Labs

Purification by Flash Column Chromatography

5.1 - Competent Chemist Rating

“Looks Are Sometimes Deceiving”

Techniques Checklist

  • Analyzing mixtures by TLC
  • Assembling a silica gel column
  • Applying crude mixtures to a silica gel column
  • Separating simple mixtures with a silica gel column

Pre-lab Discussion

  • Theory of column chromatography—Reading Zubrick chapter 28, Mohrig chapter 18
  • TLC—polarity/solvent sys—Reading Zubrick chapter 27, LLP chapter 9.3.1, Mohrig chapter 17
  • Setting up silica gel column Reading Zubrick chapter 31, LLP chapter 11.6, Mohrig chapter 18
  • Applying crude mixtures to the column
  • Running a flash column

Digital Lab Techniques Manual

Equipment

  • Flash Chromatography Column
  • Air Flow apparatus (Stopper, T-valve, Screw clamp, tubing)
  • Round-bottomed flasks—1x100-mL, 2x500-mL
  • Test tubes—18x150 mm
  • Test tube racks
  • TLC plates—cut silica/glass plates and UV lamp
  • Large plastic funnels

Goal

  • Purify a contaminated compound using silica gel flash column chromatography.

Experiment Outline

  • You will be given 2 mL of an ether/pentane solution containing 1.00 g of benzylacetone contaminated with a small amount of guaiazulene.
  • Analyze this mixture by TLC—see TLC Guide, using 10% ethyl acetate/hexanes as the solvent system.
  • Record the Rf values.
  • Prepare the column in the hood, using 10% ether/pentane and 50 g (about 5’’) of of silica gel—see Flash Column Chromatography Guide.
  • Elute the column with 10mL of pentane—Apply your sample to the column, being careful not to disturb the top layer of sand. Rinse the sample flask three times with 1 mL pentane each, and use the rinses to wash the sides of the column.
  • Run the column, monitoring the fractions by TLC—See Flash Chromatography Guide and TLC Guide.
  • Concentrate the set of fractions containing pure benzylacetone.
  • Weigh the purified compound and prepare a GC and GC-MS sample.
  • Check product with TLC and obtain a GC and GC-MS spectrum.

Results

  • To obtain your “CC Rating” in Purification by Flash Column Chromatography, you should collect at least 0.95 g of benzylacetone. This sample must be at least 95% pure as demonstrated by GC spectroscopy. Your sample must also be submitted to the TA for possible weight and purity verification.

5.2 - Expert Experimentalist

Techniques Checklist

  • Picking the correct eluent then adsorbtion of a crude mixture onto silica gel
  • Separating complex mixtures—using gradient elution

Pre-lab Discussion

  • Suggest limited list of eluent solvent systems
  • Discuss sample adsorption and gradient elution strategies

Equipment

  • Identical to CC Level

Goal

Separate mixture of three compounds using gradient elution flash column chromatography.

Chemical Data

  • Benzylacetone—FW 148.21, bp 235 °C, d 0.989
  • Benzylideneacetone—FW 146.19, mp 39–41 °C
  • 3-Methylanisole—FW 122.17, bp 175–176 °C, d 0.969

Experiment Outline

  • You will be given 1.00 g of a mixture containing 0.60 g of the major ketone, 0.20g of the minor ketone, and 0.20g of methylanisole in 20 ml of an ether/ hexane solution.
  • Analyze this mixture by TLC using various solvent systems—see TLC Guide for hints.
  • Pick an eluent.
  • Decide on the silica gel to compound ratio.
  • Prepare the column.
  • Deposit the mixture on silica gel, dry completely, then apply to the column.
  • Run the column.
  • Concentrate pure fractions.
  • Weigh the purified compounds.
  • Analyze the pure ketones by NMR and TLC.

Note

  • The ketones are somewhat volatile, and 3-methylanisole, with its low molecular weight, is much more so. Therefore, do not concentrate it (or a mixture containing it) using the vacuum pump.

Results

  • To obtain your “EE Rating” in Purification by Flash Column Chromatography, you should get at least 0.45 g of the major ketone and 0.13 g of the minor ketone, and 0.13 g of methylanisole. These samples must be at least 95% pure as demonstrated by NMR spectroscopy. Your samples must also be submitted to the TA for possible weight and purity verification.

Course Info

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