Organic chemistry questions from 1-8

1.    Give reasons for each of the following experimental techniques used in recrystallization:

a.    Using a hot filtration.

b.    Letting the substance cool to room temperature before placing it in ice.

c.    Washing the crystals with ice cold water.

2.    If a mixture is distilled rapidly, the separation of its components is poorer than if the mixture is distilled slowly. Explain.

3.    Why must you never attempt a distillation on a closed apparatus?

4.    Why is caffeine an alkaloid?

5.    The protonated form of caffeine is very soluble in water. The deprotonated form is more soluble in nonpolar solvents. Brewed tea contains tannic acid. When the tannic acid is neutralized by adding a base, the caffeine becomes less soluble in water. If you added sodium carbonate to your tea solution before the extraction, how would that affect your yield?

6.    Calculate the Rf values for the following compounds.

a.    Spot, 5.0 cm; solvent front, 20.0 cm

b.    Spot, 3.0 cm; solvent front, 12.0 cm

c.    Spot, 9.8 cm; solvent front, 12.0 cm

7.    Explain the following statements:

a.    You should avoid making the spots too large when preparing the TLC plate for development.

b.    You should avoid making the spots too small when preparing the TLC plate for development.

c.    The spots must be above the level of the development solvent in the developing chamber.

8.    A highly polar compound is moving through a gravity chromatography column too slowly. What can the chromatographer do to increase its rate of movement?