Aromaticity and Benzene Reactivity

Benzene ($C_6H_6$) is not just a simple hexagon with alternating double bonds. In reality, all six carbon-carbon bonds are of equal length, intermediate between a single and a double bond. This occurs because the six $p$-orbitals overlap to form a continuous **delocalized $\pi$-system. This 'circular' movement of electrons creates an extra layer of stability known as resonance energy** (approx. $150 \text{ kJ/mol}$). According to Hückel's Rule, for a molecule to be aromatic, it must be planar, cyclic, and have $4n+2$ $\pi$-electrons. Benzene fits this perfectly with $n=1$, giving $6$ $\pi$-electrons.

Because benzene is electron-rich, it attracts electrophiles ($E^+$). However, instead of adding across a bond, it undergoes Electrophilic Aromatic Substitution (EAS) to preserve its aromaticity. The general mechanism follows three steps: 1. Generation of the Electrophile: A catalyst (like a Lewis acid) creates a strong $E^+$. 2. Electrophilic Attack: The benzene ring donates $\pi$-electrons to the $E^+$, forming a non-aromatic sigma complex (or arenium ion). 3. Deprotonation: A base removes a proton ($H^+$) from the ring, restoring the stable aromatic $\pi$-system.

When a group is already on the ring, it influences where the next group goes. Activating groups (e.g., $-OH, -CH_3$) donate electron density into the ring, making it more reactive and directing the new group to the ortho (1,2) or para (1,4) positions. Deactivating groups (e.g., $-NO_2, -COOH$) withdraw electron density, making the ring less reactive and directing the new group to the meta (1,3) position. This is due to the stability of the resulting resonance structures in the sigma complex intermediate.