Organic Chemistry Laboratory I - Chem 341

Quiz 4e

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

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

RF = distance compound travels/distance solvent travels

So for A, 2/10 = 0.2

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

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

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. Define the term “eluate” as it pertains to chromatography. (1 pt)

The eluate is the compound that is actually moving through the chromatography system. This may be what you have already collected, or the band that is moving down the column.

3. In a distillation the end of the thermometer is placed at the head of the distillation apparatus and should be adjacent to the exit opening to the condenser. Explain the effect on the observed temperature reading if the bulb is placed above the opening of the condenser (i.e. closer to the stillpot [this should have said further from the stillpot]). (2 pts)

The observed temperature will be lower because the bulb is not in contact with the vapor entering the condenser. The vapor reaching the thermometer will have a lower concentration of the high boiling fraction. In addition, there may not be enough vapor condensing to get an accurate temperature reading.

We will also accept higher if the argument for “closer to the stillpot” is valid.

4. 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.

5. At 100 ^oC, the vapor pressures for water, methanol, and ethanol are 760, 2625, and 1694 torr, respectively. Which compound has the lowest normal boiling point? (1 pt)

High vapor pressure means that a compound easily goes into the gas phase. This results in a lower boiling point. Since methanol has the highest vapor pressure, it will have the lowest boiling point.

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.

Or

Increase the carrier gas flow