Column Chromatogrophy Organic Chem Lab questions: 1) Once the adsorbent is packe
ID: 852555 • Letter: C
Question
Column Chromatogrophy Organic Chem Lab questions:
1) Once the adsorbent is packed in a liquid chromatography column, it is important that the level of the elution solvent not drop below the top of the adsorbent. Why?
2) What precautions must be taken when you introduce a mixture of compounds to be separated onto a liquid chromatography adsorbent column?
3) What effect will the following factors have on a liquid chromatographic separation? a) too strong an adsorbent; b) collection of large elution fractions; c) very slow flow rate of the mobile phase.
4) Arrange the following compounds in order of decreasing ease of elution from a column of silica gel; a) 2-octanol; b) 1,3-dichlorobenzene; c) tert-butylcyclohexane; d) benzoic acid.
5) Why do silica gel columns with smaller particle sizes produce more effective chromatographic separations?
Explanation / Answer
1) If the elution solvent drops below the top of the adsorbent, then the entire adsorbent won't be resolved i.e. only the portion of adsorbent in contact with the elution solvent will be resolved which is improper because we require to separate all the components of adsorbent.
2) The narrow exit of the column is first plugged with glass wool or a porous plate in order to support the column packing material and keep it from escaping the tube. Then the adsorbent solid (usually silica) is tightly packed into the glass tube to make the separating column. The packing of the stationary phase into the glass column must be done carefully to create a uniform distribution of material. A uniform distribution of adsorbent is important to minimize the presence of air bubbles and/or channels within the column. To finish preparing the column, the solvent to be used as the mobile phase is passed through the dry column. Then the column is said to be "wetted" and the column must remain wet throughout the entire experiment. Once the column is correctly prepared, the sample to be separated is placed at the top of the wet column.
3) a) It will take greater time to resolve
b) Chromatography has to be done many times so as to get individual components.
c) The different components won't resolve properly and would seem to merge.
4) Decreasing ease of elution d) benzoic acid > a) 2-octanol > b) 1,3-dichlorobenzene > c) tert-butylcyclohexane
It is based on the no. of functional group substitutions. Greater the substitution bulkier the molecule and greater would be the time to elute.
5) Smaller size indicate greater porosity. So the adsorbent would pass and resolve easily such that even the larger sized particles would take time to resolve whereas the smaller sized particle will pass by easily. So chromatographic efficiency would increase greatly.
Hope my answer helps. Thank You.