Experiment 35: The Aldol Condensation Reaction:
Preparation of Benzalacetophenones (Chalcones)
The reaction. Substitute this procedure for the one in the Textbook in Experiment 35, pp
316-319. In advance, carefully determine which two reactants you will need. Also
determine the molecular weights of the two compounds. The substituted acetophenone
comes from Experiment 58.
Weigh out 1 mmol of your selected substituted benzaldehyde in a tarred conical vial.
Then weigh out 1 mmol of the acetophenone that you prepared in Experiment 58 and add
it to the vial. If you have less than 1 mmol of the substituted acetophenone, adjust the
amounts so that you maintain a 1:1 ratio of reactants. Then add 0.80mL of 95% ethanol
to the aldehyde and acetophenone mixture. Place a magnetic spin vane in the mixture
and stir 3-5 minutes to make sure it is thoroughly mixed and that no solids remain. If
solids are still present after stirring, heat may be used to dissolve them. If heat is used
wait until the mixture has returned to room temperature to continue with the next step.
Next add to the mixture 0.10mL of a concentrated sodium hydroxide solution1 from one
of the plastic disposable pipets to the mixture. Stir the mixture using the magnetic spin
vane for 25-30 minutes. Before the mixture solidifies, you may observe some cloudiness.
Wait until the cloudiness has been replaced with an obvious precipitate settling out to
the bottom of the conical vial before adding water indicated in the next paragraph. If
you don’t see an obvious precipitate after 30 minutes, try scratching the inside of the
conical vial vigorously to induce solidification of the oil. If you still don’t see a
precipitate, cap the conical vial and leave it in your locker until the next lab period.
Isolation of Crude Product
*This part of the procedure should not be used until a solid is present. If you add
water too soon, product may not form. When an obvious solid has formed, break up the
solid using a microspatula. Then transfer the mixture to a small beaker with 3 mL of ice
water. Stir the precipitate with a microspatula to break it up and then collect your solid
on a Hirsch funnel, using a vacuum filtration apparatus. Wash the solid twice, with two 1
mL portions of cold water. Remove the solid and allow it to air dry for 20 to 30 minutes
out in the open. Weigh the solid chalcone and calculate percentage yield.
Crystallization of Chalcone. Transfer your weighed crude product into a small
Erlenmeyer flask. Then add the minimum amount of boiling 95% ethanol required to just
dissolve your crude product. Put a boiling stone in with the 95% ethanol in order to
prevent bumping. Also be sure to heat and swirl your flask while adding portions of
ethanol. After dissolving your product, cork the flask and allow the mixture to cool
slowly. When it appears that crystallization is complete, place the mixture in an ice water
bath for 5 minutes. Isolate your product by vacuum filtration on a Hirsch funnel. Break
up the solid in the flask and transfer the solid to the funnel using 2 mL portions of icecold 95% ethanol to aid in the transfer. Rinse the crystals with another 2 mL portion.
Allow your product to air dry, and then remove the filter paper and allow the solid to air
dry for about 20 minutes. Weigh your chalcone, determine the percentage yield, and also
determine the melting point. Submit the purified product in a properly labeled sample
vial. How does the melting point compare to the literature value for your chalcone?
1
This concentrated sodium hydroxide solution will be prepared by the prepper in the ratio of 6g of sodium
hydroxide to 10 mL of water.