Experiment 8 Fischer Esterification Handout PDF

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9 EXPERIMENT 18 9

FISCHER ESTERIFICATION Esters, one of the common derivatives of carboxylic acids, also constitute an important functional group family on their own. Volatile esters are responsible for the flavor and fragrance of many fruits and flowers, and artificial flavors and perfumes are frequently mixtures of esters combined to reproduce the flavor and aroma of the natural extracts. Examples of simple esters with familiar odors are propyl acetate (pears), isoarnyl acetate (bananas), octyi acetate (oranges), isobutyl propionate (rum), ethyl butyrate (pineapples), and ethyl 4•bromobutanoate (apples). Many simple esters are produced commercially on a large scale as solvents, plasticizers, and starting materials for polymers, pharmaceuticals, etc. This experiment illustrates the most common method used to prepare simple esters, the fischer esterification, is named for its discoverer, the 1902 Nobel laureate Emil Fischer. Another synthesis of esters is presented in Experiment 19. An ester is formally the condensation product of an alcohol and a carboxylic acid. In the Fischer method an ester is prepared directly from those two reactants with the aid of an acid catalyst:

+ 1420

carboxylic acid

alcohol

ester

The fischer esterification method and its mechanism are discussed in your textbook, so be sure to read these sections as background for the experiment.

Equilibrium

1

9 As indicated in the equation above, the Fischer ester synthesis is reversible, and an equilibrin b soon reached between the reactants (carboxylic acid and alcohol) and products (ester and wat7). Remember that a catalyst increases the rate of a reaction but does not change the equilibrium The catalyst, usually concentrated H2S04 or H3P04 or dry HCI gas, speeds the rate at whidt attained but does not change the position of equilibrium. The classical ways in which one equilibrium are to (1) increase the concentration of one or more reactants or (2) renm products from the equilibrium. Some esterification procedures increase the yield water from the reaction mixture as it is formed, either by azeotropic distillatlm oc use d a agent. If the alcohol is methanol or ethanol, or if the acid is acetic add or vopamb told, eat18foctory

E18-1

of ester can be obtained in practice by simply using a considerable excess of the inexpensive low-

mobcular weight alcohol or acid (Le Chåtelier's principle).

O

Workup At the end of the reaction period, the ester must be separated from water, the add catalyst; and any unreacted carboxylic acid and alcohol. The first step in this separation Is oftento diluterthe' reaction mixture with water and separate the layers; most of any remaining-exæss add, being water-soluble, is removed in the water layer along With the mineral acid cät%. Washingthe ester layer with an aqueous solution of a weak base such as sodium bibarbonate thendtaws- all remaining acids into the aqueous base layer. Washing with brine (saturated Naa solution) water from the organic phase, and the last traces of water are then removed by using a solid drying æent. A final distillation separates the ester from any remaining alcohOl.

Att% one hour of reflux, cool the flask to room temperature, transfer the contents to a separatory tumd, add mL of water, and shake gently. Remove the lower aqueous layer, add more water (25 mL) to t?' wparatory tunnel, shake, and again remove the lower aqueous layer.

Next wash the ester layer with three successive 20-mL portions of 5% NaHC03 solution. Shake the fumet-carefully and vent it frequently since carbon dioxide gas is formed in this neutralization. Test the jut extractwithlitmus or pH paper to make sure that it is still basic. Wash the ester layer with two 5mL pollon•of brine, Pour the ester into an Erlenmeyer flask and dry it over anhydrous sodium sulfate or magnesium sulfate for 15-20 minutes. Decant the ester into a 50-mL round bottom flask and carry out a simple distillation, collecting the traction that boils between 135-143 oc. Record the weight and boiling range and calculate the yield. Observe the odor. Tum in your distilled ester to your instructor.

