The benzene ring
Benzene, C₆H₆, is a ring of six carbons — but it is not simply a ring of alternating double and single bonds. Its six extra p electrons are delocalised into a ring above and below the plane, so all six carbon–carbon bonds are identical, intermediate between single and double. That delocalisation makes benzene unusually stable ("aromatic"), far less reactive than an alkene.
This is why the Kekulé "alternating double bonds" picture is only a shorthand; the real structure is the delocalised ring, often drawn as a hexagon with a circle inside.
It substitutes, not adds
An alkene readily undergoes addition, which would break its double bond — but for benzene, addition would destroy the stable delocalised ring, which is energetically costly. So benzene instead undergoes electrophilic substitution: an electrophile replaces a hydrogen on the ring, and the aromatic system is preserved.
This single principle — keep the ring, swap an H — governs all of benzene's reactions.
Alkene vs arene, in one line
An alkene ADDS (its double bond is reactive and expendable). Benzene SUBSTITUTES (its delocalised ring is stable and worth preserving). Same electrophiles, opposite outcome.
The key substitution reactions
Each is an electrophilic substitution putting a new group on the ring in place of an H:
| Reaction | Reagent / electrophile | Product |
|---|---|---|
| Nitration | conc. HNO₃ / conc. H₂SO₄ (NO₂⁺) | nitrobenzene |
| Halogenation | Cl₂ or Br₂ with a halogen carrier (e.g. FeCl₃) | chlorobenzene |
| Alkylation (Friedel–Crafts) | haloalkane + AlCl₃ | an alkylbenzene (e.g. methylbenzene) |
| Sulfonation | fuming sulfuric acid | benzenesulfonic acid |
Why it matters
Aromatic compounds are everywhere in industry and biology — dyes, explosives (TNT is trinitrotoluene), drugs (aspirin, paracetamol), and plastics (polystyrene from styrene). Benzene derivatives like toluene and phenol are major feedstocks, so its substitution chemistry is worth knowing well.
A worked example
Ethene reacts with bromine by addition, but benzene does not readily add bromine. Explain the difference.
- Ethene has a localised C=C double bond that is reactive; bromine adds across it, breaking the double bond.Addition is favourable for an alkene — the product is stable.
- Benzene's electrons are delocalised into a stable aromatic ring.The ring is much more stable than a set of alternating double bonds.
- Adding bromine would destroy that delocalised ring, which is energetically unfavourable, so benzene resists addition.Losing aromatic stability costs more energy than addition releases.
- With a halogen carrier, benzene instead undergoes substitution, keeping the ring.Substitution preserves the stable aromatic system.
Ethene adds bromine (reactive localised double bond); benzene resists addition because it would destroy its stable delocalised ring, so it substitutes instead.
How electrophilic substitution works
Benzene's reactions follow one mechanism: electrophilic substitution. First a strong electrophile is generated — in nitration, concentrated sulfuric acid protonates nitric acid to make the nitronium ion, NO₂⁺. The electron-rich ring then attacks the electrophile, briefly disrupting the delocalised system, before losing an H⁺ to restore the stable ring. Restoring that aromatic stability is the driving force for substituting rather than adding.
The catalyst is regenerated at the end, and the net result is one hydrogen on the ring swapped for the new group.
Friedel–Crafts and comparing reactivity
Two important variants build carbon–carbon bonds onto the ring: Friedel–Crafts alkylation (a haloalkane with an AlCl₃ catalyst adds an alkyl group) and acylation (an acyl chloride adds an acyl group). Both make the electrophile with a halogen-carrier catalyst.
Reactivity differs across related molecules: an alkene adds readily because its localised C=C is reactive; benzene is far less reactive and only substitutes; while phenol is more reactive than benzene, because the oxygen's lone pair feeds electron density into the ring — so phenol even decolourises bromine water without a catalyst.
Why phenol beats benzene
Phenol's –OH lone pair delocalises into the ring, making it more electron-rich and so more reactive toward electrophiles than benzene — it brominates three times over with bromine water alone.