Gas+Chromatography

Zach Chandler Megan Bernard


 * Lab #3 Gas Chromatography**
 * Objective:** To track the separation behavior of a mixture and apply an understanding of the chromatography system to identify each component in the mixture.


 * Introduction:** In this experiment, Zaitsev's Rule and Gas Chromatography will be used in order to seperate and understand components in a mixture. Zaitsev's Rule states that in an elimination reaction, the most substituted, or lower potential energy, alkene product will dominate the product mixture. E1 reactions always follow Zaitsev's Rule, while E2 reactions follow Zaitsev's Rule only with a small base. Furthermore, like other forms of chromatography, Gas Chromatography is a separation method. This means that when a mixture is put through the chromatography system, the different components of the mixture will be separated by the process of adding heat and using helium as a carrier gas. Gas Chromatograph separates the components of a mixture on the basis of their boiling points. The lower boiling compound should come out of the instrument first, and the higher boiling component should come out second. Gas Chromatography is widely used in industrial and government labs, as well as in forensic labs to identify an accelerant used in arson's and in clinical medicine to identify drugs or metabolites in body fluids or blood. The reaction below is what is expected to happen. The boiling points of the two products are: 2-methlybut-2-ene at 39 degrees C, and 2-methylbut-1-ene at 31 degrees C. Nice introduction. Thanks for including the figure below--it is very helpful. Have you cited its source?




 * Procedure:**
 * 1) Draw 2 microliters of 2-methylbutan-2-ol into a gas-tight Hamilton syringe, and inject the sample smoothly and quickly through a rubber septum into the chromatograph.
 * 2) At the same moment, start the chart recorder, which will drive paper along at a constant rate, tracing any signal that shows up as time passes.
 * 3) When the sample gets through the column, take the chromatogram (the paper trace) as your data.
 * 4) Use the chromatogram to calculate the area under each curve. By doing this, cut out the curves with a pair of scissors and then weigh the two pieces. The mass of each divided by the mass of both together, x 100, equals the percent of that component.
 * 5) If your peaks are not completely separated, drop a straight line down to the baseline from the place where they cross, and cut this line to divide up the overlapping portion.


 * Observations/Data:** After the sample had went through the column in the chromatograph, the chromatogram was produced, as shown below.



After injecting the gas into the chromatograph, the chart recorder graphs the reaction progress. The reaction doesn't happen inside the chromatograph. I performed the reaction BEFORE the lab....so our sample is a mix of the products. The professor indicated that the graph recorded at 6 cm/min. Therefore the reaction time can be calculated for each peak; which is calculated from initial injection to the first and second peak. Then determine each component of the mixture, and the percent composition of each product in the mixture. To determine the percent composition the area must be calculated. In this experiment measurements of area will be calculated by cutting out each individual curve and weighing each piece. Then dividing each piece by the total to find the percent composition. After cutting the paper trace and weighing the individual pieces of paper, it was observed that the smaller of the two peaks weighed 0.003g. and the larger peak weighed 0.042g.
 * Analysis:**

First peak: 38 seconds (Lower Boiling Point, 2-methylbut-1-ene) Second Peak: 42 seconds (Higher Boiling Point, 2-methlybut-2-ene) These calculations were done using a ratio of measured distance from beginning of reaction to each peak, compared to the 6 cm/min rate.
 * Calculations:**
 * Retention Time:**

.042g + .003g = .045g
 * Total Sample weight:**

Lower BP = .003g / .045g = .067 x 100 = **6.66% Hofmann Product** Higher BP = .042g / .045g = .933 x 100 = **93.3% Zaitsev's Product**
 * Percent Composition:**

calculations are performed okay, although you have reported your answers with too many digits (sig figs errors).


 * Sources of Error:** The sample of 2-methylbutan-2-ol used has a very low boiling point, if the syringe was not emptied into the septum quickly, it is possible that some of the sample could have evaporated. While weighing the two separate peaks, the smaller of the two would not show up as any weight on the balance, but the larger peak weighed 0.003g less than the weight of both of the peaks together. So it was observed that the smaller peak must hopefully weigh the 0.003g.


 * Conclusion:** The elimination reaction that was performed, produced products that suggested that Zaitsev's Rule was operating because according to the rule the most substituted (lower potential energy) alkene product will dominate the product mixture. Primarily because 2-methylbut-1-ene is a di-substituted alkene and 2-methylbut-2-ene is a tri-substituted alkene. The Hofmann product being the lower boiling point showed up on the graph as the first peak, while the Zaitsev product appeared as the second peak because of the higher boiling point. The percent compositions also supported that Zaitsev's Rule was followed because 93.3% of the product formed was 2-methylbut-2-ene; the more substituted and lower potential energy product.

Generally, a very nice report. I am concerned, though, that you seem to think the reaction happened inside the instrument. Let me know if you don't understand my comments above--which attempt to explain that this is not the case. **References:** []