We had an experiment on ALDOL CONDENSATION – SYNTHESIS OF DIBENZALACETONE Please

Question

We had an experiment on ALDOL CONDENSATION – SYNTHESIS OF DIBENZALACETONE

Please answer the following:

1) The product formed has a pale yellow color. Why?

2) Why is C=O stretching frequency at lower wavenumber than a standard C=O frequency

Regards,

The procedure is here:

1. To a clean, dry 125 mL Erlenmeyer Flask add:
15 mL 95% Ethanol and
20 mL 3M Sodium Hydroxide (NaOH)
2. Obtain a vial of Benzaldehyde from the instructor containing approximately 2.8 mL of
Benzaldehyde
3. Weigh the vial to the nearest 0.001 g
4. To the vial containing the Benzaldehyde, add 1.0 ml Acetone
5. Weigh the vial again
6. Add the contents of the vial (Benzaldehyde and Acetone) in 2 portions, with swirling, to the
Erlenmeyer flask containing the Ethanol & NaOH.Stir the reaction mixture at room
temperature for 15-20 minutes.As the reaction progresses, a yellow precipitate should
form.

Then for Vacuum filtration
• Filter the solid product using vacuum filtration.
• Continue filtration until all liquid passes into the flask.
• Wash the product with two (2) 10 mL portions of Ethanol.
• Note: Vacuum process must be complete for each step.
• Place Vacuum Filtration Waste into the appropriate bottle in hood.
• Transfer the crystals to a 150 ml beaker.

For Recrystallization
• To the above beaker add 10 ml Ethanol.
• Heat mixture until solids dissolve completely.
• Note: For every gram of Dibenzalacetone use 5 mL of ethanol for recrystallization. It may be
necessary to add one or more 1 mL increments to effect complete dissolution.
• Remove the beaker from the hot plate and allow to cool slowly to room temperature.
• As the solution reaches to room temperature, the crystals should appear. Cool the flask in
icewater bath for 510 minutes.
• Collect the crystals by vacuum filtration. Stir the crystals occasionally to allow any
remaining ethanol to evaporate.Dry the crystals by spreading them thinly over a
pre weighed watch glass.
• Perform a TLC analysis to check the purity of the product.Take a small amount of the
product and dissolve in ethanol to be used for TLC analysis.
Use stock sample of benzaldehyde as the reactant control. A 10 ml solvent system of 90%
Hexane and 10% Ethyl acetate will be used for TLC analysis.
• Determine the Mass of the dried product.
• Calculate percent yield.
• Place the weighing tray in the class drawer and allow to dry for a week.
• Determine Melting Point of the product (M.P. Dibenzalacetone – 113oC (decomposes).

Explanation / Answer

The product is a alpha beta unsaturated ketone. The extended conjugation will reduce the homo lumo gap and the energy difference corresponds to the visible region( yellow).

The conjugation between double bond and the carbonyl bond reduces the force constant of the C=O bond. Hence the streching frequency decreases.

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