Chemistry Experiment- To Prepare a Sample of Acetanilide from Aniline
Acetanilide is used to stabilise cellulose ester varnishes and as a hydrogen peroxide decomposition inhibitor. It has also been used as an intermediary in synthesising rubber accelerators, dyes and camphor.
Table of Contents
Aim of the Experiment
Apparatus Required
Theory
Procedure
Observations
Result
Precautions
Aim of the Experiment
To prepare and submit acetanilide from aniline.
Apparatus Required
Aniline
Glacial Acetic Acid
Acetic Anhydride
Zinc Dust
Distilled Water
Round Bottom Flask
Beaker
Pipette
Reflux Condenser
Funnel
Stirrer
Bunsen Burner
Filter Paper
Electronic Balance
Theory
When aniline reacts with acetic anhydride/glacial acetic acid in the presence of zinc dust, acetanilide is formed. To obtain acetic anhydride precipitate, a mixture of aniline, glacial acetic acid, acetic anhydride, and zinc dust is refluxed under anhydrous conditions and then poured into ice-cold water. To obtain pure crystals of acetanilide, the crude precipitate of acetic anhydride is recrystallised. Zinc is used to prevent aniline oxidation in the chemical reaction. Acetanilide has medicinal applications and is used as a febrifuge.
Acetylation of aniline with acetic anhydride in concentrated hydrochloric acid can also yield acetanilide. Dissolve aniline in hydrochloric acid and stir in acetic anhydride. Pour the sodium acetate mixture into the water. Ethyl alcohol can be used to separate and recrystallise acetanilide.
Procedure
Before beginning the experiment, thoroughly clean the equipment with distilled water.
Add 10ml aniline, 20ml acetic anhydride, glacial acetic acid mixture, and zinc dust to a round bottom flask.
With the round bottom flask, secure the reflux condenser.
In an oil bath, gently heat the mixture for about 15-20 minutes.
With constant stirring, pour the hot mixture into a beaker containing ice cold water.
To hydrolyse the excess acetic anhydride, vigorously stir the mixture.
Once the acetanilide has precipitated, collect it in a Buchner funnel and filter it.
The precipitate obtained is a crude acetanilide sample. Crystallisation is required to obtain pure crystals.
Observations
Result
The yield of acetanilide is in gm.
Precautions
Do not breathe acetic anhydride fumes.
Experiments should always be carried out in a fuming chamber or near a window.
Reflux the reaction mixture using the water condenser.
Dry the acetanilide crystals before determining their weight and melting point.
Lab Manual Questions
1. Why is aniline soluble in hydrochloric acid while it is insoluble in water?
Ans. Aniline does not undergo hydrogen bonding because of the presence of benzene, which is highly hydrophobic. therefore aniline is insoluble in water. In the HCl, the amine group becomes protonated ($${{-N}{H}^{3+}}$$) and the ionic hence soluble in HCl.
2. Why is aniline a weaker base than ammonia?
Ans. Since ammonia can easily lose electron pairs while aniline cannot, thus ammonia is a stronger base than aniline.
3. In the preparation of p-nitroacetanilide another minor product is formed. What is this compound, and how can this be separated from p-nitroacetanilide?
Ans. The minor product that is o-nitroacetanilide is separated from the p-nitroacetanilide by dissolving the solution of both products in Ethanol. o-nitroacetanilide is more soluble in alcohol as compared to the p-nitroacetanilide.
4. Is it necessary to recrystallise the compound obtained from the reaction?
Ans. So recrystallisation is important after a compound is obtained from the reaction so that we get pure crystals of our compound and the yield of the compound produced from the reaction can be done accurately without any impurities.
Viva Questions
1. What is the purpose of acetanilide?
Ans. Acetanilide, a synthetic organic compound introduced in therapy in 1886 as a fever-reducing drug. Its effectiveness in relieving pain was discovered soon thereafter. It was used as an alternative to aspirin for many years in treating common complaints such as headaches, menstrual cramps, and rheumatism.
2. Which functional group is present in acetanilide?
Ans. Acetanilide has an amide functional group, meaning it has a nitrogen atom bonded directly to a carbonyl.
3. What is the pH of acetanilide?
Ans. The pH of acetanilide will be between 5 to 7.
