Esterification of an Unknown Acid: Handout
Esterification of an Unknown Acid
Required Reading
Esterification: Bruice, Section 16.15
Discussion
Esters, derivatives of carboxylic acids, can be quite fragrant and have been found to be the major
flavor and odor components of many fruits. Natural flavors and odors are usually formed by the
combination of a range of compounds. However, single esters can approximate the natural odor
associated with a plant, and are often used in the food industry for artificial flavors and fragrances.
In this experiment, you will synthesize an ester from an unknown carboxylic acid and an unknown
alcohol. You will identify the ester and prove that you successfully synthesized the ester using its
infrared spectrum, refractive index, boiling point, and odor. You will take a GC/MS of your
reaction mixture to aid in your identification. You will also be given a literature NMR of your
ester.
Special Instructions
Many of the carboxylic acids used in this reaction, as well as the catalyst (sulfuric acid), are
corrosive and may cause burns, especially when they are hot. Fumes may be irritating. The
alcohols used in this experiment are flammable and are irritants to both the skin and eyes. Dispense
all reagents into your round-bottom flask in the hood and leave capped when at your desk.
Prelab
Since you do not know which acid/alcohol combination you will be using, for the Prelab you
should present a balanced equation and calculate the theoretical yield for the synthesis of isobutyl
formate (smells of rasberries) from isobutyl alcohol and formic acid as a sample of the theoretical
yield calculation. You do not need to include a table of reagents.
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POSSIBLE ESTERS
Esterification of an Unknown Acid: Handout
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Esterification of an Unknown Acid: Handout
Procedure
Synthesis of an Unknown Ester
Preheat a sand bath (on low) to 70 °C. Set up your reflux apparatus as shown below using a roundbottom flask, water condenser, and a drying tube filled with a small amount of wetted glass wool.
Connect the tubing to allow water to circulate through the condenser.
Add 1.0 mL of your assigned alcohol (labeled A – I) to your tared 10 mL round-bottom flask;
reweigh to determine the actual mass added. Add boiling chips to the round-bottom, as well as 2.0
mL of your assigned carboxylic acid (labeled 1 – 10). Finally, add 4 drops of concentrated sulfuric
acid to the round-bottom, cap it, and reattach the round-bottom to the reflux apparatus at your
workspace. Start water flowing through the water condenser and lower the entire apparatus into the
sand bath. After a few minutes, if your combination of alcohol and acid is not refluxing, slowly
turn up the hot plate until the mixture begins to reflux.
Reflux your mixture for 90 minutes, and then cool the apparatus to room temperature. Transfer
your reaction mixture from the round-bottom flask to a screw-cap centrifuge tube. Allow the
mixture to settle. If two layers form, remove the aqueous layer using a Pasteur pipet. Place one
drop of the top layer into an automated GC/MS vial filled with ~1.5 mL of HPLC grade
dichloromethane for GC/MS analysis. Add 1.5 mL of an ice-cold solution of 10% NaHCO3 to the
organic layer. Shake or vortex the centrifuge tube to ensure mixing. Allow the layers to separate
and remove the aqueous layer. Repeat the base extraction once more. Finally, wash the organic
layer with 1.5 mL of cold saturated sodium chloride solution.
Use a fresh Pasteur pipet to transfer the organic product to a clean, dry centrifuge tube and dry the
product using anhydrous sodium sulfate. Finally, transfer the ester to a container that can be tightly
sealed with either a cap or a foil-covered cork and Parafilm.
Identification of an Unknown Ester
To analyze the ester and confirm the synthesis of an ester, you should take an IR spectrum of the
compound and analyze its odor. Determine the compound’s boiling point, index of refraction, and
density. The data from the GCMS may aid you in identifying both the ester synthesized and the
carboxylic acid starting material. You will be given a literature NMR of your ester to aid in
identification.
Report
There will be no report for this lab. However, you must correctly identify your compound to your
professor or TA and give be able to explain the identification.
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