Class 12 Chemistry Notes Chapter 10 (Preparation of Organic Compounds) – Lab Manual (English) Book

Alright class, let's delve into the practical aspects of organic chemistry as detailed in Chapter 10 of your Lab Manual, focusing on the preparation of specific organic compounds. Understanding these preparations is crucial, not just for practical exams, but also because the principles, reactions, reagents, and purification techniques involved are frequently tested in various government examinations.

Chapter 10: Preparation of Organic Compounds - Key Concepts for Exams

This chapter typically covers the synthesis, purification, and characterization of a few representative organic compounds. The core skills and knowledge you need to extract are:

1. Understanding the Reaction: Name of the reaction, type (e.g., acetylation, nitration, condensation, diazotization, haloform), balanced chemical equation.
2. Role of Reagents: Why each specific chemical is used (catalyst, reactant, solvent, dehydrating agent, oxidizing/reducing agent, base/acid).
3. Reaction Conditions: Temperature, time, pH control – why they are important for yield and selectivity.
4. Mechanism Basics: A fundamental understanding of the reaction pathway (e.g., electrophile generation, nucleophilic attack, intermediates).
5. Separation & Purification: Techniques used, primarily precipitation and recrystallization. Understand the principle of recrystallization (solubility difference at different temperatures) and choice of solvent.
6. Characterization: Determining purity, primarily through melting point determination (sharp melting point close to literature value indicates purity) and sometimes color.
7. Yield Calculation: Concept of theoretical yield (based on stoichiometry and limiting reagent), actual yield (what you obtain), and percentage yield (% Yield = (Actual Yield / Theoretical Yield) x 100).

Common Preparations Covered (Focus Points for Exams):

1. Preparation of Acetanilide from Aniline

• Aim: To prepare acetanilide by acetylating aniline.
• Reaction Type: Nucleophilic Acyl Substitution (specifically, N-Acetylation).
• Principle: Aniline acts as a nucleophile, attacking the electrophilic carbonyl carbon of the acetylating agent. The amino group (-NH₂) of aniline is converted to an acetamido group (-NHCOCH₃).
• Reactants: Aniline (C₆H₅NH₂)
• Reagents:
  • Acetic Anhydride ((CH₃CO)₂O): Primary acetylating agent. Reacts faster and more efficiently than acetic acid.
  • Glacial Acetic Acid (CH₃COOH): Can be used as a solvent and also as an acetylating agent (less reactive than anhydride). Often used with anhydride.
  • (Optional) Zinc Dust: Sometimes added to prevent oxidation of aniline by air.
• Reaction:
  C₆H₅NH₂ (Aniline) + (CH₃CO)₂O (Acetic Anhydride) --[CH₃COOH]--> C₆H₅NHCOCH₃ (Acetanilide) + CH₃COOH (Acetic Acid)
  OR
  C₆H₅NH₂ (Aniline) + CH₃COOH (Glacial Acetic Acid) --[Heat/Reflux]--> C₆H₅NHCOCH₃ (Acetanilide) + H₂O
• Procedure Highlights: Reaction mixture is usually heated, then poured into ice-cold water to precipitate the crude acetanilide (less soluble in cold water).
• Purification: Recrystallization from hot water (acetanilide is sparingly soluble in cold water but moderately soluble in hot water).
• Characterization: White crystalline solid. Melting Point: ~114°C.
• Key Exam Points:
  • Acetanilide preparation reduces the activating effect of the -NH₂ group, making it useful for controlling subsequent electrophilic substitution (like nitration).
  • Acetic anhydride is preferred over acetyl chloride because it's easier to handle and the byproduct (acetic acid) is less corrosive than HCl.

2. Preparation of Dibenzalacetone from Benzaldehyde and Acetone

• Aim: To prepare dibenzalacetone via condensation.
• Reaction Type: Claisen-Schmidt Condensation (a type of crossed Aldol Condensation followed by dehydration).
• Principle: An enolate ion generated from acetone (which has α-hydrogens) attacks the carbonyl carbon of benzaldehyde (which lacks α-hydrogens). This occurs twice, followed by dehydration, driven by the formation of a highly conjugated system.
• Reactants: Benzaldehyde (C₆H₅CHO) (2 moles), Acetone (CH₃COCH₃) (1 mole).
• Reagents:
  • Sodium Hydroxide (NaOH) solution: Base catalyst to generate the enolate ion from acetone.
  • Ethanol: Often used as a solvent to dissolve both reactants.
• Reaction:
  2 C₆H₅CHO (Benzaldehyde) + CH₃COCH₃ (Acetone) --[Aq. NaOH/Ethanol]--> C₆H₅-CH=CH-CO-CH=CH-C₆H₅ (Dibenzalacetone) + 2 H₂O
• Procedure Highlights: Reactants mixed in ethanol, base added slowly with stirring, product precipitates out.
• Purification: Recrystallization, typically from ethanol or rectified spirit.
• Characterization: Pale yellow crystalline solid. Melting Point: ~110-112°C.
• Key Exam Points:
  • It's a crossed aldol because two different carbonyl compounds react.
  • Benzaldehyde cannot undergo self-condensation as it lacks α-hydrogens.
  • Acetone provides the enolate.
  • Dehydration is rapid due to the formation of stable conjugated double bonds.

