Chemistry Research

Organic Chemistry experiments often require over two hours of heating on a hotplate. Laboratory periods are normally scheduled for three hours. It is often impossible for students to complete the experiment in the allotted time with conventional heating on a hotplate. The goal of this project was to find at least one experiment that requires a long heating period, and perform the experiment using a microwave. The experiment chosen (second attempted) was Copper (II) Ion Oxidatiten of Benzoin: Benzil taken from Microscale Organic Laboratory by Mayo, Pike, and Trumper, pages 432-433. The experiment was performed as written with heating on a hotplate for one hour at 150°C, which typically results in a 70.0% percent yield of Benzil. The experiment was then performed using a microwave for different amounts of time to determine the optimal heating time required to give the highest percent yield. The highest percent yield, 97.7%, was obtained with a microwave heating time of ten minutes.

The experiment Preparation of an Enol Acetate: Cholesta-3,5-dien-3-ol Acetate was performed according to the method on pages 414-417 of Microscale Organic Laboratory by Mayo, Pike, and Trumper [1]. 96 mg of cholesta-4-en-3-one was placed in a conical vial, and 1.0 ml acetic anhydride and 200 microliters of chlorotrimethylsilane were added. The reaction mixture was heated for 1.5 hours. The experiment was attempted three times with traditional heating on a hotplate. The products of the experiments were tested by thin-layer chromatography (TLC), and it was determined that the products were the same as the reactants each time, so it did not work. The experiment was then attempted in the microwave with the same amounts of reagents and it again did not react. At this point, it was thought that one of the ingredients could be old.

The second experiment Copper (II) Ion Oxidation of Benzoin: Benzil was performed according to the method by Mayo, Pike, and Trumper on pages 431-433 of Microscale Organic Laboratory [2]. 400 mg of benzoin and 1.4 ml of cupric acetate catalyst solution were put into a 5.0 ml conical vial. The reaction mixture was heated for one hour on a hotplate. A 61% yield of Benzil was obtained. An IR was performed on the product to verify that the product was benzil. The experiment was then performed in the microwave by putting 400 mg of benzoin and 1.4 ml cupric acetate catalyst solution into a microwave vessel. The vessel was then put into the microwave and attached to the temperature probe and pressure release valve. The computer was set to run the microwave with a ramp time of 2 minutes to 150°C, and a hold time at 150°C of 5 minutes. This resulted in an 89.9% yield of benzil, which was 98% pure according to the IR. The experiment was then performed following the same procedure in the microwave with a ramp time of 2 minutes, and hold times of 0, 2, and 10 minutes. The results were as follows, and are graphed in figure 1 on page 5.

What would happen beyond ten minutes was then questioned. It was possible that beyond 10 minutes of heating, the product would begin to decompose, thus the yield and purity would decrease. Whether or not ten minutes truly produced the maximum yield needed to be determined. The experiment was then performed in the microwave keeping all of the conditions the same except the holding time, which was changed to fifteen minutes. A 91.4% yield of benzil (98.8% purity) was obtained. The graph of this data is in figure 2 on page 5.

This graph resulted in questioning why the % yield went down with increased heating time. Did the % yield truly decrease with increased heating time? What if the maximum yield was actually below ten minutes? What about other times between 5 and 15 minutes? It was then decided to do the experiment with hold times of 7.5, 12.5, and 17.5 minutes to determine the true shape of the curve, and see what happened beyond 15 minutes. The results are as follows, and are graphed in figure 3 on page 6.

This graph did answer the questions asked so far. The % yield does decrease with increased heating, which makes sense, because some reactants as well as the product most likely decomposed with increased heating. The maximum % yield was obtained with a hold time of ten minutes.

The percent purity of benzil increases with increased heating. A graph of % purity versus time is in figure 4 on page 6. Although the increase in % purity is not statistically significant, it does gradually increase with the increased holding time. This result shows that no decomposition was taking place.

The maximum % yield of 97.7% was obtained with a holding time of ten minutes. This is much greater than the % yield of 61% obtained with heating on the hotplate, and with typical student % yields of 70%. The microwave did greatly reduce the reaction time (1 hour of heating by conventional hotplate heating decreased to 10 minutes of heating), and resulted in a higher yield than the conventional heating method.

Future experimentation may include performing more experiments at holding times between 7.5 and 12.5 minutes, to see if the maximum yield could be produced at a time other than ten minutes. The goal of this experiment has been achieved; a faster method for the copper (II) ion oxidation of benzoin to benzil was determined.

1. Mayo, D. W., R. M. Pike, and P. K. Trumper. Microscale Organic Laboratory, 4th ed., pp. 414-417. John Wiley & Sons, Inc. 2000.

2. Mayo, D. W., R. M. Pike, and P. K. Trumper. Microscale Organic Laboratory, 4th ed., pp. 431-433. John Wiley & Sons, Inc. 2000.

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Last modified: 9 January 2003. SEK.