Expt #4. Dehydrohalogenation of  2-Bromoheptane

Relevant textbook readings – Mohrig, Chapter 11, 12. Smith, Chapter 8

Overview – In this experiment, we will react 2-bromoheptane with the strong base, potassium hydroxide, to obtain an alkene product (eq 1). The expected mechanism for this reaction is E2 and three different alkenes are possible. The main goals of this experiment are to determine which alkene is actually formed as the major product and to determine amounts of any minor products that are also formed. You will also learn how to process and print out GC-MS data using either WSEARCH (PC) or OpenChrom (Mac) software, which you should download and install on your machine prior to lab using the links available off of the Chem 350 website links page.

Procedures

Safety

Wear gloves when measuring out the reactants and throughout the extraction procedures. Ethanolic KOH is very caustic and skin contact should be avoided. If you do get KOH on your skin (you will not immediately feel a burning sensation but you can tell it’s there by a slippery/soapy sensation on your skin) make sure to immediately rinse with cold water for at least five minutes.

Running the reaction

Add 0.5 g KOH, 1.0 mL 95% ethanol, and a stir bar to a 5-mL pear-shape flask (rbf). Stir or swirl briefly then attach a condenser and reflux for 5 min before adding 0.65 mL 2-bromoheptane by pipet to the rb flask. Reflux the solution for 60 min.

Work-up

Cool the solution to near room temperature and then transfer it into a reaction tube containing 1.0 mL of cold water. Cap the tube and carefully mix and shake the contents being careful to vent frequently. Let the tube stand for 5 min.

For the procedures in the following paragraph, make sure to identify the organic and aqueous layers correctly! See section 11.2 – “Practical Advice on Extractions” in Mohrig. You can also test separated aqueous layers by adding a few drops of water to them to make sure that the water dissolves in. If the presumed aqueous layer is actually organic then the added water drops will not dissolve.

Use a pipet to transfer the organic layer to another test tube. Then wash the organic layer with 1.0 mL H₂O being careful to vent as needed. Allow the layers to separate and pipet out the water layer into a waste beaker to be discarded later. (As a general rule, never discard any material from a reaction until the final purified product is obtained and verified). Wash the organic layer with 1.0 mL H₂O twice more, each time transferring the aqueous layer into the waste beaker.

Dry the organic layer over sodium sulfate. Use a Pasteur pipet to remove the liquid from the drying agent and transfer it into a dry, pre-weighed vial. Weigh the vial with the product to determine the yield.

Characterization of Product

Obtain a proton NMR spectrum of the product. Prepare a sample for GC-MS analysis by adding 1 uL of the product to approx 10 mL pentane in a clean vial. One group will also be asked to obtain a C-13 NMR of their product.

Report

Literature Spectra - Do not include literature spectra data in this report. Instead GC-MS and NMR spectra of each of the possible alkene products and the 2-bromoheptane starting material were obtained here at WSU and are being provided. (See additional details below.)

Proton NMR. Proton NMR data files for 1-heptene, E-2-heptene, and Z-2-heptene are available in the class storage folder. Process each of these spectra and summarize the data for all four compounds in a single results table (Table 1). Be aware that complex splitting patterns are expected for the Hs of the double bonds, which are expected to resonate in the vicinity of 5-6 ppm.. This is due to the (n + 1)(m +1) rule applying here because of the fact that the Hs causing splitting are not equivalent and have quite different coupling constants (J values). (see Smith chap 14.8 and Mohrig p 346).

Use the ¹H NMR data on the pure compounds to assign peaks in the ¹H NMR of the product mixture and include these findings in Table 1.

Interpretation. Pay special attention to the peaks down field of 3 ppm. (The resonances at lower frequencies than this are extensively overlapped and, therefore, of less use to us.) Use the integration values of the multiplets at 5-6 ppm to determine the ratio of 2-heptene to 1-heptene formed in your reaction. For easier comparison to the GC-MS results convert this ratio to percents. (See Mohrig p 354 where it discusses how to use the ¹H NMR of a similar product mixture to find the ratio of the product yields).

C-13 NMR. ¹³C-NMR data files for the three alkenes are also available in class storage. Process each of these spectra and summarize the data for the three compounds in a single results table (Table 2). Use the¹³C NMR spectrum obtained by the selected team as indicative of what a ¹³C NMR spectrum of your product would have looked like. Assign peaks in the above ¹³C NMR spectrum based on the spectra of the pure compounds and include these findings in Table 2.

Interpretation. Use the relative peak heights in the ¹³C NMR of the product to calculate the approximate relative amounts of the three alkenes. (¹³C NMR peak intensities depend on other factors besides sample concentration and carbon count so this approach is inherently flawed. Moreover, peak intensity is indicated by the area under the peak rather than the height but a C-13 spectrum is generally too noisy to obtain good peak integrals. However, using the peak heights is valid if the peak widths are uniform, which for these compounds is probably true. To assess the importance of these known potential sources of systematic error, we will be able to compare the results from the C-13 spectrum analysis to those provided by proton NMR and GC-MS.)

GC-MS. You will be provided with separate printouts of GC-MS results for all three alkenes and the starting material alkyl bromide. Summarize this data, including the retention time of the compound as well as the m/z values of the M+ and base peaks plus a few other strong or significant MS peaks in a table (Table 3).

Use the retention times of the pure compounds to assign the peaks in the GC chromatogram (top graph) of the product mixture and include these findings in Table 2.

Interpretation. Use the peak areas of the three alkene peaks to determine relative amounts (percents) of each.

Bonus: A somewhat unexpected product shows up at about 7.0 min in the GC-MS. Use the mass spectrum of this product to determine its identity and propose a mechanism for how it forms. Reexamine the NMR spectra to check for peaks due to this side product. Label these peaks and include the data in tables 1, 2, and 3.

Summary Table. Use Table 4 as a master table to summarize all of the most important findings of the experiment. Include the absolute yield (g), theoretical yield (g) and percent yield. Also include what the NMR, GC-MS, and IR spectra reveal about the product mixture, especially as pertains to the relative amounts of the three alkenes formed.

Discussion. Because the goal of this lab was to determine relative amounts of products (rather than to obtain a good yield of a pure product) your discussion can omit the usual discussion of yield and purity.