Experiment 6Turning up the Heat: Preparation of Isopentyl Acetate Using a Microwave Reactor with Gas Chromatographic Analysis
Robert Aslanian, Ph.D.Department of Chemistry
New Jersey City University
Introduction
For decades the microwave oven has been an indispensable method for preparing food in kitchens and dorm rooms. The genesis of using microwaves to heat food can be traced back to 1946 when Percy Spencer, then working on radar for the Raytheon Corporation, noticed that a candy bar in his pocket had melted when he stood near a microwave generator. He traced the source of heat back to the microwaves. As is often the case in science, this serendipitous observation led to an entire new way of doing things, in this case cooking food. Knowing a good thing when they saw it, Raytheon introduced the first commercial microwave oven, the Radarange, in 1967. It was much later on, in the mid 1980s, that chemists first began to examine the use of microwaves to heat reactions instead of using traditional heating techniques. After a slow start, the field has subsequently expanded greatly. Technology has improved from using home kitchen microwave ovens to fit–for–purpose instruments that can accommodate single samples to instruments that can contain multiple samples.How do microwaves produce heating? To understand this, one needs to consider both the nature of electromagnetic radiation and the nature of polar molecules. Microwaves are electromagnetic energy that consist of electric and magnetic fields oscillating at right angles to each other (Figure 1). It is the electric component of electromagnetic radiation that is important for microwave heating.
Figure 1Diagram from Clean, Fast Organic Chemistry: Microwave–assisted Laboratory ExperimentsSo what is it about the electric component of microwave energy that causes heating?The answer lies in dipolar polarization. Recall from your study of general and organic chemistry that some molecules, like water, possess a dipole moment:When molecules that possess a dipole moment are exposed to the electric field of microwave energy, the molecular dipole tries to align itself with the field. However, since the field is oscillating from plus to minus, the dipole also oscillates trying to align its positively and negatively charged poles (Figure 2). The resulting motion of the molecule causes heat by friction.Figure 2HOHNetDipole
Diagram from Clean, Fast Organic Chemistry: Microwave–assisted Laboratory ExperimentsCompared with typical thermal heating, microwave heating is more efficient, more rapid and results in shorter reaction times. For example, in a typical Fisher esterification reaction using thermal heating that we would perform in organic lab, we would reflux the reaction for one hour. In the experiment described below, the reaction is performed in eighteen minutes!Fisher EsterificationThe ester functional group is derived from the combination of a carboxylic acid and an alcohol with the loss of water. Low molecular weight esters are pleasant smelling molecules and are often used as flavors and fragrances. There are many ways to prepare esters and one of the more common methods is the Fisher Esterification.In the Fisher Esterification, a carboxylic acid is reacted with an alcohol in the presence of a strong acid like sulfuric acid. The reaction is run at reflux temperatures and water is lost during the reaction:The reaction is an equilibrium process so in order to drive the equilibrium toward the ester, an excess of one of the reagents, usually the alcohol is added. Therefore, the Fisher esterification is ROHOCarboxylic Acid+R‘OHH+ROR‘OAlcoholEster-H2O particularly useful for common alcohols like methanol and ethanol. Alternatively, one can remove the water from the reaction as it is formed to drive the equilibrium. We will utilize both these approaches.In today’s experiment, we will react isopentyl alcohol with acetic acid to produce isopentyl acetate. Isopentyl acetate smells like bananas:However, instead of using conventional heating, we will use microwave energy to promote the reaction and we will use an acidic resin, Dowex–50,in place of the mineral acid to catalyze the process. Gas Chromatographic Analysis of the Reaction MixtureIn a perfect world, when a chemist performed a reaction, it would go to 100% completion in a known amount of time. Unfortunately, it isn’t a perfect world! Therefore, chemists need to have a method for determining if their reactions are complete. There are many ways to do this including thin layer chromatography (TLC), gas chromatography (GC) and High Performance Liquid Chromatography (HPLC). Each method has its advantages and disadvantages. As part of this experiment, you will utilize gas chromatography to determine if the reaction has gone to completion.ObjectivesIn this lab experiment, you will•Conduct a Fisher esterification using microwave energy to promote the reaction.