4. Why is acetanilide soluble in hot water?
Ans. Because it has a large nonpolar benzene ring, making it insoluble in water at cold temperatures. But, the compound also has an acyl group which is polar. This would make the compound more soluble in water when it is heated.
5. What bonds are present in acetanilide?
Ans. Acetanilide has both sigma and pie bonds. It has 19 σ and 4 π bonds.
6. Is acetanilide soluble in HCl?
Ans. Acetanilide is not soluble in aqueous HCl.
7. What is a good solvent for acetanilide?
Ans. Water is a good solvent for acetanilide.
8. What type of amide is in acetanilide?
Ans. N-phenylacetamide is a member of the class of acetamides that is acetamide in which a phenyl group substitutes one of the hydrogens attached to the nitrogen. It has a role as an analgesic. It is a member of acetamides and an anilide. It is functionally related to acetic acid.
9. How is acetanilide poisoning treated?
Ans. Gently wash all affected skin areas thoroughly with soap and water. If symptoms such as redness or irritation develop, immediately call a physician and be prepared to transport the victim to a hospital for treatment.
10. What is the hydrolysis of acetanilide?
Ans. Crude acetanilide looks like grains of brown rice, while pure acetanilide forms shiny crystals in cold water. The crude solid is dissolved in the smallest possible amount of solvent of choice; in this case, the solvent is water. Acetanilide has a much higher solubility in hot water than in cold water.
Practical Based Questions
1. Aniline is
(a)Acidic
(b)Basic
(c)Neutral
(d)All of the above
Answer: (b)
2. Acetanilide was used as
(a)analgesics
(b)antipyretics
(c)both a and b
(d)None of these
Answer: (c)
3. Technique used to separate aniline from aniline water mixture is
(a)Fractional distillation
(b)Fractional crystallisation
(c)Vaccum distillation
(d)Steam distillation
Answer: (d)
4. The most convenient method to prepare primary amine containing one carbon atom less is
(a) Gabriel phthalimide synthesis
(b) Reductive amination of aldehydes
(c) Hofmann bromamide reaction
(d) Reduction of isonitriles
Answer: (c)
5. Secondary amines can be prepared by
(a) reduction of nitro compounds
(b) oxidation of N-substituted amides
(c) reduction of isonitriles
(d) reduction of nitriles
Answer: (c)
6. Which of the following amides will give ethylamine on reaction with sodium hypobromide?
(a) Butanamide
(b) Propanamide
(c) Acetamide
(d)Benzamide
Answer: (b)
7.The boiling point of acetanilide is
(a)304 degree celsius
(b)316 degree celsius
(c)320 degree celsius
(d)325 degree celsius
Answer: (a)
8. Boiling point of aniline is
(a)183 degree celsius
(b)184 degree celsius
(c)185 degree celsius
(d)184.1 degree celsius
Answer: (d)
9. Melting point of aniline is
(a)0 degree celsius
(b)-3 degree celsius
(c)-2 degree celsius
(d)-6.3 degree celsius
Answer: (d)
10. Melting point of acetanilide is
(a)115 degree celsius
(b)113 degree celsius
(c)114 degree celsius
(d)114.3 degree celsius
Answer: (d)
Conclusion
Acetanilide is a colourless, odourless solid chemical that resembles a leaf or flake. It was previously known as Antifebrin and also known as N-phenylacetamide, acetanil, or acetanilid.
Acetanilide is used to stabilise cellulose ester varnishes and as a hydrogen peroxide decomposition inhibitor. It has also been used as an intermediator in the synthesis of rubber accelerators, dyes and dye intermediates, and camphor.
When aniline reacts with acetic anhydride/glacial acetic acid in the presence of zinc dust, acetanilide is formed. To obtain acetic anhydride precipitate, a mixture of aniline, glacial acetic acid, acetic anhydride, and zinc dust is refluxed under anhydrous conditions and then poured into ice-cold water.
FAQs on To Prepare a Sample of Acetanilide from Aniline
1. What is the balanced chemical equation for the preparation of acetanilide from aniline as per the CBSE Class 12 practical syllabus for 2025-26?
