Detailed Procedure for Preparation of p-Nitroacetanilide
The preparation of p nitroacetanilide is a crucial experiment in organic chemistry, demonstrating aromatic nitration and the influence of substituents on reaction pathways. By treating acetanilide with a nitrating mixture, mainly p-nitroacetanilide is obtained, showcasing the concepts of regioselectivity and synthetic purification. In this article, we will break down the step-by-step process, underlying chemical mechanism, and the significance of each stage in the preparation of p nitroacetanilide from acetanilide.
Fundamentals of the Preparation of P Nitroacetanilide from Acetanilide
The preparation of p nitroacetanilide from acetanilide is primarily an example of aromatic electrophilic substitution, specifically aromatic nitration. Key features of this transformation include:
- Choice of acetanilide as the starting material for directed nitration.
- Nitrating mixture of concentrated nitric acid and sulfuric acid initiates the substitution.
- Formation of para and minor ortho isomers, with the para-derivative being the main product.
Chemicals and Reaction Equation
- Acetanilide ($C_6H_5NHCOCH_3$) as substrate
- Concentrated nitric acid ($HNO_3$) and sulfuric acid ($H_2SO_4$) as nitrating reagents
- Product: p-nitroacetanilide ($p-$NO$_2$-$C_6H_4$-$NHCOCH_3$)
The overall balanced equation for the preparation is:
$$ C_6H_5NHCOCH_3 + HNO_3 \xrightarrow[H_2SO_4]{\text{cold}} p{-}NO_2C_6H_4NHCOCH_3 + H_2O $$
Step-by-Step Procedure for Preparation of P Nitroacetanilide
- Dissolve acetanilide in glacial acetic acid, gently warming if needed.
- Prepare the nitrating mixture by carefully adding concentrated sulfuric acid to concentrated nitric acid under cooling.
- Slowly add the cold nitrating mixture to the acetanilide solution with constant stirring, keeping the temperature below 10oC.
- After complete addition, continue stirring for optimal reaction completion.
- Pour the mixture onto crushed ice to precipitate the crude p nitroacetanilide.
- Filter and wash the solid thoroughly with cold water to remove acids.
- Purify the product by recrystallization from ethanol, yielding colorless crystals of p nitroacetanilide.
Mechanism of Nitration (Preparation of P Nitroacetanilide from Acetanilide Mechanism)
The acetamido group in acetanilide is an ortho/para-directing group, which influences where the nitro group is introduced during the reaction. The process proceeds via:
- Generation of the nitronium ion ($NO_2^+$) by reaction of $HNO_3$ and $H_2SO_4$.
- Electrophilic attack of $NO_2^+$ at the para position of the benzene ring due to less steric hindrance.
- Re-aromatization and restoration of the aromatic ring to give p nitroacetanilide as the primary product.
Key Purification and Analysis Steps
- Recrystallization ensures high purity and separation from o-nitroacetanilide (minor product).
- Melting point determination checks purity and confirms product identity.
- Calculation of percent yield validates the efficiency of the preparation.
Significance and Applications
- Demonstrates directed nitration techniques in the lab.
- Illustrates how protecting groups such as acetyl can direct incoming groups during aromatic substitution.
- Used as a classic experiment for teaching laboratory skills and product purification.
For those interested in a deeper understanding of chemical purification and analysis methods, explore the concept of stationary phase in chromatography. Understanding intermolecular forces in reactions is also relevant, as explained in interaction between forces in chemistry. To reinforce your broader science foundation, you may also review basics of chemistry.
In summary, the preparation of p nitroacetanilide from acetanilide is a fundamental example of aromatic nitration, demonstrating both the influence of substituents and regioselectivity in organic synthesis. This experiment not only shows the practical aspects of reaction setup and purification but also provides insight into mechanistic organic chemistry—especially the role of the acetamido group in directing nitration to the para position. Mastery of these steps is valuable whether preparing lab reports, tackling viva questions, or building a strong theoretical base for organic synthesis.
