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Experiment 8 The Aldol Condensation: Preparation of Dibenzalacetone Objectives Carry out a mixed aldol condensation between acetone and benzaldehyde. Characterize the product using IR and NMR spectroscopy, and melting point. Use proton NMR coupling constants to determine double bond configuration. NaOH Adapted from Williamson, Kenneth L. and Masters, Katherine M., Macroscale and Microscale Organic Experiments, 6th edition. Belmont, CA: Brooks/Cole, 2011, pp486-7. Procedure Twenty-five mL of a 1.25 M solution of NaOH in aqueous ethanol (provided) is placed in a 125 mL Erlenmeyer flask along with a 1-inch magnetic stirring bar and a thermometer. The solution is stirred while the calculated volume of benzaldehyde is added. (Note temperature and other changes.) The calculated volume of acetone is then added and the mixture is stirred for 30 minutes. Water (10 mL) is added to the reaction mixture and the solid product is isolated by vacuum filtration on a Buchner funnel. The solid is washed three times with 25 mL portions of water and air dried for several minutes. A small amount of solid is placed in an open test tube for melting point analysis and the remainder is recrystallized from 95% ethanol. (while crystallizing is taking place, the contents of the filtration flask are transferred into the appropriate aqueous waste container. The flask is rinsed with water and the Buchner funnel is rinsed with acetone.) The recrystallized product is collected by vacuum filtration and washed with a small amount of chilled 95% ethanol. The crystals are air-dried until the next lab period and characterized by melting point (on crude and final product), and IR and NMR spectroscopy. The coupling constant, J, for the alkenyl protons is determined using the appended instructions.
Expert Answer
Solution:
In the IR spectrum of pure dibenzalacetone, the carbonyl peak C=O stretching vibration occur at a lower frequency than the 1720 cm-1 ketone C=O peak in acetone because in dibenzalacetone carbonyl group is conjugated with the aromatic ring and conjugation of the carbonyl group with the aromatic ring shifts the band to the 1685 – 1649 cm-1 region. a medium intetensity band at 1600 cm-1 appears due to C=C stretching band. Further band due to aromatic vibrations appear at around 3025- 2050 cm-1 and 1580- 1588 cm-1The student spectrum of synthesized dibenzalacetone can be compared to the IR spectra of the pure dibenzalacetone pure acetone and pure benzaldehyde. IR spectrum of the synthesized dibenzalacetone should not contain any additional bands excluding the above mentioned band. In case of presence of additional bands the product synthesized may be contaminated.
The 1H NMR spectrum of dibenzalacetone exhibit a multiplet around 7.4-7.5 ppm, which represents the meta- and para- protons on the benzene rings. The peak around 7.6 ppm represents the ortho- protons on the benzene rings. The peaks around 8.8-8.9 ppm represent the protons nearest to the carbonyl. The peaks around 7.0-7.1 ppm represent the protons between the carbonyl carbon and benzene ring. Similar IR spectrum, the 1H NMR spectrum of synthesized dibenzalacetone can be compared to the NMR spectrum of pure dibenzalacetone, benzaldehyde and acetone. Appearance of additional peaks besides the above mentioned peaks in indicates the presence of impurity.
