Preparation of Synthetic Banana Oil



Preparation of Synthetic Banana Oil

Introduction

In pharmaceutical terms, an oil is an odiferous principle found in various plant parts. While some oils consist mainly of one chemical substance, many oils contain dozens, if not hundreds, of chemicals. In this experiment, we will synthesize the major component that gives bananas their characteristic odor. We will use a standard procedure for reacting a carboxylic acid with an alcohol, to produce an ester, with water as a byproduct. The procedure involves heating the carboxylic acid with the alcohol in the presence of a strong acid catalyst: without the catalyst, no significant amount of product is produced.

Reaction Scheme

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Precautions

Sulfuric acid and acetic acid are corrosive, so don’t spill them. If you spill some on the counter, wipe it up with wet paper towels. If you spill them on yourselves, wash them off quickly with lots of water.

Experimental Procedure

Reaction

In your 100 mL round-bottomed, place 15 mL of 3-methyl-1-butanol (isoamyl alcohol or isopentyl alcohol), 15 mL of acetic acid, and 1 dropper-full of sulfuric acid. Add a small magnetic stirring bar, and set up a reflux apparatus. Have an instructor check your apparatus, turn on the water going through the condenser, and set the variable transformer to a setting to about 8. Allow the reaction to heat at reflux for at least an hour. Remove the thermowell, and allow the reaction flask to cool to close to room temperature. Remove the condenser, remove the magnetic stir bar, and cork the flask with a real cork, not a ground-glass stopper. Allow the corked flask to sit in your drawer until you do the next part of the experiment.

Reflux Apparatus

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Separation

Pour the reaction mixture into your separatory funnel, and add 50 ml of water to the separatory funnel. Shake and vent 3-4 times, then carefully drain off the lower water layer. Add 25 mL of sodium bicarbonate solution to the separatory funnel. Swirl the funnel until all of the bubbling stops, then carefully shake and vent 3-4 times, and then carefully drain off the lower aqueous layer. Add 25 mL of sodium bicarbonate solution to the separatory funnel. Swirl the funnel until all of the bubbling stops, then carefully shake and vent 3-4 times, and then carefully drain off the lower aqueous layer. Pour the ester layer out of the top of the separatory funnel into your clean dry 50 mL round-bottomed flask. Add two boiling stones to the flask.

Purification

Set up a simple distillation apparatus, and have it approved by an instructor. Collect everything that distills at less than 132º in a beaker, and discard it. Collect everything that distills between 132-143º in a preweighed product vial: record your actual boiling range. DO NOT DISTILL TO DRYNESS, however: leave a small amount in the flask. Reweigh the vial to determine the amount of ester you made. Label your vial as isoamyl acetate, the weight of product, and your names.

Simple Distillation Apparatus

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Analysis

Give the vial to your instructor for Gas Chromatography analysis. See the diagram below.

The column has an absorbent material, similar to the material on the TLC plate. After the sample is injected, it is vaporized, and the carrier gas pushes it into the column. The absorbent material binds the impurity least strongly, so it comes out of the column first. When the impurity hits the detector, a signal is sent to the recorder, and a peak is drawn. Isoamyl alcohol binds tighter to the absorbent material, so the peak for it comes out next. When the isoamyl acetate hits the detector, another signal is sent to the recorder, and another peak is drawn. The area of the peaks is proportional to the concentration of each substance in your sample, so you can determine what percent of your sample is isoamyl acetate.

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Sample Gas Chromatography Chromatogram for a Banana Oil Sample

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The chromatogram shows three peaks. The first peak took 0.804 minutes to come out of the GC, and is an impurity, probably water. The second peak is isoamyl alcohol. I know this because I injected a known sample of isoamyl alcohol into the GC, and found out how long it took to come out. The third peak that took 1.6 minutes to come out is isoamyl acetate. The table that prints out after the chromatogram lists the peaks in the order that they came out of the GC. The impurity peak in this sample was about 1.3% of the whole sample, the isoamyl alcohol peak was about 4.8 % of the sample, and isoamyl acetate peak was about 93.9 % of the sample.

Report Format for Preparation of Synthetic Banana Oil

Title Page

1. Descriptive Title with 10-20 words

2. Your Name

3. Your Partner’s name

4. Dates experiment was done

5. Course and Section Number

Data (Start a new page):

1. Weight of Empty Product vial

2. Weight of product vial and product

3. Weight of product

4. Odor of product

5. Percent ester in your product

Calculations: You must show your work! Label things and don’t forget your units.

1. Calculate the number of moles of isoamyl alcohol that you used.

2. Calculate the number of moles of acetic acid that you used.

3. What is the limiting reagent? Explain your reasoning.

4. What is the maximum number of grams of isoamyl acetate that you could have made? This is called the theoretical yield.

5. What was your percent yield?

Questions

1. After the reaction was over, you transferred the reaction mixture to your separatory funnel, and washed it with water. What did the water remove?

2. After the water wash, you washed the product with sodium bicarbonate solution. What two chemicals did the sodium bicarbonate washes remove? What gas was produced? Write balanced chemical equations for the reactions of these chemicals with sodium bicarbonate.

3. What combinations of carboxylic acid and alcohol would you react together to form each of the following esters?

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