FAQs on How to Prepare p-Nitroacetanilide in the Laboratory
1. What is P Nitroacetanilide and how is it prepared?
P Nitroacetanilide is an organic compound obtained by the nitration of acetanilide using a mixture of concentrated acids. Preparation involves:
- Mixing acetanilide with concentrated sulphuric acid (H₂SO₄) and nitric acid (HNO₃)
- Allowing the mixture to react to introduce a p-nitro group
- Isolating and purifying the p-nitroacetanilide product
This is a classic example of an electrophilic aromatic substitution reaction relevant to CBSE Chemistry.
2. What is the principle behind the preparation of p-nitroacetanilide?
The principle involves nitration of acetanilide under controlled conditions, favouring para-substitution due to the acetamido group's directing effect. Key points:
- Acetanilide acts as the aromatic substrate
- Mixed acid (conc. H₂SO₄ and HNO₃) generates the nitronium ion (NO₂⁺) as electrophile
- The reaction predominantly gives p-nitroacetanilide because the –COCH₃ group is a para-directing substituent
3. Which chemicals are required for the lab preparation of p-nitroacetanilide?
The following chemicals are needed for preparing p-nitroacetanilide:
- Acetanilide
- Concentrated sulfuric acid (H₂SO₄)
- Concentrated nitric acid (HNO₃)
- Distilled water
- Ice (for cooling and crystallisation)
These reagents are used under controlled temperature to ensure a safe and selective reaction.
4. Describe the procedure for preparation of p-nitroacetanilide.
The preparation involves careful nitration of acetanilide.
- Dissolve acetanilide in concentrated sulphuric acid in a beaker placed in an ice bath.
- Slowly add chilled concentrated nitric acid with constant stirring to maintain temperature below 10°C.
- After complete addition, pour the mixture over crushed ice to precipitate p-nitroacetanilide.
- Filter, wash and recrystallize the crude product from hot alcohol.
Temperature control is essential to favour the para isomer and avoid side reactions.
5. What is the chemical reaction involved in preparing p-nitroacetanilide from acetanilide?
The main reaction is an electrophilic aromatic substitution (nitration):
- Acetanilide + HNO₃ + H₂SO₄ → p-Nitroacetanilide + H₂O
The acetamido group directs the incoming nitro group to the para position on the benzene ring.
6. What are the precautions to be taken during the preparation of p-nitroacetanilide?
Precautions ensure safety and desired product formation:
- Always add acids slowly with stirring in a cold environment.
- Maintain low temperature (below 10°C) to avoid excessive nitration.
- Handle concentrated acids with gloves and goggles.
- Work in a well-ventilated lab or fume hood.
- Dispose chemicals as per lab protocols.
7. What is the role of acetanilide in the nitration reaction?
Acetanilide acts as the substrate and orienting group:
- The acetamido group (-NHCOCH₃) is an electron-donating, para-directing group
- It activates the benzene ring, favouring nitration at the para position
- Helps produce predominantly p-nitroacetanilide with minimal ortho product
8. Why is the temperature kept low during the nitration of acetanilide?
Low temperature controls the reaction and product selectivity:
- Prevents formation of unwanted by-products like dinitroacetanilide
- Ensures selective formation of p-nitroacetanilide
- Reduces risk of decomposition and side reactions
Always use an ice bath to maintain optimal temperature.
9. What are the uses of p-nitroacetanilide?
P-Nitroacetanilide has several applications, including:
- Intermediate for synthesis of dyes and drugs
- Research in organic synthesis and mechanism studies
- Teaching example in studying electrophilic aromatic substitution reactions
10. How can you distinguish between o-nitroacetanilide and p-nitroacetanilide?
They can be distinguished by physical and chemical properties:
- Melting point: p-nitroacetanilide has a higher melting point than ortho isomer
- Solubility differences: Para isomer less soluble in water, more soluble in organic solvents
- Crystalline appearance: Para isomer generally forms colourless, needle-like crystals
