Organic Chemistry II  | Lecture | Laboratory

Organic Chemistry Laboratory II
Synthesis of Banana Oil (Esterification)
Experimental Procedure


Introduction
In this experiment, students will work in pairs to generate isoamyl (isopentyl) acetate, the ester responsible for the scent of bananas, from isopentyl alcohol and acetic acid. The ester product is generated  from a carboxylic acid and an alcohol, but the reaction must be catalyzed to to increase the rate of the reaction. The preparation of isoamyl acetate will use concentrated sulfuric acid as a catalyst.  The scheme for this reaction is outlined in Figure 1 below.


Procedure
Synthesizing the Ester
Clamp a 100ml round-bottomed flask to a ring stand and place the reaction flask into a snug-fitting, 100ml  heating mantle.  Position the reaction set-up on the side of the hood with the faucet labeled "distillation set-up".  Set the heating mantle on a stirrer/hotplate.  Using a funnel, dispense 15ml of isopentyl alcohol and 20 ml of glacial acetic acid into a the  flask (Figure 2). Slowly add 4 ml concentrated sulfuric acid, add a magnetic stir bar and gently begin to stir the reaction mixture by turning on the magnetic stirrer. Connect the heating mantle to a Variac.  Insert a condenser tube into the reaction flask and attach the water hoses (in at the bottom, out at the top) (Figure 3). Insert a drying tube into the top of the reflux apparatus.
 

 
 


 
 

Figure 2:  Using a powder funnel to add reagents to the flask

Figure 3:  Reflux apparatus

Plug the Variac into the wall outlet and set the dial to between 50 and 60V. Heat the reaction mixture to reflux (i.e, until the contents is boiling and the vapors condense and drip back into the flask) for one hour.  While the mixture is heating, dispense 100ml of deionized water into a clean 200ml Erlenmeyer flask, clamp it to a second ring stand and cool it in an ice bath.

Crude Purification of the Reaction Mixture in a Separatory Funnel
Allow the reaction mixture to cool to room temperature. Pour the cooled mixture into a 125 ml separatory funnel set in a ring, clamped to a ring stand. Check that the stop cock on the funnel is in the closed position and add 55 ml of the cold deionized water to the funnel.  Add 10ml of the cold deionized water to the reaction flask, swirl and add the water to the separatory funnel.  Stopper the funnel, invert and shake the mixture for 25-30 seconds.  Dispense the lower (aqueous) layer into a 200 ml beaker, labeled "aqueous layer".  The organic layer containing the ester product will remain in the separatory funnel.

To remove any leftover acetic acid from the ester solution (organic layer), wash the organic layer by adding 25ml 5% aqueous sodium bicarbonate to the separatory funnel.  Stopper the funnel, invert and shake for 25-20 seconds, being sure to vent the funnel frequently by opening the stopcock. Drain the lower, aqueous layer into the flask labeled “aqueous layer”. Wash again with a second 25 ml aliquot of 5% sodium bicarbonate. Check the pH of the “water layer” flask. If the solution is basic, move on to the next step. If the solution is acidic, wash one more time with the 5% sodium bicarbonate. Add 25ml of saturated NaCl solution to the separatory funnel.  Stopper the funnel, invert and shake the mixture for 25-30 seconds.  Dispense the lower (aqueous) layer into a 125 ml flask, labeled "aqueous layer".  The organic layer containing the ester product will remain in the separatory funnel.

Drain  the organic layer remaining in the funnel into a clean, dry 125 ml Erlenmeyer flask. Add ~2g MgSO4 to the flask and cover the flask with Parafilm. Swirl gently until the liquid is clear. If the cloudiness persists, add an additional 0.5g MgSO4. Decant the solution into a clean, dry Erlenmeyer flask, cover with Parafilm and keep the ester solution in the lab drawer until week 2.

Weight and Percent Yield
Inspect your liquid product, weigh it and calculate the percent yield. 

Boiling Point Determination of Isopentyl Acetate
Calibrate a thermometer in ice and boiling water. Using the calibration values, adjust the experimental boiling point of the isopentyl acetate product.  Record the calibrated boiling point of the product.

IR Spectroscopic Analysis of Isopentyl Acetate
Place 2 drops of the isopentyl acetate product between two salt plates (Figure 5) and use the IR spectrophotometer to generate an IR spectrum. Place two drops of the known isoamyl acetate product between two salt plates and use the IR spectrophotometer to generate an IR spectrum. Run a background scan if one has not yet been for the lab. Compare the spectrum of the experimental product to the spectrum of the known isopentyl  acetate. Record all the major peaks in the spectrum.

Waste Disposal
Dispose of the water layer in the basic waste container. Used Parafilm, MgSO4 and organic solids should be disposed of in the solid waste container.