Preparation of Esters Notes

Preparation of Esters

Introduction

• The unique flavor of food is a combination of taste and smell.
• The first taste perception comes from the aroma of volatile organic compounds.
• In fruits, esters are the primary flavor and fragrance ingredients.

Concepts

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

Background

• Organic chemistry is organized around functional groups, which give organic compounds unique properties.
• Compounds in the same functional group class have similar solubility, boiling points, and chemical reactions.
• Ester Functional Group:
  • Structure: R-COO-R', where R and R' are alkyl groups (any combination of carbon and hydrogen atoms).
  • Alkyl groups can form chain or ring structures.
• Esters are widely distributed in nature.
  • Low-molecular weight esters provide the pleasant odor of fruits and flowers.
  • Used in natural and artificial flavors.
  • Many esters are designated as GRAS (generally regarded as safe) by the FDA, allowing use as food additives without testing.
  • Natural banana flavor is due to four esters.
• Esters are derivatives of carboxylic acids, prepared by reacting a carboxylic acid with an alcohol.
• Ethyl Acetate Preparation:
  • Illustrates ester synthesis.
  • Important industrial solvent and flavor ingredient in apples and bananas.
  • Obtained by heating ethyl alcohol and acetic acid with a strong acid catalyst (e.g., sulfuric acid).
  • Reaction: $CH<em>3COOH + CH</em>3CH<em>2OH \rightleftharpoons CH</em>3COOCH<em>2CH</em>3 + H_2O$
• Ester synthesis demonstrates the functional group concept: most carboxylic acids and alcohols undergo esterification under proper conditions.

Equilibrium

• Ester synthesis is reversible, indicated by the double arrow in the equation.
• The reverse reaction (hydrolysis) splits the ester functional group with water.
• Under heat and an acid catalyst, the reaction reaches equilibrium quickly, with significant amounts of reactants and products, limiting ester yield.
• Equilibrium constant (Kc) for ethyl acetate formation at 25°C is approximately 4.
  $K<em>c = \frac{[CH</em>3CO<em>2CH</em>2CH<em>3][H</em>2O]}{[CH<em>3CO</em>2H][CH<em>3CH</em>2OH]} = 4$
• Starting with one mole of ethyl alcohol and one mole of acetic acid, the maximum ester yield at equilibrium is 0.67 mole (67%).
• Yield can be increased by shifting the equilibrium to the right (Le Châtelier's principle) using:
  • Excess ethyl alcohol.
  • Removing water as it forms.

Naming Esters

• Esters are named as derivatives of an alcohol and carboxylic acid.
• Two-word name: first word from the alcohol, second word from the carboxylic acid (changing -ic to -ate).

Experiment Overview

• Purpose: to investigate the preparation of organic esters starting with alcohols and carboxylic acids.
• Esters will be identified by their odors.

Materials

• Alcohols (4 mL each):
  • Ethyl alcohol ($CH<em>3CH</em>2OH$)
  • Isoamyl alcohol ($(CH<em>3)</em>2CHCH<em>2CH</em>2OH$)
  • Methyl alcohol ($CH_3OH$)
  • Octyl alcohol ($CH<em>3(CH</em>2)<em>7CH</em>2OH$)
  • Propyl alcohol ($CH<em>3CH</em>2CH_2OH$)
• Carboxylic acids:
  • Acetic acid ($CH<em>3CO</em>2H$, 4 mL)
  • Benzoic acid ($C<em>6H</em>5CO_2H$, 2 g)
  • Propanoic acid ($CH<em>3CH</em>2CO_2H$, 4 mL)
  • Salicylic acid ($HOC<em>6H</em>4CO_2H$, 2 g)
• Sodium bicarbonate solution ($NaHCO_3$, saturated, 8 mL)
• Sulfuric acid ($H<em>2SO</em>4$, concentrated, 2 mL)
• Water, distilled
• Equipment: Beaker, boiling stones, pipets, cotton balls, graduated cylinders, hot plate, Pasteur pipet, test tube clamp and rack, test tubes, thermometer, wash bottle, watch glasses, wax marking pencil

