Arenes

Benzene has six carbons and three apparent double bonds, yet it refuses to react like an alkene. This topic explains why: its electrons are spread around the whole ring, which makes it unusually stable and steers it towards substitution rather than addition. It also covers the reactions that put new groups onto the ring, and why phenol reacts so much more readily.

The structure of benzene

Organic compounds are either aliphatic (straight or branched chains, and rings that are not aromatic) or aromatic, containing one or more benzene rings. Compounds containing a benzene ring are called arenes.

Benzene, C₆H₆, is a flat hexagonal ring. Each carbon forms σ bonds to two other carbons and one hydrogen, with bond angles of 120°. That leaves each carbon with one electron in a p orbital at right angles to the ring. The six p orbitals overlap sideways all the way round, forming rings of π electron density above and below the plane: the six π electrons are delocalised over all six carbons rather than held in three separate double bonds. The ring is drawn as a hexagon with a circle inside.

Evidence against Kekulé's structure

In 1865 Kekulé proposed a ring of alternating single and double bonds, cyclohexa-1,3,5-triene. Three pieces of evidence show that this is wrong:

Bond lengths: X-ray diffraction shows all six C–C bonds are the same length, 0.139 nm, between a C–C single bond (0.154 nm) and a C=C double bond (0.134 nm). Kekulé's structure would have three of each.
Enthalpy of hydrogenation: hydrogenating cyclohexene, with one C=C, releases 120 kJ mol⁻¹, so three C=C bonds should release 3 × 120 = 360 kJ mol⁻¹. Benzene actually releases only 208 kJ mol⁻¹. Benzene is 152 kJ mol⁻¹ more stable than Kekulé's structure: this is the delocalisation energy.
Reactivity: alkenes decolourise bromine water quickly by addition, but benzene does not react with bromine water. Addition would break up the stable delocalised system.

Naming aromatic compounds

Simple derivatives are named with benzene as the parent: methylbenzene, chlorobenzene, nitrobenzene, benzenecarboxylic acid (C₆H₅COOH), benzaldehyde (C₆H₅CHO).
With two or more substituents, number the ring to give the lowest numbers, and list different groups alphabetically: 1,3-dimethylbenzene, 1-chloro-4-methylbenzene, 2-methyl-1,3,5-trinitrobenzene (TNT).
When the ring is attached to a more important group, it is called the phenyl group, C₆H₅–: phenylamine (C₆H₅NH₂), phenylethene, phenylethanone (C₆H₅COCH₃), phenyl ethanoate.
Toxicity: benzene is a carcinogen, so methylbenzene is used instead in school laboratories.

Combustion and hydrogenation

Combustion: C₆H₆ + 7½O₂ → 6CO₂ + 3H₂O. Arenes burn with a very smoky, sooty flame because their high carbon-to-hydrogen ratio leaves unburnt carbon.
Hydrogenation: with hydrogen over a nickel catalyst at about 200 °C and 30 atm, benzene adds three molecules of H₂ to give cyclohexane. This is one of the few addition reactions of benzene, and it needs far harsher conditions than for an alkene.

Electrophilic substitution

The delocalised π electrons give benzene a region of high electron density that attracts electrophiles. But addition would destroy the delocalisation, so instead benzene undergoes electrophilic substitution: one hydrogen is replaced and the stable ring is restored. Every mechanism follows the same three stages:

Making the electrophile: a catalyst turns the reagent into a positive ion, E⁺.
Attack: a pair of π electrons from the ring forms a bond to E⁺. This gives a positively charged intermediate in which the delocalisation covers only five carbons (drawn as a horseshoe from carbon 2 to carbon 6), with the carbon that was attacked holding both H and E.
Loss of H⁺: the electrons of the C–H bond return to the ring, restoring the full delocalised system, and H⁺ is released, usually regenerating the catalyst.

Reaction	Reagents and conditions	Electrophile and how it forms	Product
bromination (halogenation)	bromine with an iron(III) bromide catalyst (or iron filings, which form FeBr₃); room temperature. Chlorine with AlCl₃ or FeCl₃ works the same way.	Br⁺: Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻; then H⁺ + FeBr₄⁻ → FeBr₃ + HBr	bromobenzene + HBr
nitration	concentrated nitric acid and concentrated sulfuric acid, below about 55 °C (higher temperatures add more nitro groups)	NO₂⁺, the nitronium ion: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻; then H⁺ + HSO₄⁻ → H₂SO₄	nitrobenzene
sulfonation	fuming sulfuric acid (SO₃ in concentrated H₂SO₄), heated under reflux	SO₃, whose sulfur carries a large δ+ charge from its three oxygens	benzenesulfonic acid, C₆H₅SO₃H (used for detergents, dyes and drugs)
Friedel–Crafts alkylation	a halogenoalkane with an anhydrous aluminium chloride catalyst, heated under reflux	R⁺: CH₃CH₂Cl + AlCl₃ → CH₃CH₂⁺ + AlCl₄⁻; then H⁺ + AlCl₄⁻ → AlCl₃ + HCl	ethylbenzene + HCl
Friedel–Crafts acylation	an acyl chloride with an anhydrous AlCl₃ catalyst, heated under reflux at about 50 °C	RCO⁺, the acylium ion: CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻	phenylethanone, C₆H₅COCH₃, + HCl

Friedel–Crafts reactions matter in synthesis because they form a new carbon–carbon bond to the ring. Nitration is the first step towards phenylamine, used to make dyes, and towards explosives such as TNT.

