Jtdylktuy

Recently, I carried out the synthesis of dibenzylideneacetone. I had a lot of problems with the purification process, especially the part involving recrystallisation. I was instructed to carry out the recrystallisation using an "ethanol/water" pair. That means to say, I was supposed to dissolve the crude product in ethanol until just enough dissolved, then add a small volume of water until some of the non-polar dibenzylideneacetone precipitated out of the solution and lastly, add another small volume of ethanol to re-dissolve the precipitate that had just formed. Once done, the beaker is removed from the hotplate and left to cool at room temperature. Subsequently, the crystals were washed with cold ethanol to finally obtain the final product. It is similar to what was done in this video.

During the practical I had to repeat the recrystallisation step multiple times for a variety of reasons. Firstly, there were some instances when the dibenzylideneacetone melted to form a yellow oily liquid. Secondly, there were also some instances when the crystals "crashed out" of solution upon cooling. This resulted in poor-quality crystals which clumped together. I am just wondering if anyone has any tips on how I could have improved my procedure so that I don't have to perform recrystallisation multiple times and yet still obtain poor-quality crystals.

Armarego's Purification of Laboratory Chemicals suggests using dichlormethane–hexane pair instead of ethanol–water; it also lists crystallization from warm ethyl acetate, ethanol or petroleum ether as alternative [1, p. 342].

Dibenzalacetone [DBA, bda, trans-trans-1,5-diphenyl-1,4-dien-3-one, 1,5-(bisphenyl)-penta-1E,4E-diene-3-one] [538-58-9] $ce{C17H14O}$, $M~234.3$, $m~107°$, $111°$, $112°$, $113°$, $120-122°$.
Purify the ketone by flash chromatography (150 mesh $ce{Al2O3}$ deactivated with $6%$ v/w $ce{H2O}$) using petroleum ether $40-60°/ce{EtOAc}$ ($4/1$, v/v), and the yellow solid is recrystallised by layering a concentrated $ce{CH2Cl2}$ solution with hexane (i.e. $ce{CH2Cl2}$/hexane, $1:4$, v/v).
Also recrystallise the ketone from hot ethyl acetate $(pu{2.5 ml/g})$ or $ce{EtOH}$.
It has been also crystallised from petroleum ether b $30-60°$.

References

1. Armarego, W. L. Purification of Laboratory Chemicals; Elsevier: Boston, MA, 2017. ISBN 978-0-12-805457-4.

Ethyl acetate is the solvent of choice in Org. Synth. 1932 article. The procedure was submitted and checked. The following is the the full procedure stated in Organic Syntheses.; Collective Volume (Ref.1):

A cooled solution of 100 g. of sodium hydroxide in 1 l. of water and 800 cc. of alcohol (Note 1) is placed in a 2-l. wide-mouthed glass jar which is surrounded with water and fitted with a mechanical stirrer. The solution is kept at about 20–25° and stirred vigorously (Note 2) while one-half of a mixture of 106 g. (1 mole) of benzaldehyde and 29 g. (0.5 mole) of acetone is added (Note 3). In about two or three minutes a yellow cloud forms which soon becomes a flocculent precipitate. After fifteen minutes the rest of the mixed reagents is added, and the container is rinsed with a little alcohol which is added to the mixture. Vigorous stirring is continued for one-half hour longer, and the mush is then filtered with suction on a large Büchner funnel. The product is thoroughly washed with distilled water (Note 4) and then dried at room temperature to constant weight. The yield is 105–110 g. (90–94 per cent of the theoretical amount) (Note 5) of a product which melts at 104–107°.

The crude dibenzalacetone may be recrystallized from hot ethyl acetate, using 100 cc. of solvent for each 40 g. of material. The recovery in this purification is about 80 per cent; the purified product melts at 110–111°.

The Notes (1-5) are as follows:

1. Sufficient alcohol is used to dissolve the benzaldehyde rapidly and to retain the benzalacetone in solution until it has had time to react with the second molecule of aldehyde. Lower concentrations of base slow up the formation of the dibenzalacetone and thus favor side reactions which yield a sticky product. Higher concentrations of base give added difficulty in washing. These concentrations were suggested by, and are approximately the same as, those used in the preparation of benzalacetophenone described in Org. Syn. Coll. Vol. I, 1941, 78.


2. Only temperatures between $20$ and $25^{circ}$ were tried; it was assumed that a change of temperature would have the same effect that it has in the preparation of benzalacetophenone mentioned above. Stirring is essential, as it makes considerable difference in the uniformity of the product.


3. The benzaldehyde was u.s.p. quality which had been washed with sodium carbonate solution and distilled. Commercial c.p. acetone was used. The theoretical quantities are used, since an excess of benzaldehyde results in a sticky product while an excess of acetone favors the production of benzalacetone. The mixture is prepared before addition in order to ensure additions of equivalent quantities.


4. Since the product is practically insoluble in water, large amounts can be used in the washing. Sodium compounds are probably the chief impurities. The dried product contains some sodium carbonate which results from the failure to remove the sodium hydroxide completely. There remain also the impurities insoluble in water. However, the product is pure enough for use in most reactions.


5. If the mush is allowed to stand several hours, chilled, and filtered cold, a slightly larger yield is obtained, but this is not worth while. The filtrate may be used as a medium for a second run in which about 93 per cent of the theoretical yield is obtained. The melting point of the second product is slightly lower.

I think Note 4 should be important to OP, because it avoids product purification by chromatography.

Reference:

Charles R. Conard, Morris A. Dolliver, "dibenzalacetone [1,4-pentadien-3-one, 1,5-diphenyl-]," Org. Syn. Coll. Vol. 2, 1943, 167 (http://www.orgsyn.org/demo.aspx?prep=CV2P0167) and Org. Synth. 1932, 12, 22 (DOI: 10.15227/orgsyn.012.0022).