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Pharmaceutical Organic Chemistry II (BP301T)

Unit I – Benzene & Aromaticity

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1. Introduction to Benzene

Benzene is an aromatic hydrocarbon with molecular formula C₆H₆. It is a planar cyclic molecule containing six carbon atoms arranged in a hexagonal ring with delocalized π-electrons.

2. Structure of Benzene

Benzene exhibits resonance, and its structure is best represented by resonance hybrids rather than alternating single and double bonds. All C–C bond lengths are equal due to electron delocalization.

3. Aromaticity

Aromaticity refers to the unusual stability of cyclic, planar, conjugated systems containing (4n + 2) π-electrons, as per Huckel’s rule.

4. Huckel’s Rule

A compound is aromatic if it:

5. Electrophilic Aromatic Substitution

Benzene undergoes electrophilic substitution reactions instead of addition reactions to preserve aromatic stability.

6. Activating and Deactivating Groups

Substituents on benzene ring influence reactivity and orientation. Electron-donating groups activate the ring, while electron-withdrawing groups deactivate it.

7. Orientation in Aromatic Substitution

8. Pharmaceutical Importance

Many pharmaceutical compounds contain aromatic rings. Understanding aromaticity is essential for predicting chemical reactivity and drug design.