The aluminium chloride must be anhydrous: water would react with it and destroy the catalyst. In the mechanism, the curly arrow for the attack starts from the circle (the delocalised electrons), not from a single bond.

How side groups change the ring

Electron-releasing groups such as alkyl groups, –OH and –NH₂ push electron density into the ring, making it more attractive to electrophiles. Methylbenzene therefore nitrates more readily than benzene, under milder conditions.

When an atom with a lone pair (O, N or Cl) is bonded directly to the ring, its lone pair overlaps with the π system and becomes partly delocalised into it. This changes both the ring and the side group:

Chlorobenzene: the C–Cl bond gains some double-bond character and becomes stronger, so chlorobenzene does not undergo the substitution and elimination reactions of halogenoalkanes; the electron-rich ring also repels nucleophiles.
Phenol: the C–O bond becomes stronger and the O–H bond weaker, so phenol is more acidic than an alcohol and is not oxidised like one.
Phenylamine: the nitrogen lone pair is partly delocalised, so it is less available to accept a proton, and phenylamine is a much weaker base than aliphatic amines (see Topic 19).

Phenol

In phenol, C₆H₅OH, the –OH group is bonded directly to the ring. (C₆H₅CH₂OH, with a CH₂ in between, is an ordinary alcohol, phenylmethanol.)

Phenol as an acid

The phenoxide ion is stabilised because its negative charge is partly delocalised into the ring, so phenol is a weak acid: stronger than alcohols and water, but weaker than carboxylic acids.

Reagent	Ethanol	Phenol	Ethanoic acid
sodium	reacts: H₂	reacts: C₆H₅OH + Na → C₆H₅O⁻Na⁺ + ½H₂	reacts: H₂
sodium hydroxide	no reaction	reacts: C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O (the solid dissolves, as sodium phenoxide is soluble)	reacts
sodium carbonate	no reaction	no reaction: phenol is too weak an acid	fizzes: CO₂

The sodium carbonate test therefore tells a phenol from a carboxylic acid.

Phenol with bromine water

Phenol decolourises bromine water at room temperature, with no catalyst, giving a white precipitate of 2,4,6-tribromophenol:

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

The oxygen lone pair delocalised into the ring raises its electron density so much that it polarises bromine molecules directly, without needing a halogen carrier, and three hydrogens are substituted rather than one. Benzene needs FeBr₃ and pure bromine, and substitutes only once. Phenols are used to make plastics, resins, antiseptics and disinfectants.

Quick checks

Which observation is the best evidence that benzene's six C–C bonds are identical?

Kekulé's structure would have three short C=C and three long C–C bonds. One intermediate length shows the π electrons are spread evenly round the ring.

The enthalpy of hydrogenation of cyclohexene is −120 kJ mol⁻¹ and of benzene −208 kJ mol⁻¹. What is the delocalisation energy of benzene?

Three C=C bonds would release 3 × 120 = 360 kJ mol⁻¹; benzene releases 208, so it is 360 − 208 = 152 kJ mol⁻¹ more stable. 88 is 208 − 120, which compares with only one double bond.

Why does benzene undergo substitution rather than addition with electrophiles?

Losing H⁺ at the end restores the full ring of delocalised electrons and its delocalisation energy.

What is the electrophile in the nitration of benzene, and how is it formed?

Sulfuric acid, the stronger acid, protonates nitric acid, which then loses water to give the nitronium ion. Sulfuric acid is regenerated at the end.

Which reagents would convert benzene into phenylethanone, C₆H₅COCH₃?

Friedel–Crafts acylation: AlCl₃ forms the acylium ion CH₃CO⁺ from ethanoyl chloride. Chloroethane would give ethylbenzene by alkylation.

How can you distinguish phenol from ethanoic acid?

Both react with sodium and with sodium hydroxide, but phenol is too weak an acid to release CO₂ from a carbonate.

Why does phenol react with bromine water without a catalyst, while benzene does not?

The extra electron density makes the ring so attractive to electrophiles that Br₂ is polarised directly, and substitution happens at three positions to give 2,4,6-tribromophenol.

In the mechanism of electrophilic substitution, what does the "horseshoe" in the intermediate show?

Two of the six π electrons have formed the bond to the electrophile, so the other four, with the positive charge, are delocalised over the remaining five carbons. The sixth carbon carries both H and E.

Interactive diagrams

Switch between the Kekulé model and real benzene and compare the bond lengths. Tick Show both on the diagram: real benzene releases 152 kJ mol⁻¹ less than Kekulé predicts, so it is more stable.

Pick a reaction, then drag the Step slider from 1 to 4. Notice how the electrophile is made, where each curly arrow starts and ends, and how the catalyst is regenerated.