E2 Brutus? Part IV

We did not remove the solvent because the amount of liquid product would have been too small to do analyses like RI and IR. Instead, the students performed a Br2 qualitative test with the upper (organic) phase. This was a little problematic as well. The students were instructed to count how many drops it took to make the bromine solution colorless. It turns out that the end point of this titration is somewhat subjective. Some students added a considerable amount of their upper layer to the test tube. The students should have had been instructed to observe the color change after adding a fixed amount of drops.

We ran the product on a Restek Rtx- 5 Column (5% diphenyl 95% dimethyl polysiloxane
30 M 0.53 mmID  5.0 micromdf).  The retention times for hept-1-ene, E-hept-2-ene, and Z-hept-2-ene were 10.6, 11.2, and 11.6 minutes respectively for this particular program. Generally, this was baseline separation for the three peaks. There was a pesky peak between 10.6 and 11.2 which might have been hept-3-ene or heptane. The starting material, 2-bromoheptane came out at 22.4 minutes. I was surprised that many student samples had a significant amount of starting material left. We refluxed the reaction mixture for nearly 45 minutes in a thermwell. The only thing that I can think of is that this may be due to solubility issues that may arise because of poor mixing. We did not follow this quantitatively.

The methoxide experiment gave a peak at 18.0 minutes which may be the substitution product. The ethoxide experiment gave a similar peak at 20.2 minutes which may be the substitution product. The substitution product was not seen for the tert-butoxide base. We did not follow this quantitatively either.

In the lab report the students were asked to describe the trends as the bases changed from methoxide to ethoxide and tert-butoxide.  Then the students were asked to explain these trends based on what we had learned about E2 reactions. This is a good example where a reaction does not give a single predicted product but gives a mixture of 3 different products. The ratio of those three products is sensitive to the structure of the base as shown in this experiment. It is quite likely that the ratio of these three products is also sensitive to other factGC-FID_elminationors related to the reaction conditions.


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