Chapter 15: Benzene and Aromaticity
Naming substituted benzenes
Benzene derivatives are named on a system that mixes systematic construction with a small set of names retained for historical reasons. A ring carrying a single substituent is normally named by prefixing the substituent to the parent word benzene, giving bromobenzene, nitrobenzene, and ethylbenzene. Several monosubstituted benzenes, however, retain common names that IUPAC continues to accept and that also act as parent names when the ring carries additional groups: toluene for methylbenzene, phenol for hydroxybenzene, aniline for aminobenzene, benzaldehyde, benzoic acid, anisole for…
Benzene is planar, symmetric, and unusually stable
Benzene has the molecular formula C₆H₆ and therefore four degrees of unsaturation, a figure that a chain triene would also satisfy. Its chemistry, however, is not the chemistry of a triene. Benzene does not decolorize a solution of Br₂ in the absence of a Lewis acid catalyst, does not add HBr, and resists oxidation by KMnO₄ under conditions that destroy ordinary alkenes. When benzene does react with Br₂ and FeBr₃, the product is bromobenzene — a substitution product in which the ring survives — rather than a 1,2-dibromide. The structural evidence resolves the apparent contradiction. Benzene…
Six p orbitals, six π molecular orbitals, one closed shell
The number of molecular orbitals equals the number of atomic orbitals combined, so benzene's six p orbitals give six π molecular orbitals. Three lie below the energy of an isolated p orbital and are bonding; three lie above it and are antibonding. The lowest orbital, ψ₁, has no node cutting through the π system and places bonding overlap between every adjacent pair of carbons. The next two orbitals, ψ₂ and ψ₃, each carry one node and are degenerate, meaning they have exactly the same energy; one has its node passing through two opposite carbons and the other through two opposite bonds. The…
Hückel's rule: cyclic, planar, conjugated, and 4n + 2 π electrons
Aromaticity is defined by four requirements that must be checked in order, because failing an earlier one makes the electron count irrelevant. The π system must be cyclic. The ring must be planar or close enough to planar that the p orbitals overlap continuously. Every atom in the ring must contribute a p orbital to that system, which for carbon means sp² hybridization and for a heteroatom that means it must have an available p orbital. Only then does the electron count decide the outcome: 4n + 2 π electrons, with n a whole number, identifies an aromatic ring, which makes 2, 6, 10, and 14 the…
Charged rings obey the same rule
Hückel's rule is stated in terms of π electrons, not of neutrality, so cyclic ions are examined in exactly the same way. What changes is the count: removing an electron pair from a ring lowers the π count by two, and adding one raises it by two. Cyclopentadiene illustrates the anionic case. The neutral molecule has two double bonds and one sp³ CH₂ carbon; because that carbon carries no p orbital, the conjugated system does not close and the molecule is not aromatic. Removing a proton from the sp³ carbon leaves behind a lone pair. If that pair occupies a p orbital and the carbon rehybridizes…
Pyridine and pyrrole: where the nitrogen lone pair sits
A heterocycle is a cyclic compound containing one or more atoms other than carbon in the ring. Aromatic heterocycles are judged by the same four requirements as hydrocarbons, with one additional question: whether each heteroatom lone pair occupies a p orbital and joins the π system or occupies an in-plane orbital and does not. The answer is determined by how many π electrons the ring needs. Pyridine is benzene with one CH replaced by nitrogen. All six ring atoms are sp²-hybridized, and each contributes one electron in a p orbital perpendicular to the ring, so the π system holds six electrons…
Fused rings: naphthalene and beyond
Polycyclic aromatic compounds are built from two or more rings fused along a shared carbon–carbon bond. Naphthalene, C₁₀H₈, is the simplest example: two six-membered rings sharing two carbons, giving a planar framework in which every carbon is sp²-hybridized and carries a p orbital perpendicular to the molecular plane. Because the two fusion carbons belong to both rings, the correct way to count is by p orbitals rather than by rings; naphthalene has ten sp²-hybridized ring atoms, each contributing one p orbital, so those ten orbitals form one continuous fused π system holding ten π electrons;…
Recognizing an aromatic ring in a spectrum
The signature of an aromatic ring appears in every spectroscopic method introduced in the preceding chapters, and the ¹H NMR evidence is the most distinctive. Hydrogens bonded to an aromatic ring resonate between δ 6.5 and 8.0, far downfield of alkene hydrogens near δ 5, and benzene itself absorbs at δ 7.37. The explanation is not electronegativity but the ring current. When the molecule is placed in the applied field, the delocalized π electrons circulate around the ring, and this circulation induces a magnetic field of its own. Inside the ring the induced field opposes the applied field;…
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