Aromaticity Aromatic (stable) Anti-aromatic (unstable)

Nitration of Chlorobenzene vs. Nitrobenzene

The directing effects of substituents on electrophilic aromatic substitution (EAS) determine the product distribution during nitration.

Nitration of Chlorobenzene

Substituent: Chlorine (-Cl)
Electronic Effects:
- -I Effect: Electron-withdrawing inductively, deactivating the ring.
- +M Effect: Electron-donating via resonance (lone pair delocalization), activating ortho and para positions.
Directing Effect: Overall ortho/para director. The resonance effect dominates in directing the incoming electrophile, making ortho and para positions more electron-rich than meta.
Products: Major products are ortho-nitrochlorobenzene and para-nitrochlorobenzene. The para isomer is typically favored due to less steric hindrance.

Nitration of Nitrobenzene

Substituent: Nitro group (-NO$_2$)
Electronic Effects:
- -I Effect: Strong electron-withdrawing inductively.
- -M Effect: Strong electron-withdrawing via resonance (pulls electron density from ring into nitro group).
Directing Effect: Strong meta director. The nitro group strongly deactivates the ortho and para positions, leaving the meta position as the "least deactivated" site for electrophilic attack.
Products: Major product is meta-dinitrobenzene.

Summary of Substituent Effects in Nitration

Substituent	Type of Group	Directing Effect	Major Products
Chlorine (-Cl)	Weak electron-withdrawing (-I), electron-donating (+M)	Ortho/para	o-nitrochlorobenzene, p-nitrochlorobenzene
Nitro (-NO$_2$)	Strong electron-withdrawing (-I and -M)	Meta	m-dinitrobenzene

Cyclopropane Stability and Banana Bond Theory

Cyclopropane, despite its high ring strain due to $60^\circ$ bond angles (deviating significantly from $109.5^\circ$), exhibits unique stability explained by the Banana Bond Theory (Coulson-Moffitt modification).

Key Concepts of Banana Bond Theory

Bond Angles and Strain: The $60^\circ$ internal bond angles in cyclopropane create substantial angle strain, forcing bonds to deviate from ideal geometry.
Bent Bonds (Banana Bonds): To reduce this angle strain, the carbon-carbon bonds are "bent" or "banana-shaped".
- The $sp^3$ hybrid orbitals of the carbon atoms overlap in a curved manner.
- This bending redistributes electron density away from the internuclear axis, alleviating angle strain.
Reduced Orbital Overlap: This bending results in less efficient overlap between $sp^3$ orbitals compared to normal sigma bonds, leading to weaker C-C bonds and contributing to cyclopropane's characteristic reactivity (e.g., ring-opening reactions).
Delocalization of Electrons: Electron density is distributed outside the direct internuclear axis, including towards the center of the ring, which contributes to the molecule's relative stability.