We performed an Aldol condensation of acetone with benzaldehyde to form dibenzal

Question

We performed an Aldol condensation of acetone with benzaldehyde to form dibenzalacetone. NaOH was used as base, in ethanol solvent. In the work-up, we used water to remove acetone and NaOH and the water was removed by a syringe to practice decanting. We, then, added ethanol while the reaction vial was in an ice bath and let it stir for 2 minutes. We removed the ethanol and added more ethanol but the reaction vial was put in a hot bath (about 90 degrees celcius) under reflux until the solution became clear (1-2 minutes). Then we set it down and let it cool to room temp and let the crystals form. We then removed the ethanol and added cold 0.5mL ethanol and gently agitated the mixture to wash the crystals in an ice bath.

So, I know that putting in the warm bath under reflux until clear and letting it cool down is the recrystallization portion and I know why recrystallization for this is a good purification method. What I don't understand is why we had room temp ethanol wash before that and cold ethanol wash afterwards. Thank you in advance!

Explanation / Answer

Recrystallization is a method of purification, it works based on solubility difference of a compound in certain solvent in cold and hot condition. The compound should be sparingly soluble in particular solvent at room temperature or cold conditions and it should be clearly soluble in hot conditions, This is the basis for recrystallization.

Coming to the problem, initially our purpose was to dissolve the compoud completely in ethanol by heating. Any way we are going to heat the mixture so ther is no point in taking cold ethanol. Where as after the crystals are developed, the task was to filter the crystals to collect them as they are pure and the filtrate will have other impurities. At this stage if we use hot ethanol or rooom temprature ethanol, there is a risk of compound going dissolved into the ethanol so we ultimately loose some of the comopound in filtrate if we use room temperature or hot ethanol. Therefore we had to use cold ethanol for washing the crystls because the compound is almost insoluble in ethanol in cold conditions.

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