• The structure of benzene puzzled chemists for years because it behaves as unsaturated (it can add hydrogen or chlorine) but usually reacts by substitution not addition; and all six C–C bond lengths are equal and intermediate between a single and a double bond.
• Kekulé proposed a hexagonal ring with alternating single and double bonds; modern theory says the six π electrons are delocalised over the whole ring, so there are no real separate single/double bonds.
• This is represented by a hexagon with a circle inside.
• All six carbons (and hydrogens) are identical, so a substituent can attack any carbon.

• Explosives (TNT). Polynitro organic compounds are explosive: each molecule contains its own "fuel" (carbon) and the oxygen of the nitro groups; the weak N–O bond (≈ 201 kJ/mol) breaks and very strong bonds form (the C=O bonds of CO₂ and especially the N≡N bond of N₂, 941 kJ/mol), releasing a great deal of energy and large volumes of gas suddenly. TNT (2,4,6-trinitrotoluene) is prepared by nitrating toluene with a 1:1 mixture of concentrated nitric and sulphuric acids:
• C₆H₅CH₃ + 3HNO₃ —(conc. H₂SO₄)→ C₆H₂(NO₂)₃CH₃ + 3H₂O

R–C₆H₄–SO₃H + NaOH → R–C₆H₄–SO₃⁻Na⁺ + H₂O

• The detergent molecule has a long hydrocarbon tail (hydrophobic, dissolves in oil/grease) and an ionic head (hydrophilic, dissolves in water) — this lets it lift grease into water.