL beaker and clamp, as in the Preparation step 3.
4. Place the three-way adapter vertically in the neck of the distilling flask.
5. Insert the thermometer into its adapter.
6. Place the thermometer and its adapter in the top of the three-way adapter so that the thermometer bulb is just below the side arm.
7. Connect the condenser, with attached tubing, to the side arm of the three-way adapter and clamp it to a ring stand.
8. Connect the outlet adapter to the condenser. Add rubber bands to secure the condenser connections.
9. Weigh a clean, dry 125-mL Erlenmeyer flask on an analytical balance. Record the mass in the data table.
10. Place the Erlenmeyer flask under the outlet condenser. Check all fittings to make sure all connections are secure.
11. Attach the condenser inlet tubing to the water source and place the outlet tubing in the drain.
12. Slowly run cold water through the condenser.
13. Turn the hot plate on and heat the round-bottom flask in the hot water bath. Heat until the ethyl acetate is gently boiling.
14. As the ethyl acetate vapors start to carry over and condense, record the temperature of the vapors in the data table. Record this temperature at the beginning and end of the distillation in the data table.
15. Distill the ethyl acetate until no more distillate comes over. There should be some liquid remaining in the round-bottom flask. Never distill to dryness!
16. Turn off the hot plate.

Identification of Ethyl Acetate

1. Remove the 125-mL Erlenmeyer collection flask and weigh the flask plus ethyl acetate on an analytical balance. Record the mass in the data table.
2. In the hood, add about 2 mL of the distilled ethyl acetate to a beaker with about 200 mL of distilled water.
3. Swirl the mixture and carefully smell the ester by wafting some of the vapors toward the nose. Record the odor and fragrance in data table.
4. Dispose of the ethyl acetate as directed by your instructor.

Student Worksheet PDF

10538_Student1.pdf