3. Preparation of p-Nitroacetanilide from Acetanilide

• Aim: To prepare p-nitroacetanilide by nitrating acetanilide.
• Reaction Type: Electrophilic Aromatic Substitution (Nitration).
• Principle: The acetamido group (-NHCOCH₃) in acetanilide is an ortho-, para- directing group. Nitration using nitrating mixture generates the electrophile NO₂⁺ (nitronium ion), which attacks the activated benzene ring. The para product predominates due to steric hindrance at the ortho positions.
• Reactant: Acetanilide (C₆H₅NHCOCH₃).
• Reagents:
  • Nitrating Mixture: Concentrated Nitric Acid (Conc. HNO₃) + Concentrated Sulphuric Acid (Conc. H₂SO₄).
  • Glacial Acetic Acid: Often used as a solvent.
• Role of H₂SO₄: Acts as an acid and dehydrating agent, facilitating the generation of the nitronium ion (NO₂⁺) from HNO₃.
  HNO₃ + 2 H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2 HSO₄⁻
• Reaction:
  C₆H₅NHCOCH₃ (Acetanilide) + HNO₃ --[Conc. H₂SO₄, Glacial Acetic Acid]--> p-NO₂-C₆H₄-NHCOCH₃ (p-Nitroacetanilide) + H₂O (Minor ortho product also formed)
• Procedure Highlights: Acetanilide dissolved in glacial acetic acid, cooled in an ice bath. Nitrating mixture added slowly with stirring while maintaining low temperature (to control the reaction rate and prevent side reactions/di-nitration). Poured into ice water to precipitate the product.
• Purification: Recrystallization from ethanol.
• Characterization: Pale yellow crystalline solid. Melting Point: ~214-216°C.
• Key Exam Points:
  • Why nitrate acetanilide instead of aniline directly? Direct nitration of aniline is problematic: (a) Strong oxidation of -NH₂ group by HNO₃. (b) Protonation of -NH₂ in acidic medium forms anilinium ion (-NH₃⁺), which is meta-directing and deactivating. Acetylation protects the amino group and makes it moderately activating and o/p directing.
  • Low temperature control is crucial.
  • Para product is major due to steric factors.

4. Preparation of Aniline Yellow (p-Aminoazobenzene) from Aniline

• Aim: To prepare the azo dye, Aniline Yellow.
• Reaction Type: Diazotization followed by Azo Coupling.
• Principle:
  • Step 1: Diazotization: Aniline reacts with nitrous acid (HNO₂, generated in situ from NaNO₂ and HCl) at low temperature (0-5°C) to form benzenediazonium chloride (C₆H₅N₂⁺Cl⁻).
  • Step 2: Azo Coupling: The electrophilic benzenediazonium ion attacks another molecule of aniline (which acts as the coupling component) at the electron-rich para position to form p-aminoazobenzene (Aniline Yellow).
• Reactant: Aniline (C₆H₅NH₂).
• Reagents:
  • Sodium Nitrite (NaNO₂): Reacts with HCl to generate HNO₂ in situ.
  • Hydrochloric Acid (HCl): Provides acidic medium and reacts with NaNO₂.
  • Aniline (C₆H₅NH₂): Used for both diazotization and as the coupling component.
• Conditions: Diazotization MUST be done at 0-5°C as diazonium salts are unstable at higher temperatures. Coupling occurs in mildly acidic conditions.
• Reactions:
  • Diazotization: C₆H₅NH₂ + NaNO₂ + 2 HCl --[0-5°C]--> C₆H₅N₂⁺Cl⁻ (Benzenediazonium chloride) + NaCl + 2 H₂O
  • Coupling: C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ --[Mildly acidic, ~pH 4-5]--> C₆H₅-N=N-C₆H₄-NH₂ (p-Aminoazobenzene) + HCl
• Procedure Highlights: Careful temperature control during diazotization. Slow addition of diazonium salt solution to aniline solution. Precipitation of the dye.
• Purification: Recrystallization from ethanol or methylated spirit.
• Characterization: Yellow/Orange crystalline solid (Azo Dye). Melting Point: ~123-126°C.
• Key Exam Points:
  • Importance of low temperature (0-5°C) for diazotization.
  • Benzenediazonium ion acts as an electrophile.
  • Coupling occurs at the para position of the activated aniline ring.
  • Azo compounds (-N=N- linkage) are typically colored (dyes).

