FYUGP B.Sc Chemistry Semester 1: Unit 6 - Electronic Effects in Organic Molecules

This unit explores the electronic effects that influence the reactivity and stability of organic molecules. Topics include electrophiles, nucleophiles, inductive effects, resonance, conjugation, and delocalization. These notes are designed to help students understand the electronic nature of organic reactions and their mechanisms.

1. Concept of Electrophiles and Nucleophiles

Electrophiles and nucleophiles are key players in organic reactions:

a. Electrophiles

• Definition: Electron-deficient species that seek electrons.
• Examples: H⁺, BF₃, NO₂⁺.
• Behavior: Attack electron-rich sites (nucleophiles).

b. Nucleophiles

• Definition: Electron-rich species that donate electrons.
• Examples: OH^-, NH₃, CN^-.
• Behavior: Attack electron-deficient sites (electrophiles).

Fig 1: Electrophiles and Nucleophiles in Organic Reactions

2. Inductive Effects

The inductive effect is the polarization of σ bonds due to differences in electronegativity between atoms. It can be:

a. Electron-Withdrawing Inductive Effect (-I)

• Atoms or groups pull electron density away from the rest of the molecule.
• Examples: -NO₂, -CN, -F.

b. Electron-Donating Inductive Effect (+I)

• Atoms or groups push electron density toward the rest of the molecule.
• Examples: -CH₃, -C₂H₅.

Fig 2: Inductive Effect in Organic Molecules

3. Resonance

Resonance is the delocalization of π electrons within a molecule, leading to stabilization. Key points:

• Resonance Structures: Different representations of a molecule with the same arrangement of atoms but different electron distributions.
• Resonance Hybrid: The actual structure is an average of all resonance structures.
• Example: Benzene has two resonance structures.

Fig 3: Resonance Structures of Benzene

4. Conjugation and Delocalization

Conjugation involves the overlap of p orbitals across adjacent atoms, allowing π electrons to delocalize. Key points:

• Conjugated Systems: Alternating single and double bonds (e.g., 1,3-butadiene).
• Delocalization: Electrons are spread over multiple atoms, increasing stability.
• Example: In 1,3-butadiene, the π electrons are delocalized over all four carbon atoms.

Fig 4: Conjugation in 1,3-Butadiene

5. Practical Applications

• Reactivity Prediction: Understanding electronic effects helps predict the reactivity of organic molecules.
• Drug Design: Electronic effects influence the biological activity of pharmaceutical compounds.
• Material Science: Conjugated systems are used in organic semiconductors and LEDs.

These notes provide a comprehensive understanding of electronic effects in organic molecules. Practice problems and reaction mechanism analysis will help reinforce these concepts.