Concept 05 / 07Hydrocarbons
Benzene and Aromaticity: Structure, Resonance, Huckel's Rule and Preparation
Kekule structure and its limitations, resonance and molecular orbital picture of benzene, equal C-C bond lengths and resonance energy from heats of hydrogenation, Huckel's (4n+2) pi electron rule for aromaticity with aromatic, antiaromatic and non-aromatic examples, preparation of benzene, and addition and combustion reactions.
Benzene, , is highly unsaturated on paper (DBE ) yet does not decolourise bromine water or Baeyer's reagent; it prefers substitution to addition. This special stability is aromaticity.
Structure. Kekule (1865) proposed a six-membered ring with alternating single and double bonds, rapidly oscillating between two forms. But benzene gives only one ortho-disubstituted product, and all six C-C bonds have the same length, (between C-C and C=C ). The modern description: benzene is a resonance hybrid of the two Kekule structures. In orbital terms, each carbon is (planar, ), and the six unhybridised p orbitals overlap all round the ring to form a continuous cloud above and below the plane, holding six delocalised electrons. It is drawn as a hexagon with a circle.
Resonance energy (see figure). Hydrogenating cyclohexene releases ; a hypothetical cyclohexatriene would release . Benzene releases only about , so it is about more stable: the resonance (delocalisation) energy.
Huckel's rule. A compound is aromatic if it is cyclic, planar, fully conjugated (every ring atom has a p orbital) and contains electrons (: , , , ).
Aromatic: benzene (), naphthalene (), anthracene (), cyclopentadienyl anion (), cycloheptatrienyl (tropylium) cation (), cyclopropenyl cation (), pyridine (), pyrrole, furan and thiophene (, including a lone pair on the heteroatom).
Antiaromatic (cyclic, planar, conjugated with electrons, extra unstable): cyclobutadiene (), cyclopentadienyl cation ().
Non-aromatic: not fully conjugated (cyclopentadiene, with an carbon) or non-planar (cyclooctatetraene, electrons, adopts a tub shape to avoid antiaromaticity).
Preparation of benzene.
Decarboxylation: (soda lime).
Reduction of phenol by zinc dust: .
Cyclic polymerisation of ethyne over red-hot iron at .
Industrially from coal tar and by catalytic reforming (aromatisation) of petroleum fractions.
Reactions other than substitution.
Addition under drastic conditions: (Ni, high temperature and pressure); (UV light, ; benzene hexachloride, BHC, whose isomer is the insecticide gammaxene or lindane).
Combustion with a sooty flame (high carbon content): .
Key Takeaways:
- All C-C bonds ; six delocalised electrons; resonance energy about .
- Huckel: cyclic, planar, fully conjugated, electrons.
- Cyclopentadienyl anion and tropylium cation are aromatic; cyclobutadiene is antiaromatic; cyclooctatetraene is non-aromatic.
- Benzene prefers substitution, adding only under forcing conditions.
- Pitfall: count lone pairs in the ring system only if they sit in a p orbital perpendicular to the ring (pyrrole's N lone pair counts; pyridine's does not).
Brain-Teaser 1: Classify as aromatic, antiaromatic or non-aromatic: cyclopentadienyl anion, cyclopentadienyl cation, cyclopentadiene, tropylium cation.
Solution: Cyclopentadienyl anion: electrons, planar, conjugated: aromatic (hence cyclopentadiene is unusually acidic). Cation: electrons: antiaromatic. Cyclopentadiene: one carbon breaks conjugation: non-aromatic. Tropylium cation: electrons over seven carbons: aromatic.
Brain-Teaser 2: How many electrons does pyrrole have in its aromatic sextet, and why is pyrrole a much weaker base than pyridine?
Solution: Pyrrole has four electrons from two C=C bonds plus the N lone pair ( total), so its lone pair is part of the aromatic sextet; protonating it would destroy aromaticity, making pyrrole a very weak base. In pyridine the N lone pair lies in the ring plane in an orbital, outside the system, and is available for protonation.
Brain-Teaser 3: How many monochloro and dichloro derivatives of benzene are possible, and why does this support the resonance picture?
Solution: One monochlorobenzene (all hydrogens equivalent) and three dichlorobenzenes (ortho, meta, para). If Kekule's fixed structure were real, there would be two ortho isomers (Cl atoms across a single or across a double bond). Only one is found, consistent with all C-C bonds being identical.
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