Benzene has the SMILES c1ccccc1; all C-C bonds equal 1.39 Å; planar hexagonal ring; 6 delocalized pi electrons.

Benzene's bond length (1.39 Å) is intermediate between single (1.54 Å) and double (1.34 Å), confirming delocalization.

Kekule structures are resonance contributors only — not the actual structure. The true benzene is the resonance hybrid.

Huckel's Rule: A compound is aromatic if (1) planar + (2) cyclic + (3) fully conjugated + (4) (4n+2) pi electrons (n = 0, 1, 2...).

Benzene satisfies Huckel's Rule: 6 pi electrons, n=1 → (4×1+2)=6.

Anti-aromatic: planar + cyclic + conjugated + 4n pi electrons → destabilized. Example: cyclobutadiene (4 pi $e^{-}$).

Non-aromatic: fails any structural criterion. COT (8 pi $e^{-}$) is non-aromatic because it is non-planar (tub shape), not anti-aromatic.

EAS mechanism: (1) $E^{+}$ generation → (2) $E^{+}$ attacks pi cloud → arenium ion (RDS) → (3) $H^{+}$ loss → rearomatization.

The arenium ion (sigma complex / Wheland intermediate) is non-aromatic; the $sp^{3}$ carbon carries both E and H temporarily.

Step 2 of EAS (electrophilic attack forming the arenium ion) is the rate-determining step.

Five EAS reactions: Halogenation, Nitration, Sulfonation, FC Alkylation, FC Acylation.

Only sulfonation is reversible: $ArSO_{3}H$ → ArH by heating with dilute $H_{2}SO_{4}$ at high temperature.

Nitration electrophile: $NO_{2}^{+}$ (nitronium ion), generated by conc. $H_{2}SO_{4}$ + $HNO_{3}$.

FC Acylation advantages: acylium ion (R$CO^{+}$) is resonance-stabilized → no rearrangement; -COR product deactivates ring → no polyacylation.

FC Alkylation problems: $R^{+}$ rearranges to more stable carbocation; monoalkyl product activates ring → polyalkylation.

Ortho-para directors (activating): -OH, -$NH_{2}$, -OR, -$NR_{2}$, -$CH_{3}$ (alkyl) — donate electrons by +M or +I/hyperconjugation.

Halogens are ortho-para directors (via +M) but deactivating (via dominant -I). The ONLY deactivating o/p directors.

Meta directors (all deactivating): -$NO_{2}$, -CN, -CHO, -COOH, -COR, -$SO_{3}H$ — withdraw via -M effect; deplete o/p > meta.

FC reactions fail on strongly deactivated rings (e.g., nitrobenzene).

Protonated aniline (-$NH_{3}^{+}$) loses its lone pair availability and becomes a meta director and deactivating group.