L. Scheffler
Preparation of Esters
Introduction:
Carboxylic acids are a very important class of organic compounds. Carboxylic acids and their derivatives
appear widely in nature. Amino acids are the building blocks or proteins. Fats and oils have carboxylic acids
and glycerol as their main components.
The reaction of a carboxylic acid and an alcohol produces an ester. Esters are known for their pleasant
"fruity" aroma. They are found in most fruits and are the constituents of several artificial flavoring agents.
The perception of flavor is a phenomenon that incorporates the senses of taste, smell and texture. One cannot
taste the flavor of spearmint chewing gum, or raw onion, if the nostrils are held closed. Esters are a class of
compound that can help to illustrate the correlation between smell and flavor. Many esters occur naturally
and can be derived from a variety of plants. Many esters are prepared synthetically and are on the "GRAS"
list (Generally Recognized as Safe). Besides being used as food additives to enhance flavor, they are used in
synthetic fragrances, industrial solvents, and as starting materials for plastics such as plexiglass.
Esters may be prepared by the direct esterificiation of a carboxylic acid with an alcohol. The yields in such
processes are generally substantially less than 100 %, since esters are frequently hydrolyzed by the water
produced in the reaction to form the alcohol and the acid again. A couple of drops of concentrated sulfuric
acid is used to catalyze the reaction. Sulfuric acid also absorbs water readily helping to prevent the reverse
reaction from occurring. Sometimes the cheaper of the two reagents is used in excess to force the equilibrium
to the right and favor ester production as well.
Esters are named essentially as derivatives or salts of carboxylic acids, with the regular acid ending changed
to -ate. The alkyl or aromatic group of the alcohol is named first. For example:
acetic acid + ethyl alcohol à ethyl acetate + water
Some common odors associated with fruits and flavorings are listed below:
amyl butyrate (pear)
amyl acetate (banana)
n-butyl acetate (banana)
methyl salicylate (winter-green)
methyl butyrate (pineapple)
Some simple esters which can be formed in the laboratory include the following:
butyric acid (1.0 mL) + amyl alcohol (1.5 mL)
acetic acid (1.0 mL) + amyl alcohol (1.5 mL)
acetic acid (1.0 mL) + n-butyl alcohol (1.5 mL)
salicylic acid (1.8 g) + methyl alcohol (1.0 mL)
butyric acid (1.0 mL) + methyl alcohol (0.8 mL)
Safety:
• Concentrated sulfuric acid is very corrosive to flesh and causes burns. Wear goggles at all times. Wash
spills immediately with large amounts of water.
• Butanoic acid (Butyric acid) has a very foul odor. Use carefully in the fume hood
Procedure:
1. Obtain two test tubes, 10 cm3 test tubes two one-holed rubber stoppers, #2; and two disposable large
bore Beral pipets, to act as air condensers.
2. Cut off the end of the Beral pipet and slip it over the end of the test tube to in sure that has a snug fit .
See the diagram below
3. Refer to the reactants list on the previous page. After deciding what two esters you would like to
make, obtain the appropriate acids and alcohols. Add the designated amount of alcohol to the test
tubes and then add the designated amount of carboxylic acid.
4. Add 2-3 drops of concentrated sulfuric acid and one small piece of stone boiling chip to the test tubes.
Do not use marble chips since they will react with the acid
5. Stopper the test tube with a pre-assembled air condenser. Place both test tubes, with condensers
attached, into a 250-mL beaker 1/2 full of water. Heat the water to boiling (hot plates should be used
due to the flammability of organics). Continue to heat for 10 to 15 minutes.
6. Cool for 10 minutes or place in a cold water bath. Use a wafting technique to note the odor (if any) of
the product.
For Discussion:
1. What are the names and formulas of the esters you synthesized?
2. Describe the odors of the esters.
3. Compare the odors of the esters to those of the real fruits and flavorings displayed on the lab table.