CHM-DSC-253: Section B (Organic Practical)
Paper Name: Practical (Inorganic, Organic and Physical Chemistry)
Section Marks: 15 (Preparation) + 15 (Purification) = 30 Marks
Table of Contents
1. Principles of Organic Preparation
Organic preparation involves the synthesis of a specific organic compound through chemical reactions like substitution, oxidation, or condensation.
- Theoretical Yield: The maximum amount of product that can be formed from the given reactants.
- Percentage Yield: Calculated as (Actual Yield / Theoretical Yield) x 100.
- Crude Product: The initial product obtained before any purification steps are performed.
2. Key Organic Preparations
The syllabus lists several fundamental organic reactions used for synthesis [cite: 764-770].
Nitration Reactions
The introduction of a nitro group (-NO2) into an organic molecule.
- Examples: Nitration of acetanilide, nitrobenzene, or salicylic acid.
- Reagent: Typically a mixture of concentrated Nitric Acid (HNO3) and Sulfuric Acid (H2SO4).
Bromination Reactions
The substitution of hydrogen atoms with bromine atoms.
- Examples: Bromination of phenol or aniline.
- Observation: Often results in the formation of white precipitates (e.g., 2,4,6-tribromophenol).
Iodoform Preparation
The haloform reaction of acetone or ethanol with iodine and alkali.
- Product: Yellow crystals of Iodoform (CHI3) with a characteristic medicinal smell.
3. Specialized Reactions
Some preparations involve structural rearrangements or condensation [cite: 766-769].
- Azomethyne: Formation of Schiff bases through the condensation of amines and aldehydes.
- Benzil from Benzoin: An oxidation reaction using concentrated nitric acid.
- Benzilic Acid from Benzil: A classic rearrangement reaction involving the addition of strong alkali (KOH) to benzil.
4. Purification Techniques: Recrystallization
Recrystallization is the most common method for purifying solid organic compounds based on differences in solubility.
Steps in Recrystallization
- Selection of a suitable solvent where the solute is soluble at high temperature but insoluble at low temperature [cite: 775-777].
- Preparation of a saturated solution in hot solvent.
- Hot filtration to remove insoluble impurities.
- Cooling the filtrate to allow crystal formation.
- Cold filtration to collect pure crystals and washing.
Specific Applications
- Decolorization: Use of animal charcoal to remove colored impurities from crude sulphanilic acid.
- Solvents: Boiling water is used for acetanilide; ethanol is used for naphthalene or m-dinitrobenzene.
5. Purification Techniques: Sublimation
Sublimation is used for substances that pass directly from solid to vapor phase upon heating.
- Compounds: Naphthalene, camphor, and phthalic acid.
- Process: The crude solid is heated in a porcelain dish covered with a perforated filter paper and an inverted funnel to collect pure "sublimate" on the cool walls of the funnel.
6. Exam Focus: Laboratory Tips & Viva
Practical Exam Tips
- Safety: Always perform nitration in a fume hood or well-ventilated area due to acid fumes.
- Drying: Ensure crystals are completely dry before weighing them to calculate the yield.
- Melting Point: The purity of your purified product should be checked by determining its melting point; a sharp melting point indicates high purity.
Common Viva Questions
Q: Why is concentrated H2SO4 used in nitration?
A: It acts as a dehydrating agent and helps in the generation of the nitronium ion (NO2+), which is the active electrophile.
Q: What is the role of animal charcoal in recrystallization?
A: It adsorbs high molecular weight colored impurities from the solution.
Q: Define a Schiff Base.
A: It is an azomethine compound formed by the condensation of a primary amine with a carbonyl compound.