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Synthesis of Esters (handwritten)
Esters are an important class of organic compounds which are characterized by the following generic formula.
Low molecular weight esters have pleasant odors and are responsible for many of the distinctive odors in a wide
variety of fruits and flavorings. Esters can be readily prepared from their corresponding carboxylic acid via a
condensation reaction with an alcohol. Generally this reaction is catalyzed by strong acids in solution.
The equilibrium constant for the reaction between primary alcohols and unhindered carboxylic acids is
approximately four. If equal quantities of acid and alcohol are used the reaction gives a product yield of only
67%. In order to make esters in high yields the equilibrium must be shifted using LaChatelier’s principle. In this
experiment you will synthesize a series of low molecular weight esters from their corresponding acids and
alcohols. In order to generate the esters efficiently you will use an excess of alcohol and in some cases you will
remove the water as a low boiling azeotrope. You will then attempt to identify the characteristic odor of each of
the esters you have made. Use the table of esters and structures on the following page to assist you in determining
which acids and alcohols to use.
Many esters have the characteristic property of a pleasant odor. As such, they are used as artificial flavors and
perfumes. Have you ever tasted peppermint gum or banana-flavored ice cream? These flavors are most likely due
to an organic compound known as an ester. Not all esters have odors, some are odorless, too. An example of an
odorless ester would be vegetable oil.
Esters are formed by the process of dehydration synthesis using an alcohol with an acid. Their names are formed
from the very alcohol and acid used in their making. For example, when making the ester methyl salicylate (oil of
wintergreen), the alcohol and acid used would be methanol and salicylic acid, respectively. This ester has an
agreeable “peppermint” odor. Another example would be the ester butyl formate which is produced when
reacting butyl alcohol with formic acid.
When an organic acid (RCOOH) reacts with an alcohol (R’OH), water is removed and an ester is formed. This
process is an example of dehydration synthesis.
Some artificial flavors are formed by the mixing of esters in an attempt to duplicate the flavor or odor of a natural
substance. For example, the odor of raspberries is due to a mixture of isobutyl formate and isobutyl acetate. In
this experiment you will make three different esters having characteristic odors of banana, peppermint, and fruit
essences. The product formed will be quite crude; however, you will be able to detect the three different odors, as
well as experience the basic method of producing esters.
MATERIALS
4 culture tubes 6 x 50 mm
4 graduated pipets with 1 inch of barrel cut-off
50 mL beaker
Use a hot plate to heat solutions.
Concentrated Sulfuric Acid (in the black fume hood at the back of the room. BE CAREFUL with this acid.)
Use the chemicals listed below.
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*Acetic Acid (Vinegar) with Ethanol (Ethyl Alcohol)
*Salicylic Acid with Methanol (Methyl Alcohol)
PROCEDURE
1. PUT ON YOUR GOGGLES NOW! HAVE YOUR INSTRUCTOR DEMONSTRATE THE
PROPER PROCEDURE FOR DETECTING ODORS OF REAGENTS. (Wave your hand over the
test tube and to bring the odor to your nose. DON’T stick you nose in the test tube.)
2. Fill a 50 mL beaker with water. Set the beaker on the hot plate. Bring the water to a gentle boil.
3. Examine the structure of the ester you wish to synthesize. Use the chemical with a “*” by them
from the “Materials” list above for the acid and the alcohol you need to use. Do the two labeled
with a “”.
4. In a 6 x 50 mm culture tubes (small test tube about 2.5 inches long) obtain two drops of the
carboxylic acid. If the carboxylic acid you wish to use is a solid, use about the amount which would
fit inside this letter “O” Add six drops of alcohol and one drop of concentrated sulfuric acid. BE
VERY CAREFUL WITH THE SULFURIC
ACID.
5. Do this lab at the back left lab table near the Black
Hood. Turn on the hood while working. Make sure
you turn it off when you finish for the day.
6. Wave the fumes from this mixture to your nose and
carefully note and record the odor of this reaction.
7. Place the cut-off graduated pipet into the culture
tube. It should form a tight seal. See diagram.
8. Boil the mixture for 3-5 minutes and observe and
record any changes which take place in the reaction
mixture.
9. Remove the reaction assembly from the boiling
water.
10. Remove the pipet from the culture tube and
carefully force out some of the vapor. Wave the
fumes to your nose and carefully note and record the
odor of the ester you synthesized.
11. Occasionally, the vapor is too concentrated and
“over powers” your nose. Add some water to the
pipet and smell again.
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Questions: Read the introduction to your lab. 5 pts each
1. In some case the odor of the synthesized ester does not exactly match the odor of the fragrance found in
nature. What might be a possible explanation for this?
2. What is a characteristic of esters that smell good?
3. What two types of chemicals are combined to make an ester?
4. Identify the branches and functional groups from the structural formulas on the next page.
5. Name the two esters you were required to make.
6. Look up the structural formula for aspirin (an ester) and write an equation of its synthesis.
7. Identify the odor of your ester and determine its structure. Draw the structure.
8. Name four products that you have used or know about that have esters in them. These should different
from your partners. (Think of things that smell fruity or sweet but have no fruit in them.)
9. Define low boiling azeotrope.
10. What type of reaction produces esters? Describe why it has this name.
11. Which flavor worked the best for you?
12. Write a balanced equation for each ester below: Partner 1 do odd-Partner 2 do even. 5 pts each
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