Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium bicarbonate solution, saturated: To prepare 100 mL of saturated solution, add 150 g of sodium bicarbonate to 75 mL of distilled or deionized water. Stir to dissolve and then dilute to 100 mL with water. Mix well before dispensing, decant the supernatant liquid and discard the excess solid.

Safety Precautions

Perform this experiment in a hood or well-ventilated lab only and avoid inhaling any vapors. Do not breathe vapors, mist or spray. Concentrated sulfuric acid causes severe skin burns and eye damage. Wear protective gloves and clothing. Wash thoroughly after handling. Clean up all acid spills, even a few drops, immediately. Acetic acid is corrosive to skin and body tissue. It is a moderate fire risk and toxic by ingestion and inhalation. Methyl alcohol is extremely flammable, a dangerous fire risk and toxic by ingestion. Keep away from heat, sparks and open flames. Ethyl alcohol is a flammable solvent and toxic by ingestion. Propanoic acid is a flammable liquid and causes severe skin burns and eye damage. It may be harmful if swallowed and has a rancid odor. Salicylic acid is moderately toxic by ingestion. Propyl alcohol is irritating to skin and eyes and is slightly toxic by ingestion. Benzoic acid is slightly toxic by ingestion and causes serious eye irritation. Avoid contact of all chemicals with skin and eyes. Do not use any flames in the laboratory when working with alcohols and other flammable liquids. Keep away from all sources of ignition. Volatile organic liquids, such as the low–molecular weight alcohols and carboxylic acids used in this experiment, may cause drowsiness or dizziness and may be harmful if inhaled. Do not breathe vapors, mist or spray. Instruct students in the proper procedure for smelling chemical odors—carefully waft the vapors to the nose—do NOT “sniff.” Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your 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. Aqueous mixtures produced in the synthesis of esters should be neutralized if necessary, and may be rinsed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The laboratory work for this experiment can be completed in less than two hours. Divide up the possible alcohol and carboxylic acid combinations among different student groups so that all of the possible esters shown in Table 2 may be observed. To avoid sensory overload in the lab from too many different products, the number of esters to be investigated was reduced from the 20 possible combinations to 12. All of the possible acid and alcohol combinations shown in Table 2 will work and the esters, with the exception of octyl benzoate and octyl salicylate, are biodegradable, have pleasant odors, and are “generally regarded as safe” (GRAS) by the FDA.
• Although many butanoate and pentanoate esters have pleasant and easily identifiable odors, we chose not to use either butanoic (C₄) or pentanoic acid (C₅) in this lab. Both acids emit a piercing stench—the odors may linger for weeks.
• Concentrated sulfuric acid is severely corrosive. For best results, dispense concentrated sulfuric acid on a spill tray in the hood or other central location.
• Methyl alcohol is extremely flammable and a severe fire risk. Keep away from heat, flames and other sources of ignition. Do NOT under any circumstances use Bunsen burners to heat the water baths.
• Perform this experiment in a hood or well-ventilated lab only. With so many different esters being prepared, it may be difficult for students to differentiate and identify the various odors. Keep a box of unscented tissues in the lab for students to cleanse their noses before smelling a new compound. Practice and encourage strict chemical hygiene techniques. Students should wash their hands carefully before handling tissues. If ventilation is a problem, you may want to choose one ester and have all students synthesize the same product. Methyl salicylate and isoamyl acetate are easily identifiable and good choices.
• The most commonly used esters for artificial flavors are ethyl butyrate, methyl salicylate, isoamyl acetate, ethyl propionate, ethyl anthranilate, butyl butyrate and isobutyl acetate. Esters are also widely used as fragrances to make perfumes. The Good Scents Company website is a good source of information about the use of esters. Simply click on the search tab and enter the name of a specific ester to obtain odor descriptions and information about the natural occurrence, physical properties and perfumery uses. (Accessed 2014.) Even though most of the esters prepared in this lab are classified as GRAS, they must still be treated as potentially harmful laboratory chemicals. Do not allow any chemicals to be removed from the lab.
• Students may wish to compare the chemical scents (esters) prepared in the lab versus natural sources of the same scent. This can be done by placing orange, banana, pear, etc., slices in plastic bags. Compare fragrance intensity and purity.
• Food chemists rarely use one pure compound in artificial flavors or fragrances. Imitation pineapple flavor, for example, might consist of 10 esters and carboxylic acids.

