I want to separate my target compound, 1, 3-phenylene bis[4-{[4-(pent-4-en-1-yloxy)phenyl]diazenyl}benzoate (ester) from the reactant 4-{[4-(pent-4-ene-1-yloxy) phenyl] diazenyl} benzoic acid. I used the ethyl acetate and hexane (3:1) as eluent to separate them.Even though the separation of these two compounds over the silica gel is good (they are two distinctive bands with long distance between them, plus there is only one spot observed on tlc), i realized that the compound is still not pure because the ftir shows the functional group of carboxylic acid. The reactant acid should be stuck at the top of column. Can anyone tell me what happened and suggest me a better way to purify my compound?
Check out the purification of a similar compound Ethyl 4-{[4-(but-3-en-1-yloxy)phenyl]diazenyl}benzoate
How dry is your silica? if there is some residual water, the acidity of the silica might allow for some hydrolysis. You could try oven drying some silica to see if that makes a difference. Also, rather than chromatography, you might also try just stirring a solution of the mixture over a small amount of silica and filter off the silica. The acid should stick to the silica more than the ester and provide some purification.
I second William Hartners suspicion of hydrolysis on silica. Another way to check this is to run a 2D-TLC of your product: cut a square TLC plate and apply one (and only 1) spot of your product in a corner. Run the TLC, which should result in one or more spots on a vertical line. Then tilt the plate 90°, those spots are now on a horizontal line, which is the 'new' baseline. Run the TLC once more in the new direction, starting from the 'new baseline'. If there's no breakdown of product(s) on the silica, all resulting spots should be on a diagonal line on the plate. In case of product breakdown, you'll see spots on other places than the diagonal of the plate.
Thomas Verbrugghen William Harter Jürgen Wintner thank you for your insights.i will try it first.
Washing with a dilute aqueous base solution is a viable option if the ester is stable enough. It will depend on time and temperature as well as the scale that you intend on running. Sodium bicarbonate is only soluble in water to ~10g/100mL at ~20°C so a 50% solution isn't possible, but a saturated solution might work on a lab scale. On a pilot plant scale, that may not work as well. Alternatively, a basic amine resin could absorb traces of acid.
Thomas Verbrugghen i already tried the 2D TLC and the product is stable over the silica. William Harter The intensity of ester bond and acid bond increases and decreases, respectively. It shows show removal of ester from acid compound. However, stirring the solution over silica and purify the compound by using column are the same thing right? but why i cannot remove it by chromatography? The acid compound should be stuck in silica.
Column chromatography and stirring the solution over silica are similar but not the same. The biggest difference is the contact time with the silica. Chromatography has a much longer residence time than stirring the material over silica, giving a great chance to hydrolize the ester. If you stir silica with your material, be careful not to rinse the silica too much to avoid eluting the acid from the silica.
Separate ester from carboxylic acid by using chromatography can be achieved by SiO2 gel column chromatography using ethyl acetate as a solvent. Ethyl acetate will be eulated first while acid later.
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