

Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
Chem 322 experiment procedure and queries prepared by Dr. Laurie S. Starkey
Typology: Lab Reports
1 / 3
This page cannot be seen from the preview
Don't miss anything!
Introduction:
In this experiment, you will perform a type of base-catalyzed crossed aldol condensation called the Claisen-Schmidt reaction. The aldol reaction is used extensively to synthesize new C-C bonds. In a crossed aldol synthesis, two different aldehydes or ketones (or one ketone and one aldehyde) react in the presence of dilute base to yield β-hydroxyaldehydes or β-hydroxyketones. In most base-catalyzed aldol reactions, the end product is an α,β-unsaturated aldehyde (or ketone) and a separate molecule of water. The initial product results from the attack of an α- carbon (in the form of an enolate ion) of aldehyde (or ketone) on an electropositive carbonyl of a second aldehyde (or ketone). A Claisen-Schmidt reaction product always has at least one double bond that is conjugated to both a carbonyl group and an aromatic ring. In this particular experiment, you will be preparing dibenzalacetone, a compound that once was used as an active ingredient in some sunscreen formulations because of its ability to absorb UV radiation. See the overall reaction below:
The two organic starting materials have many uses outside of chemistry. Benzaldehyde is found in almonds and almond paste which can be used in lots of cookies and nutritional bars recipes. Some uses of acetone are 1) a paint and varnish remover; 2) a solvent in many varnishes, rubber cements, lacquers, etc.; and 3) an indicator of metabolic disorders, like diabetes, if present at high concentrations in the bloodstream because acetone is a natural metabolic byproduct normally found in limited quantity in the body.
Normally, crossed aldol condensations produce 4 different products. However, several precautions have been taken to make this reaction successful.
2.2 moles of benzaldehyde to 1 mole of acetone will be used instead of a 2.0 : 1 molar ratio of benzaldehyde to acetone as required by the stoichiometry. A slight excess is important because:
In this experiment, students work as pairs to test two different procedures. In synthesis route A, only half of the initial ketone-aldehyde mixture will be able to react with the base in the beginning of the reaction. In synthesis route B, a solution of acetone in water will be slowly dripped into a mixture of the base and benzaldehyde.
Experimental Procedure:
******Decide with your partner which synthesis route each person will be doing. One student should do Synthesis Route A, and one student should do Synthesis Route B. ******
Synthesis Route A:
Add 20 mL of water to a 100 mL beaker. In another 100 mL beaker, weigh out 2.2 g of NaOH pellets. With good mixing, slowly add the NaOH pellets (a couple of pellets at a time over a couple minutes) to the 20 mL of water. Note: NaOH is hygroscopic so weigh this out as quickly as you can without spilling the pellets. If any pellets spill, clean them up immediately. Note: this is an exothermic reaction, so do not turn the heat on. You may need to cool the beaker to control the temperature as the NaOH pellets go into solution. When the NaOH pellets are completely dissolved, bring the solution to room temperature, if necessary, and then add 16 mL of 95% ethanol. Use a micropipetter to prepare a solution of 2.26 mL benzaldehyde and 0.73 mL reagent grade acetone. Thoroughly mix the components in a small test tube equipped with a stopper.
Use a Pasteur pipette to transfer half the benzaldehyde/acetone mixture to the NaOH/ethanol/water solution (re-stopper the test tube). Stir the reaction mixture at room temperature for 15 minutes. Transfer the remaining benzaldehyde/acetone mixture into the reaction mixture. Wash the small test tube with about 1 mL of 95% ethanol, and add this wash to the reaction mixture. Keep stirring the reaction mixture at room temperature for an additional 15 minutes.
Synthesis Route B:
Add 18 mL of water to a 100 mL beaker. In another 100 mL beaker, weigh out 2.2 g of NaOH pellets. With good mixing, slowly add the NaOH pellets (a couple of pellets at a time over a couple minutes) to the 18 mL of water. Note: NaOH is hygroscopic so weigh this out as quickly as you can without spilling the pellets. If any pellets spill, clean them up immediately. Note: this is an exothermic reaction, so do not turn the heat on. You may need to cool the beaker to control the temperature as the NaOH pellets go into solution. When the NaOH pellets are completely dissolved, bring the solution to room temperature, if necessary, and then add 17 mL of 95% ethanol. Continue stirring the mixture. Use a micropipetter to add 2.26 mL of benzaldehyde to the NaOH/ethanol/water solution. Use a micropipetter to obtain 0. mL of reagent grade acetone, and another micropipetter to obtain 0.73 mL of DI water. Combine these with good mixing into a small test tube equipped with a stopper. Use a Pasteur pipette to add the acetone/water solution to the mixture of benzaldehyde/NaOH/ethanol/water dropwise (about 4 -5 drops a minute. Rate of addition is important!). Cap the solution of acetone and water in-between additions to minimize evaporation. Rinse the small test tube with approx. 1 mL of DI water, and add this to the reaction mixture. Stir the reaction mixture at room temperature for 30 min (begin timing at first sign of cloudiness).