(SEM III) THEORY EXAMINATION 2023-24 PHARMACEUTICAL ORGANIC CHEMISTRY II

SECTION A – Short Answer Section

This section contains short conceptual questions related to aromatic compounds, phenols, oils and fats, and cycloalkanes. These questions test the basic knowledge of directing effects, chemical tests, and simple organic chemistry concepts used in pharmaceutical chemistry.

Question 1: Mention ortho, meta and para directing groups with examples.

Answer:

In electrophilic substitution reactions, substituents on benzene ring influence the position where new groups attach.

Ortho–Para Directing Groups:
These groups direct substitution to the ortho and para positions.
Examples: –OH, –NH₂, –CH₃.

Meta Directing Groups:
These groups direct substitution to the meta position.
Examples: –NO₂, –COOH, –SO₃H.

Question 2: Why are Aromatic Amines Less Basic than Aliphatic Amines?

Answer:
Aromatic amines are less basic because the lone pair of electrons on nitrogen is delocalized into the benzene ring through resonance.

This reduces the availability of the electron pair for protonation, making aromatic amines weaker bases compared to aliphatic amines.

Question 3: Mention Two Qualitative Tests for Phenol.

Answer:

Ferric Chloride Test:
Phenol gives a violet or purple color with ferric chloride solution.

Liebermann’s Test:
Phenol reacts with sodium nitrite and sulfuric acid producing deep blue or green color.

SECTION B – Long Answer Section

This section requires detailed explanations of aromatic reactions, synthesis of hydrocarbons, and theoretical concepts of ring strain in cycloalkanes.

Question 1: Discuss Aromatic Electrophilic Substitution Reactions.

Answer:

Aromatic electrophilic substitution is a reaction where an electrophile replaces a hydrogen atom in an aromatic ring.

Common reactions include:

1. Nitration:
Benzene reacts with nitric acid in the presence of sulfuric acid to form nitrobenzene.

2. Friedel–Crafts Alkylation:
An alkyl group is introduced into benzene using alkyl halide and AlCl₃ catalyst.

3. Friedel–Crafts Acylation:
An acyl group is introduced using acyl chloride and AlCl₃ catalyst.

These reactions proceed through formation of an arenium ion intermediate.

Question 2: Write the Synthesis and Uses of Anthracene.

Answer:

Anthracene is a polynuclear aromatic hydrocarbon with three fused benzene rings.

Synthesis:
It is obtained from coal tar during fractional distillation.

Uses:

Manufacture of dyes such as alizarin

Used in organic synthesis

Used in pharmaceutical and chemical industries.

Question 3: Discuss 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 bond angle of 109.5°.

Deviation from this angle causes strain in cyclic molecules.

Example:
Cyclopropane has high strain due to 60° bond angle.

Limitations:

Does not explain stability of larger rings.

Assumes molecules are planar.

Later, Coulson and Moffitt modifications explained non-planar structures of cycloalkanes.

SECTION C – Descriptive Section

This section tests deeper understanding of aromatic structure, organic acids, hydrocarbons, and properties of oils.

Question 1: Write Analytical and Synthetic Evidence for Structure of Benzene.

Answer:

Evidence supporting benzene structure includes:

Analytical Evidence:

Molecular formula C₆H₆

High degree of unsaturation.

Synthetic Evidence:

Benzene undergoes substitution reactions rather than addition reactions.

Only one monosubstituted product is formed.

These observations support the hexagonal ring structure with delocalized electrons.

Question 2: Give Structure, Preparation and Properties of Benzoic Acid.

Answer:

Structure:
C₆H₅COOH

Preparation:
Benzoic acid can be prepared by oxidation of toluene using potassium permanganate.

Properties:

White crystalline solid

Slightly soluble in water

Used as a preservative and antifungal agent.

Question 3: Explain Hydrolysis, Rancidity and Drying of Oils.

Answer:

Hydrolysis:
Breaking down of fats and oils into fatty acids and glycerol.

Rancidity:
Spoilage of oils due to oxidation, producing unpleasant odor and taste.

Drying of Oils:
Certain oils react with oxygen forming hard films used in paints and coatings.

These processes affect the quality and stability of oils in pharmaceuticals and food products.