Day Fifteen: Chromatography

It is at this point in the process, with Interim swiftly coming to a close, that I should say our polymerization will not be happening by the end of the month.  There is a good chance that my colleagues will be able to continue their syntheses over the next semester, but since I will be studying abroad, I won't get to complete it myself.

In light of these developments, we spent today learning useful separation techniques in an attempt to isolate our products from the Grignard reaction.  The first technique we explored was thin-layer chromatography (TLC).  Essentially, a plastic-backed paper is coated with a polar compound - in this case, a silicon oxide.  A drop of product is placed about one centimeter from the bottom of the paper, and the paper is set in a container with a small amount of solvent.  This solvent is absorbed onto the TLC paper and catches the drop of product as it travels.  Depending on the attractions between the solvent and the product, the product will move a certain distance along the paper, until the paper overcomes that attraction.  If multiple components exist in the solution, they will separate based on their polarities.  Analysis of the TLC is possible using UV radiation (if the product contains pi bonds) or solid iodine.

The purpose of the TLC was to determine what composition of solvent would best serve to separate our compounds in the product solution.  In my case, I was attempting to separate 2-benzylcyclohexanol, some 2-chlorocyclohexanol, and a small amount of bibenzyl.  Out of some varied combinations of hexane (nonpolar) and ethyl acetate (fairly polar), we determined that a 4:1 ratio was optimal overall.

The second technique we utilized was column chromatography.  Assembling the column itself was probably the most interesting part of the process.  First, we stuffed the bottom of the column right above the stopcock with a swab of cotton and poured in about 3 cm of hexane.  Next, we measured enough silica gel powder to fill 20 cm of the column, dissolved it in hexane, and poured it into the column with excess hexane.  After draining out the hexane and removing any air bubbles by tapping the column with a rubber hose, I poured a sample of my product dissolved in hexane and ethyl acetate into the column and let it settle into the silica layer.  After that, it was a matter of preparing 50-mL hexane and ethyl acetate solvents of varying compositions - lower to higher concentrations of ethyl acetate.

We let the liquid elute for almost two hours, catching 5-10 mL at a time in test tubes and replacing the solvent when necessary.  I tested for eluting product by placing small dots on TLC paper but not developing it, rather relying on UV radiation to confirm the presence of a benzyl product.  There was no visible dot left by the end of the column chromatography process.

In the morning, I will develop TLC plates for every fourth test tube out of a total thirty-five and run appropriate gas chromatography samples from today's separation.