Synthesis of Dibenzalacetone: Microscale Procedure
IN THIS EXPERIMENT an ethanolic solution of acetone and benzaldehyde is to aqueous
sodium hydroxide. The product, dibenzalacetone, crystallizes after minutes. The product
is filtered from the mixture, washed, pressed dry, and realized from an ethanol-water
mixture. This very important reaction is easily carried out.
Into a 10 X 100-mm reaction tube place 2 mL of 3 M sodium hydroxide solution. To this
solution add 1.6 mLof95% ethanol and 0.212 g of benzaldehyde. Then add 0.058 g of
acetone to the reaction mixture. Alternatively, your instructor may provide a solution that
contains 58 mg of acetone in 1.6 mL of ethanol the tube immediately and shake the
mixture vigorously. The benzaldehyde, initially insoluble, goes into solution, resulting in
a water-clear, pale-yellow solution. After a minute or so it suddenly becomes cloudy, and
a yellow precipitate product forms. Continue to shake the tube from time to time for the
next 30 min. If the product fails to crystallize, open the tube and scratch the inside of the
tube with a glass rod. Remove the liquid from the tube using a Pasteur pipette by
squeezing the bulb of the pipette, pressing the tip against the bottom of the tube, and
bringing the liquid into the pipette, leaving the crystals in the tube (Pig. 37.1). Add 3 mL
of water, cap, and shake the tube vigorously. Remove the wash liquid as before and wash
the crystals twice more with 3-mL portions of water.
After the final washing, add 3 mL of water to the tube and collect the crystals on a Hirsch
funnel using vacuum nitration. Use the filtrate to complete the transfer of the crystals.
Squeeze the product between sheets of filter paper to dry it and then recrystallize the
crude dibenzalacetone from a 70:30 ethanol-water mixture. During the recrystallization
process (see Chapter 4), insert a wooden boiling stick to promote even boiling when
heating the solute in the solvent. Remove the tube from the hot sand bath and place it in
an insulated container to cool slowly to room temperature. Should the product separate as
an oil, obtain a seed crystal, heat the solution to dissolve the oil, and add the seed crystal
as the solution cools. If the product continues to oil out, add a bit more ethanol. After
cooling the tube for several minutes in ice, collect the product by removing the solvent
with a pipette (Fig. 37.1) and washing once with about 0.5 mL of ice-cold 70% ethanol
while the tube is in ice. Alternatively, collect the crystals on a Hirsch funnel (Fig. 37.2) or
Wilfilter and wash once with ice-cold 70% ethanol. Dry the product under vacuum by
attaching the tube to an aspirator (Fig. 37.3). Determine the melting point a pure sample
will have a melting point between 110.5-112°C. The yield should be around 0.10 grams.