The Aldol Condensation Puzzle CH344 Bruce A. Hathaway

Crossed Aldol Condensations One variation of this reaction uses a ketone with α-hydrogens reacting with an aldehyde that does not have α-hydrogens. This reaction, called the Claisen-Schmidt reaction, often gives good yields of the conjugated ketone products. An example is shown below.

The Reaction Today Sign up for a particular combination of Aldehyde (A, B, C, or D) and Ketone (1, 2, 3, or 4). The ketones have droppers marked at the 0.5 mL mark. Use your graduated cylinders to measure 2.0 mL of aldehyde.

Reaction Procedure In a 50 mL round-bottomed flask, place in this order the ketone (0.5 mL), the aldehyde (2.0 mL), 95% ethanol (12 mL – use the ethanol to rinse your 10 mL graduated cylinder), and 2M aqueous sodium hydroxide (10 mL). Stir or shake the reaction mixture until no more precipitate is observed to form. Some reactions form precipitate almost immediately; others require several hours.

Reaction Procedure If no precipitate has formed after 15 minutes, you have a slow-reacting mixture. Attach a reflux condenser, and heat the reaction mixture at reflux for about 30 minutes, then allow the reaction to cool, and see if crystals form. Magnetically stirring the reaction mixture also speeds up the rate of reaction.

Isolation of the Product Once crystallization is complete at room temperature, cool the reaction in an ice bath, and then collect the product by suction filtration. Wash the product consecutively with ice-cold, 5 mL portions of: 1. 95% ethanol, 2. 5% acetic acid in 95% ethanol, 3. 95% ethanol.

Recrystallization and TLC Recrystallize a small portion of your product from either 95% ethanol or toluene (whichever solvent works the best). Check the purity of your crude and recrystallized products by TLC. Dissolve tiny amounts of each in ethyl acetate in separate small test tubes, and spot each solution on a small silica gel TLC plate.

Recrystallization and TLC Use toluene dichloromethane as a developing solvent, and visualize the plate with UV light to see the spots if needed. When you have demonstrated that your recrystallized product is pure, you can obtain the NMR spectrum of your product from your lab instructor.

Next Week Student Evaluation. Weigh your crude product. Take melting point ranges of your crude and recrystallized products. Turn in your product. Check out of lab.

Lab Final Exams Thursday Lab Final is Tuesday, May 11, 2PM, in Magill 148! Friday Lab Final is Wednesday, May 12, at noon, here (after the lecture final). If you are in the Friday lab, and would prefer to take you lab final on Tuesday, let me know no later than Friday, May 7.