An Overview of Class 12 Chemistry To Prepare A Sample Of Dibenzalacetone Experiment
Dibenzalacetone is a crystalline solid frequently used as a ligand in the preparation of organometallic complexes that are then used as catalysts in coupling reactions. It is also a common ingredient in some sunscreens because it absorbs harmful UV light.
Table of Contents
Aim of the Experiment
Apparatus Required
Theory
Procedure
Observations
Result
Precautions
Aim of the Experiment
To prepare a sample of dibenzalacetone.
Apparatus Required
Benzaldehyde
Acetone
Sodium Hydroxide Solution
Methylated Spirit
Dilute Hydrochloric Acid
Ether
Beaker
Funnel
Conical Flask
Filter Papers
Theory
In the presence of an alkali, aromatic aldehyde condenses with aldehyde or ketone containing alpha hydrogen atoms. This is known as the Claisen-Schmidt reaction. According to Claisen, in the presence of sodium hydroxide, aldehydes can condense with another aldehyde or ketone, thereby removing a water molecule. Dibenzal acetone is formed when moles of benzaldehyde condense with one mole of acetone.
Procedure
Obtain a conical flask. 10ml freshly distilled benzaldehyde and 20ml acetone are added.
Place the flask in a cold water bath and gradually add 2.5ml sodium hydroxide while stirring constantly.
Keep the temperature set at $${{30}^{o}{C}}$$.
Stir the mixture for 2 hours after adding all the sodium hydroxide.
Transfer the reaction mixture to a 250ml separating funnel after adding dilute hydrochloric acid.
Shake the mixture thoroughly after adding 20 ml of chloroform/ether.
Shake the mixture thoroughly, remove the organic layer, and repeat twice.
In ice water, cool the mixture. Dibenzyl acetone separates as a fine emulsion first, then as yellow crystals.
Under pressure, distil the residual portion and collect the fraction boiling at $${{150}^{o}{C}}$$.
The pale yellow crystals should be washed with cold water, dried, and crystallised with ethanol.
Observations
Result
The yield of DibenzalAcetone is in gm.
Precautions
When a vigorous reaction occurs inside the flask, periodically release the pressure by opening the flask's cork.
The temperature should not be raised above $${{30}^{o}{C}}$$.
Keep ethanol and acetone away from the flame.
Lab Manual Questions
1. Which solvent was used for the recrystallization of dibenzalacetone?
Ans. The dry product from the previous lab period will be recrystallised by using about 20 mL of hot, boiling 95% ethanol. After dissolving, crystals should reform as soon as the solution cools to room temperature (without ice).
2. Which functional group is present in dibenzalacetone?
Ans. Dibenzalacetone contains a ketone and two alkene functional groups.
3. Is it necessary to recrystallise the dibenzalacetone?
Ans. Yes, it is important to recrystallise a compound obtained from a reaction.
4. Is dibenzalacetone soluble in water?
Ans. Dibenzalacetone is insoluble in water.
Viva Questions
1. What kind of isomerism is involved in the synthesis of dibenzalacetone?
Ans. This dibenzalacetone can show cis-trans geometrical isomerism due to a double bond.
2. What is the density of dibenzalacetone?
Ans. The density of dibenzalacetone is 1.1±0.1 g/$${{cm}^{3}}$$.
3. What side products are formed in the aldol condensation of dibenzalacetone?
Ans. $$\beta$$-hydroxy aldehyde is the product of the aldol condensation of dibenzalacetone.
4. Why is ethanol used in the recrystallization of dibenzalacetone?
Ans. Sufficient alcohol is used to dissolve the benzaldehyde rapidly and to retain the benzalacetone in the solution until it has had time to react with the second aldehyde molecule.
5. What is the intermediate formed in the synthesis of dibenzalacetone?
Ans. Benzaldehyde is the intermediate product in the synthesis of benzalacetone.
6. Is dibenzalacetone soluble in acetone?
Ans. Yes. Benzalacetone is soluble in acetone.
7. What is the limiting reagent in aldol condensation?
Ans. Acetone is the limiting reagent because it produces the least dibenzalacetone.
8. Why can benzaldehyde and acetone undergo double aldol condensation?
Ans. Acetone has $$\alpha$$-hydrogens (on both sides) and thus can be deprotonated to give a nucleophilic enolate anion. As aldol condensation is a type of nucleophilic reaction.
9. What is the molar ratio of benzaldehyde and acetone required to make dibenzalacetone?
Ans. To prepare dibenzalacetone 2:1 molar ratio of benzaldehyde and acetone is required.
10. What is the purpose of dibenzalacetone by the aldol condensation?
Ans. Aldol condensation is an important route of organic synthesis because it provides an efficient way to form carbon-carbon bonds. In this condensation, an enol or enolate ion reacts with a carbonyl compound to form a β-hydroxy ketone or β-hydroxyaldehyde, followed by dehydration. As dibenzalacetone is an organic compound, it is required to undergo aldol condensation.
