Expt #4. Dehydrohalogenation of  2-Bromoheptane

Procedure revised on 11/9/2018.

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

Overview – In this experiment, we will react 2-bromoheptane with a strong base, KOC(CH₃)₃ to obtain an alkene product. The combination of a strong base with a secondary halide points to an E2 mechanism with three different alkene products possible, 1-heptene, E-2-heptene and Z-2-heptene. 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.

Procedures

Safety

Wear gloves when measuring out the reactants and throughout the extraction procedures.

Running the reaction

Add 400 mg KO-t-Bu, 2.0 mL 2-methyl-2-propanol, and a stir bar to a 5-mL pear-shape flask (rbf). Stir or swirl briefly then attach a condenser and a drying tube and then reflux for 5 min before adding 0.50 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 2.0 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 most of the aqueous layer to another test tube. Then wash the remaining organic layer with 2.0 mL H₂O being careful to vent as needed. Allow the layers to separate and pipet out the water layer into a waste container (large test tube) 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 2.0 mL H₂O three times 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 in CDCl₃.

Report

Literature Spectra - Do not include literature spectra data in this report. Instead 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, Z-2-heptene, and 2-bromoheptane 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 little 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 and 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).