Purpose: Salicylic acid is converted to the liquid ethyl salicylate by addition of ethanol. Yields of salicylic acid (week 1) and ethyl salicylate will be found, as well as the overall yield of the 2-step conversion of acetyl salicylic acid to ethyl salicylate. Introduction:
Esters are formed from the reaction of acids and alcohols:
R-C-OH + HO-R’ → R-C-OR’ + H2O
acid alcohol ester water The pleasant smelling oil of ethyl salicylate is made by reacting salicylic acid (week 1) with excess ethanol. An acid catalyst is also needed.
salicylic acid (SA) ethanol ethyl salicylate (ES) The overall yield is determined from multiplying yields of the two reactions: 1. ASA + HCl(aq) →SA and 2. SA + ethanol → ES + water Apparatus
The reaction takes place by refluxing (gentle boiling) in a 100-mL round bottom flask. This time the ice-cooled condenser is
essential, otherwise the ethanol reactant (b.p.79˚C) would
evaporate before the reaction was over.
Melting points of acetyl
salicylic acid and salicylic
acid (dried from week 1) are
taken with MelTemp devices.
If you have never used one
your instructor will
OH H OC H2CH3
Part A: Esterification of Salicylic Acid with Ethanol
1. Before starting remove a spatulaful (that will be < 0.1 g) of SA (salicylic acid) and place in a labeled plastic vial to use for melting points.
2. In a 100 mL round bottom flask, mix at least 5 g dry salicylic acid (0.043 mol) with 20 mL 100% ethanol. The salicylic acid should be weighed out to two decimal places and recorded on the data sheet. The alcohol is in excess. It can be poured from a graduated cylinder, and should not be scaled down.
Note: Use more salicylic acid if you have more from the first week. You should have between 7 and 8 g. Adjust amount of ethanol accordingly, say to 30 mL. 3. Add approximately 1/2 gram of boric acid catalyst that we use as a substitute for concentrated sulfuric acid in this particular esterification. 4. Place entire assembly on a hot plate and heat until the alcohol is boiling. A setting of around 3 on the hot plate can be tried and then adjusted up or down as needed. You may need to add more ice.
Note: While the refluxing is taking place, you can work on Part B, in which the materials you have already made are characterized by taking their melting points. 5. Reflux for a minimum of 1 hour and 30 minutes or longer as time allows. Cool until the flask is no longer hot, then in an ice bath, until the mixture is at room temperature. Transfer to a clean separatory funnel.
6. Add 10 mL 2M NaOH(aq) .The mixture turns milky as approximately 2-3 mL of ester appears at once as an insoluble bottom layer. Ethyl salicylate is an oily liquid with a density of 1.1 g/mL. Wait a few minutes until the cloudy emulsion begins to clear up. You can estimate the yield by marking the funnel and measuring an equal volume of water. Record this. You will probably get about 1/2 mL for every 1 gram of salicylic acid. 7. You should be able to recover about 1.5 mL of the ester oil by separating the layers using a separatory funnel. Allow the bottom layer to pour into a vial. This layer contains mostly ethyl salicylate and should have a very pleasant odor, something like wintergreen. It will also include a little NaOH solution and thus will not be clear and sparkling. If you have enough liquid you can attempt the next step.
8. Dry by mixing with a small amount of anhydrous magnesium sulfate. See your instructor to get help with this. Pour off the clear ethyl salicylate.3 Part B: Melting Points
1. Measure melting points of the dry...