Aldol Condensation

<>Introduction:

The purpose of this experiment is to synthesis dibenzalacetone (trans, trans-1,5-diphenyl-1,4-pentadien-3-one) through the aldol condensation of acetone with benzaldehyde.  The synthesis begins by using strong base to generate the acetone enolate ion.  Water (not shown) is formed as a byproduct. The equilbrium position of this reaction strongly favors the starting acetone, and the amount of acetone enolate formed is quite small;  however the enolate is extremely nucleophilic


Being a very strong nucleophile, this enolate attacks the carbonyl of benzaldehyde and forms a B-carbonyl alkoxide ion.  This alkoxide ion abstracts a proton from water to form a beta hydroxy ketone. Sodium hydroxide abstracts another acidic alpha H to form a stabilized carbanion.  The electron pair on carbon is used to eliminate the hyroxide ion, forming a alpha-beta unsaturated ketone in an irreversible step.   This is an example of an E1CB mechanism.   Note that in this reaction, the intermediate alcohol is dehydrated under basic conditions, unlike most alcohol dehydrations, which are generally E1 mechanisms under acid conditions.   The E1CB mechanism is made possible by the presence of the carbonyl, which stabilizes the intermediate carbanion.

Since this newly formed ketone still posses alpha hydrogens, it too can undergo the same enolate condensation reaction with a second mole of benzaldehyde to form the final product:

 


Procedure
:

Before you come into lab, make sure you have filled in your table of reagents and products.  You will need these values (particularly the molecular formula and molecular weight) to determine the identify of your products and to calculate your final yield.  Take care to determine the coefficients of the balance chemical reaction when determining the theoretical yield.  .You will also need to come to lab with IRs of your starting materials and expected product(s) already in your notebooks (General Chemistry Links).  Finally, it is important that you know exactly what you are going to be doing so you can work more efficiently:

  1. Pipet into a 250mL Erlenmeyer flask the following quantities:  1.43g acetone and 5.25g benzaldehyde (densities will be needed to determine the proper volumes). Note:  it is important to maintain a 1:2 molar ratio of acetone to benzaldehyde.  You may have to adjust your volumes accordingly.  
  2. Add a magnetic stirring bar to the Erlenmeyer flask and set the motor for the faster stirring possible WITHOUT splatter.
  3. In a 100 mL beaker, thoroughly mix 20 mL of 6 M sodium hydroxide and 20 mL of 95% ethanol.
  4. Pour this mixture into the Erlenmeyer flask and adjust the stirrer motor to maintain maximum stirring.
  5. Continue to stir for an additional 15 minutes.  Note any color changes that may occur.
  6. Isolate the product by suction filtration and wash with COLD ethanol (remember:  it takes a while to cool the ethanol so plan ahead).
  7. Continue the suction filtration for an additional 5-10 minutes to allow the product to air dry.
  8. Weigh the crude product and determine its melting point.
  9. Recrystalize your product by dissolving it in 15 mL of ethyl acetate.  Use low heat on your hot plate and stir until all of the product dissolves.  Stir in 15 mL of 95% ethanol and remove from heat.  Continue to stir constantly until crystals begin to form.
  10. Collect the crystals by suction filtration and allow to air dry for 5-10 minutes.
  11. Estimate the purity of your product using IR and melting point data

 


Conclusions
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 (Created by C.R. Snelling
(Updated by J. Neilan, 8/7/06))