The reaction involves the acetylation of aniline using acetic anhydride in the presence of glacial acetic acid. The balanced chemical equation is:
C₆H₅NH₂ (Aniline) + (CH₃CO)₂O (Acetic Anhydride) → C₆H₅NHCOCH₃ (Acetanilide) + CH₃COOH (Acetic Acid)
This reaction is an example of nucleophilic acyl substitution.
2. For the board practicals, what is the specific role of adding a small amount of zinc dust during the acetylation of aniline?
Adding zinc dust is an important precautionary step in this experiment. Its primary roles are:
- To prevent oxidation: Aniline can be easily oxidised during the reaction, which results in coloured impurities (like tar) in the final product. Zinc dust prevents this oxidation.
- To ensure a colourless product: By preventing the formation of coloured impurities, zinc helps in obtaining a purer, white or colourless sample of acetanilide.
3. Why is recrystallisation using hot water an important final step in preparing a pure sample of acetanilide?
Recrystallisation is a crucial purification technique in this experiment. It is effective because of the specific solubility properties of acetanilide:
- Acetanilide is sparingly soluble in cold water but highly soluble in hot water.
- The crude product is dissolved in a minimum amount of boiling water to form a saturated solution.
- As the solution cools, the solubility of acetanilide decreases drastically, causing it to crystallise out as a pure solid, while the more soluble impurities remain dissolved in the mother liquor.
4. Aniline is a weak base, but the product acetanilide is almost neutral. What structural change during the reaction accounts for this significant decrease in basicity?
This is a key conceptual question. The decrease in basicity is due to the change in the availability of the lone pair of electrons on the nitrogen atom.
- In aniline, the lone pair on the nitrogen atom is available for donation (protonation), making it basic.
- In acetanilide, the lone pair on the nitrogen is delocalised over the highly electronegative acetyl group (C=O) due to resonance. This makes the lone pair less available for donation, thus making acetanilide a much weaker base and effectively neutral.
5. What are two essential precautions a student must take while handling the reagents for the synthesis of acetanilide?
For a safe and successful experiment, students must observe the following precautions:
- Handle Acetic Anhydride with care: Acetic anhydride has pungent, irritating fumes and is corrosive. The experiment should be performed in a well-ventilated area or a fume hood.
- Use Anhydrous Conditions: The round-bottom flask and all apparatus should be perfectly dry. Acetic anhydride readily reacts with water (hydrolyses) to form acetic acid, which would reduce the yield of the desired product.
6. Why is acetic anhydride generally preferred over acetyl chloride for the laboratory preparation of acetanilide in a school setting?
While both can be used for acetylation, acetic anhydride is preferred for several practical reasons:
- Reaction Vigor: The reaction with acetyl chloride is extremely vigorous and hard to control. The reaction with acetic anhydride is smoother and more manageable.
- Byproduct Handling: The reaction with acetyl chloride produces corrosive HCl gas as a byproduct, which is difficult and hazardous to handle. The reaction with acetic anhydride produces acetic acid, which is a liquid and much easier to handle and separate from the product.
7. How would you identify the final pure product in this experiment? Mention the expected colour and physical state of acetanilide.
After successful preparation and purification by recrystallisation, the final product can be identified by its physical characteristics. Pure acetanilide is a colourless or white, crystalline solid that often appears as shiny flakes or leaflets. Its melting point is also a key identifier, which is approximately 114.3°C.
8. Explain why the starting material, aniline, is soluble in dilute HCl but only sparingly soluble in water.
The solubility of aniline is a classic example of its basic nature.
- In dilute HCl: Aniline acts as a Lewis base and reacts with the acid to form a salt, anilinium chloride (C₆H₅NH₃⁺Cl⁻). This ionic salt is readily soluble in water.
- In water: Aniline is only sparingly soluble because the large, nonpolar phenyl group (C₆H₅) is hydrophobic (water-repelling) and counteracts the polar effect of the amino (-NH₂) group.
9. What are the primary uses of acetanilide, and why is it no longer a commonly used painkiller?
Acetanilide has several industrial applications, including its use as an intermediate in the synthesis of dyes, rubber accelerators, and pharmaceuticals like penicillin. It was historically used as a fever-reducing and pain-relieving drug under the name 'Antifebrin'. However, its use as a common analgesic has been discontinued due to its toxicity and significant side effects, primarily methemoglobinemia (a blood disorder). It has been replaced by safer alternatives like paracetamol.
