(SEM III) THEORY EXAMINATION 2022-2023 PHARMACEUTICAL ORGANIC CHEMISTRY-II

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.