Esterification

Nucleophilic acyl substitution is one of the central reactions in organic chemistry. In this experiment you will synthesize an ester by acid-catalyzed nucleophilic substitution of a carboxylic acid by an alcohol. This synthetic procedure is also known as a Fischer esterification. An example of a Fischer esterification of benzoic acid using ethanol is shown below (the mechanism can be found on page 782 of McMurry).

Many small molecular weight esters (like the ones prepared in this experiment) have characteristic odors associated with fruits. You will be assigned an acid and an alcohol to use in this experiment.

Alcohols to use include: isopentyl alcohol (3-methyl-1-butanol), 1-propanol, isobutyl alcohol (2-methyl-1-propanol).

Carboxylic acids to use include: glacial acetic acid, propanoic acid.

Procedure

Use graduated pipets to measure 15 mmol of your assigned alcohol and 38 mmol of your assigned carboxylic acid into a 10-mL round-bottomed flask. Carefully, with swirling, add several drops of concentrated sulfuric acid to the flask. Also add a boiling stone.

Assemble a reflux apparatus using a water-cooled condenser and heat (using a heating mantle) the reaction mixture at reflux for 50-60 minutes. Remove the apparatus from the heating mantle, let it cool briefly, then put the flask in a beaker of water until the reaction mixture reaches room temperature.

Transfer the reaction mixture to a centrifuge tube and add 5 mL of water. Shake the mixture vigorously and discard the water layer. Wash the organic phase remaining in the conical vial with four 2.5-mL portions of saturated sodium bicarbonate solution. Vent the vial frequently by loosening the cap. After each washing, remove the lower aqueous phase. Wash the organic layer a last time with 2 mL of 4M sodium chloride solution. Remove the aqueous layer and add a small amount of anhydrous calcium chloride. Allow the product to dry for 10-15 minutes.

Transfer the dried product to a dry 5-mL conical vial containing a spin vane. Assemble the apparatus for microscale distillation, using an air condenser above the Hickman still head. Turn on the stirrer and heat the apparatus using an aluminum block. Place the distillate in a tared vial. Record the boiling point. Weigh your product and determine the percent yield.

Report

Your report should include a discussion of the identity of your ester, how pure your ester appears to be, what you could do to improve the purity and percent yield. Be sure to include your analysis of the NMR and IR of your product.