Organic Chemistry Laboratory I

Organic Chemistry Laboratory I - Chem 341

Quiz 4a

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 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 more polar eluant (solvent) affect the RF value? (1 pt)

A more polar eluant would cause both compounds to move faster (i.e. further) since they would adhere less 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 increase for a more polar eluant and decrease for a less polar eluant.

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

The adsorbent is the stationary phase. Typically this is a polar material such as silica gel or alumina.

3. Why is it important to never let the solvent level fall below the alumina 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 vapor at 65 ˚C. (2 pts)

at 65 ^oC the vapor is about 82% acetone and 18% ethyl acetate.

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

Decrease the temperature of the oven.

Decrease the carrier gas flow rate