(SEM II) THEORY EXAMINATION 2017-18 PHARMACEUTICAL ORGANIC CHEMISTRY I

SECTION A – Short Answer Section

This section contains short conceptual questions that test the basic understanding of aromatic compounds, organic reactions, fats and oils, and chemical tests. Students must answer briefly using key definitions and concepts of pharmaceutical organic chemistry.

Question 1: State Huckel’s Rule of Aromaticity.

Answer:
Huckel’s rule states that a compound is aromatic if it contains (4n + 2) π electrons, where n is an integer (0,1,2,3…). Aromatic compounds are cyclic, planar, and have a conjugated system of π electrons.
Example: Benzene has 6 π electrons (n=1) and is aromatic.

Question 2: What is Friedel–Crafts Acylation?

Answer:
Friedel–Crafts acylation is a reaction in which an acyl group (R–CO–) is introduced into an aromatic ring using an acyl chloride or acid anhydride in the presence of a Lewis acid catalyst such as AlCl₃.
Example: Benzene + Acetyl chloride → Acetophenone.

Question 3: What is Saponification?

Answer:
Saponification is the hydrolysis of fats or oils with alkali (NaOH or KOH) to produce soap and glycerol.
Example: Fat + NaOH → Soap + Glycerol.

SECTION B – Long Answer Section

This section requires detailed explanations of aromatic compounds, benzene reactions, and analytical constants of fats and oils.

Question 1: Explain the Structure of Benzene.

Answer:
Benzene (C₆H₆) is a planar hexagonal ring with six carbon atoms. Each carbon atom undergoes sp² hybridization forming sigma bonds with adjacent carbons and hydrogen atoms.

Key features:

All carbon-carbon bond lengths are equal.

The π electrons are delocalized over the ring, giving stability.

Resonance structures explain the equal bond lengths.

Benzene satisfies Huckel’s rule (6 π electrons) and therefore shows aromatic character.

Question 2: Explain Nitration, Sulphonation and Halogenation of Benzene.

Answer:

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

Reaction:
Benzene + HNO₃ → Nitrobenzene + H₂O

2. Sulphonation:
Benzene reacts with concentrated sulfuric acid to form benzene sulfonic acid.

3. Halogenation:
Benzene reacts with chlorine or bromine in the presence of FeCl₃ or FeBr₃ catalyst to form chlorobenzene or bromobenzene.

These reactions are examples of electrophilic substitution reactions.

Question 3: Explain Analytical Constants of Fats and Oils.

Answer:

Analytical constants help determine the quality and composition of fats and oils.

1. Acid Value:
It measures the amount of free fatty acids present in fats or oils.

2. Saponification Value:
It indicates the amount of alkali required to hydrolyze fats.

3. Iodine Value:
It measures the degree of unsaturation in fats and oils.

These values are important in pharmaceutical and food industries.

SECTION C – Descriptive Section

This section tests deeper understanding of aromatic reactions, phenols, fats and oils, and organic reaction mechanisms.

Question 1: Explain the Acidity of Phenols.

Answer:
Phenols are more acidic than alcohols because the phenoxide ion formed after losing a proton is stabilized by resonance.

Factors affecting acidity:

Electron-withdrawing groups increase acidity.

Electron-donating groups decrease acidity.

Example: Nitro group increases phenol acidity.

Question 2: Explain Baeyer’s Strain Theory.

Answer:
Baeyer’s strain theory explains the stability of cycloalkanes based on angle strain.

According to this theory:

Carbon prefers a bond angle of 109.5°.

Deviation from this angle causes strain in cyclic compounds.

For example:

Cyclopropane has high strain due to small bond angles.

Cyclopentane is more stable.

Limitations:
It cannot explain the stability of larger ring structures.

Question 3: Write Structure and Uses of DDT and Saccharin.

Answer:

DDT (Dichloro Diphenyl Trichloroethane):
Structure: C₁₄H₉Cl₅
Use: Insecticide used to control mosquitoes and agricultural pests.

Saccharin:
Structure: Artificial sweetener compound.
Use: Used as a non-caloric sugar substitute in medicines and food products.