Answers to Prelab Questions

1. Ester formation is a very versatile reaction—many different carboxylic acids and alcohols may be used. In this experiment, five alcohols and four carboxylic acids will be available as starting materials (see Table 2). Fill in the blanks with the name of the ester that would be obtained from each combination of an alcohol and a carboxylic acid. The combinations that are marked with a line will not be used and may be left blank.
   {14023_PreLabAnswers_Table_2_Preparation of Esters from Alcohols and Carboxylic Acids}
2. Draw the structures of (a) ethyl propanoate and (b) methyl benzoate.
   {14023_PreLabAnswers_Figure_4}
3. Write a balanced chemical equation for the preparation of (a) octyl acetate and (b) methyl salicylate from the necessary starting materials in this experiment. Note: Salicylic acid is 2-hydroxybenzoic acid.
   {14023_PreLabAnswers_Figure_5}
4. What is the purpose of concentrated sulfuric acid in ester synthesis? What precautions are necessary when working with concentrated sulfuric acid?

   Sulfuric acid is used as an acid catalyst to speed up the rate of ester formation. Sulfuric acid is severely corrosive and will cause skin and eye burns. Exercise extreme care when using sulfuric acid and work with sulfuric acid only in a designated area. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Notify the instructor immediately if any acid is spilled.

Sample Data

Laboratory Report

Alcohols to be tested: ___Isoamyl alcohol and octyl alcohol___
Carboxylic acids to be tested: ___Acetic acid and propanoic acid___

Answers to Questions

1. Write a chemical equation for the formation of each ester in test tubes 1–4 and write the name of each ester product next to its structure.
   {14023_Answers_Reaction_1}
2. Describe in general terms the odor or fragrance of the esters that were prepared by you and others in the class. Compare the odors of the esters to the odors of the starting materials.

   Most of the esters have pleasant smells. The alcohols have medicinal smells, while the acids are sharp, biting and unpleasant. The odors of all of the possible ester products are described here:

   {14023_Data_Table_5}
3. Were any of the esters easily identified as a specific fragrance (e.g., apple or banana)? In cases where a specific fragrance was detected, how does the odor compare to the natural fragrance? Give one reason for any difference between the synthetic fragrance and the natural fragrance.

   There were some very distinct odors that were easy to identify. Isoamyl acetate (banana) and methyl salicylate (wintergreen) stand out. The odors of these compounds, while very distinct, were stronger and less subtle than the natural fragrances. The natural fragrances are more of a blend of several different odors. Natural fragrances are made up of many different compounds. Apple flavor, for example, is a blend of at least 29 volatile compounds.

4. Sodium bicarbonate solution was added to the product mixtures in step 12 to remove any unreacted acid. Write a balanced chemical equation for the reaction of acetic acid with sodium bicarbonate.

   CH₃CO₂H(aq) + NaHCO₃(aq) → CH₃CO₂Na(aq) + H₂O(l) + CO₂(g)

5. Ethyl propanoate and propyl acetate are isomers. Compare the molecular formulas and the structures of these compounds and write a definition of isomers based on this comparison.

   Ethyl propanoate and propyl acetate have the same molecular formula—C₅H₁₀O₂. They have different structures, however, as shown.
   Isomers are compounds that have the same formula but different structures. Isomers are different compounds and have different physical properties.

   {14023_Answers_Figure_6}

Introduction

What do you “taste” when you bite into an apple or a banana? The unique flavor of any food is due to a combined sense of both taste and smell. Indeed, the first taste perception of a food comes from the aroma or fragrance of volatile organic compounds. In the case of fruits, the primary flavor and fragrance ingredients are organic compounds called esters. What are esters and how can they be prepared in the lab?

