Organic Chemistry Laboratory I - Chem 341

Quiz 4d

 

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 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 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. Draw the structure of the compound that came off of the alumina column last in the column chromatography separation last week.  (1 pt)

 

         

Fluorenone was the most polar compound, therefore it travels more slowly down the column.

       

                  

                    

Fluorenone 

 

 

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 below the opening of the condenser (i.e. closer to the stillpot).  (2 pts)

 

 

The observed temperature will be higher than it should be because the bulb is too close to the stillpot.  Therefore, the vapor that is condensing has more of the high boiling component than further up towards the condenser. 

 

 

4. Would a simple or fractional distillation be more appropriate for separating drinking water from sea water?  (1 pt)

 

 

Simple distillation would be most appropriate.

 

 

5. Define the term ÒeluateÓ 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.

 

 

 

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