Experiment 18 - Synthesis of Esters

Chem 22 Spring 2010 Experiment 18 — Synthesis of Esters _____________________________________________________________________________ Pre-lab preparation. Be sure to leave enough time for the pre-lab work; you will not be able to start this experiment until you have developed a viable procedure. (1) Write a balanced equation for the reaction that your group will be doing. Show structures clearly, as always. (2) Find the relevant physical properties of the acid and alcohol reactants and of the ester you will be making. What do you expect your ester to smell like? Cumquat? Pomegranate? A fresh spring breeze, perhaps? (3) Read the procedure and then determine the quantities of acid and alcohol reactants needed to produce 2.0g of ester. (4) Write the mechanism of the reaction. (5) Write an outline of the procedure that you will use to make and isolate your ester. Include a flow chart showing each step of the extraction. Many esters have pleasant fragrances and are partly responsible for the flavors and aromas of fruits and flowers. Many are made industrially and used in the food and perfume industries — either alone or in combination with other esters and compounds of different classes — to produce various flavors and aromas. The most common method for preparing esters is acid-catalyzed reaction of a carboxylic acid and an alcohol, a process known as Fischer esterification. The reaction is reversible and requires excess reactant and/or removal of water to drive it to completion. Each group will prepare one of the following esters: n-butyl acetate, isobutyl acetate, npropyl acetate, isopentyl acetate, benzyl acetate, n-octyl acetate, or one of the 6 corresponding propanoate esters. Assignments were made during the last lab. Procedure. You are responsible for writing your own procedure this time. Determine the quantities of reagents needed to make a theoretical yield of 2.0 g of ester. To drive the equilibrium toward the ester, an excess of one reagent is normally used. A 3-fold excess should do the trick. (Well, at least this increases the amount of ester relative to the limiting reagent; "driving the equilibrium" to the right might be overstating it a bit.) 1 Experiment 17 Spring 2010 Whichever reagent you decide to use in excess will need to be removed when the reaction is over. Most esters are not appreciably water-soluble but should be quite soluble in diethyl ether. If one of your reactants is water-soluble, that can be removed from the ester by extraction. That's the one you should use in excess. If neither is water-soluble, perhaps one or the other would become soluble by reaction with aqueous acid or base. Sulfuric acid has traditionally been used as the catalyst for these reactions, but we will be using p-toluenesulfonic acid, or "tosic acid" ("TsOH"), instead. This is sort of a friendlier, dripproof, crystalline analog of sulfuric acid. But keep in mind that it is just as strong as sulfuric acid, so handle it carefully — leave the bottle in the fume hood, and don't scatter the solid around where it will get on other things like glassware, notebooks, sleeves, etc. Only a few mg will be necessary. Don't weigh it out — just estimate. You'll need to heat the mixture to get the reaction to occur. Use an IR lamp, and don't forget a boiling stone. A reflux condensor will allow you to boil the mixture without losing either reactant. Half an hour at reflux should be sufficient. Cool the mixture before attempting the work-up. Dissolve the mixture in about 20 ml of Et2O. If your excess reagent is water-soluble, wash it out of the ether with a few portions of water. We want to be sure to remove the last traces of tosic acid, as well as any carboxylic acid that may be present before concentrating the Et2O solution to isolate the ester, so wash with a portion or two of 5% NaHCO3. Caution: If this reacts with an acid, it will form CO2 gas, so start shaking gently, and vent the separatory funnel frequently, into the hood, to relieve pressure. Your final bicarbonate wash will be basic. Test it. If it's not basic, then it's not your final bicarb wash, is it? 5 or 10 ml of sat'd aq NaCl will suck most of the water out of the Et2O. Dry the solution over MgSO4, filter, and remove the solvent by rotary evaporation ("rotavap"). Determine the yield. Think ahead. You'll end up with a flask full of liquid — the mass of the flask is not part of the yield. Hmmm… Are the mass, appearance, and odor what you expected? The first ones to make each ester need to make up NMR samples. We'll run one spectrum of each ester and distribute these during the next lab period. But don't wait for the spectrum to finish your report. Write it up and turn it in at the end of the period. Experiment adapted from S. Selfe Lab Manual for General, Organic, and Biochemistry, 2000, pp 157-163. 2