Organic Chemistry Laboratory I

Organic Chemistry Laboratory I - Chem 341

Quiz 4b

1. Consider the following TLC plate and answer the questions below.

a) Calculate the RF value for compound B. (1 pt)

RF = distance compound travels/distance solvent travels

So for B, 6/10 = 0.6

b) Which compound is less polar, A or B? (1 pt)

Compound B is less polar because it travels further and spends less time on the stationary phase. In this case the silica gel is very polar, so A sticks more tightly since it is more polar than B.

c) How would a less polar eluant (solvent) affect the RF value? (1 pt)

A less polar eluant would cause both compounds to move slower (i.e. less distance) since they would adhere more tightly to the polar stationary phase. However, the solvent would move up the plate at the same rate regardless of the polarity of the solvent. Therefore, the RF values would decrease for a less polar eluant and increase for a more polar eluant.

2. Why should the developing chamber for a TLC plate not be open to the atmosphere? (1 pt)

If the cover is left off the developing chamber, then the solvent vapor in the chamber is not constant throughout. In fact, there would be very little solvent vapor at the top of the chamber. So when the solvent line of the TLC plate starts to reach the top of the plate (and top of the chamber), then the solvent will start to evaporate. So the solvent front will not move very much since it is evaporating. Meanwhile, the compound that are being eluted (the eluate) is still moving up the plate. Thus, it will appear that the eluate moves further than if should have, and the RF values will be too high.

3. Why is it important to never let the solvent level fall below the adsorbent level? (1 pt)

Cracks might develop because air would get into the column. Once cracks form they create channels that your eluates can pass through very rapidly. The net effect would be that your migratory bands would be very broad and may not separate completely.

4. Why should you never heat a closed distillation system? (1 pt)

When molecules of a liquid are heated, they will enter the gas phase, resulting in a greater pressure on the walls of the vessel. This increased vapor pressure could lead to an explosion.

5. Consider the temperature-composition diagram for a mixture of acetone and ethyl acetate, and estimate the composition of the liquid at 65 ˚C. (2 pts)

at 65 ^oC the liquid is about 20% acetone and 80% ethyl acetate.

6. Consider a gas chromatography experiment and describe one way in which you could decrease the retention time on the GC column. (2 pts)

Increase the temperature of the oven.

Increase the carrier gas flow rate