AQA A-Level Chemistry Paper 2, 2018: Question 6

💡 Key Knowledge (The 3 Stages)

A top-level response must cover all three stages coherently:

• Stage 1: Bonding
  • Each C forms three covalent (σ) bonds (two to C, one to H).
  • Unpaired electron on each C sits in an unhybridised p-orbital.
  • p-orbitals overlap sideways to form a delocalised π electron cloud above and below the ring.
• Stage 2: Shape
  • Planar hexagonal ring.
  • All C–C–C and H–C–C bond angles are 120°.
  • All C–C bond lengths are equal (intermediate between C–C single and C=C double bonds).
• Stage 3: Stability & Data Comparison
  • Expected ΔH° of hydrogenation for theoretical cyclohexa-1,3,5-triene: 3 × (-120) = -360 kJ mol⁻¹.
  • Experimental ΔH° of hydrogenation of benzene is -208 kJ mol⁻¹.
  • Benzene is 152 kJ mol⁻¹ less exothermic (lower in energy / more stable) than expected due to delocalisation.

📐 Step-by-Step Energy Calculation

Step 1: Calculate theoretical value for Kekulé structure

3 × (-120 kJ mol⁻¹) = -360 kJ mol⁻¹

Step 2: Compare with experimental benzene data

ΔH°(benzene) = -208 kJ mol⁻¹

Step 3: Calculate the delocalisation energy difference

Difference = (-360) - (-208) = -152 kJ mol⁻¹

Interpretation: Benzene releases 152 kJ mol⁻¹ less energy when hydrogenated, meaning it sits 152 kJ mol⁻¹ lower on an energy diagram (more thermodynamically stable).

✅ Model Full-Mark Response

Bonding: Each carbon atom uses three valence electrons to form three single covalent sigma (σ) bonds: two with neighbouring carbon atoms and one with a hydrogen atom. The remaining fourth valence electron on each carbon occupies a p-orbital perpendicular to the ring plane. Adjacent p-orbitals overlap sideways with each other around the entire ring to produce a delocalised π electron system spread above and below the carbon ring.

Shape: Benzene is a planar, regular hexagonal molecule. Each carbon atom has three areas of electron density giving trigonal planar geometry with bond angles of 120°. Because of delocalisation, all six carbon–carbon bonds are equal in length (139 pm), intermediate between a standard C–C single bond (154 pm) and a C=C double bond (134 pm).

Stability Comparison: Cyclohexene has one double bond and an enthalpy of hydrogenation of -120 kJ mol⁻¹. If benzene were the hypothetical cyclohexa-1,3,5-triene containing three isolated C=C bonds, its expected enthalpy of hydrogenation would be 3 × (-120) = -360 kJ mol⁻¹. The experimental value for benzene is -208 kJ mol⁻¹, which is 152 kJ mol⁻¹ less exothermic than expected. This shows that benzene is 152 kJ mol⁻¹ lower in energy and therefore significantly more stable than the hypothetical cyclohexa-1,3,5-triene due to delocalisation energy.

❌ Common Errors & Lost Marks

• Imprecise language about energy: Saying benzene "has 152 kJ mol⁻¹ more energy" rather than saying it is lower in energy / more stable.
• Mixing up enthalpy signs: Stating the enthalpy of hydrogenation is "+152" or confusing an exothermic release with an endothermic requirement.
• Omitting bond lengths: Forgetting to mention that all C–C bonds are identical in length and intermediate between single and double bonds.
• Failing to identify the orbitals: Writing "electrons overlap" rather than "p-orbitals overlap sideways".

✅ Mark Scheme Requirements

Mark 1 (Value): Any numerical value in the range -239 to -121 kJ mol⁻¹ .

Mark 2 (Structure): The double bonds are separated by exactly one single bond (alternating double and single bonds) / shown in a drawn conjugated structure.

Mark 3 (Orbital interaction): This arrangement allows some sideways overlap of p-orbitals / partial delocalisation of π electrons.

Note: Mentioning "double bonds are close enough to allow delocalisation" scores both Mark 2 and Mark 3.

🧠 Exam Technique & Logic

1. 1 Baseline: Two isolated C=C bonds (like two cyclohexenes) would be 2 × (-120) = -240 kJ mol⁻¹.
2. 2 Deduce effect: Cyclohexa-1,3-diene is a conjugated diene ( -C=C-C=C- ). Conjugation gives extra stability.
3. 3 Direction of value: Extra stability means the hydrogenation reaction is less exothermic than -240 kJ mol⁻¹ (i.e. a number between -239 and -121 kJ mol⁻¹, often suggested as around -232 kJ mol⁻¹).
4. 4 Boundary limit: It must still be more exothermic than a single double bond (-120 kJ mol⁻¹), hence the lower limit of -121 kJ mol⁻¹.

❌ Common Pitfalls

• Sign errors: Omitting the negative sign (e.g. writing "230 kJ mol⁻¹") results in a Contradiction Error (CE = 0 marks).
• Suggesting an impossible magnitude: Giving a value more exothermic than -240 (e.g., -250 kJ mol⁻¹) indicates a misunderstanding that conjugation stabilizes the molecule.
• Confusing diene isomers: Cyclohexa-1,4-diene has double bonds separated by two -CH₂- groups (non-conjugated, ΔH° ≈ -240 kJ mol⁻¹). Cyclohexa-1,3-diene has alternating bonds, allowing overlap.

💡 Quick Summary Formula

For cyclic alkenes:

• Cyclohexene (1 C=C): -120 kJ mol⁻¹
• Cyclohexa-1,4-diene (isolated): 2 × (-120) = -240 kJ mol⁻¹
• Cyclohexa-1,3-diene (conjugated): between -239 and -121 kJ mol⁻¹ (typically ~ -232 kJ mol⁻¹)
• Benzene (fully delocalised): -208 kJ mol⁻¹ (much less exothermic than 3 × -120 = -360 kJ mol⁻¹)