15 Chapter 15 – Benzene and Aromaticity Solutions to Problems

The electronic descriptions of pyridine and benzene are very similar. The pyridine ring is formed by the σ overlap of carbon and nitrogen sp² orbitals. In addition, six p orbitals, perpendicular to the plane of the ring, hold six electrons. These six p orbitals form six π molecular orbitals that allow electrons to be delocalized over the π system of the pyridine ring. The lone pair of nitrogen electrons occupies an sp² orbital that lies in the plane of the ring.

A compound that can be described by several resonance forms has a structure that can be represented by no single form. The structure of the cyclopentadienyl anion is a hybrid of all of the above structures and contains only one kind of carbon atom and one kind of hydrogen atom. All carbon–carbon bond lengths are equivalent, as are all carbon– hydrogen bonds lengths. Both the ¹H NMR and ¹³C NMR spectra show only one absorption.

The heterocyclic thiazolium ring contains six π electrons. Each carbon contributes one electron, nitrogen contributes one electron, and sulfur contributes two electrons to the ring π system. The thiazolium ion is aromatic because it has 6 π electrons in a cyclic, planar, conjugated system.

Azulene is aromatic because it has a conjugated cyclic π electron system containing ten π electrons (a Hückel number).

Purine is a ten-π-electron aromatic molecule. The N–H nitrogen atom in the five-membered ring donates both electrons of its lone pair to the π electron system, and each of the other three nitrogens donates one electron to the π electron system.

Molecules with dipole moments are polar because electron density is drawn from one part of the molecule to another. In azulene, for example, electron density is drawn from the seven-membered ring to the five-membered ring, satisfying Hückel’s rule for both rings and producing a dipole moment. The five-membered ring resembles the cyclopentadienyl anion in having six π electrons, while the seven-membered ring resembles the cycloheptatrienyl cation. The electrostatic potential map shows that the five-membered ring is more electron-rich (red) than the seven-membered ring.

The circled bond is represented as a double bond in four of the five resonance forms of phenanthrene. This bond has more double-bond character and thus is shorter than the other carbon–carbon bonds of phenanthrene.

If o-xylene exists only as structure A, ozonolysis would cause cleavage at the bonds indicated and would yield two equivalents of pyruvaldehyde and one equivalent of glyoxal for each equivalent of A consumed. If o-xylene exists only as structure B, ozonolysis would yield one equivalent of 2,3-butanedione and two equivalents of glyoxal. If o–xylene exists as a resonance hybrid of A and B, the ratio of ozonolysis products would be glyoxal : pyruvaldehyde : 2,3-butanedione = 3:2:1. Since this ratio is identical to the experimentally determined ratio, we know that A and B contribute equally to the structure of o-xylene. Note that these data don’t distinguish between the resonance hybrid structure and the alternate possibility, equilibrium between two isomeric o-xylenes.

The product of the reaction of 3-chlorocyclopropene with AgBF₄ is the cyclopropenyl cation C₃H₃⁺. The resonance structures of the cation indicate that all hydrogen atoms are equivalent, and the ¹H NMR spectrum, which shows only one type of hydrogen atom, confirms this equivalence. The cyclopropenyl cation contains two π electrons and is aromatic according to Hückel’s rule. (Here, n = 0.)

Indole, like naphthalene, has ten π electrons in two rings and is aromatic. Two π electrons come from the nitrogen atom.

Only structure I shows a six-proton singlet at 2.30 δ, because it contains two identical benzylic methyl groups unsplit by other protons. The presence of four protons in the aromatic region of the ¹H NMR spectrum confirms that I is the correct structure.

Data in Section 15.7 show that an IR absorption of 745 cm^–1 corresponds to an o-disubstituted benzene, and an absorption of 825 cm^–1 corresponds to a p-disubstituted benzene.

Protonation of 4-pyrone gives structure A, which has resonance forms B, C, D, E and F. In E and F, a lone pair of electrons of the ring oxygen is delocalized into the ring to produce a six π electron system, which should be aromatic according to Hückel’s rule.

The cycloheptatrienyl cation has six π electrons (a Hückel number) and is aromatic.

The ortho and para products predominate because the intermediate carbocation is more stabilized. The third resonance form drawn for ortho-para attack places the positive charge at the methyl-substituted carbon, which is a more stable tertiary carbocation.