Edexcel IAL Chemistry · Organic Chemistry
Arenes: Benzene, Substitution & Phenols
Benzene's electrons are delocalised in a ring of continuous π density, which makes it stable and resistant to addition — so it reacts by electrophilic substitution instead, swapping out an H atom rather than breaking its ring apart.
Summary — What This Chapter Covers
- Benzene's real structure isn't Kekulé's alternating single/double bonds — it's a planar ring of sp² carbons with a delocalised π system above and below the ring.
- Four types of evidence (hydrogenation enthalpy, bond lengths, resistance to bromine water, IR spectroscopy) all point to delocalisation, not alternating double bonds.
- Because the ring is so stable, benzene doesn't do addition reactions like alkenes do — it undergoes electrophilic substitution instead.
- Key substitution reactions: halogenation (needs a halogen carrier), nitration (conc. HNO₃ + conc. H₂SO₄), Friedel–Crafts alkylation & acylation, and sulfonation.
- Every electrophilic substitution mechanism follows the same three-step pattern: generate the electrophile → electrophilic attack (forms a horseshoe intermediate) → restore aromaticity.
- Phenol reacts far more readily than benzene because oxygen's lone pair feeds extra electron density into the ring — so phenol reacts with bromine water directly, no catalyst needed.
1. Benzene — Structure & Stability
The Kekulé Problem
Back in the 1800s, Kekulé proposed that benzene (C₆H₆) was a hexagonal ring of carbon atoms with alternating single and double bonds — basically three "mini alkenes" fused into a ring. It's a neat idea, and it does explain the molecular formula. The problem is: if that were true, benzene should behave like an alkene. Alkenes react eagerly with things like bromine water via electrophilic addition, because their C=C double bond is a small, localised pocket of high electron density that practically begs an electrophile to attack it.
But benzene doesn't decolourise bromine water on its own. It just sits there. So either the Kekulé structure is wrong, or something else is going on. Spoiler: the structure is wrong (or at least incomplete).
The Real (Delocalised) Structure
Here's the modern picture. Every carbon in the ring is sp² hybridised: it forms three σ (single) bonds — two to neighbouring carbons and one to a hydrogen — all lying flat in the same plane, with bond angles of 120°. That accounts for the "hexagon" shape you always draw.
But each carbon also has one leftover electron sitting in a p orbital, sticking up above and below the plane of the ring like a little dumbbell. Because all six of these p orbitals are right next to each other, they don't stay as six separate, isolated electrons — they overlap sideways with their neighbours and merge into one continuous cloud of electron density that stretches all the way around the ring, forming a "doughnut" above the plane and a matching one below it.
This is called a delocalised π system. Instead of three fixed double bonds sitting in three fixed places, you have six electrons smeared evenly around all six carbons. That's why we draw modern benzene as a hexagon with a circle inside it — the circle represents this delocalised ring of electron density, not any specific bond.
Key Idea
no true C=C double bonds
The Four Pieces of Evidence
why
① Enthalpy of hydrogenation.