Introduction: In this experiment, a mixture of two compounds, cyclohexane and toluene, was separated into fractions by the techniques of simple and fractional distillation. The individual fractions that were gathered from the distillation were analyzed using gas chromatography (GC) and used to compare the efficiencies of the two different distillation techniques. The ultimate goal of this experiment was to determine whether simple or fractional distillation was the more efficient means of separating volatile compounds.
Key Experimental Details, Observations, and Results:
Key Experimental Details and Observations:
Simple and Fractional Distillation set up:
To set up distillation apparatus:
1.For simple distillation: Connect a distilling flask to the bottom of the side-arm joint. For fractional distillation: Connect a distilling flask to the bottom of the fractionating column, wrapped in aluminum foil. Connect the top of the fractionating column to the bottom of the side-arm flask. 2.Insert the thermometer into the top of the side-arm joint through the rubber stopper. The bottom tip of the thermometer should rest within the side-arm joint in a position slightly lower than the bottom of the condenser. 3.Attach the side of side-arm joint to the end of the condenser. 4.Attach the remaining end of the condenser to the vacuum adaptor. 5.Connect the receiving flask to the bottom of the vacuum adaptor. 6.Connect water hose from sink faucet to condenser water intake nozzle. Connect a second water hose from condenser exhaust nozzle to sink for drainage. 7.Stabilize the entire apparatus with clamps.
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To prepare gas chromatography injections:
1.Dilute fractions by combining 10µL of fraction sample with 990µL of dichloromethane (DCM).
2.Prepare syringe by filling syringe entirely with DCM and discharging it into waste container. Do this ten times. Afterwards fill the syringe with the diluted fraction sample and discharge into waste container five times. 3.Draw 1µL of air into syringe, followed by 1µL of the fraction sample, then followed by 1-2 µL more of air. The 1µL of sample should be suspended between the air pockets. 4.Insert syringe into injection port and discharge quickly and smoothly. Remove syringe promptly. 5.Repeat for remaining fractions once gas chromatography for previous sample has completed. GC Instrument Number:
simple distillation fractions for this lab were analyzed using gas chromatography machine #2. Fractional distillation fractions for this lab were analyzed using gas chromatography machine #2.
The compounds used in this experiment were different in two ways that were vital to this experiment. First, cyclohexane (C6H12) has a literature boiling point of 80.7°C, while toluene (C7H8) has a literature boiling point of 111.0°C. The difference between the boiling points of these two compounds can be attributed to the intermolecular forces of each one. Toluene is a polar compound, while cyclohexane is non-polar. This results in an increase in the boiling point due to the increased strength of intermolecular forces associated with polarity. Further, the three double-bonds around the six-carbon ring in toluene add to the strength of the intermolecular forces due to the presence of pi bonds. As the fractional distillation progressed, a reference temperature-versus-time graph with distinct “steps” should have been seen.
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However, the graph created from the experimental data displayed a more linear line with less than perfect “steps”. This inconsistency with the reference graph was likely due to human error, specifically poor thermometer placement or progressing through the distillation too quickly or not catching the right drops. A reference fractional distillation should have looked similar to the graph of the simple distillation except with a slight dip present after the first “step” of the graph. The experimental simple distillation displayed a shape much more similar to the reference. After the first temperature spiked and somewhat leveled out, the temperatures continued to rise steadily. Comparing these two graphs, it was apparent that the fractional distillation fractions were taken around very similar temperatures with the simple distillation fractions. Further, analysis of the graph showed that only the simple distillation reached the boiling point of toluene, and that was just barely.
The boiling point depression associated with mixing compounds can explain why toluene vaporized at such a lower temperature than its literature boiling point. The retention time peaks for the simple distillation were very close to the standard values provided. As compared to the cyclohexane (Peak 1) standard value of 0.377 minutes, experimental values of 0.382 minutes, 0.377 minutes, and 0.377 minutes were obtained. These very similar data values verify that the peak viewed at that time is the cyclohexane within each fraction. As compared to the toluene (Peak 2) standard value 1.06 minutes, experimental values of 1.061 minutes, 1.054 minutes, and 1.054 minutes were obtained. These similar data values verify that the peak viewed at that time is the toluene within each fraction. The retention time peaks for the fractional distillation were also extremely close to the standard values provided.
As compared to the cyclohexane (Peak 1) standard value of 0.377 minutes, experimental values of 0.375 minutes, 0.379 minutes, and 0.372 minutes were obtained. As compared to the toluene (Peak 2) standard value of 1.06 minutes, experimental values of 1.053 minutes, 1.057 minutes, and 1.049 minutes were obtained. These similar data values for each fraction verify that the first and second peaks viewed at that those times are cyclohexane and toluene respectively. Cyclohexane consistently had shorter retention times than Toluene because of its lower boiling point and nonpolarity. Toluene traveled through the GC much slower than cyclohexane because due to its polarity which caused it to interact more with the liquid stationary phase within the GC. The percent area for peaks 1 and 2 from the simple distillation were 64.5% and 35.5% for the first fraction, 49.4% and 50.6 % for the second fraction, and 17.0% and 83.0% for the third fraction.
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The percent area for peaks 1 and 2 from the fractional distillation were 74.4% and 25.6% for the first fraction, 49.3% and 50.7% for the second fraction, and 1.4% and 98.6% for the third fraction. Based on this data obtained by the chromatograms, fractional distillation was strongly supported as the more efficient technique for purifying volatile compounds. In both the first and third fractions where one would expect to see fractions almost entirely of cyclohexane and toluene respectively, fractional distillation produced fractions with much greater purity. The data suggests that fraction distillation is a more efficient method of separating volatile compounds. Fractional distillation makes use of a fractionating column, while simple distillations do not. This addition allows for more thoroughly separated mixtures, as the fractionating column causes vapors to condense and evaporate several times as they ascend the column. This separates compounds more thoroughly based on their vapor pressure and boiling point than using the simple distillation alone.
