Question

Based on solubility, density, and polarity information what will happen if a caffeine extraction and sublimation experiment is carried out at 100°C? (Note: show the outline, the experiment might not work perfectly in the actual lab, but a hypothetical outline is needed).

Procedure below:

1. Boil 20mL DI water. Place 1^st tea bag in beaker taken off heat and cool to handling temperature.
2. Squeeze tea from bag without breaking it. Boil the mixture. Repeat steps with 2^nd teabag.
3. Cool to room temp. Add 2g sodium carbonate ONLY when water is cool. Stir to dissolve.
4. Pour solution in centrifuge tube and cool in ice bath.
5. Add 2mL dichloromethane to tube and then cap. Invert several times and allow tube to vent occasionally.
6. Let sit and separate.
7. Pipette the lower, organic layer (the one with the caffeine) into a small flask. Repeat 5-7 2x more.
8. When you are sure all the dichloromethane is removed, flush tea/aqueous/top layer down the drain. Pour the pure extract into a centrifuge tube. Rinse the flask with dichloromethane and dump into the tube.
9. Add 4mL NaCl solution to tube and invert several times.
10. Allow to separate.
11. Pipette organic layer into new flask and add a small amount of sodium sulfate. Let sit for 5 min until dry.
12. Weigh a filter flask. Pipette dichloromethane extraction into the flask.
13. Rinse the sodium carbonate with dichloromethane and dump into the flask.
14. Evaporate solvent by gently heating in sand bath.
15. Yellow-green residue will form. Let cool and weigh.
16. Purify by sublimation.
    1. Use only ~20g
    2. Caffeine sublimation temp is ~180°C
    3. Collect pure caffeine and conduct MP experiment. Find % yield.

Answer 1

Ans:    Based on the described experimental procedure the caffeine will be extracted up to its crude solid form (step#15) only. But the purification by sublimation cannot happen at 100 ºC, as it has no appreciable vapor pressure to get sublimed.

Hence, the caffeine cannot be purified by sublimation at 100 °C.

            Alternate procedure, recrystallization can be attempted to purify it. Since, we know that the caffeine is soluble in both hot water and polar organic solvents and significantly less soluble in non-polar solvents, a combination of suitable solvent mixture would be very useful for its purification.

1. Take crude caffeine about 5 g in to a 50 mL beaker.

2. Add 20 mL of acetone and warm to ~ 40 °C (Acetone B.P. 56 °C) to dissolve the crude caffeine.

3. Transfer the solution to a 50 mL Erlenmeyer flask for recrystallization.

4. Add a few drops of petroleum ether until you reach the cloud point.

5. Caffeine is less soluble in this mixed solvent and is just beginning to precipitate.

6. Cool the solution.

7. Caffeine crystal separates out.

8. Filter, dry in air and then record the m.p. and yield as done in the mentioned lab experiment.