The Ester Lab!
Purpose:
- To prepare at least five esters using proper laboratory techniques illustrated below
- To identify the ester produced from the characteristic odor it emits
- To identify the unknown carboxylic acid and alcohol from the ester produced
- To draw the complete chemical reactions for each ester produced, illustrating the carboxylic acid, the alcohol, and the ester produced.
Background Information:
The esters are a group of organic compounds best known for their interesting odors. Esters are one of the most widely used organic compounds, making up fats, oils, fragrances, flavorings, pheromones, DNA backbones, explosives, plastics, and clothings. Esters are commonly created by combining a carboxylic acid with an alcohol in the presence of a strong acid. This process is referred to as acid-catalyzed esterification, of sometimes Fischer Esterification, after the German chemist who first isolated the technique.
A general equation for the formation of esters is:
The R and R represent alkyl groups such as methyl, ethyl, or propyl. The esters are named after the compounds from which they are formed. The first part of the name comes from the alcohol, and the second part of the name comes from the carboxylic acid, with the “oic” changed to “oate”. Thus when ethyl alcohol (ethanol) combines with acetic acid, the resulting ester is named ethyl ethanoate.
The synthesis of an ester must be done in the presence of an acid in order to push the reaction closer to completion. The reaction can be reversed by adding a strong base, such as NaOH. The acid that you will be using as a catalyst in this experiment is sulfuric acid.
Mechanism of Fischer Esterification
General Mechanism: The first step involves protonation of the carbonyl oxygen, followed by the nucleophilic attack of the alcohol, finished off with the loss of a proton. Again protonation of one of the alcohols, followed by loss of water occurs. Electrons from the oxygen come down to form the double bond. Loss of a proton yields the ester.
Many of the aromas of natural fruits and flowers are due to simple esters. Esters are also used for shampoo, soap, and perfume fragrances, as well as topical and oral anesthetics.
COMMON AND POPULAR ESTERS
Materials:
600 mL beaker, 5 test tubes, thermometer, various alcohols, various carboxylic acids, safety goggles, concentrated H2SO4, hot plate, test tube rack, evaporating dish, ice bath
Safety Precautions
• Concentrated sulfuric acid is a strong oxidizing agent. It will start a fire if mixed incorrectly with any of the alcohol or other acids used in this experiment. Use exactly as directed.
• All of the liquid acids used in this experiment are corrosive to skin, eyes, and clothing. While working on this experiment, wear safety goggles. Wash spills and splashes off your skin and clothing immediately, using plenty of water. Call your teacher if there is any type of spill!
• The alcohols and the organic acids used in this experiment are all flammable. Be sure all burners and other flames in the laboratory are extinguished before you start this experiment.
• You should always detect odors with caution. Breathing the vapors of some of these esters can cause sore throat, dizziness, headache, and drowsiness. Pour the contents of your test tube into an evaporating dish. Hold the evaporating dish 30 cm away and 15 cm below your nose. Waft the odor toward your nose, sniffing cautiously, once or twice. Do not breathe deeply while sniffing.
Procedure:
1. Put on your safety goggles.
2. Label the test tubes 1, 2, 3, 4, and 5 and place them on the test tube rack. Make sure you know which test tubes contain which combinations of alcohols and acids.
3. Into the appropriate test tube, pour the correct amount of an unknown alcohol and then add an unknown carboxylic acid. Make sure you use the appropriate amount of each substance – USE 1 mL (20 drops) IF IT IS A LIQUID, 1 GRAM IF IT IS A SOLID. CLOSE THE LID WHEN YOU ARE DONE WITH THE ACID OR ALCOHOL!
Any combination of the following that has a “yes” in the box will produce an ester:
Carboxylic Acid
Alcohol / H / I / J / K / L / MA / yes / yes / yes / yes
B / yes / yes / yes
C / yes
D / yes
E / yes / yes / yes
F / yes
G / yes
4. Carefully add four drops of concentrated sulfuric acid to each test tube.
5. Put about 400 mL of water in a 600 mL beaker. Place the test tubes into the water and heat the water on a hot plate to a temperature of 60°C. Leave the test tubes in the hot water bath for 15 min. Ensure that the water bath remains at approximately 60°C for that period of time. You may have to remove the water bath from the direct heat.
6. Cool the test tubes on the counter for a few minutes and then immerse them in an ice bath.
7. Add 5 mL of distilled water – this amounts to a small squirt from your squirt bottle - to each of the test tubes. This step is included to dilute the ester and soften its odor. You might have to pour out the substance into an evaporating dish to waft the odor – do this carefully!
8. Pour the contents of your test tube into an evaporating dish. Carefully note the odor of the contents of each of the test tubes. Remember to waft the odors of the evaporating dish toward your nose. Do NOT smell the test tubes directly.
9. Dispose of all materials down the sink with copious amounts of water.
10. Before leaving the laboratory, wash your hands thoroughly with soap and water. Remember to clean the surface of your bench and put away everything as you found it.
Conclusions:
1. Identify each ester according to its odor. The skeleton drawings for each ester are provided above. Now, work backwards, and identify the acid and alcohol that made that ester.
2. Draw the chemical reaction for each ester, complete with the carboxylic acid, the alcohol, and the acid catalyst. You only need to write out the net reaction – not the entire mechanism! Make sure you draw the chemical structure for the ester, the chemical structure for the alcohol and acid that created it. Label each with an appropriate name as well.
HERE IS AN EXAMPLE:
propanol ethanoic acid propyl ethanoate water