Relevant textbook readings – Mohrig, Chapter 11, 12. Klein,
Chapter 8 Overview
– In this experiment, we will react 2-bromoheptane with a
strong base, KOH in ethanol, 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.
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.
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 3.
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. |