HYDROLYSIS OF METHYL SALICYLATE

HYDROLYSIS OF METHYL SALICYLATE

PURPOSE

Hydrolyze the ester known as methyl salicylate under base conditions in order to synthesize salicylic acid, a carboxylic acid.

TECHNIQUES

• Hydrolysis of an ester

• Heating under reflux

• Filtration

• Crystallization

• Melting point

THEORY

Methyl salicylate is an ester easily recognized by its odor and is known as oil of wintergreen because of its natural source. This ester will be treated with aqueous base. The hydrolysis reaction that occurs will form methanol, water, and the sodium salt of salicylic acid. Salts of organic compounds usually are soluble in water or will dissolve in water with a bit of heating. Later in the work-up, the salt is acidified with sulfuric acid to convert the organic salt into the protonated carboxylic acid. Therefore, the major organic products of this reaction are methanol and salicylic acid. The salicylic acid is a solid that can be isolated and purified by crystallization. The chemical equations that describe this experiment are:

Because the phenolic hydroxyl group is acidic, it is also converted to the corresponding sodium salt during the basic hydrolysis. In the subsequent acidification, this group also becomes reprotonated.

PROCEDURE

APPARATUS

1) Assemble a reflux apparatus, using a 250 mL round-bottom flask and your watercooled condenser with the wide bore inside tube (Fig. 23.1, p. 204, Survival Manual). Be sure to use a small amount of stopcock grease to seal the glass joint between the round bottom and condenser. Use a heating mantle to heat, setting its height so that the mantle can be dropped down when cooling is desired.

REACTION MIXTURE

2) Dissolve 10 g sodium hydroxide into 50 mL of deionized water in a 250 mL Erlenmeyer flask. While this solution cools a bit, remove the round-bottom flask from your reflux set up. Weigh 5.0 g of the methyl salicylate liquid into a small beaker, then pour this liquid into your round-bottom. Add one or two small boiling stones to the round bottom. Finally, add the sodium hydroxide solution to this same round-bottom. Reassemble the apparatus for reflux.

REFLUX

3) Start water flowing through the condenser, turn on the mantle, and bring the mixture to boil. Continue heating under vigorous reflux for 30 minutes. Your "reflux ring" should be about ½ to 2/3 the way up your condenser. After the thirty minutes of reflux, stop heating, lower the heating mantle if possible, and shut down the water flow.

CRYSTALLIZATION AND FILTRATION

4) When the solution has cooled for about five minutes, transfer it to a 250 mL beaker. Try to leave the boiling stones "behind" when you decant the reaction mixture from the round-bottom to the beaker.

5) Carefully add enough 1 M sulfuric acid to make the solution acidic when tested with litmus paper (blue litmus turns red). It may be necessary to add as much as 150 mL of the acid, so you can pour in large quantities before checking with litmus each time. When the litmus turns pink, add an additional 15 mL of 1 M sulfuric acid to guarantee an acidic environment. The acid addition should cause the salicylic acid to precipitate from the solution.

6) Cool the mixture in an ice bath to a temperature of 5 °C. Set up a vacuum filtration using a Buchner funnel and filter paper. Start the vacuum on the Buchner system, swirl the beaker containing the precipitate to loosen it, then carry out the filtration. Use a stirring rod or spatula to transfer as much solid as possible to the filter. Since you will be recrystallizing the product "from water" in the next step, you need not get it very dry at this time.

RECRYSTALLIZATION AND RECOVERY

7) Prepare the following equipment so it is ready immediately when we need it in Step 9. You will prepare for a hot filtration (see image on next page). Use a powder funnel (plastic is OK, glass is preferred), find filter paper that will fit it, then flute the paper to increase its surface area. Add just enough water to the bottom of a 250 mL Erlenmeyer so that the entire bottom is wet. Place the fluted paper in the funnel, place the funnel in the flask, and set it aside for now.

8) [Note that the recrystallization procedure below is NOT typical – we normally dissolve the solid in a MINIMUM of boiling solvent.] Place 100 mL of deionized water in a beaker on the hot plate and bring it to a boil. Transfer the semi-dry product from the Buchner into a 250 mL Erlenmeyer flask. Using a powder funnel with its large opening might make this task easier. Without burning your fingers, pour the 100 mL of boiling water into the flask so the solid can dissolve. If the solid does not dissolve, add a small amount of extra water. Do not be concerned if a few small pieces fail to dissolve; these might be insoluble impurities.

9) Place the flask/ fluted paper/ funnel set-up on a hot plate and heat so that the water boils gently and causes a hot environment within the flask and funnel bottom. This will help prevent solids from crystallizing prematurely in the funnel in the next part of the procedure. Your product from the previous step should still be totally dissolved in the hotplate solution. Using a heat safe glove or using a paper towel wrap, pour small amounts of hot boiling product solution into your preheated funnel system. Continue until all product solution is filtered. Hopefully, very little product formed on the filter paper, as this will be discarded. As soon as the filtration is complete, remove the bottom Erlenmeyer from the hot plate (now containing your product as filtrate) and set it aside. You must allow this flask to cool very slowly. After a while, beautiful needles of salicylic acid should form. Once the flask reaches near room temperature, place it in an ice bath to increase the amount of product that leaves the solution.

10) Collect the crystals by vacuum filtration. When mostly dry, transfer the crystals to a beaker or watch glass and allow them to dry in your lab drawer until the next lab period.

11) During a slow period in next week's lab, weigh your dry solid, take a melting point, and submit your sample in a properly labeled vial. Don't forget to calculate theoretical and percent yield.

RESULTS

In your notebook Conclusion, report your percent yield and appearance of your product. Discuss and compare the literature melting point and the value you observed. Discuss problems you encountered and suggestions you may have to improve the procedure.