If the product is colored, the elution can be followed by observing the loss of color of the support. Colored by-products are usually formed in these condensations which are more strongly held than the desired product. Methanol or ethanol will wash the KOH off the support. The purity of the eluted product can be followed using TLC or HPLC if available. If product isolation is desired, evaporate the elution solvent and recrystrallize the product from the appropriate solvent.
Reaction can remain for longer periods of time, up to one week, but often side products increase.
It is important that the aldehydes used be free of any corresponding acids. We routinely shake benzaldehyde or liquid substituted benzaldehydes with bicarbonate solution before use. Solid substituted benzaldehydes should be recrystallized or otherwise purified to remove any suspected acid impurities. Wash the resin several times in the solvent to be used before running the reaction. SPE tubes with 8 ml reservoirs, frits and stopcocks may be purchased from Alltech.