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Experiment #6...

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Gas Chromatography: Identifying Unknown Ketones Preston Hawkins Lab Partner: Sam Lyon Performed on 3/7/2019 Submitted 3/14/2019

Grade=98 Nearly perfect! Purpose of Experiment: The purpose of this experiment was to record and analyze the retention times of a mixture of four ketones as they passed through a Vernier mini GC, and to then use these “standard” retention times to determine the composition of an unknown mixture of ketones. Theory and Principle: Gas chromatography (GC) is known to be a powerful tool for the separation and identification of compounds. GC operates under the same principle as all other chromatography; it involves a mobile phase which travels along a stationary phase. In the case of this experiment, the stationary phase is a long, coiled, metal column inside of the mini gas chromatograph that contains a liquid coating, and the mobile phase is simply hot compressed air that runs through the column, this is known as liquid GC (Gas chromatography [Wikipedia]). When a solution of multiple compounds is injected into a GC, it is vaporized and then flows through the column to a sensor which will produces peaks on a chromatogram as each compound flows out of the column. This graph can then be used to determine the retention time of each compound, and also to identify a compound by its retention time. Several factors affect retention time in GC including boiling point and molecular weight. In general, it is expected that the lower the boiling point of a compound, the lower its retention time, this is because compounds with higher boiling points will tend to want to remain in the liquid (stationary) phase and will not flow out of the column as readily as those with lower boiling points (Gas Chromatography [Khan Academy]). Below is a table of the compounds used in this experiment and their suspected elution order, 2-hexanone was not used in this experiment.

Compound

Boiling Point (°C)

Predicted Elution Order

2-butanone

79.6

2

2-hexanone

127.6

(5)

4-methyl-2-pentanone

117-118

4

acetone

56-57

1

2-pentanone

101-105

3

Table 1. Expected Elution Order of Ketones (Provided by D. Paschal) Summary of Experimental Procedure: The Vernier mini GC was connected to a laptop in order to produce a chromatogram. A glass syringe was then prepared by “rinsing” with a solution containing all four of the ketones listed in Table 1. Once the GC had warmed up to the correct temperature, approximately 0.2 microliters of the standard solution were injected into the GC, and data collection was run for approximately 7 minutes to obtain a complete chromatogram of the solution. The peaks on the chromatogram were then analyzed to determine the retention times of the compounds in the known mixture. The glass syringe was then “rinsed” again with an unknown solution X that contained three out of the four listed ketones. Once the GC had warmed up again, approximately 0.2 microliters of the unknown solution were injected into the GC, and data collection was run for 7 minutes. The chromatogram for the unknown solution was then compared to the graph of the standard solution to determine which ketones were present in the unknown solution.

Figure 1. The Vernier Mini GC (Left) and the Glass Syringe (Right)

Data: Compound

Boiling point (°C)

Actual Elution Order

Retention Time (min)

2-butanone

79.6

2

1.240

2-hexanone (Not Used)

N/A

N/A

N/A

4-methyl-2pentanone

117

4

3.210

Acetone

56.5

1

0.790

2-pentanone

103

3

2.230

Table 2. Experimental Data

Peak #

Retention Time (min)

1

0.810

2

1.265

3

3.320

Table 3. Retention Times of Unknown solution

Figure 2. Chromatograph of Known Solution

Figure 3. Chromatograph of Unknown Solution X

Data Analysis Questions: 1. In the Pre-Lab table, you predicted the elution order of the ketones. Did the chromatogram of the ketone mixture support your prediction? Explain. Yes, four peaks were produced on the chromatogram and each peak was proportional to the expected elution order based on boiling point. 2. From the graph of the known ketone mixture, measure the retention time for each peak in the chromatogram. Identify the ketone that produced each peak. The first peak was acetone with a retention time of 0.790 min, second was 2-butanone (1.240 min), third was 2-pentanone (2.340 min), and the forth peak was 4-methyl-2pentanone (3.210 min). 3. 2-hexanone and 4-methyl-2-pentanone are isomers, thus their molecular weights are equal. Suggest reasons for their differing boiling points and GC retention times. The difference in the boiling points of two isomers can be explained by branching that is present in the compounds. 4-methyl-2-pentanone has more branching, therefore each molecule cannot approach each other as closely as in 2-hexanone, which leads to weaker intermolecular forces, meaning that it requires less energy to break the molecules apart, thus leading to a lower boiling point. 4. Identify the ketones that are present in your unknown mixture. Support your identification. The ketones present in the unknown mixture were acetone, 2-butanone, and 4-methyl2-pentanone. This identification was supported by the retention times of the known mixture of ketones. Peak one on Figure 2 had a retention time of 0.810 min; this was within error of the retention time for acetone from the known mixture of 0.790 min. Peak two had a retention time of 1.265 min which is close to the time observed for 2butanone. And peak 3 had a retention time of 3.320 min, again within error to the observed value for 4-methyl-2pentanone of 3.210 min.

5. Based on the results of your testing, predict the retention time of diethyl ketone. Explain your prediction. The retention time of diethyl ketone would be predicted to be approximately 2.230 min, the retention time of 2-pentanone that was found in this experiment. This outcome is expected because both compounds have the same chemical formula of C5H10O, and therefore the same molecular weight. Conclusion: The purposes of this experiment were fulfilled; a known mixture of ketones was analyzed on a gas chromatograph, and the composition of an unknown mixture of ketones was determined from the retention times of the known mixture. Gas chromatography was demonstrated to be a powerful tool for separating and identifying compounds, as was evidenced by the “closeness” of the retention times that were obtained for the known and unknown mixtures of ketones.

References: D. Paschal, unpublished private communication. “Gas Chromatography,” https://www.khanacademy.org/test-prep/mcat/chemicalprocesses/separations-purifications/v/gas-chromatography. “Gas chromatography,” https://en.wikipedia.org/wiki/Gas_chromatography....