LAB_ Preparation of Fragrant Esters

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University of Guelph *

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1040

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Chemistry

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Dec 6, 2023

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LAB: Preparation of Fragrant Esters /19 TOTAL Introduction The general reaction for the esterification of an organic acid with an alcohol is R-COOH + HO-R’ R-CO-OR’ + H 2 O In this general reaction, R and R’ represent hydrocarbon chains, which may be the same or different. Unlike many organic chemical compounds, esters often have very pleasant, fruitlike odours. Many of the odours and flavourings of fruits and flowers are due to the presence of esters in the essential oils of these materials. Generally, a fruit or flower may only contain a few drops of ester, giving a very subtle odour. When prepared in the laboratory, in larger amounts, the ester may have a pronounced chemical odour. In order to identify the correct smell in this lab, we need to keep in mind microscale amounts. Safety Precautions ● Protective eyewear ● Most of the organic compounds used or produced in this lab are highly flammable. All heating will be done with a hotplate, and no flames will be permitted in the laboratory ● The acids in this lab are dangerously corrosive! Acids should not come in contact with skin, clothes or lab desks.. ● When identifying the odour of the ester, DO NOT deeply inhale. Excessive inhalation of the products may cause headaches or dizziness. Merely waft a small amount of vapour from the ester toward your nose. ● All waste must be placed in the Organics bin located in the fume hood. DO NOT pour any chemicals down the sink. Apparatus/Reagents Required Hotplate Thermometer graduated cylinder 6M H 2 SO 4 500 mL beaker 4 test tubes test tube holder watch glass eye droppers vent hood graduated cylinder ethanol propanoic acid methyl salicylate 20 % (mass/Volume) NaOH Procedure Condensation Reaction ● Set up a water bath using the hotplate and 500 mL beaker (2/3 full of tap water) ● Heat the water to 70 °C ● To synthesize the ester, mix 10 drops of the propanoic acid and 20 drops of the ethanol in a clean dry test tube ● Add 2 drops of the concentrated sulphuric acid ( Caution! )

● Heat the mixture for ten minutes ● Using a dropper, place some warm water into a watch glass ● Place a few drops of your ester on the water. Make any observations ● Waft the vapour towards your nose and note the odour ● Dispose of all waste in the organics disposal containers located in the fume hood Hydrolysis Reaction ● Obtain 5 drops of methyl salicylate in a clean dry test tube ● Note the odour of the ester ● Add 2 mL of water followed by 5 drops of 20% NaOH (in the fume hood). Make any observations ● Heat the mixture for 15 minutes ● Note the odour of the resultant mixture Name: _____________________________________________ Partner: _____________ Observations: Set up a table/chart for observations and use it to record experimental observations. /3 Observation Condensation Reaction Hydrolysis Reaction Temperature of water bath Heated to 70°C using hotplate and 500 mL beaker N/A Mixture appearance Clear liquid Clear liquid Odor of starting materials Propanoic acid: sharp, vinegar-like smell Ethanol: fruity, alcoholic smell Methyl salicylate: sweet, wintergreen smell Odor of product Sweet, fruity smell Salicylic acid: no distinct odor Methanol: sharp, slightly sweet smell Solubility in water Insoluble in water, forms a layer on top of water Soluble in water

pH N/A Basic (due to the presence of NaOH) Color change N/A Mixture turned light pink Other observations Ester ﬂoated on top of the water in the watch glass.Vapor smelled sweet and fruity Mixture bubbled and produced heat when NaOH was added.Odor changed from sweet to slightly medicinal after heating Name: _____________________________________________ Partner: ___________ Post lab Questions: Inquiry /12 1. Using condensed structural formulae outline the esterification reaction and the hydrolysis reaction. Write the IUPAC name of each reactant and product. /4

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Answer the following questions in complete sentences. 2. What was the function of sulphuric acid in these reactions? /1 The catalyst utilized is sulfuric acid, which functions to accelerate reaction rates without interfering with the reaction itself as well as helping to remove H20! 3. What experimental evidence is there that the carboxylic acids used in this investigation are soluble or insoluble in aqueous solution? Explain this evidence in terms of molecular structure of the acids. /3

The fact that the propanoic acid and methyl salicylate were able to dissolve in ethanol in the condensation reaction suggests that they are soluble in organic solvents. However, in the hydrolysis reaction, both esters were hydrolyzed in the presence of water and sodium hydroxide. This suggests that they are also soluble in aqueous solutions. Carboxylic acids, in general, are polar molecules with a hydrophilic carboxyl group (COOH) and a hydrophobic alkyl chain. This allows them to be soluble in both organic and aqueous solvents due to their ability to form hydrogen bonds with water molecules. 4. What experimental evidence is there that the esters synthesized in this investigation are soluble or insoluble in aqueous solution? Explain this evidence in terms of molecular structure of the esters and confirm based on your observations. /3 From the observations table, we can see that the synthesized ester floats on top of the water and does not dissolve, confirming that it is insoluble in water. However, when the ester is added to ethanol, it dissolves readily, indicating that it is soluble in organic solvents. This is consistent with the molecular structure of esters, which are typically insoluble in water but soluble in organic solvents. Esters are typically less polar than carboxylic acids and alcohols because they have a smaller dipole moment. They also have a nonpolar portion of the molecule, which is typically a hydrocarbon chain. Therefore, based on the observations from the observation table, we can confirm that the esters synthesized in this investigation are insoluble in aqueous solution, but soluble in organic solvents. 5. Identify the odours of the synthesized ester. /1 The odors were fruity, primarily with a pineapple-like smell. Communication: /4 CRITERION LEVEL 1 LEVEL 2 LEVEL 3 LEVEL 4 Written Communicat ion Scientific terminology is seldom used and is often correct. There are abundant spelling errors. Answers are rambling and much of it is off topic or brief and uninformative. Answer is disjointed and hard to follow. Scientific terminology is sometimes used and is not always correct. There are frequent spelling errors. Answers are long and not always on topic or brief and incomplete. The flow of information is not well organized. Scientific terminology is usually used and is correct. There are few spelling errors. Answers are fairly concise and mostly on topic. The flow of information is fairly logical Scientific terminology is used meticulously and is correct. There are no spelling errors. Answers are concise, yet complete and on topic. There is a logical flow of information.