Experimental Procedure (microscale) Weigh (tare) an empty 5-mL conical vial and record its weight. Place isoamyl alcohol in the vial using an automatic pipet. Reweigh the viat containing the alcohol and subtract the tare weight to obtain an accurate weight for the alcohol. Add '1.4 mL,mglaciabacetic •a" using an automatic pipet. Using a disposable Pasteur pipet, add 2-3 drops of concentrated sulfuric acid and add a small boiling chip. Assemble a reflux apparatus using a water-cooled condenser. Heating with a sand bath or aluminum block on a hot plate, bring the mixture to a boil (sand bath at 150-160 cc) and continue heating under reflux for 60-75 minutes. Remove the heat source and allow the reaction mixture to cool to room temperature. Disassemble the apparatus and remove the boiling stone with a forceps. Add 1.0 mL of 5% aqueous NaHC03to the mixture, using a calibrated Pasteur pipet, and stir the mixture with a microspatula until C02 evolution is no longer vigorous. Then cap the vial and shake gently, with venting, until the evolution ot gas is complete. Remove the bwer aqueous layer with a filter-tip pipet and discard it.

Repeat the bicarbonate wash two more times, using a fresh 1.0-mL portiomoféodium bicarbonate solution each time. 0 TrarEfer the ester to a dry conical vial using a pipet and add a small amount of anhydrous sodium sulfate to dry the ester. Allow to stand for 15 minutes with occasional swirling, then use a pipet to transfer the ester to a 3-mL conical vial. Distill the ester using a Hickman still and a water-cooled condenser. Distill until only a few drops of liquid remain in the distilling flask. Transfer the distillate to a tared vial. Determine the weight of the ester, calculate the percent yield, and determine the boiling point using the microscale boiling point method (seeTechnique T4.H).

Questions 1. Write a detailed mechanism for this Fischer esterification. 2. What is the IUPAC name of isoamyl alcohol? Of isoamyl acue? 3. Write an equation for the Fischer preparation of (a) benzyl isobutyrate (b) isobutyl benzoate

Alternative ester preparations Acetates of other simple alcohols can be prepared using essentially the same procedure, i! the instructor wishes to vary the experiment. The properties of a few representative examples with convenient boiling points are given below. Methyl and ethyl esters are easily prepared by a similar procedure, this time using the alcohol in excess. Borne convenient esters to prepare are listed below.

MethylALCOHOL pivalatepropanoate Methyl 1 -Propanol Ethyl pivalate Ethyl propanoate 1 -Butanol Isobutyl alcohol 1 -Pentanol

80 97

1 17 108 138

ACETATE Ethyl chloroacetate Methyl valerate n-Propyl acetate Ethyl valerate Methyl chloroacetate 145.5 130-132 n-Butyl acetate 126 Isobutyl acetate n-Amyl acetate 149

Esterification - Worksheet CHEM 21 OOL

Mass of 100 mL beaker

Name

64.94 g

Mass of vial and isoamyl alcohol

68.84 g

Mass of isoamyl alcohol

4 mL

Nfass of clean vial (for product)

Mass of vial and isoamyl acetate (product).

Mass of isoamyl acetate.

68.284

3.342 g

Percent yield of isoamyl acetate (Show calculation)

(3.342 g / 5.905 g) * 100 = 56% yield of isoamyl acetate

Boiling point of isoamyl acetate.

142 C

) Write out a balanced equation showing the acid catalyzed esterification of isoamyl alcohol with acetic acid to give

isoamyl acetate.

+ ) Draw structures for the following:

a. benzyl propanoate



b. n-propyl benzoate

c. n-butyl hexanoate

) What alcohol and what acid would be used to form the esters in question 2 (give names and structures)?

a. benzyl alcohol with propionic acid

+ b. esterification of benzoic acid with propanol

+ c. esterification of hexanoic acid with n-butyl alcohol

+ ) How would your esterification be effected if 2 mL of water were added to the vial prior to heating? Provide an

explanation for your statement. This would result in the esterification of isoamyl acetate to be smaller than it already is. The isoamyl acetate would effectively be diluted by the extra 1 mL of water....