Concepts

• Ester functional group
• Carboxylic acids and alcohols
• Esterification reaction
• Equilibrium

Background

The study of organic chemistry is organized around functional groups—groups of atoms, bonded together in a specific pattern, that give organic compounds their unique physical and chemical properties. For example, compounds belonging to the same functional group class show similar trends in their solubility and boiling points and also undergo characteristic types of chemical reactions.

The structure of the ester functional group is shown in Figure 1. The “R” groups (R and R′) represent any combination of carbon and hydrogen atoms and are called alkyl groups. The C atoms in the alkyl groups may be bonded together to form either chain or ring structures.

Organic esters are widely distributed in nature. Low–molecular weight esters are responsible for the pleasant odor or fragrance of many fruits and flowers, and they are important ingredients in natural and artificial flavors. More than 100 esters (and all of the esters that will be made in this lab) are designated by the Food and Drug Administration as GRAS—generally regarded as safe. This means that the esters may be used as food additives without going through a testing and approval process. Natural banana flavor, for example, is due to four esters (see Figure 2). Notice the ester functional groups and the variety of alkyl groups attached to the ester linkages. Esters are considered derivatives of carboxylic acids; they may be prepared by the reaction of a carboxylic acid with an alcohol. The preparation of ethyl acetate illustrates the general principles of ester synthesis. Ethyl acetate is an important industrial solvent. It is used in many consumer items, such as nail polish, and is also a naturally occurring “flavor ingredient” in apples and bananas. Ethyl acetate is obtained by heating a solution of ethyl alcohol and acetic acid in the presence of a strong acid catalyst, such as sulfuric acid (Equation 1). The synthesis of esters demonstrates the usefulness of the functional group concept in organic chemistry. With few exceptions and under the proper conditions, most carboxylic acids and alcohols will undergo this type of esterification reaction. The presence of the double arrow in Equation 1 indicates that ester synthesis is reversible. The reverse reaction of water with ethyl acetate splits apart the ester functional group and is called hydrolysis. Under typical conditions, with heat and an acid catalyst, the forward and reverse reactions will quickly reach equilibrium. Both reactants and products may be present in significant amounts at equilibrium, limiting the yield of ester that may be obtained. The equilibrium constant for the formation of ethyl acetate illustrates this problem. At 25 °C, the value of the equilibrium constant for the synthesis of ethyl acetate is approximately four (Equation 2). Starting with one mole of ethyl alcohol and one mole of acetic acid, the maximum amount of ester at equilibrium will be 0.67 mole (67% yield). The yield of ester may be increased by shifting the equilibrium to the right through the application of Le Chatelier’s principle. Typical strategies for improving the amount of ester produced include using excess ethyl alcohol and/or removing water from the reaction mixture as it forms. Esters are named as derivatives of an alcohol and carboxylic acid. The name of an ester is always two words, where the first word comes from the name of the alcohol and the second word from the name of the carboxylic acid by changing the –ic ending to –ate. The examples in Table 1 illustrate the names and formulas of some naturally occurring esters.

Experiment Overview

The purpose of this experiment is to investigate the preparation of organic esters starting with alcohols and carboxylic acids. The esters will be identified by their characteristic odors.

Materials

Prelab Questions

1. Ester formation is a very versatile reaction—many different carboxylic acids and alcohols may be used. In this experiment, five alcohols and four carboxylic acids will be available as starting materials (see Table 2). Fill in the blanks with the name of the ester that would be obtained from each combination of an alcohol and a carboxylic acid. The combinations that are marked with a line will not be used and may be left blank.
   {14023_PreLab_Table_2_Preparation of Esters from Alcohols and Carboxylic Acids}
2. Draw the structures of (a) ethyl propanoate and (b) methyl benzoate.
3. Write a balanced chemical equation for the preparation of (a) octyl acetate and (b) methyl salicylate from the necessary starting materials in this experiment. Note: Salicylic acid is 2-hydroxybenzoic acid.
4. What is the purpose of concentrated sulfuric acid in ester synthesis? What precautions are necessary when working with concentrated sulfuric acid?