Safety Precautions

• Perform the experiment in a well-ventilated area.
• Concentrated sulfuric acid causes severe burns; wear protective gear and clean spills immediately.
• Acetic acid is corrosive and a moderate fire risk.
• Methyl alcohol is extremely flammable and toxic by ingestion.
• Ethyl alcohol is a flammable solvent and toxic by ingestion.
• Propanoic acid is flammable and causes severe skin burns; harmful if swallowed and has a rancid odor.
• Salicylic acid is moderately toxic by ingestion.
• Propyl alcohol is irritating to skin and eyes and 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 flames when working with flammable liquids.
• Volatile organic liquids may cause drowsiness or dizziness; do not breathe vapors.
• Waft vapors carefully to smell products.
• Wear chemical splash goggles, chemical-resistant gloves, and a lab coat.
• Wash hands thoroughly before leaving the laboratory.

Procedure

1. Heat water in a 400-mL beaker to about 80°C on a hot plate.
2. Choose two alcohols and two carboxylic acids.
3. Label four clean, dry test tubes with reagent names.
4. Obtain 4 mL of each alcohol and acid (2 g for solids) in the labeled test tubes.
5. Label a second set of four test tubes 1-4 and record which alcohol and carboxylic acid will be combined in each.
6. Transfer 2 mL of alcohol and 2 mL (or 1 g for solids) of carboxylic acid into the appropriate test tubes.
7. Place all test tubes in a test tube rack.
8. Add 10 drops of concentrated sulfuric acid to each test tube (Caution!).
9. Place the test tubes in the 80°C hot water bath.
10. Heat the reaction mixtures for about 10 minutes. Record any changes in appearance.
11. Remove the test tubes from the hot water bath and allow them to cool for about 5 minutes. Record observations.
12. Add 2 mL of saturated sodium bicarbonate solution to each test tube. Record observations.
13. Remove 1-2 drops of liquid from the upper layer with a Pasteur pipet and add them to a cotton ball on a watch glass. Waft vapors to smell the product.
14. Describe the odor of the ester product.
15. Detect the odors of the ester products in test tubes 2-4.
16. Dispose of the contents of the test tubes according to instructions.

Laboratory Report

• Alcohols to be tested:
• Carboxylic acids to be tested:
• Observations of Ester and Odor:

1. Write a chemical equation for the formation of each ester in test tubes 1-4 and write the name of each ester product:

   • Octyl alcohol + acetic acid → octyl acetate + water
     • $C<em>8H</em>{17}OH + CH<em>3COOH \rightarrow CH</em>3(CH<em>2)</em>7O<em>2CCH</em>3 + H_2O$
   • Ethyl alcohol + benzoic acid → ethyl benzoate + water
     • $C<em>2H</em>5OH + C<em>6H</em>5COOH \rightarrow C<em>6H</em>5CO<em>2C</em>2H<em>5 + H</em>2O$
   • Methyl alcohol + salicylic acid → methyl salicylate + water
     • $CH<em>3OH + C</em>7H<em>6O</em>3 \rightarrow C<em>8H</em>8O<em>3 + H</em>2O$
   • Isoamyl alcohol + acetic acid → isoamyl acetate + water
     • $C<em>5H</em>{11}OH + CH<em>3COOH \rightarrow CH</em>3CO<em>2C</em>5H<em>{11} + H</em>2O$

2. Describe in general terms the odor or fragrance of the esters that were prepared.

3. Were any of the esters easily identified as a specific fragrance, e.g., apple or banana?

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

   • $CH<em>3COOH + NaHCO</em>3 \rightarrow CH<em>3COONa + H</em>2O + CO_2$

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: $C<em>5H</em>{10}O<em>2$; $CH</em>3CH<em>2COOCH</em>2CH_3$
   • Propyl acetate: $C<em>5H</em>{10}O<em>2$; $CH</em>3COOCH<em>2CH</em>2CH_3$
   • Isomers are compounds that have the same molecular formula but different structures.

Isoamyl Acetate

• Isoamyl alcohol (clear liquid) + acetic acid (clear liquid) → clear liquid
• Addition of sulfuric acid turns the bottom of the test tube yellow
• During heating, the yellow color darkens, forming a dark layer on top and a lighter yellow layer on the bottom
• After heating, the bottom portion is a reddish-brown color
• After $NaHCO_3$ addition, the liquid becomes lighter and cloudier with bubbling
• Odor: Sweet Hawaiian Banana