Organic Chemistry Laboratory I - Chem 341

Quiz 4c

 

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 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 first in the column chromatography separation last week.  (1 pt)

 

         

Fluorene was the least polar compound, therefore it travels more rapidly down the column.

       

                    

Fluorene 

 

 

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. Would a simple or fractional distillation work better for separating benzene (b.p.~80ūC) from toluene (b.p.~111ūC)?  (1 pt)

 

 

Fractional distillation since the b.p. for these compounds is fairly close.  The fractional has more theoretical plates and is therefore more efficient.

 

 

5. Consider the temperature-composition diagram for a mixture of acetone and ethyl acetate, and estimate the composition of the vapor at 60 ūC.  (2 pts)

 

 

at 60 oC the vapor is about 96% acetone and 4% 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.

 

                  Or

 

Decrease the carrier gas flow rate