Chemistry 3112, Spring 1999. Experiment 4: Fractional Distillation

Name ___________________________ Date _________________ TA ___________________

Read pp 109-113 in Fessenden and Fessenden, Organic Laboratory Techniques, 2nd edition.

Pre-Lab

1. What is the purpose of the "packing" in a fractional distillation column?

 

 

 

 

2. What is "flooding" and how can it affect the separation achieved in a fractional distillation.

 

 

 

 

3. Why should a distillation flask never be more that 3/4 full before starting a distillation?

 

 

 

Procedure

1. Take the condenser having small glass indentations at the bottom, and carefully slide the Raschig rings into the condenser to create a fractionating column. Add 25 mL of hexane, 25 mL of toluene and 2-3 boiling stones to a 100-mL one-necked round-bottomed flask clamped to a ring stand and supported by a 100-mL thermowell heater elevated on a large metal ring. Assemble the remaining equipment as pictured on p 110 of your text, but with a 50 mL graduated cylinder as the receiver. Heat the flask (Variac setting about 55) noting the boiling point for every 2-3 mL of distillate. Adjust the heat to maintain a distillation rate of about 1 drop per 2-4 seconds. When finished pour the distillate and the liquid remaining in the distillation flask in the solvent waste can. Unpack the fractionating column and return the packing material to the TA.

volume collected (mL) bp (oC)

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

________________ ______________

 

2. Plot a graph of bp versus total volume of distillate in mL for the fractional distillation. Label the axes properly. Look up the boiling points of hexane and toluene in the CRC Handbook of Chemistry and Physics or the Aldrich Chemical Co. catalog (both available in the laboratory) and mark them on the y-axis.

Plot a graph of bp versus total volume of distillate for the simple distillation of Experiment 3 on the same graph. Label the two plots.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Observations

Record any important observations you make during the course of the distillation, such as temperature fluctuations, appearance of the vapor front as it moves through the apparatus, rate of distillation, hot spots, flooding of fractionation column, need for column insulation, size of column holdup, etc.

 

 

 

Questions

1. From the results of your graph discuss how well the fractionating column improved the separation of hexane from toluene. If you wanted to collect pure hexane and pure toluene, what volume from the middle of the distillation would you reject because it is a mixture?

 

 

 

 

 

 

 

 

2. Due to the larger apparatus, more heat is needed for a fractional distillation than for a simple distillation. This can sometimes cause problems with thermal decomposition of the compound. How might one overcome this problem?

 

 

 

 

3. Identify the cause of these possible disasters that could happen during distillation and tell what should be done about it.

a. The liquid in the flask boils over in one great eruption.

 

 

b. The liquid in the flask begins to smoke and turn brown as the temperature is increased.