5. Preparation of Iodoform (Triiodomethane) from Ethanol or Acetone

• Aim: To prepare iodoform.
• Reaction Type: Haloform Reaction.
• Principle: Compounds containing the CH₃-C=O group (methyl ketones like acetone) or CH₃-CH(OH)- group (alcohols oxidizable to methyl ketones, like ethanol) react with iodine in the presence of a base (like NaOH) to give a haloform (iodoform, CHI₃, in this case) and the sodium salt of a carboxylic acid with one less carbon atom than the starting carbonyl compound/alcohol.
• Reactants: Ethanol (CH₃CH₂OH) or Acetone (CH₃COCH₃).
• Reagents:
  • Iodine (I₂)
  • Sodium Hydroxide (NaOH) solution (or sometimes Na₂CO₃ solution)
• Overall Reactions (Simplified):
  • From Ethanol: CH₃CH₂OH + 4 I₂ + 6 NaOH --[Heat]--> CHI₃ (Iodoform) + HCOONa (Sodium formate) + 5 NaI + 5 H₂O
  • From Acetone: CH₃COCH₃ + 3 I₂ + 4 NaOH --[Heat]--> CHI₃ (Iodoform) + CH₃COONa (Sodium acetate) + 3 NaI + 3 H₂O
• Mechanism Hints: Base promotes oxidation of ethanol to acetaldehyde, or forms enolate from acetone. Repeated iodination occurs at the α-methyl group. Finally, cleavage by OH⁻ yields CHI₃ precipitate and carboxylate salt.
• Procedure Highlights: Reactants are mixed and usually warmed gently. Formation of a yellow precipitate (iodoform).
• Purification: Washing with water, followed by recrystallization from ethanol or rectified spirit.
• Characterization: Yellow crystalline solid with a characteristic antiseptic smell. Melting Point: ~119-121°C.
• Key Exam Points:
  • The Haloform reaction is a diagnostic test for methyl ketones and secondary alcohols with -OH on the second carbon (like ethanol, propan-2-ol).
  • Iodoform is the specific product when iodine and base are used.
  • Recognize the structural requirement for a positive iodoform test.

General Exam Tips:

• Pay attention to the limiting reagent when thinking about theoretical yield.
• Understand the principle of recrystallization: Dissolve impure solid in minimum amount of hot solvent, filter hot (if needed to remove insoluble impurities), cool slowly to crystallize the pure compound, filter cold, wash with cold solvent, dry. The compound should be highly soluble in the hot solvent and sparingly soluble in the cold solvent. Impurities should ideally remain soluble in the cold solvent or be insoluble in the hot solvent.
• Know the melting points and physical appearance (color, state) of the prepared compounds – they are key identifiers.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the concepts discussed, suitable for government exam preparation:

1. The preparation of acetanilide from aniline using acetic anhydride is an example of:
   a) Electrophilic substitution
   b) Nucleophilic substitution
   c) Nucleophilic acyl substitution
   d) Electrophilic addition

2. In the preparation of p-nitroacetanilide, concentrated H₂SO₄ is used along with concentrated HNO₃ primarily to:
   a) Act as a solvent
   b) Act as an oxidizing agent
   c) Facilitate the generation of the nitronium ion (NO₂⁺)
   d) Prevent side reactions

3. The Claisen-Schmidt condensation is used to prepare dibenzalacetone. Which of the following acts as the nucleophile in the initial step?
   a) Benzaldehyde
   b) Enolate ion of benzaldehyde
   c) Acetone
   d) Enolate ion of acetone

4. Diazotization of aniline to form benzenediazonium chloride must be carried out at low temperatures (0-5°C) because:
   a) The reaction is highly exothermic
   b) Aniline is volatile
   c) Benzenediazonium chloride is unstable at higher temperatures
   d) Sodium nitrite decomposes at higher temperatures

5. Which of the following compounds will give a positive iodoform test?
   a) Methanol
   b) Propan-1-ol
   c) Propan-2-ol
   d) Benzaldehyde

6. Aniline Yellow (p-aminoazobenzene) is formed by the coupling reaction between benzenediazonium chloride and aniline. The diazonium ion acts as:
   a) A nucleophile
   b) An electrophile
   c) A base
   d) A reducing agent

7. The most common method used for the purification of solid organic compounds like acetanilide or p-nitroacetanilide prepared in the lab is:
   a) Distillation
   b) Steam distillation
   c) Chromatography
   d) Recrystallization

8. Why is aniline usually acetylated before nitration?
   a) To increase the reactivity of the benzene ring.
   b) To make the amino group meta-directing.
   c) To protect the amino group from oxidation and control the substitution (o/p directing).
   d) To make the final product more soluble.

9. In the preparation of dibenzalacetone, benzaldehyde does not undergo self-condensation because:
   a) It is sterically hindered.
   b) It lacks alpha-hydrogens.
   c) It is less reactive than acetone.
   d) The base catalyst is too weak.

10. The characteristic color and smell of iodoform (CHI₃) are:
    a) White crystalline, odorless
    b) Yellow crystalline, antiseptic smell
    c) Colorless liquid, sweet smell
    d) Orange powder, pungent smell

Answer Key:

1. c
2. c
3. d
4. c
5. c (Propan-2-ol has the CH₃-CH(OH)- group)
6. b
7. d
8. c
9. b
10. b

Study these preparations thoroughly, focusing on the reaction, reagents, conditions, and purification principles. Good luck with your preparation!