Part 1: Benzene: bonding, evidence and resistance to addition

Use orbital overlap, bond lengths and hydrogenation data to explain benzene’s delocalised structure and its preference for substitution.

From a useful formula to an evidence-based bonding model

Benzene has molecular formula C₆H₆ and a planar six-carbon ring. Each carbon makes three σ bonds: two to neighbouring carbons and one to H. The local bond angles are about 120°. One p orbital remains on each carbon, perpendicular to the ring plane, containing one electron available for the π system.

The six adjacent p orbitals overlap sideways around the ring. Six π electrons are delocalised over the whole ring in electron density above and below the plane. Delocalised means not confined to an individual pair of carbon atoms. The ring-with-a-circle representation indicates this shared system; the circle is not a moving electron orbit.

Kekulé’s alternating single/double-bond formula is useful for bookkeeping, including mechanisms, but a molecule does not switch back and forth between two sets of short and long bonds. The delocalised structure is one bonding arrangement. In a mechanism use a Kekulé form or delocalisation arc in a way that correctly shows where electron density is lost and restored.

Equivalent bonds test the alternating-bond prediction

A localised alternating-bond model predicts three C–C single bonds and three shorter C=C double bonds. Measurements instead show all six C–C bonds are the same length, about 0.140 nm, intermediate between typical C–C single (about 0.154 nm) and C=C double (about 0.134 nm) bonds.

The relevant evidence is equality and intermediate length, not merely that a ring exists. It is consistent with equivalent bonds in a delocalised system and inconsistent with a fixed arrangement of three ordinary single plus three ordinary double bonds. Given data in a question, quote those values rather than assuming all bonds in all molecules match these approximate examples.

Worked energy comparison uses a common final product

Hydrogenating one C=C bond in cyclohexene to cyclohexane releases about 120 kJ mol⁻¹. A hypothetical localised cyclohexa-1,3,5-triene is therefore estimated to release 3 × 120 = 360 kJ mol⁻¹ on complete hydrogenation. Actual benzene releases only about 208 kJ mol⁻¹ to give the same cyclohexane product.

Because both routes end at the same product, the less exothermic actual hydrogenation means benzene starts at a lower enthalpy than the hypothetical localised structure. The stabilisation magnitude is 360 − 208 = 152 kJ mol⁻¹ for these illustrative approximate values. The enthalpy change from hypothetical localised material to benzene would be −152 kJ mol⁻¹.

Do not say benzene needs 152 kJ mol⁻¹ to hydrogenate, or that a smaller exothermic value proves a slower rate. This thermodynamic comparison concerns relative energy; kinetic resistance is discussed through an activation barrier. On an energy diagram, the localised model is highest, benzene is lower, and cyclohexane is lower still.

Compare the electron density and what the reaction would destroy

An alkene’s π electrons are concentrated between two carbon atoms and readily polarise Br₂. Electrophilic addition then opens that localised π bond. Benzene’s π electrons are spread around the ring, and addition would remove the stabilisation associated with the continuous delocalised system. It therefore resists bromination under the mild conditions that decolourise bromine with an alkene.

Benzene reacts with bromine using a halogen carrier such as FeBr₃. The catalyst strongly polarises/activates bromine, providing a sufficiently reactive electrophile. Substitution replaces one H with Br and restores the aromatic π system after a temporary disruption. It is not enough to say “benzene has no double bonds” or “benzene has no electrons available”.

Benzene also burns in air, often with a smoky/sooty flame under conditions of incomplete combustion. Its high carbon-to-hydrogen ratio favours soot formation when oxygen mixing is insufficient. Complete combustion is still represented by CO₂ and H₂O; soot is not a product of the complete-combustion equation.

Generate the electrophile, attack, then restore the ring

A simplified Edexcel electrophile-generation equation is Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻. This is mechanistic shorthand for strongly activated bromine; it does not imply a bottle of freely isolated Br⁺. Draw a curly arrow from the ring π-electron system to the electrophilic Br atom.

The intermediate has a new C–Br bond at one ring carbon that still carries its original H. It is positively charged and no longer fully aromatic; show the positive charge delocalised over the remaining five-carbon region, not a complete circle passing through the substituted tetrahedral carbon.

A base removes H⁺ from that carbon while a curly arrow from the C–H bond returns its electron pair to the ring. Aromaticity is restored. FeBr₄⁻ + H⁺ → FeBr₃ + HBr regenerates the catalyst. Check that the overall reaction replaces H with Br and does not add Br to every ring carbon.

Quick checks

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

Sources