Practical Based Questions
1.IUPAC name of dibenzal acetone is
(a)1,5-Diphenylpenta-1,4-dien-3-one
(b)2,5-Diphenylpenta-1,4-dien-3-one
(c)1,5-Diphenylpenta-2,4-dien-3-one
(d)1,5-Diphenylpenta-1,3-dien-3-one
Answer: (a)
2.What is the odour of dibenzalacetone
(a)Sweet almond odour
(b)Bitter almond odour
(c)Sweet odour
(d)Bad odour
Answer: (b)
3. Boiling point of dibenzalacetone is
(a)150 degree celsius
(b)125 degree celsius
(c)130 degree celsius
(d)145 degree celsius
Answer: (c)
4. Melting point of dibenzalacetone is
(a)100 degree celsius
(b)110 degree celsius
(c)111 degree celsius
(d)99 degree celsius
Answer: (c)
5. Benzalacetone is
(a)Acidic
(b)Basic
(c)Neutral
(d)None of the above
Answer: (c)
6. Which of the following is the simplest member of organic compounds?
(a) Formic acid
(b) Formaldehyde
(c) Methane
(d) Methanol
Answer: (c)
7.Synthesis of dibenzalacetone from benzaldehyde known as
(a)Claisen-Schmidt reaction
(b)Swarts reaction
(c)Cannizzaro Reaction
(d)Hoffmann Bromamide Reaction
Answer: (a)
8. Which of the following bonds is made up of many organic compounds?
(a) Metallic bond
(b) Dipolar bond
(c) Ionic bond
(d) Covalent bond
Answer: (d)
9.Which of the following is a classification of Organic compounds?
(a) alicyclic compounds and acyclic compounds
(b) Cyclic compounds and alicyclic compounds
(c) Open chain compounds and acyclic compounds
(d) Open chain compounds and linear chain compounds
Answer: (a)
10. Which of the following is not a class of organic compounds?
(a) Amides
(b) Electro compounds
(c) Nitro compound
(d) Carbonyl compound
Answer: (b)
Conclusion
Dibenzylideneacetone, also known as dibenzalacetone, is an organic compound with the formula $${{C}_{17}{H}_{14}{O}}$$.
It is a pale yellow solid that is soluble in ethanol but insoluble in water.
Dibenzalacetone is a crystalline solid frequently used as a ligand to prepare organometallic complexes that are then used as catalysts in coupling reactions.
It is also a common ingredient in some sunscreens because it absorbs harmful UV light.
FAQs on Class 12 Chemistry To Prepare A Sample Of Dibenzalacetone Experiment
1. What is the main chemical reaction for preparing dibenzalacetone in the Class 12 practical exam?
The preparation of dibenzalacetone is a classic example of the Claisen-Schmidt condensation. In this reaction, two molecules of benzaldehyde (an aromatic aldehyde with no α-hydrogen) react with one molecule of acetone (an aliphatic ketone with α-hydrogens) in the presence of a base like sodium hydroxide (NaOH).
2. What is the role of each reactant—benzaldehyde, acetone, and sodium hydroxide—in this synthesis?
For the CBSE Class 12 practical, understanding the role of each reactant is crucial for viva questions. Each has a specific function:
- Benzaldehyde: Acts as the electrophile. It provides the aromatic rings and the carbonyl groups that will be attacked.
- Acetone: Acts as the nucleophile after the base removes an acidic α-hydrogen to form an enolate ion.
- Sodium Hydroxide (NaOH): Acts as the catalyst. It is a base that helps form the acetone enolate, which is essential for the reaction to start.
3. How is the percentage yield of dibenzalacetone calculated, and what is a typical result for this experiment?
Calculating the yield is a key part of scoring well in practicals. The percentage yield is found using the formula: Percentage Yield = (Actual Yield / Theoretical Yield) × 100. First, you identify the limiting reagent (usually acetone). Then, you calculate the theoretical yield from the balanced equation. Finally, weigh your dry, pure product for the actual yield. A good result in a school lab is typically between 60% and 80%.
4. What are the key precautions a student must follow to get a good result in the dibenzalacetone experiment?
To ensure safety and a high-quality product, you must:
- Handle sodium hydroxide carefully as it is corrosive.
- Stir the mixture constantly for efficient mixing.
- Maintain a low reaction temperature (20-25°C) to prevent the Cannizzaro reaction, a common side reaction.
- Wash the final crystals with cold water to remove any leftover base, which can affect purity.
5. Why does the dibenzalacetone product appear as a pale-yellow solid?
The yellow colour is an important characteristic and is due to the molecule's structure. Dibenzalacetone has an extensive conjugated system, which is a series of alternating single and double bonds. This system absorbs light in the blue-violet region of the spectrum, causing the compound to appear in the complementary colour, which is yellow.
6. What would happen if the temperature is not controlled or the mixture is not stirred properly during the reaction?
These are common experimental errors that examiners often ask about.
- High Temperature: If the mixture gets too warm, benzaldehyde can undergo a side reaction called the Cannizzaro reaction, which will lower the final yield of your desired product.
- Poor Stirring: If the reactants are not mixed well, the reaction will be slow and incomplete, resulting in a very low yield of dibenzalacetone.
7. Why is recrystallization an important final step when preparing a pure sample of dibenzalacetone?
Recrystallization is a critical purification technique that is often tested. The initial crude product contains impurities like unreacted chemicals or by-products. By dissolving the crude solid in a minimum amount of hot ethanol and letting it cool, pure dibenzalacetone forms crystals, while most impurities stay dissolved. This step ensures you get a pure sample with a sharp melting point, which is a sign of high purity.
