Question

In the lab manual you are required to keep a 2 – 3 mm layer of acetone and then add water. A student vaporized the acetone absolutely and state that he can use the residue for the later measurement. What is the biggest defect of doing this comparing to the correct procedure? How the calculated yield and measured boiling range will be affected?

Experiment 4: Extraction of an Antibiotic Introduction The extraction of compounds from plant and animal sources is vital direction used by the pharmaceutical industry in search of new medications. Often the natural products uncovered have undesirable side-effects or lower than desired efficacy. The modification of the structures of natural products can lead to improved pharmokinetics or improved specificity. Extractions are performed by many of us on a daily basis as we brew coffees and teas. Essentially we are using hot water to extract the tasty water soluble esters and flavors from the solids (and some caffeine). Lichens are a symbiotic relationship between a fungus and an algae. The fungi were originally considered be only useful in the absorption of water, to the point they were considered almost parasitic. However, it has become clear that the fungus is responsible for the generation of a very important chemical defense agent, usnic acid. Several lichens native to New England have been shown to have antibiotic properties and usnic acid has been shown to be the agent responsible for this biological activity. Lichens have been used medicinally for thousands of years. There is evidence the Egyptians viewed lichens as medicines. Usnic acid has been found to useful as an antibiotic with activity against several Gram-positive bacteria including skin infections and tuberculosis. There have been reports that usnic acid can cause liver problems.

Experimental Procedure 1. Weigh 4 grams of lichen and place in a 150 ml beaker. 2. Add 50ml of acetone to the 150ml beaker. 3. Stir and crush the lichen in the acetone with a stir-rod or spatula for 30 minutes. 4. Remove the pieces of lichen from the acetone by filtering through a glass funnel with a folded filter paper (15cm) into a 125ml Erlenmeyer flask. 5. Clean the 125ml beaker and transfer the contents of the Erlenmeyer flask. 6. Add two boiling sticks and place the beaker on a hotplate (in the hood) and warm to boiling (setting #4 is usually sufficient). 6. Evaporate almost all the acetone. A layer approximately 2-3mm deep at the bottom of the beaker is all that is required. Some solid may be noted at the meniscus. 22 7. Remove the beaker from the heat and remove the boiling sticks. Return to your bench area. 8. Add 2-4 drops of water until a slightly cloudiness is noted. 9. After the flask has cooled on the bench-top for approximately 3 minutes, filter the small yellow crystals using a Buchner funnel, a vacuum flask (125ml) and an aspirator to apply a vacuum. Use the illustration on page 24 as a guide. 10. Weigh the crystals and calculate a percentage of the lichens that is usnic acid. Note the melting point of the crystals. Results and Calculations. 1. Yield should be noted by weighing the crystals formed and comparing to the initial weight of the lichens. 2. Melting range of the usnic acid should be noted. Waste Management The usnic acid generated in this experiment is not regulated as hazardous waste. Acetone is regulated by the EPA as hazardous wastes because they are ignitable below 140 deg F. 1. The filter paper with ground lichens, from Step 4 of the experiment, is non-hazardous and can be disposed of in the trashcan. 2. The filtrate from step 9 of the experiment contains acetone and is flammable. Dispose of as hazardous waste in the waste container labeled with the respective chemical contents. 4. The usnic acid crystals are non-hazardous so can discarded into normal trash. 5. The vacuum flask used for filtration can be washed with soap and water to remove residue. Any acetone used to rinse should be discarded of as hazardous waste.

Answer 1

When you treat lichen in acetone all of the organic compounds present in the lichen will be soluble in acetone. Lichen contains not only Usnic acid but also many other organic compounds as minor product. At the end when there are 2-3 mm of acetone that acetone contains Usnic acid along with other minor products. The solubity of Usnic acid in acetone is good but in water its not soluble. So when you will add water in this acetone solution then Usnic acid will be crystalize whereas other oganic chemicals will not be crystalize because it has much higher solubity in acetone and water mixture. So once you will evaporate all the acetone then you will not able to get the nice sigle crystal of Usnic acid. You will get impure Usnic acid and as result the percentage yield will be different than the expected. Also the melting point of an impure compound showed a wronge melting point range usually very long melting point range than the usual (Pure compound has sharp meltic point range). So you will get yield and melting point both incorrect data. For structure of Usnic acid see below