Safety Precautions

Perform this experiment in a hood or well-ventilated lab. Concentrated sulfuric acid causes severe skin burns and eye damage. Wear protective gloves and clothing. Wash thoroughly after handling. Notify the instructor and clean up all spills, even a few drops, immediately! Acetic acid is corrosive to skin and body tissue. It is a moderate fire risk and toxic by ingestion and inhalation. Methyl alcohol is extremely flammable, a dangerous fire risk and toxic by ingestion. Keep away from heat, sparks and open flames. Ethyl alcohol is a flammable solvent and toxic by ingestion. Propanoic acid is a flammable liquid and causes severe skin burns and eye damage. It may be harmful if swallowed and has a rancid odor. Salicylic acid is moderately toxic by ingestion. Propyl alcohol is irritating to skin and eyes and is slightly toxic by ingestion. Benzoic acid is slightly toxic by ingestion and causes serious eye irritation. Avoid contact of all chemicals with skin and eyes. Do not use any flames in the laboratory when working with alcohols and other flammable liquids. Keep away from all sources of ignition. Volatile organic liquids, such as the low–molecular weight alcohols and carboxylic acids used in this experiment, may cause drowsiness or dizziness and may be harmful if inhaled. Do not breathe vapors, mist or spray. To smell a product, carefully waft the vapors to your nose—do NOT “sniff!” Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Fill a 400-mL beaker two-thirds full with hot tap water and add a boiling stone. Heat the water to about 80 °C on a hot plate.
2. Choose two alcohols and two carboxylic acids from the list of materials. Write the names of the alcohols and acids selected for testing in the data table. Note: In order to test all possible combinations, the instructor may select the alcohols and carboxylic acids that will be tested by each group.
3. Obtain four medium test tubes. Make sure each test tube is clean and dry. Label the test tubes with the name or an abbreviation for each reagent selected (e.g., MeOH for methyl alcohol, BA for benzoic acid).
4. Obtain about 4 mL of each alcohol and acid to be tested in the corresponding test tube. Note: In the case of solids, obtain about 2 grams of the compound.
5. Label a second set of four test tubes 1–4. In the data table, write in the names of the alcohol and the carboxylic acid that will be combined in each test tube. Example: Methyl alcohol and benzoic acid in test tube 1, methyl alcohol and salicylic acid in test tube 2.
6. Using a clean graduated Beral pipet or a spatula, transfer 2 mL of the appropriate alcohol and 2 mL (or 1 g for a solid) of the appropriate carboxylic acid into test tube 1. Example: Place 2 mL of methyl alcohol and 1 g of benzoic acid into test tube 1.
7. Repeat step 6 for test tubes 2, 3 and 4. Place all of the test tubes in a test tube rack.
8. Take the test tube rack to a central location where concentrated sulfuric acid is being dispensed. Using a glass eyedropper or Pasteur pipet, carefully add 10 drops of concentrated sulfuric acid to each test tube 1–4. Caution: Exercise extreme care when working with concentrated sulfuric acid.
9. Return the test tube rack to the lab bench and place each test tube 1–4 into the hot water bath at 80 °C (step 1).
10. Heat the reaction mixtures in test tubes 1–4 in the hot water bath for about 10 minutes. Record any changes in the appearance of the materials.
11. After 10 minutes, use a test tube clamp to remove the test tubes from the hot water bath. Place the test tubes in the test tube rack and allow them to cool for about 5 minutes. Record any observations in the data table.
12. Add 2 mL of saturated sodium bicarbonate solution to each test tube. Record observations in the data table.
13. Using a glass Pasteur pipet, remove 1–2 drops of liquid from the upper layer in test tube 1. Add the drops to a cotton ball on a clean watch glass. Carefully waft the vapors from the watch glass to your nose to smell the product. To detect the odor, first hold your breath briefly and then fan your fingers across the cotton ball. Inhale slightly (see Figure 3).
    {14023_Procedure_Figure_3}
14. In the data table, describe the odor of the ester product. Some odors will be easy to identify. Adjectives that fragrance chemists use to describe odors include floral, fruity, earthy, pungent, musky, sweet, herbal and green.
15. Wait a few minutes, then use a similar technique to detect the odors of the ester products in test tubes 2–4.
16. Dispose of the contents of the test tubes according to your instructor’s directions.

Student Worksheet PDF

14023_Student1.pdf