CHEM106 Benzilic acid Synthesis - CHEMEDU Home

[Pages:2]Chamras

Glendale Community College

Chemistry 106 Laboratory Experiment: Multi-Step Synthesis of Benzilic Acid

Part 3: Synthesis and Purification of Benzilic Acid

For your pre-laboratory protocol: Learn and write the mechanism for this reaction.

Procedure:

a) Synthesis: Mass approximately 2.00 g of pure benzil and in a 50mL Kjeldahl flask. Add 6 mL of 95% ethanol and two pieces of Teflon boiling chips. In the fume hood, assemble a reflux apparatus by connecting a water-jacketed condenser and a heating mantle of the appropriate size. Since the reaction will be carried out at high temperature and basic condition, you will need to apply silicon grease to the male joint of the condenser before assembling it with the reaction flask, in order to avoid fusing the glassware pieces during the reaction. To grease the junction, simply take a small amount (pea size) of the grease onto a wooden applicator, and transfer it as a straight vertical line along the side of the ground glass joint on the condenser. Begin heating the mixture. As soon as all of the solids have dissolved, add 5 mL of potassium hydroxide solution to the reaction mixture dropwise. Bring the reaction mixture to a gentle boil and keep it boiling for 15 minutes. If you notice solids formed but pushed up on the walls of the flask, and are separated from the reaction mixture, gently shake the apparatus to mix them in. Carefully record your observations of the color changes during the heating period. After the 15-minute period, remove the heat source while running the water through the condenser, and allow the reaction mixture to cool down to a warm, but comfortable to touch temperature.

b) Purification and Isolation:

Disassemble the apparatus, and transfer the warm reaction mixture into a beaker. Cool the beaker and its contents in an ice bath to further enhance crystallization. The crystals are potassium benzilate, not benzilic acid. Why?

Once the mixture has cooled in the ice bath, separate the crystals via vacuum filtration. Wash with four 3mL portions of cold 95% ethanol. This step is expected to remove most of the color from the crystals. Transfer the solids, to a 100mL Erlenmeyer flask containing 55 mL of 70?C water. Stir the mixture until all of the solids have dissolved or until it appears that the remaining solid will not dissolve. Any remaining solid will likely form a fine suspension. If solid does remain in the flask, gravity filter the hot solution through fast fluted filter paper until the filtrate is clear. Dispose of the filter paper by placing it in dry waste container. Place a magnetic stir bar in the clear filtrate, and add 1.5mL of12M hydrochloric acid in dropwise fashion, while stirring on a stir plate. At this point, crystals of crude benzilic acid will start to form. Monitor the pH using universal pH paper, and continue adding the acid down to pH of 2. At this pH, you will obtain the largest amount of crude benzilic acid crystals. Cool the crystallized mixture in an ice bath, and then vacuum filter to separate. Wash the crude benzilic acid crystals with eight 5mL portions of cold deionized water. This is to remove potassium chloride salts from the benzilic acid matrix.

Recrystallize using boiling hot water. Filter, separate, and allow to dry until next laboratory period.

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For your report: 1. Report the % Yield 2. Measure the melting point 3. Obtain the IR spectrum 4. Predict the 1H-NMR spectrum.

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