CHM-DSM-251 Unit 2: Organic Preparation and Purification

Course Code: CHM-DSM-251

Paper Name: Practical (Inorganic, Organic and Physical Chemistry)

1. Qualitative Organic Analysis

Systematic qualitative analysis is performed to identify the functional group present in an unknown organic compound.

• Scope: Identification of mono-functional groups including carboxylic acids (-COOH), phenols, aldehydes, ketones, amides, nitro groups, and amines.
• Methodology:
  • Preliminary Tests: Physical state, color, odor, and solubility in water/NaOH/NaHCO3.
  • Detection of Elements: Lassaigne’s test to detect Nitrogen, Sulphur, and Halogens.
  • Functional Group Tests: Confirmatory chemical tests (e.g., Litmus test for acids, FeCl3 test for phenols, 2,4-DNP test for carbonyls).

2. Organic Preparations and Reactions

Practical synthesis involves performing a chemical reaction to create a specific derivative or compound.

Key Synthetic Reactions

• Nitration: Introduction of a nitro group into acetanilide, nitrobenzene, or salicylic acid.
• Bromination: Electrophilic substitution of bromine into phenol or aniline.
• Haloform Reaction: Synthesis of Iodoform using acetone.
• Condensation: Preparation of oximes or 2,4-dinitrophenylhydrazones of aldehydes/ketones.
• Rearrangement: Synthesis of Benzil from benzoin and subsequently Benzilic acid from benzil.
• Acylation: Benzoylation of phenol or aniline.

3. Purification Techniques: Recrystallization

Recrystallization is the primary method for purifying crude solid products by dissolving them in a suitable solvent at high temperatures and allowing them to crystallize upon cooling.

Standard Procedures

• Decolorization: Adding animal charcoal to crude sulphanilic acid during boiling to remove colored impurities, followed by hot filtration.
• Solvent Selection:
  • Boiling Water: Used for purifying Acetanilide.
  • Hot Water/Ethanol: Used for purifying Benzoic acid.

4. Purification by Sublimation

Sublimation is used for solid compounds that have high vapor pressure and can bypass the liquid phase.

• Target Compounds: Naphthalene, camphor, and phthalic acid.
• Technique: The substance is heated in a porcelain dish; vapors are condensed on a cool surface (like an inverted funnel) to yield pure crystals.

5. Exam Focus: Tips and Viva Questions

Practical Exam Tips

• Yield Calculation: Always weigh your final purified product to calculate the percentage yield.
• Purity Check: The quality of your purification is often assessed by the sharpness of the melting point.
• Filtration: Use "fluted filter paper" for hot filtrations to increase the surface area and prevent premature crystallization in the funnel.

Frequently Asked Questions

Q: What is the purpose of adding NH4Cl before NH4OH in cation analysis?
A: (Refresher) To suppress the ionization of NH4OH through the common ion effect, ensuring only Group III hydroxides precipitate.

Q: Why is Iodoform preparation used as a test?
A: It specifically identifies the presence of methyl ketones (CH3CO-) or alcohols that can be oxidized to methyl ketones.

Q: What is the benefit of sublimation over recrystallization?
A: It is a solvent-free method that is very effective for substances that decompose at their melting point but have high vapor pressure.