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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.