last updated Wednesday, October 22, 2014

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 of the compounds to be separated (see below). In addition, silica is a very fine powder that can cause lung diseases (silicosis).

b. The sand is placed on the bottom between the cotton and the silica to prevent that the silica gets pushed into the tip of the burette. Sand is also placed on top of the silica to reduce disturbance of the top when more mobile phase is added. It also reduces the chance that the top of the stationary phase dries out.

c. Since silica is polar, a non-polar solvent should be used to wet the column (i.e., hexane, petroleum ether) to maintain the activity of the stationary phase.

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 column chromatography step. The student would elute mainly 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 and the epoxide, fraction #5 seems to contain only the epoxide and fraction #8 apparently only the aldehyde. The student should run a second TLC plate with the fractions #3, #4, #6 and #7 to evaluate those as well for the presence of epoxide.

2. a. Any solvents 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. This complicates 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 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 resulting in a very poor ¹³C-NMR spectrum and also 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.

b. The concentration of the GC sample should be 1 mg/mL in terms of epoxide. Higher concentrations lead to an overload of the GC column resulting in poor peak separation and inaccurate quantitation and identification. One drop of the NMR solution is diluted with hexane to reach the proper concentration.

c. The sample cannot contain water or any solids. Water causes the destruction of the column and solids can clog up the injection syringe.

d. 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.).