Chemistry · Class 12 · C-12-D-21 to C-12-D-29
Aromatic Hydrocarbons
Structure, resonance, reactivity and the reaction mechanisms of benzene — explained step by step, with video lessons, notes and a solved-exercise set.
1Shape of the benzene molecule
Benzene, C₊Hₖ, is a planar, hexagonal molecule in which every carbon is sp² hybridised. Each carbon forms three sigma bonds — two to neighbouring carbons and one to a hydrogen — all lying in the same plane at 120° bond angles.
Molecular orbital picture: each carbon has one unhybridised p-orbital standing perpendicular to the ring plane. These six p-orbitals overlap sideways to form a continuous pi molecular orbital system above and below the ring, spreading the six pi electrons equally over all six carbons rather than locking them into three fixed double bonds.
This delocalised pi cloud is why every carbon-carbon bond in benzene is identical, with a bond length between that of a single and a double bond.
2Resonance, resonance energy and stability
Resonance
Benzene cannot be represented accurately by a single structure. It is treated as a hybrid of two equivalent Kekule structures, with the double bonds shifted between alternating positions. The true molecule is a blend (resonance hybrid) of these contributing structures, not a rapidly interconverting mixture of them.
Resonance energy
Resonance energy is the extra stability benzene has compared with a hypothetical single Kekule structure with three isolated double bonds. It is measured experimentally by comparing the actual heat of hydrogenation of benzene with the value calculated for the hypothetical structure — the difference (roughly 150 kJ/mol) is the resonance energy.
Relative stability
Because delocalisation of the pi electrons lowers the overall energy of the molecule, benzene is significantly more stable than a non-aromatic triene would be, and resists reactions (like addition) that would destroy this delocalised system.
3Reactivity: benzene vs alkanes vs alkenes
- Alkanes are largely unreactive, undergoing only free-radical substitution (e.g. halogenation under UV light) because they contain only strong sigma bonds.
- Alkenes are highly reactive toward addition reactions because their exposed, localised pi bond is an easy target for electrophiles.
- Benzene sits in between: its delocalised pi system makes it far less reactive toward addition than alkenes (addition would destroy the stabilising resonance), so instead it favours electrophilic substitution, which lets the ring regenerate its aromatic stability after the reaction.
4Halogenation: chlorination and bromination
In the presence of a Lewis acid catalyst (FeCl₃ or FeBr₃, AlCl₃), chlorine or bromine undergoes electrophilic aromatic substitution with benzene.
Mechanism: the catalyst polarises the halogen molecule, generating an electrophilic halogen species. The benzene pi cloud attacks this electrophile, forming a resonance-stabilised carbocation intermediate (the arenium/sigma complex) in which aromaticity is temporarily lost. A base then removes a proton from the sp³ carbon, restoring the aromatic ring and giving the substituted product plus HX.
Directing effects and isomers: once a substituent is already on the ring, it directs the incoming halogen to specific positions:
- Ortho/para-directors (e.g. -OH, -NH₂, -CH₃, halogens) push the new group mainly to the 2- and 4- positions relative to themselves.
- Meta-directors (e.g. -NO₂, -COOH, -SO₃H) push the new group mainly to the 3- position.
The major product is predicted by identifying the directing group already present and applying its ortho/para or meta preference, while also factoring in steric hindrance (which usually favours para over ortho for bulky groups).
5Nitration
Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid to introduce a -NO₂ group.
Mechanism: sulfuric acid protonates nitric acid, which then loses water to generate the electrophile NO₂⁺ (the nitronium ion). Benzene’s pi electrons attack this nitronium ion, forming a resonance-stabilised arenium (sigma) complex. Loss of a proton to HSO₄⁻ restores aromaticity and regenerates the sulfuric acid catalyst, giving nitrobenzene.
As with halogenation, any substituent already on the ring directs where the -NO₂ group is installed, and the major product follows the same ortho/para vs meta rules.
6Friedel-Crafts alkylation and acylation
Alkylation attaches an alkyl group to the ring using an alkyl halide and a Lewis acid catalyst such as AlCl₃. Acylation attaches an acyl group (-COR) using an acyl chloride and the same type of catalyst.
Role of the Lewis acid: AlCl₃ accepts a lone pair from the halide, weakening the carbon-halogen bond and helping generate a strong electrophile — a carbocation for alkylation, or an acylium ion for acylation.
Mechanism: the electrophile is attacked by the benzene ring, forming the usual arenium (sigma complex) intermediate. Loss of a proton restores aromaticity, and the regenerated catalyst is released, giving an alkylbenzene or a phenyl ketone as the major product.
Acylation is generally preferred synthetically because the acylium ion cannot rearrange and the resulting carbonyl group deactivates the ring, preventing unwanted multiple substitutions — a common complication in alkylation.
7Side-chain oxidation
Alkyl side chains attached to a benzene ring can be oxidised using strong oxidising agents such as hot acidic or alkaline KMnO₄.
Mechanism (outline): the oxidant attacks the benzylic carbon-hydrogen bonds of the side chain, progressively removing hydrogens and inserting oxygen. Regardless of the original chain length, the entire side chain is degraded down to a single carboxylic acid group directly attached to the ring, so the reaction always converts an alkylbenzene into benzoic acid (provided the benzylic carbon carries at least one hydrogen).
The aromatic ring itself is untouched by this oxidation — only the side chain is broken down.
8Hydrogenation of benzene
Benzene can be reduced by hydrogen gas under high pressure in the presence of a finely divided metal catalyst such as Ni, Pt or Pd.
Role of the metal catalyst: the catalyst surface adsorbs both the hydrogen molecules and the benzene ring, weakening the H-H bonds and allowing hydrogen atoms to add across the ring in a stepwise, syn fashion.
All three formal double bonds are saturated, converting the planar aromatic ring into the fully saturated, non-planar cyclohexane ring — the major (and only) product of this addition.
9Electrophilic aromatic substitution — the general picture
Every reaction above (halogenation, nitration, Friedel-Crafts alkylation/acylation) follows the same general pathway, called electrophilic aromatic substitution (EAS):
- Step 1 — generation of the electrophile: a catalyst or reagent combination produces a strongly electrophilic species (Cl⁺/Br⁺-type species, NO₂⁺, a carbocation, or an acylium ion).
- Step 2 — attack and sigma-complex formation: the delocalised pi electrons of the ring attack the electrophile, forming a new sigma bond and producing a resonance-stabilised, non-aromatic carbocation intermediate (the arenium ion / sigma complex).
- Step 3 — re-aromatisation: a base removes the proton from the sp³ carbon that formed in step 2, restoring the continuous pi system and the ring’s aromatic stability.
Predicting the major product: once a directing group is present on the ring, compare its effect (activating ortho/para-director vs deactivating meta-director) to determine where the new electrophile is most likely to attack, then account for steric effects to decide between ortho and para where both are possible.
🎥 Watch the video lessons
These lectures walk through the concepts above with worked examples on the board.
📄 Notes and practice
Download the notes and try the solved exercise set to test your understanding.