Download Experiment 7

Ester Synthesis & the Chemistry of Smell
INTRODUCTION:
Whether it’s a fresh nectarine or a rotting tomato, a floral rose or funky flatulence, the scent of an object comes
down to chemistry. In order for you to sense a smell, a molecule has to travel through the air and ultimately land on
an olfactory receptor in your nasal cavity. This has several implications in the theory of olfaction (sense of smell).

To be capable of reaching your nose, the odorous substance must be volatile—meaning it has weak enough
intermolecular forces to get into the gas phase. This usually limits the substance to relatively small
molecular sizes.

Olfactory receptors are made up of proteins, which consist of strings of amino acid molecules. The unique
shapes and electronic structures of these receptors make it possible to sense the arrival of smelly molecules
based on their own 3D positioning & electronic arrangement. Similar to a lock and key mechanism, odorants
(acting as keys) bind to specific receptors (acting as locks) to excite a neuron and trigger the sensation of
particular smells.
Organic molecules can be classified based on the atoms they contain and how these are arranged. Patterns in
atomic arrangement and bonding are called functional groups, which typically share similar properties & reactivity.
For example, alcohols (ROH) are characterized by a hydroxyl group (an oxygen—hydrogen bond) attached to a
carbon atom. A carboxylic acid (RCO2H), on the other hand, has a hydroxyl group in addition to a second oxygen
atom that is connected through a double bond to the same carbon atom (see reaction below).
Carboxylic Acid
Alcohol
Ester
Water
Note: In the equation above, “R” serves as placeholder. It could represent a chain of carbon atoms, or perhaps a ring.
It provides a generalized method for representing the rest of a molecule. In this way, organic chemists can ignore the
other complex features of a molecule and focus on the highlighted functional groups. The numbers (R1, R2, etc.) help
distinguish between these groups, indicated they don’t have to be the same.
In this lab experiment, we will react different alcohols & carboxylic acids together (see tables below) to synthesize
various esters. Esters (RCO2R’) are a sort of hybrid functional group—they have two oxygen atoms, including the
C=O double bond of a carboxylic acid, but lack the oxygen—hydrogen bond of an alcohol. They are formed when an
alcohol condenses with a carboxylic acid releasing a water molecule and generating the new functional group—a
chemical reaction called an esterification. Given their different electronic structures & 3-dimensional shapes, esters
smell characteristically different from carboxylic acids or alcohols.
Alcohols
Carboxylic Acids
salicylic acid
ethanoic acid (aka acetic acid)
propanoic acid (aka propionic acid)
butanoic acid (aka butyric acid)
methanol
ethanol
1-propanol
1-butanol
1-pentanol
1-octanol
PROCEDURE:
Wear safety glasses at all times!
Avoid contact with all chemical reagents and dispose of reactions using appropriate waste containers.
**Exercise particular caution with butyric acid, the smell is STRONG and can linger on clothing for hours.
Work with a partner. Each group should synthesize two esters:
A) Methyl salicylate is an ester having the signature fragrance of wintergreen, and is synthesized from
salicylic acid and methanol.
B) Synthesize a custom ester of your choice & describe the fragrance. You’ll need to choose a starting
carboxylic acid & alcohol to prepare the desired fragrance.
1. Acquire 2 graduated test tubes, a 100-mL beaker, test tube rack and hot plate from the reagent cart.
2. Fill the 100-mL beaker with 40 mL of water and heat on the hot plate to approximately 65 oC (a setting of about
3 on the hot plate). Carefully monitor the temperature—be sure to hold the bulb of the thermometer off the
bottom of the beakers to avoid artificially high readings.
3. A) In the case of the salicylic acid, add 0.1 grams into one of the test tubes. B) In the case of the liquid carboxylic
acid, place approximately 4 drops of one of the acids into the other test tube.
4. Cautiously sniff/waft the test tube to detect the smell of the acids. The acids will have a strong acrid smell. Use
caution when wafting the odors of the acids. Do not stick the test tube directly below your nose. Butanoic
acid is particularly strong so use appropriate caution.
5. Add approximately 1 mL of an alcohol to the same test tube containing the carboxylic acid (fill with alcohol to
the graduation mark).
6. Repeat steps 3 through 5 until you have the two test tubes with two unique reactant mixtures: A) one making
methyl salicylate, B) one making the ester of your choice.
7. Add a single drop of concentrated sulfuric acid to each of the test tubes. Your instructor will bring the sulfuric
acid to your workstation once you have prepared both samples.
8. Heat the reactions in the water bath for about 15 minutes. For those reactions involving methanol as the
alcohol, heat these in a lower temperature water bath provided for the class. Methanol has a substantially
lower boiling point than the other alcohols and will boil off in the higher temperature bath.
9. Cautiously sniff/waft the test tube to detect the smell of the ester produced.
10. Make a table of the odors of the alcohols, carboxylic acids and esters used in the synthesis.
CLEAN UP:
Pour all of the ester waste into the container marked organic waste. Rinse the test tube with a small
amount of water and dispose of in the organic waste. Deposit the test tubes in the container marked
“Rinsed Test Tubes Only.” Wash your hands with soap and water before leaving.