Fat Bases Want the Aisle Seat

During the chemistry Collaborations Workshops & Community of Scholars (cCWCS) participants performed an elimination reaction with 2-bromoheptane and two different bases: sodium methoxide and potassium t-butoxide. The overall goal was to compare the product ratios of the resulting alkenes: 1-heptene, E-2-heptene, and Z-2-heptene which can be separated on a GC column. According to most textbook explanations, the smaller methoxide should give primarily the most stable product E-2-heptene and the larger t-butoxide should give primarily 1-heptene. The general trends held up pretty well for the combined class data. For the methoxide, the average ratio of 1-heptene, E-2-heptene, and Z-2-heptene was 1.5 / 4.1 / 1.0 respectively. For the t-butoxide the same sequence of ratios was 7.2 / 2.0 / 1.0. In the discussion following the experiment it was pointed out that the alkyl group on the base was not the only variable that was changed between the two versions of the same experiment. One should also take into consideration the different cation, reaction temperatures, and base strengths between the two reactions. To make things even more messy, a fourth alkene showed up in the GC chromatogram of the product mixture. This was quite mysterious until the 2-bromoheptane starting material was found to be contaminated with a small percentage of 3-bromoheptane. In conclusion, a rather straightforward verification of a textbook prediction ends up to generate a very rich (read complex) data set, especially when the whole class’ data was considered. This experiment may be found in Modern Projects and Experiments in Organic Chemistry: Miniscale and Standard Taper Microscale. 2nd ed. Mohrig, Hammond, Schatz, & Morrill. An interesting discussion of this experiment may be found in the Journal of Chemical Education: “Positional and geometrical orientation in eliminations from 2-bromoalkanes induced by sodium methoxide-methanol, potassium tert-butoxide-tert-butyl alcohol, and potassium tert-butoxide-dimethyl sulfoxide.”

Advertisements

One Trackback/Pingback

  1. […] is an experiment that I performed as part of a Chemistry Collaborations Workshops & Community of Scholars (cCWCS) workshop during the summer of 2009. I had planned on incorporating the experiment the next time I taught […]

Post a Comment

You must be logged in to post a comment.
%d bloggers like this: