Gas+Chromatography


 * Week 4: Gas Chromatography**

In elimination reactions, if more than one alkene is produced, the more stable alkene is the major product. In a compound, the carbon to carbon double bond that is more highly substituted (carbons attached directly to the double bond) is more stable. This is according to Zaitsev’s rule, named after Russian chemist Alexander Mikhailovich Zaitsev. In the (E1) elimination reaction of 2-methyl-2-butanol there are two possible products, 2-methylbutene and 2-methylbut-2-ene (Picture #1.). By using Zaitsev’s rule, the more stable product can be determined. 2-Methylbutene’s carbon to carbon double bond is di-substituted (has two carbons attached) and 2-Methylbut-2-ene is tri-substituted (has three carbons attached), so the 2-methylbut-2-ene product will dominate the product mixture. To test Zaitsev’s rule, a Gas Chromatography (GC) will be used for this lab. The GC separates and analyzes the components of mixtures based on interactions between the components, vaporizing them without decomposition. In this case it will be used to identifying the products. In GC there are two phases, the mobile phase and the stationary phase. In the mobile phase, gas sweeps the components through the column to the detector. The gas used is helium, since it is unreactive. The stationary phase is the material inside the column. For this GC the column is lined with wax-coated crushed firebrick. This allows the substance to slow down through a gas-phase by allowing it to momentarily condense on the wax. Of the two products in this case, the one that has the higher boiling point will be condensed on the wax for more time than the lower boiling point. Therefore the compound with the lower boiling point will hit the detector first. The chart recorder documents the amount of gas passing over the detector by graphing it. The results appear as peaks on the printout, the larger peak representing the most abundant product and the smaller peak representing the less prevalent product.The order in which the products pass the detector and the magnitude of the peak it creates on the printout together tell which product is indeed most prevalent. Very nice!!
 * Introduction:**


 * Procedure:**

1. Draw 2 micro-liters of sample into a Hamilton syringe (these are small, glass, gas-tight syringes) 2. Inject sample into the chromatograph through the rubber septum. Be sure to inject quickly and smoothly, as to not let any sample escape. 3. At that same moment, start the chart recorder. Recorder is set at 6cm per minute. 4. When sample has finished going through the chromatograph and the chart recorder is no longer recording changes, stop the recorder and take the graph as the data. 5. Make at least one copy of the data printout. 6. Cut out (with scissors!) the two curves of the printout and weigh each section separately and weigh the two together. Be sure to cut accurately.If the two peaks overlap, be sure to cut the place where the peaks meet perpendicularly to the x-axis of the graph to get the most accurate measurement possible. 7. Calculate the percentage of each peak's mass to the total mass.



The initial prediction was that the more stable Zaitsev product would dominate the composition of the product mixture. The predicted product was 2-methylbut-2-ene, because it is the more stable of the two potential products according to Zaitsev's Rule. It is tri-substitued while the alternative, 2-Methylbutene is only di-substituted. The chromatogram below shows the results given by the chromatograph. Note: according to Dr. Higginbotham, the various skips of the needle creating the unsightly irregularity of the baseline is instrument ill-temperedness and not experimenter error. Did I really say "ill tempered?" silly. Yes, it was an electrical issue, I believe.
 * Data/Analysis:**

calculations are done correctly, data is nicely organized, though you have made errors with sig figs.

After analyzing the data, the GC has proved to confirm Zaitsev’s rule, and the initial prediction.The smaller peak represents the product with the lower boiling point because it passed through the detector first. Comparing the boiling points of the two potential products in reference material (CRC), it is discovered that 2-methylbutene has the lower boiling point and therefore is represented by the smaller peak. 2-Methylbut-2-ene is tri-substituted, stayed in the column longer since it has the higher boiling point and dominates the product mixture with a value of 92.0%. All though this test proved effective in proving Zaitsev’s rule there can still be room for error. The mixture itself could have contained impurities. The injection of the 2 micro-liters of mixture into the rubber septum must be done quickly or some of the gas could be lost. The chart recorder must be turned on at the exact moment of injection. When observing the chart in this procedure the hills after the large peak should not all be there, except for one that may have shown the solvent in this mixture. This shows that the chart recorder had some technical difficulties. Cutting and weighing out the chart peaks can result in calculation errors.
 * Conclusion:**


 * Works Cited:**

"Gas Chromatography." //Wikipedia, the Free Encyclopedia//. Web. 10 Feb. 2012. [].

Higginbotham, Carol. Ph.D. “Gas Chromatography Lab” CH-335: Organic Chemistry Course. COCC/UO: Blackboard, Winter 2012. Web. 10 Feb 2012.

Straumanis, Andrei. "ChemActivity 14: Elimination." //Organic Chemistry: A Guided Inquiry : A Process Oriented Guided Inquiry Learning Course//. Boston, MA: Houghton Mifflin, 2009. 200-10. Print.

Weast, Robert C. //CRC Handbook of Chemistry and Physics: A Ready-reference Book of Chemical and Physical Data//. 56th ed. Cleveland, OH: CRC Pr., 1975. C224. Print.