Spring 2004
Chemistry 3719L – Week 10
Aldol condensation – Synthesis of Dibenzalacetone
Pre-lab reading from Zubrick:
Chapter 13:
Chapter 12:
Whole Chapter – review recrystallization
Pages 87-92 – review melting points
Aims
The aldol reaction is one of the most studied in all of Organic Chemistry and it also plays a major
role in the formation of C-C bonds in Biochemistry (i.e. biosynthesis). This experiment highlights the
formation of two new C-C bonds via deprotonation alpha to the carbonyl in acetone to generate the
enolate, which then attacks the carbonyl of (nonenolizable) benzaldehyde. Overall the aldol reaction
happens twice followed by loss of water to give the highly conjugated enone system of the product.
Since two different carbonyl precursors are used this is known as a crossed aldol reaction.
Reaction
Acetone (1) has six alpha protons, which are all equivalent. Deprotonation with base generates the
corresponding nucleophilic enolate, which then attacks the electrophilic benzaldehyde (2) to form a new
carbon-carbon bond. This can happen again at the other alpha position such that one molecule of acetone
has reacted with two molecules of benzaldehyde. Loss of two molecules of water is favored since the
product is very highly conjugated dibenzalacetone (3).
H
+
1
H
O
O
O
H
NaOH
EtOH, H2O
H
H
3
2
Procedure
• Dissolve sodium hydroxide (5g) in water (50 mL) and ethanol (40 mL) in a 250 mL Erlenmeyer
flask. Make sure the mixture is at room temperature.
• Mix benzaldehyde (5 mL) and acetone (1.9 mL) in a separate flask (small Erlenmeyer) and then add
half of this mixture to the sodium hydroxide solution. Stir at room temperature for 15 minutes.
• Add the rest of the acetone/benzaldehyde mixture and wash out the container with a couple of mL of
ethanol, adding the washing to the reaction flask.
• Stir the mixture at room temperature for a further 30 minutes then collect the solid product by
vacuum filtration. Use the correct size of paper to avoid losing product.
• Wash the product with distilled water (4 times, each with 50 mL of water) to remove any sodium
hydroxide that remains in the product.
• Air dry the solid for ~15 minutes then recrystallize the product using ~10 mL of ethanol for every 4g
of product.
• Record the weight of pure dibenzalacetone and obtain its melting point.
• You should hand the product to your TA in a sealed, labeled test tube for grading.
Synthesis Report = 10 pts
Adapted from Williamson, Macroscale and Microscale Organic Experiments, Houghton Mifflin.