1. Benzene Structure & The Disproof of the Kekule Model
Benzene has the molecular formula C6H6. In 1865, Friedrich Kekule proposed that benzene was a six-membered carbon ring containing alternating single and double carbon-carbon bonds (cyclohexa-1,3,5-triene). However, experimental evidence conclusively disproved this structure.
Three Critical Pieces of Evidence Disproving the Kekule Model
- Identical Carbon-Carbon Bond Lengths: X-ray diffraction shows that all six C-C bonds in benzene are of exactly equal length: 0.139 nm. This is strictly intermediate between a standard C-C single bond (0.154 nm) and a C=C double bond (0.134 nm). If Kekule were correct, the ring would be an irregular, alternating hexagon.
- Thermochemical Enthalpy of Hydrogenation:
- Hydrogenation of cyclohexene (one C=C double bond) has delta H = -120 kJ mol-1.
- Theoretical cyclohexa-1,3,5-triene with three isolated double bonds would have delta H = 3 * (-120) = -360 kJ mol-1.
- Experimental hydrogenation of benzene releases only -208 kJ mol-1.
- Benzene is 152 kJ mol-1 more stable than predicted by the Kekule structure. This 152 kJ mol-1 energetic stabilization is called the delocalisation resonance energy.
- Chemical Unreactivity towards Addition: Benzene does not decolourise bromine water at room temperature and does not undergo typical alkene electrophilic additions. It undergoes substitution reactions to preserve its energetically stable aromatic ring.
2. Hydrogenation Enthalpy Energy Diagram
3. Why Benzene Undergoes Electrophilic Substitution
Each carbon atom in benzene uses three of its valence electrons to form three planar sp2 hybrid sigma bonds (two to adjacent carbons, one to hydrogen), with 120 degree bond angles. The fourth electron resides in an unhybridised 2p orbital perpendicular to the ring plane.
The six p-orbitals overlap sideways in all directions above and below the plane of the carbon ring, forming a continuous delocalised pi electron system containing six electrons. This doughnut-shaped pi cloud has high electron density, attracting electrophiles.
However, addition of an electrophile would permanently disrupt the continuous pi ring, sacrificing the 152 kJ mol-1 delocalisation stability. Therefore, benzene undergoes electrophilic substitution, where a hydrogen atom is replaced while the continuous delocalised ring is regenerated.
4. Nitration of Benzene
Benzene reacts with a nitrating mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at 50 to 55 degrees C to form nitrobenzene:
C6H6 + HNO3 -> C6H5NO2 + H2O
Generation of the Nitronium Ion Electrophile (NO2+)
Concentrated sulfuric acid acts as a catalyst and a Bronsted-Lowry acid by protonating nitric acid:
HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
Mechanism: Electrophilic Aromatic Substitution
- The high electron density of the benzene pi ring attacks the positive nitronium ion (NO2+). A curly arrow goes from the ring to the nitrogen of NO2+.
- This breaks the aromatic ring, forming an unstable, positively charged horseshoe intermediate (arenium ion). The horseshoe open face must point directly towards the carbon carrying the -NO2 group and -H atom.
- A curly arrow goes from the C-H bond into the ring, releasing H+ and reforming the stable delocalised aromatic pi electron ring.
- Catalyst regeneration: H+ + HSO4- -> H2SO4.
The temperature is maintained strictly at or below 55 degrees C. Higher temperatures result in multiple substitutions, producing 1,3-dinitrobenzene and explosive 1,3,5-trinitrobenzene.
5. Friedel-Crafts Acylation of Benzene
Acylation introduces an acyl group (R-CO-) into the benzene ring to yield an aromatic ketone. Reagents: an acyl chloride and an anhydrous aluminum chloride (AlCl3) catalyst.
Generation of the Acylium Ion Electrophile
AlCl3 acts as a Lewis acid halogen carrier, accepting a chloride ion from the acyl chloride:
RCOCl + AlCl3 -> R-C+=O + AlCl4-
Reaction with Benzene
The strongly electrophilic acylium ion (R-C+=O) attacks benzene via electrophilic substitution. The intermediate loses a proton to form an aromatic ketone (e.g. phenylethanone from ethanoyl chloride), and the catalyst is regenerated:
AlCl4- + H+ -> AlCl3 + HCl(g)
6. Worked Mechanism & Thermochemical Calculation
Calculate the expected enthalpy of hydrogenation for the non-delocalised Kekule model of benzene, and determine the delocalisation energy given the experimental value of benzene is -208 kJ mol-1.
Step 1: Calculate theoretical hydrogenation for 3 double bonds:
Cyclohexene has 1 double bond: -120 kJ mol-1.
3 isolated double bonds: 3 * (-120 kJ mol-1) = -360 kJ mol-1.
Step 2: Calculate delocalisation stability:
Delocalisation energy = |-360 - (-208)| = 152 kJ mol-1.
Benzene is 152 kJ mol-1 lower in energy (more stable) than the theoretical non-delocalised triene.
Reactants: Benzene + Ethanoyl chloride (CH3COCl)
Catalyst: Anhydrous aluminum chloride (AlCl3)
Electrophile: CH3-C+=O (ethanoyl cation / acylium ion)
Equation: C6H6 + CH3COCl -> C6H5COCH3 + HCl
Product: Phenylethanone (an aromatic ketone) and steamy fumes of hydrogen chloride.