Expt 4. Preparation of
Dibenzalacetone vial Aldol Condensation
Relevant textbook reading - https://www.proton.guru/part-vi-lessons,
Lesson VI.17
Overview – In this experiment, you will carry
out a double crossed aldol condensation reaction. The
reaction uses the simple starting materials, acetone and
benzaldehyde, to form the 17-carbon product commonly
known as dibenzalacetone. The product will be
recrystallized (if time allows) and characterized by
NMR, and melting point determination.
Procedures
Running the reaction.
- In a 50-mL Erlenmeyer flask equipped with a
magnetic stirring bar, prepare a mixture of 7.5 mL
EtOH, 1.5 mL 10% NaOH, and 1.5 mL water.
- To the stirring solution add 14 mmol benzaldehyde.
- Add 6.5 mmol acetone and note the time. (You will
want to time your observations throughout the
reaction.)
- Continue stirring for 30 min.
Work-up procedures.
- Add 10 mL water, and then vacuum filter the
mixture.
- Wash the solid three times with 25-mL of water
each time.
- Allow the vacuum to pull air through and press the
solid dry by pressing a filter paper or rubber glove
to squeeze out more water.
- Measure the crude yield and save a small sample
for melting point determination.
- Recrystallize the crude product from ethanol.
- Allow the crystals to dry thoroughly on the vacuum
and pressing as before.
- Measure the final yield.
Characterization
of Product
Obtain the 1H and 13C NMR
spectrum using CDCl3 as the solvent.
Obtain an IR spectrum using the dry film method.
Obtain the mp of both the crude and final
products.
Assigned
Questions
1. Why is it important to add the benzaldehyde
first and then the acetone rather than in the
opposite order?
2. What is the purpose of the water washes in step 2
of the work up procedure?
3. What does the coupling constant observed for the
alkene proton doublets reveal about the
stereochemistry of the product?
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