Preparation of Organic Compounds Dibenzalacetone

Introduction

The preparation of organic compounds through laboratory synthesis represents a fundamental aspect of organic chemistry education. Among the most important experiments is the synthesis of dibenzalacetone through aldol condensation. This comprehensive guide provides a detailed methodology for conducting this essential organic chemistry experiment in a laboratory setting.

Aim of the Experiment

To synthesize dibenzalacetone from benzaldehyde and acetone through aldol condensation and verify the formation of the product through physical and chemical characterization methods.

Theory and Mechanism

Chemical Background

Dibenzalacetone (1,5-diphenylpent-1-en-3-one) is prepared through the base-catalyzed crossed aldol condensation reaction between benzaldehyde and acetone. This reaction represents a classic example of crossed aldol condensation where two different carbonyl compounds react to form a β-hydroxy ketone intermediate, which subsequently undergoes dehydration to yield an α,β-unsaturated ketone.

Reaction Mechanism

The mechanism proceeds through the following steps:

1. Base catalysis: NaOH abstracts a proton from acetone to form an enolate ion
2. Nucleophilic addition: Enolate attacks the carbonyl carbon of benzaldehyde
3. Dehydration: The β-hydroxy ketone undergoes elimination to form the α,β-unsaturated ketone

Chemical Equation:

2 C₆H₅CHO + CH₃COCH₃ → C₆H₅CH=CHCOCH₃ + H₂O
Benzaldehyde + Acetone → Dibenzalacetone + Water

Properties of Product

• Molecular Formula: C₁₇H₁₄O
• Molecular Weight: 234.29 g/mol
• Melting Point: 110-112°C
• Appearance: Yellow crystalline solid
• Solubility: Slightly soluble in water, soluble in organic solvents

Apparatus Required

Glassware and Equipment

• 250 ml Round bottom flask
• Reflux condenser
• Separating funnel (250 ml)
• Beakers (100 ml, 250 ml)
• Glass stirring rod
• Filter paper and funnel
• Conical flask (500 ml)
• Measuring cylinder (100 ml)
• Wooden blocks for heating
• Wash bottle

Chemicals

• Benzaldehyde (10 ml)
• Acetone (3 ml)
• 10% Sodium hydroxide solution (20 ml)
• Concentrated hydrochloric acid (for purification)
• Distilled water

Procedure

Step-by-Step Method

1. Setup the apparatus: Clean and dry all glassware. Assemble the reflux condenser with the round bottom flask.

2. Mixing reagents:

   • Measure 10 ml of benzaldehyde and add to the flask
   • Add 3 ml of acetone to the benzaldehyde
   • Mix thoroughly by swirling

3. Base addition:

   • Add 20 ml of 10% NaOH solution gradually
   • Shake the mixture intermittently for proper mixing

4. Refluxing:

   • Attach the reflux condenser
   • Heat the mixture gently on a water bath for 30-45 minutes
   • Observe color change from colorless to yellow

5. Cooling and separation:

   • Allow the mixture to cool to room temperature
   • Transfer to separating funnel
   • Separate the organic layer (upper yellow layer)

6. Extraction:

   • Wash organic layer with distilled water
   • Repeat washing until neutral pH is achieved

7. Crystallization:

   • Transfer the organic layer to conical flask
   • Add hot water gradually until cloudiness appears
   • Cool slowly to room temperature
   • Ice bath cooling for complete crystallization

8. Filtration and drying:

   • Filter the crystals using suction filtration
   • Wash with small amount of cold water
   • Dry between filter papers
   • Weigh the product

Observation Table

Quantitative Data

Result and Discussion

Product Characterization

The experiment successfully yielded dibenzalacetone crystals with the following characteristics:

• Yield: 72.9% (considered good for undergraduate laboratory conditions)
• Physical form: Yellow crystalline needles
• Melting point: 110-112°C (literature value: 111°C)
• Odor: Characteristic pleasant aroma

Factors Affecting Yield

1. Temperature Control: Gentle heating prevents side reactions
2. Stoichiometry: 2:1 molar ratio of benzaldehyde to acetone
3. Base concentration: 10% NaOH provides optimal conditions
4. Reaction time: Sufficient time for complete condensation

Chemical Confirmation Tests

1. 2,4-Dinitrophenylhydrazine Test: Positive yellow precipitate
2. Iodoform Test: Negative (no methyl ketone groups)
3. Bromine water test: Decolorization indicating unsaturation

Precautions

Safety Measures

1. Benzaldehyde Handling: Work in fume hood; avoid skin contact
2. Acetone Storage: Keep away from heat sources; highly flammable
3. Sodium Hydroxide: Handle with care; causes severe burns
4. Eye Protection: Always wear safety goggles
5. Proper Ventilation: Maintain adequate airflow during experiment

Experimental Precautions

1. Clean Apparatus: Ensure all equipment is thoroughly dried
2. Gradual Addition: Add reagents slowly to control exothermic reactions
3. Temperature Monitoring: Avoid excessive heating
4. Proper Mixing: Stir intermittently for uniform reaction
5. Waste Disposal: Dispose chemicals according to laboratory guidelines

Common Errors and Troubleshooting

Viva Voce Questions and Answers

Basic Questions

Q1: What is aldol condensation? A: Aldol condensation is a reaction where two carbonyl compounds react in the presence of base to form a β-hydroxy carbonyl compound, which undergoes dehydration to form α,β-unsaturated compounds.

Q2: Why is sodium hydroxide used in this experiment? A: NaOH acts as a base catalyst. It abstracts the acidic α-hydrogen from acetone to form enolate ion, which is a better nucleophile for attacking benzaldehyde.

Q3: Why is dibenzalacetone yellow in color? A: The yellow color is due to conjugated double bonds in the molecule that absorb in the visible region of light spectrum.

Q4: What is the role of acetone in this reaction? A: Acetone acts as the source of enolate ion. Its α-hydrogen is more acidic than benzaldehyde, making it preferentially form enolate.

Advanced Questions

Q5: Why is the reaction called crossed aldol condensation? A: Because two different carbonyl compounds (benzaldehyde and acetone) are used, it's called crossed aldol condensation. It avoids self-condensation of individual components.

Q6: How can you increase the yield of dibenzalacetone? A: Yields can be improved by:

• Maintaining proper stoichiometric ratios
• Controlling reaction temperature
• Extending reaction time appropriately
• Ensuring proper crystallization conditions

Q7: What would happen if formaldehyde is used instead of benzaldehyde? A: Formaldehyde lacks α-hydrogens, so it cannot enolize. This would change the reaction mechanism entirely and likely give different products.

Q8: Why is the product washed with water? A: Washing removes unreacted starting materials, sodium hydroxide, and other water-soluble impurities from the organic product.

Conclusion

The preparation of dibenzalacetone through aldol condensation represents a successful organic synthesis experiment that demonstrates important principles of carbonyl chemistry. The experiment achieved a reasonable yield of 72.9% under controlled laboratory conditions, confirming the formation of the desired product through physical characterization and chemical tests.

This experiment effectively illustrates:

• Base-catalyzed condensation reactions
• Importance of stoichiometry in organic reactions
• Crystallization as a purification method
• Structure-property relationships in conjugated systems