Column chromatography works on the same principle as TLC The adsorbent (alumina or silica gel) should be packed with a stream of air or nitrogen to drive out air pockets in the column. This leads to better separation.

Selection of the eluting solvent is an important factor in a good separation The more polar the eluant, the faster compounds will move through the column. If a solvent is too “fast”, everything will come out with the solvent front.

More polar compounds travel more slowly through the column. Compounds with more polar groups will adhere to the adsorbent (alumina or silica gel) more strongly than less polar molecules.

The column should have a level surface so that the bands stay even as they travel through the column. The sample should be applied to the column in a minimum amount of solvent. Wide band widths lead to poor separation.

Narrow bands traveling through the column prevent overlap.

Isolating the separated compounds Run TLC’s of the fractions after the column to decide which ones to combine.

The C=O bond has some single bond character.

If Z is an electron-withdrawing group, then resonance structure 2 becomes less important

The wavenumber is the inverse of the wavelength. It is directly proportional to energy.

Expt. 16 – Separation of an Alkane Clathrate Preparation: You will add urea dissolved in methanol to a mixture of 2,2,4-trimethylpentane and hexadecane. After a white solid forms, cool with an ice/water bath until crystallization of the clathrate is complete.

Confirm identity of the hydrocarbon by comparison of its IR spectrum to that of hexadecane and 2,2,4-trimethylpentane

Clathrates in the News Methane hydrate deposits on the ocean floors are twice the size of the known coal and gas reserves on earth. Could they be tapped as an energy source? Methane is a potent greenhouse gas and could contribute to the global warming phenomenon.