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Transcript

SPECIAL SYNTHESIS

Presented by: Sandy Thea San Carlos

Discussion

Introduction

2,4 DNP test: positive

Iodoform test: negative

Condensation Reactions

Tollens' test: negative

Enolates

Mixed Aldol Condensation

MP: 103- 108 C

Theoretical Yield: 0.91 g

Percent Yield: 102.46%

Percent Recovery: 59.14%

  • occur between the alpha carbons of two carbonyl- containing functional groups
  • common: aldehydes, ketones and esters
  • aldehydes (Aldol condensation) ; aldehydes and ketones (Mixed Aldol condensation); esters (Claisen condensation)
  • typically catalyzed either by acid or base
  • carbonyl- containing functional groups that contain at least one alpha hydrogen atom that are susceptible to reaction with base to generate an anion
  • nucleophilic reaction intermediates that are stabilized via resonance with the carbonyl pi bond
  • react readily with electrophiles
  • reactive enolate in a base- catalyzed condensation reaction is derived from a ketone and the reactive carbonyl- carbon is part of an aldehyde group
  • synthesis: reaction between an enolate derived from acetone and electrophilic carbonyl carbon of benzaldehyde
  • benzaldehyde: has no alpha hydrogens (cannot serve as enolate); carbonyl- carbon is more electrophilic than carbonyl carbon of acetone (less steric crowding around carbonyl group)

Mixed Aldol Condensation

Synthesis of Dibenzalacetone

The purpose of this experiment was to synthesize dibenzalacetone via an aldol condensation reaction between acetone and benzaldehyde. This was done by mixing the two reactants with NaOH and ethanol, then allowing the reaction to sit for thirty minutes. The crystals were then washed with water three times and recrystallized using ethanol. It was then characterized using melting point analysis. The percent recovery for this reaction was 59.14%. This was due to loss of crystals during recrystallization and during solvent removal from the reaction mixture. The observed melting point was 103 – 108°C, compared to a literature value of 110°C. The lower and broader observed melting point may have been due to the product still being wet. It may also be due to unevaporated ethanol or other impurities in the product. However, the observed melting point was close to the literature value, and it can thus be concluded that the product was dibenzalacetone. Thus, the aldol condensation reaction was successful.

Mechanism

Results

Action

  • benzaldehyde + acetone
  • mixture + ethanol

layers: top oily liquid and clear colorless bottom

  • mixture + NaOH
  • formed b-hydroxyketone that was formed from the reaction continues to react under the conditions in the experiment
  • two additional reactions occur due to the presence of excess base and excess benzaldehyde
  • second enolate is generated (same mechanism as first enolate formation) with excess benzaldehyde
  • b-hydroxyketones and other b-hydroxycarbonyl compounds are often not stable in basic media and are susceptible to E2 elimination reaction
  • remaining a-hydrogen reacts with base while b-hydroxyl group serves as leaving group
  • when condensation products undergo elimination, resulting product is called a,b- unsaturated carbonyl (pdt: a,b- unsaturated ketone) which are very stable (resonance stabilization)

clear colorless mixture

  • stand at room temp

pale yellow mixture

  • precipitate

creamy yellow mixture

  • recrystallized product

pale yellow crystalline solid

yellow green crystals

Procedure

  • Place 0.9 g of benzaldehyde into 10 mL round bottom flask
  • add 0.3 mL of acetone
  • Add 5 mL of 95% ethanol. Stir the mixture in an ice bath.

for 15 minutes, slowly add 2 mL of 40% NaOH solution to the mixture while stirring

  • Remove ice bath. Stir the mixture at room temperature for another 10 minutes.
  • Place 10 g of crushed ice in a small beaker. Pour the reaction mixture into the ice and stir until the precipitation occurs.
  • Collect the precipitate by vacuum filtration.
  • Wash the filtrate with distilled water. Test the last drop of water wash with red litmus

paper, continue the washing if it is still basic.

  • Transfer the precipitate into an Erlenmeyer flask.

Recrystallize the product using ethanol and water.

Cool the flask in ice bath for 5-10 minutes.

  • Collect the crystals by vacuum filtration
  • Determine the melting point
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