Joseph K. Myers
2-17-2004
In simple distillation, there must be a boiling point difference of at least 40-50 degrees Celsius between components of a mixture. The relatively pure extract of the more volatile substance may be obtained as a distillate from the less volatile substances.
In fractional distillation, the vapors will contain elements of some of each of the volatile compounds. The total vapor pressure is equal to the sum of the partial vapor pressures of each compound. The preferred method to distill such constiuents from each other is to use a fractional distillation column apparatus. All of the vapors are passed through a long column, with the a temperature difference between the upper end of the column (lower temperature) and the lower end of the column (higher temperature). The vapors continue to circulate among each other, repeatedly condensing and revaporizing. Thus, the equivalent of performing a number of simple distillations is performed within the column. Vapors at each step become increasingly richer in the more volatile component; the condensate flowing down correspondingly becomes increasingly richer in the less volatile component.
In this particular experiment, this theory will be applied to enable the distillation of individual and combined organic components of cyclohexane or cyclohexane (C6H12) and toluene (C7H8).
None.
| Compound | MW | mp(C) | bp | fp | d(g/cm3) |
|---|---|---|---|---|---|
| toluene | 92.139 | -94.95 | 110.63 | ? | 0.8668 |
| cyclohexane | 84.159 | 6.59 | 80.73 | ? | 0.7739 |
Set up. 10 mL of cyclohexane in round-bottom flask.
Add spinvane to flask for smooth boiling.
Assemble distillation apparatus. (Position the thermometer in the stillhead so top of thermometer bulb is level with the bottom of the sidearm of the distillation head.
Begin heating stillpot (without flame).
Regulate heat as soon as boiling begins, to obtain a rate of 2-4 drops/sec.
Record temperature as soon as the distillation rate is adjusted and the head temperature is constant. Periodically record temperature as distillation proceeds.
Discontinue when only 2-3 mL of impure cyclohexane remains.
Record volume of distilled cyclohexane.
Set up. 10 mL of cyclohexane and 20 mL of toluene in round-bottom flask w/ spinvane. 3 clean and dry 25-mL containers, labeled A, B, and C.
Equip it for fractional distillation.
Pack distillation column.
Insulate fractionating column by wrapping with glasswool.
Top of mercury thermometer bulb should be level with bottom of sidearm of distillation head.
Place receiver A so the tip of vacuum adapter extends inside the neck of the container.
Begin heating stillpot.
Distillation begins at 81 degrees C.
Regulate heat to 1 drop/1-2 sec.
As soon as temperature deviates from 81 by more than + or - 3 degrees C, change to receiver B and increase heat. More liquid will distill.
Leave receiver B in place until 110 C, and change to receiver C.
Continue now until 1-2 mL of liquid remains in pot.
Record volumes of the distillate collected in each receiver by using a graduated cylinder. Allow the liquid in the column to drain into the distillation flask, then record volume of the pot residue.
Save samples of each fraction A, B, and C.
The results for this experiment are given in [1].
The pot residue was 5.62 mL after distillation. The sample of toluene was probably inadequate--more toluene could have been purified from the discarded residue.
The temperature was too low when the final toluene fraction was started. There was still too much cyclohexane evaporating at 90 degrees Celsius, due to the poor quality of the fractional distillation apparatus in terms of theoretical plates. Beads were too few and the column was too short.
There were four fractions taken in all.
The first drip occurred at 41 degrees C. Temperature went up to 50, and then without interference in the heat supply, cooled down to 47. The fraction was removed and placed into a sample bottle for fraction #1, containing .62 mL. The composition of this fraction was cyclohexane as well as acetone residue.
The second fraction was made up of the product as distillation continued. The temperature went on falling until it reached 40 degrees C. Then it began to increase after .3 mL of product had distilled since the beginning of the second fraction. It reached 60, and stabilized at 72. When it dropped to 70, 2.43 mL of sample was saved to make fraction #2.
The third fraction began with a continued temperature drop to 68 degrees C. It proceeded to increase to 75. Then when it went to 90, 5.61 mL was harvested of the distillation product into fraction #3.
The fourth fraction began at 90 degrees C, and concluded at 100, after a progressive increase in temperature. 2.80 mL was taken into fraction #4.
1. Myers JK. Gas Chromatography. 2004. Second Edition. [Results].