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Introduction
Methyl salicylate (oil of wintergreen) undergoes basic hydrolysis to give salicylic acid according to the equation below.
Procedure
1. Using a 5 cm3 graduated pipette, add 2.2 cm3 of methyl salicylate into a 100 cm3 round-bottomed flask (rbf).
2. Using separate 25 cm3 measuring cylinders, add 22 cm3 of ethanol and 22 cm3 of 2 mol dm–3 NaOH into the rbf. Place a magnetic stirring bar into the rbf.
3. Set up the apparatus as shown in the diagram. Gently heat and stir the mixture under reflux for 15 min.
4. Carefully remove the hot round-bottomed flask from the set-up [CAUTION: The flask is HOT!] and place it on the rbf holder. Cool the rbf under a running tap if necessary.
5. Place about 25 cm3 of 2 mol dm–3 hydrochloric acid into a measuring cylinder. Add the acid into the mixture using a dropper, swirling until no more precipitate appears.
6. Using vacuum filtration, filter the crude product and wash the residue with a little cold water. Transfer the crude product into a
100 cm3 conical flask.
7. Dissolve the crude solid product in a minimum volume of hot water in a 100 cm3 conical flask. This is done by heating up some distilled water in a separate 100 cm3 conical flask and using a dropper, add the hot water dropwise into another 100 cm3 conical flask containing the crude product until the crude product just dissolves.
8. Cool the saturated solution in an ice-bath to recrystallize the solid.
9. Filter the mixture under vacuum filtration to obtain the purified crystals. Wash the crystals with a small amount of cold water and allow them to dry in a desiccator. Weigh the purified crystals.
10. Measure the melting point of the dried crystals to estimate their purity. If the crystals melt sharply close to 159 oC, it is pure. If the crystals start melting below 159 oC and over a large range of temperatures, the crystals are not pure.
Photos and videos showing the different stages mentioned in the experimental procedure
Empty round bottom flask with methyl salicylate & micropipette.
Introduction of reflux set-up & adding methyl salicylate, ethanol & NaOH into the round-bottom flask.
Question: What reaction occurs when NaOH is added into solvated methyl salicylate? What is the likely identity of the white ppt at the end of the above video? Why does the precipitate appear slowly?
The appearance of the content in the round bottom flask after leaving the mixture to stand for a while.
Set-up before reflux.
The appearance of the content after 5 min.
The appearance of the content during reflux.
Letting the round-bottom flask to air-cool after reflux.
Question: What is the major organic species present in the colourless mixture? Why does it appear colourless? Why does the white ppt present in the mixture after adding NaOH 'disappears'?
The appearance immediately after reflux. HCl(aq) is added to the cooled mixture from the above. Why does precipitation occur?
Extension 1: Mechanism of the basic hydrolysis of methyl salicylate
The hydrolysis of methyl salicylate to give salicylate occurs via a four-step mechanism:
acid-base reaction between OH– and the phenolic hydroxyl group;
OH– attacks the carbonyl carbon to form a tetrahedral alkoxide intermediate;
the p bond of the C=O is reformed, together with the heterolytic cleavage of C-O leading to the expulsion of methoxide;
the methoxide undergoes an acid-base reaction with the carboxylic group formed from the 3rd bullet point.
Suggest the mechanism for the basic hydrolysis of methyl salicylate. Show any relevant lone pairs, dipoles and charges, and indicate the movement of electron pairs with curly arrows.
Performing suction (vacuum) filtration.
The appearance of the crude product.
Performing recrystallisation.
The time-lapsed videos below showcase the slow cooling of a saturated solution of salicylic acid. Take note of the formation of the needle-shaped crystals (compare its appearance from the crude product obtained). Many thanks to the two groups of students for the 2021 batch of students for sharing the video with us :)
Beautiful needle-shaped crystals after recrystallization.
How do we know that the products obtained at the end are indeed salicylic acid? Besides performing a melting point test (#10 of the procedure), we can also do characterization using spectroscopic techniques. What do you think are the limitations of a melting point test?
Safety hazard and precaution
Methyl salicylate and 2.0 mol dm-3 NaOH are corrosive. Wear gloves to prevent direct contact with the skin.
Organic compounds are volatile and flammable. Use a water bath to heat instead of using Bunsen burner (no open flame)
Extension 2: Characterization using NMR and MS
The 1H NMR, IR and Mass spectra of methyl salicylate are shown below. (All spectra images were taken from the SDBS database: https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi)
The 1H NMR, IR and Mass spectra of salicylic acid are shown below. (NMR spectrum image was taken from Chegg Study: https://www.chegg.com/homework-help/questions-and-answers/given-nmr-spectrum-salicylic-acid-identify-individual-peaks-represent-identify-peak-corres-q18996712;IR and Mass spectra images were taken from the SDBS database: https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi)
How can we leverage on the various spectra of methyl salicylate and salicylic acid to help us to determine if the basic hydrolysis has been successfully completed?
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