Benzene and extended organic reactions (HL)

The big idea: Benzene, C₆H₆, is the simplest arene (aromatic ring). It is a flat, regular hexagon of six carbon atoms, each bonded to one hydrogen.

Every carbon is sp² hybridised, so it forms three σ bonds in the plane (to two carbons and one hydrogen) at 120°.

That leaves one electron in a p-orbital on each carbon, perpendicular to the ring. These six p-orbitals overlap sideways to form a delocalised π system — two ring-shaped clouds of electrons, one above and one below the plane of the ring.

Key terms: - sp² hybridised — three hybrid orbitals in a plane at 120°, leaving one unhybridised p-orbital. - Delocalised electrons — electrons shared over several atoms, not fixed between two (here, spread around the whole ring). - π (pi) system — the cloud formed by the sideways overlap of p-orbitals, sitting above and below the σ framework. - Aromatic — describes the special stability of a ring with a continuous, delocalised π system.

All six C–C bonds are equal: Because the π electrons are spread evenly around the ring, every C–C bond is identical — each one is intermediate between a single (C–C) and a double (C=C) bond, at about 140 pm.

There are no alternating long-and-short bonds: benzene has one kind of carbon–carbon bond, not two.

How do we know the π electrons are delocalised rather than fixed in three double bonds? Two classic pieces of experimental evidence point the same way.

The hydrogenation argument, step by step: - Cyclohexene (one C=C) releases about −120 kJ mol⁻¹ when hydrogenated. - A Kekulé benzene (three separate C=C) would therefore be expected to release 3 × (−120) ≈ −360 kJ mol⁻¹. - Real benzene releases only about −208 kJ mol⁻¹. - The shortfall (≈ 150 kJ mol⁻¹ less energy released) means real benzene started lower in energy — it is more stable than the imaginary Kekulé molecule by that amount. This is the delocalisation (resonance) stabilisation.

Chemists therefore draw benzene two ways. The Kekulé structure (alternating double bonds) is handy for counting electrons; the delocalised structure (a circle inside the hexagon) is the accepted picture because it matches the evidence.

An ordinary alkene has a localised C=C double bond that is a reactive, electron-rich target, so alkenes undergo rapid addition (e.g. with bromine, the orange colour vanishes at once).

Benzene protects its ring: Benzene's stability comes from its delocalised π system. An addition reaction would force two of those π electrons into a new σ bond and break up the delocalisation — destroying the very stability that makes benzene special.

So benzene strongly resists addition. Instead it tends to undergo substitution: a hydrogen atom on the ring is swapped for another atom or group, and the delocalised ring stays intact.

Set-up only — the mechanism is separate: Here we only explain why benzene substitutes (to preserve the stable ring). The detailed electrophilic substitution mechanism (the curly arrows for nitration / halogenation of benzene) is taught in micro 6.4.3.

How this is tested: At HL, organic chemistry is assessed as reaction pathways: you are given a starting material and a target, and asked to deduce the route — the reagents, conditions and intermediate(s) for a two- or three-step functional-group interconversion.

- Paper 2 typically asks you to deduce a product, state the reagents/conditions for each step, or construct the overall scheme. - The skill is recognising what each functional group can be converted into, then chaining the steps in the right order.

A toolkit of common functional-group interconversions you can chain together:

Planning a route: (1) Identify the functional group in the start and the target. (2) List the conversions that bridge them. (3) Give the reagent and conditions for each step (a step with no reagent earns no mark). (4) Check each intermediate is sensible before moving on.