last updated Wednesday, October 14, 2015

1. a. Silica is used as the stationary phase in this step because it is polar. Therefore, it is able to separate polar and medium polar compounds. Silica is acidic, which can cause decomposition (i.e., rearrangement) of the compounds to be separated (see below). In addition, silica is a very fine powder that can cause lung diseases (i.e., silicosis).

b. The dry packing is preferential because it is very difficult to transfer the slurry/sludge into the burette (through the small opening) without making a mess. The transfer of the dry stationary phase is much easier.

c. Hexane (or a different hydrocarbon like petroleum ether) should be used to wet the column. The wetting process is required to check for cracks and bubbles. Any higher polarity solvent will cause problems later because it interferes with the separation.

d. Silica (SiO₂*n H₂O) itself is slightly acidic because it is a dehydrated form of silicic acid ("H₄SiO₄"). If the stationary phase is not pretreated, it causes most of the epoxides to rearrange during the chromatographic step. The student would elute an aldehyde or a ketone (see reader). The 1 % triethylamine (in hexane) solution neutralizes the majority of the acidic groups on the silica, which reduces this undesired side reaction.

e. Fraction #2 contains the alkene only, fraction #4 seems to contain the epoxide and the alkene and fraction #6 apparently only the epoxide or the aldehyde. The student should run a second TLC plate with the fractions #5, #6 and  #7 to evaluate those as well for the presence of epoxide.

2. a. Any solvents (even water) that are left in the NMR tube will give rise to signals in the NMR spectrum. Often times, these signals are larger than the signals of the compound being investigated. Their presence will complicate the analysis of the NMR spectra significantly.

b. Deuterochloroform (CDCl₃) is used as solvent for the epoxide because the NMR spectrometer needs a lock on to the solvent signal which serves as a reference in the magnetic field. If a non-deuterated solvent was used, the NMR spectrometer would be lacking this reference point and would (most likely) generate a random spectrum, which in most cases is rather meaningless. Generally, about 0.5 mL of CDCl₃ is sufficient to prepare a sample, which should ideally contain about 50-100 mg of the epoxide as well. Deuterochloroform is a selected carcinogen and should only be handled under the hood with proper PPE.

c. The solution level in the NMR tube should be as close as possible to 5 cm. Larger amounts of solvent dilute the sample unnecessarily resulting in a very poor ¹³C-NMR spectrum and causes problems when the NMR spectrum is acquired (logging/shimming process takes much longer!).

d. The NMR tube should be labeled in the upper third with a clearly identifiable label with a Sharpie. The same label should be used for the NMR tube and the sign-in sheet. The label should include the name, the unknown, the section and "Chem30CL". A label "ABC123" is not acceptable here!

3. a. The preferential solvent here is hexane and not CDCl₃, which is slightly acidic and would cause a slow rearrangement of the epoxide. However, a very small amount of CDCl₃ is not a problem. The concentration  should be 1-2 mg/mL.

b. The gas chromatograms are acquired using a chiral stationary phase. In Chem 30CL, a modified version of b-cyclodextrin is used.

c. Since the GC spectra are acquired using a chiral stationary phase, the GC portion allows the students (hopefully) to determine the degree of enantioselectivity of the reaction. The mass spectra allow to identify the unknown alkene via its epoxide (i.e., molecular ion peak, fragmentation, etc.).