•Utilize the mini–GC to monitor the extent of the reaction by determining the retention times of the starting materials, product and reaction mixture.EquipmentVernier Mini–GCCEM Mars 6 MicrowaveH3COHOH3CCH3OH+H+-H2OH3CCH3OCH3OIsopentyl Acetate
Procedure Reagent MWmmol Mass(g)Density(g/mL)Volume (mL)Glacial Acetic Acid60704.21.04943–Methyl–1–butanol882320.8092.5Dowex–500.3Silica Gel0.4Safety: Diethyl ether and 3–methyl–1–butanol are flammable and should not be used near sources of ignition. Glacial acetic acid can cause burns.Glacial acetic acid (4.0 mL, 70 mmol), 3–methyl–1–butanol (2.5 mL, 23 mmol), Dowex–50 (0.3 g) and silica gel (0.4 g) are added to a microwave vessel. A stir bar is added. The vessel is fitted with a vessel top and disk and a torque wrench is used to tighten the top in place. The vessel is placed in the microwave carousel, making note of the position. The carousel is placed in the microwave and heated using the programed conditions (ramp to 130°C over six minutes, hold at 130°C for 12 minutes). Do not remove your sample until the microwave vessel has cooled sufficiently.While the microwave is running, set up the Mini–GC and run the reaction standards (acetic acid, 3–Methyl–1–butanol and a known sample of isopentyl acetate):1.Turn on the GC and connect the USB cable to the computer.2.Turn on the computer and launch Logger–Pro.3.Tap the Collect arrow (u) and input the following parameters:Start Temperature40°CHold Time1 minRamp Rate5°C/minFinal Temperature70°CHold Time6 min
Total Length13 minPressure15 kPaSelect done and allow the Mini GC to warm up to the proper conditions. When it is ready for an injection, the dialog box will say “Inject and select Collect simultaneously”. The LED will turn green.While the GC is warming up, clean the syringe by flushing it several times with methanol as demonstrated by your instructor. Do not pull the syringe plunger out past the 0.5 μL mark or you risk damaging it.Once the LED has turned green, fill the syringe with 0.2 μL of one of the standards. Insert the needle through the septum on the Mini–GC. Simultaneously press the syringe plunger and “Collect” on the computer. The Mini–GC will start the run and automatically stop when the time is reached. When the run is complete, analyze the chromatogram. Choose “Peak Integration” from the Analyze Menu. Select the peak bystarting fromthe left side of the peakand dragging across the peakusing your finger. Record the retention time and peak area. Perform the same analysis with the other standards.At this point, the reaction should be cooled sufficiently to work up. The solids should have settled to the bottom of the reaction vessel. Remove 0.2 μL of the crude reaction mixture with the GC syringe and injectit into the mini GC. Decant the remaining liquid into a separatory funnel and dilute with 8 mL of ether. Gently rinse the reaction vessel with 2 mL of ether being careful not to disturb the solid, and transfer it to the separatory funnel. Carefully wash the ether sequentially with saturated sodium bicarbonate two times (10 mL X 2) and water(10 mL).Be careful when adding the sodium bicarbonate as CO2gas will vigorously evolve. Pour the ether layer into a clean Erlenmeyer flask and dry with sodium sulfate. Decant the ether off the sodium sulfate into a pre–weighed flask and evaporate it as directed by your instructor. Obtain the weight of product and determine your percent yield. Inject a 0.2 μL sample of your isopentyl acetate into the Mini–GC. Analyzethe results of you GC injections.
Questions1.
The three standards have different retention times on the Mini–GC. What physical characteristics of the molecules do you think contributes to this?
2.What role does the acid play in the Fisher esterification?
3.What role does the silica gel play in the reaction?
