Gas+Chromatography+Lab

=Introduction=

In this lab, gas chromatography will be used to determine whether the Zaitsev product or Hofmann product is presented in a greater quantity from an elimination (E1) reaction between 2-Methyl-2-butanol and H3 O + in heat. According to Zaitsev's rule, in an elimination reaction the more stable alkene product, containing the more highly substituted C=C pi bond, will be the major product. As a result of this rule, 2-methyl-2-butene is expected be the major product. The least substituted alkene product in an elimination reaction is referred to as the Hofmann product. In this reaction, the Hofman product is expected to be 2-methyl-1-butene. These two products have different boiling points, therefore, a gas chromatoraph can be used to separate them in order to decipher the relative concentrations of each. The elimination reaction that produced the products is shown below. Thanks for the mechanism--nice!

Gas chromatography is a separation method that is often used to analyze environmental samples. However, there are many variables that can be adjusted to provide analyses in many different applications. The chromatography system used in this lab separates different components of a mixture based on boiling point. The product mixture is injected into the chromatograph and swept up into a stream of inert Helium gas. This gas pushes the mixture though a long tube, the chromatography column, which is surrounded by a controlled oven. The wax-coated crushed firebrick inside the column causes the separation to occur as the compound with the highest boiling point will spends more time condensed on the wax, allowing the compound with the lowest boiling point to exit the device first. Before exiting the chromatograph, the separated mixture travels past a detector that recognizes when any substance other than helium that passes through it. When the components of the mixture travel past the detector, it produces a signal which is then received by a strip-chart recorder, and a curve concentration vs. time is plotted.

=Procedure=

In small groups, we will go into the instrument room and run our samples through the instrument. GC systems are very fussy, so if we struggle to get a nice plot, don’t despair. I will make sure you have some data to examine, even if I have to supply you with a plot that I made already.

Before we inject your sample, I will show you the inside of another GC that is not currently running. You can look at the column inside the oven, see the oven components, etc..

Then we will have one person from your group draw 2 microliters into a gas-tight Hamilton syringe, and inject the sample smoothly and quickly through a rubber septum into the chromatograph. At that moment we will start the chart recorder, which will drive paper along at a constant rate, tracing any signal that shows up as time passes.

When we get the sample through the column, you can take the chromatogram (the paper trace) as your data. Do the following things with your chromatogram:

1. Make a photocopy so that you have a copy to put into your lab report, on the wiki.

2. One copy will be sacrificed to determine the area under each curve. In math, this is Integration, right calculus students? In our lab, we have a cheezy but sufficiently accurate way to get the area under the curve with no fancy math. You will cut out the curves--with a 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.

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. (Higginbotham, 2012)

Relevant Compounds:

 * Name:** 2-Methyl-1-butene
 * Cas. #:** 563-46-2
 * Molecular Formula:** C 5 H 10


 * Name:** 2-Methyl-2-butanol
 * Cas. #:** 75-85-4
 * Molecular Formula:** C 5 H 12 O


 * Name:** 2-Methyl-2-butene
 * Cas. #:** 513-35-9
 * Molecular Formula:** C 5 H 10

=Data=

Below are the boiling points for the Zaitsev and Hoffman products of the elimination reaction between 2-Methyl-2-butanol and H 3 O + with heat. A citation would be appropriate for the data below, to make clear these values come from the literature.



2 µL of an unknown mixture of 2-Methyl-1-butene and 2-Methyl-2-butene was injected into the Gas Chromatograph. Below is the resulting chromatogram.



The curves were prepared via the instructions in the procedure and the results are as follows.



=Analysis=

The % composition of the mixture was found using the following formulas.



Very nice. =Conclusion=

Gas chromatography was used to separate the Zaitsev and Hofmann products of the elimination reaction of 2-Methyl-2-butanol. The Hofmann product, 2-methyl-1-butene, has a boiling point of 31°C, and the Zaitsev product, 2-methyl-2-butene, has a boiling point of 39°C. Therefore, the Hofmann product, having a lower boiling point, spends less time condensed on the column and is represented by the first curve to exit the device. The second, and largest, curve is representative of the Zaitsev product as it has the higher boiling point and exited the device last. Visually, it is easy to see that Zaitsev's rule was in fact in effect as the elimination reaction progressed. By using the % composition equations found in the analysis section, the mixture was found to be 91% 2-methyl-2-butene (Zaitsev) and 9% 2-methyl-1-butene (Hofmann). This data confirms that Zaitsev's rule applies to the elimination reaction between 2-Methyl-2-butanol and H 3 <span style="font-family: Arial,Helvetica,sans-serif;">O + <span style="font-family: Arial,Helvetica,sans-serif;">in heat.

The most notable source of error in this lab is due to the the down and dirty approach to the integration of the curves. The curves were weighed, therefore, any inconsistencies in how they were cut would show in the results. Also, the rate at which the sample was pumped into the device could effect the chromatograph. This effect would most likely be amplified with substances that would spend a short amount of time on the column.

Good job.

References:
//2-methyl-1-butene//. (2011). Retrieved from http://www.wolframalpha.com/entities/chemicals/2-methyl-1-butene/us/zm/oq/

//2-methyl-2-butanol//. (2012, February 09). Retrieved from http://en.wikipedia.org/wiki/2-Methyl-2-butanol

//2-methyl-2-butene//. (2011, October 26). Retrieved from http://en.wikipedia.org/wiki/2-Methyl-2-butene

<span style="font-family: 'Arial','sans-serif'; font-size: 13.3333px;">Higginbotham, C. (2012). //<span style="font-family: 'Arial','sans-serif';">Gas chromatography lab //. Unpublished manuscript, Chemistry, OSU Cascades, Bend, Oregon. Retrieved from https://bb.cocc.edu/bbcswebdav/pid-424976-dt-content-rid-2591699_1/courses/CH242-10406_WI12/ChromatographyLab.pdf