Part 1: Benzene, delocalisation and the evidence

Use bond lengths, hydrogenation and reactivity to evaluate structural models, then name substituted aromatic molecules unambiguously.

A continuous π system changes the bonding

Benzene, C₆H₆, is a planar six-carbon ring. Each carbon forms three σ bonds, to two neighbouring carbons and one hydrogen, leaving a p orbital perpendicular to the ring. Sideways overlap of all six p orbitals creates a continuous delocalised π system above and below the plane.

The six π electrons are spread across the ring rather than confined to three independent C=C bonds. The circle in a hexagon represents that delocalisation; it is not an extra atom or six electrons physically orbiting on a circular track. Carbon valency still has to be respected when a substituent replaces H.

The Kekulé representation uses alternating single and double bonds. It is a useful way of drawing electron rearrangements, but a model of three isolated alkene bonds does not explain the full experimental behaviour. Do not describe the actual molecule as alternating rapidly between two separate structures.

Three different observations test the model

All six C–C bonds have the same length, intermediate between typical C–C and C=C bonds. A static alternating-bond model would predict two lengths. Equalisation is explained by the continuous π system.

Hydrogenating one isolated C=C in a cyclohexene-like comparison releases roughly 120 kJ mol⁻¹. Three independent such bonds would suggest about −360 kJ mol⁻¹, whereas benzene hydrogenation is about −208 kJ mol⁻¹. The smaller energy release to the same saturated product implies benzene begins at a lower enthalpy: delocalisation stabilises it.

The difference, about 152 kJ mol⁻¹ in this conventional comparison, is a model-based stabilisation estimate, not three extra bonds being broken. Benzene also resists ordinary bromination compared with alkenes. A good evaluation connects each observation to a prediction of the competing structural models.

Why substitution preserves the aromatic ring

An alkene has a localised electron-rich π bond that can polarise Br₂ sufficiently for addition under ordinary test conditions. Benzene’s π density is distributed over the ring and does not polarise bromine as effectively at one site. A halogen carrier produces a more powerful electrophile.

Electrophilic substitution temporarily disrupts aromaticity but then restores it when H⁺ is lost. An addition product would leave the aromatic π system disrupted. Explain both the initial susceptibility to attack and the restoration of the ring; “benzene has no double bonds” alone is not an adequate reactivity explanation.

Worked example: number positions before naming

Name a single substituent with benzene as the parent where appropriate: chlorobenzene, nitrobenzene or methylbenzene. With multiple substituents, number the ring so positions are specified unambiguously and follow the appropriate parent and lowest-locant rules. A phenyl group, C₆H₅–, is a benzene ring acting as a substituent attached through a ring carbon.

For an original structure with a methyl group taken as position 1 and nitro groups at positions 2 and 4, the name 2,4-dinitromethylbenzene fixes their relationship. Rotating the drawing on the page does not create a new isomer. For 2-phenylethan-1-ol, the OH-bearing carbon is carbon 1 in the two-carbon parent chain and the ring is attached to carbon 2.

Before predicting a product, label the carbon already bearing a substituent and count around the ring. Most naming and directing errors become obvious when the structure, locants and carbon count are checked together.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources