About this previous year paper
SECTION A – Short Answer Section
This section contains short conceptual questions related to aromatic compounds, fats and oils, phenols, and analytical constants used in pharmaceutical organic chemistry.
Question 1: What is Huckel’s Rule?
Answer:
Huckel’s rule states that a compound is aromatic if it contains (4n + 2) π electrons, where n is an integer (0,1,2…).
Conditions for aromaticity:
Cyclic structure
Planar structure
Conjugated π electron system
Example: Benzene (6 π electrons).
Question 2: What is Acid Value? Give its Use.
Answer:
Acid value is the number of milligrams of potassium hydroxide (KOH) required to neutralize free fatty acids present in 1 gram of fat or oil.
Use:
It is used to determine the quality and purity of fats and oils.
Question 3: What are Fats and Oils? Give Examples.
Answer:
Fats and oils are esters formed from glycerol and fatty acids.
Difference:
Fats are solid at room temperature.
Oils are liquid at room temperature.
Examples:
Fat → Butter
Oil → Olive oil.
SECTION B – Long Answer Section
This section requires detailed explanations of cycloalkanes, aromatic structures, and theories explaining stability of ring compounds.
Question 1: Explain Baeyer’s Strain Theory.
Answer:
Baeyer’s strain theory explains the stability of cycloalkanes based on angle strain.
According to the theory:
Carbon atoms prefer a bond angle of 109.5°.
Any deviation from this angle causes strain in cyclic compounds.
Example:
Cyclopropane (60° bond angle) has high strain and is less stable.
Limitations:
Assumes all rings are planar.
Cannot explain stability of larger rings like cyclohexane.
Question 2: Explain Sachse-Mohr Theory.
Answer:
Sachse-Mohr theory explains that cyclohexane rings are not planar but exist in non-planar conformations.
Two important conformations:
Chair form – Most stable form.
Boat form – Less stable due to steric strain.
This theory explains the actual stability of cycloalkanes better than Baeyer’s theory.
Question 3: Explain Evidence for Structure of Benzene.
Answer:
Evidence supporting benzene structure:
Analytical Evidence:
Molecular formula C₆H₆.
High degree of unsaturation.
Chemical Evidence:
Benzene undergoes substitution reactions instead of addition reactions.
Physical Evidence:
All carbon–carbon bond lengths are equal.
These evidences support the hexagonal ring structure with delocalized electrons.
SECTION C – Descriptive Section
This section tests deeper understanding of aromatic compounds, phenols, aromatic amines, and analytical methods used in fats and oils.
Question 1: Why Phenols are Acidic?
Answer:
Phenols are acidic because the phenoxide ion formed after losing a proton is stabilized by resonance.
Resonance spreads the negative charge over the benzene ring, making phenols more acidic than alcohols.
Effect of substituents:
Electron-withdrawing groups increase acidity.
Electron-donating groups decrease acidity.
Question 2: Define Aromatic Amines and Explain Resonance of Aniline.
Answer:
Aromatic amines are compounds where an amino group (–NH₂) is attached to an aromatic ring.
Example: Aniline (C₆H₅NH₂).
In aniline, the lone pair of electrons on nitrogen participates in resonance with the benzene ring, which stabilizes the molecule.
This resonance affects the basicity and reactivity of aniline.
Question 3: Principle of Estimation of Acid Value of Fat/Oil.
Answer:
Principle:
The acid value is determined by titrating free fatty acids in the fat or oil with a standard KOH solution.
Procedure:
Dissolve fat or oil in alcohol.
Add phenolphthalein indicator.
Titrate with KOH until a pink color appears.
The acid value indicates the degree of rancidity or degradation of fats and oils.