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