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.

Most Reactive ———————> Least Reactive

For acetanilide, resonance delocalization of the nitrogen lone pair electrons to the aromatic ring is less favored because the positive charge on nitrogen is next to the positively polarized carbonyl group. Resonance delocalization to the carbonyl oxygen is favored because of the electronegativity of oxygen. Since the nitrogen lone pair electrons are less available to the ring than in aniline, the reactivity of the ring toward electrophilic substitution is decreased, and acetanilide is less reactive than aniline toward electrophilic substitution.

The circled resonance forms are unfavorable, because they place two positive charges adjacent to each other. The intermediate from meta attack is thus favored.

Treatment with KMnO₄ oxidizes the methyl group but leaves the tert-butyl group untouched.

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.

Thus, it isn’t possible to synthesize m-nitrotoluene by any route that we have studied in this chapter.

Six of these compounds are illustrated and named in Problem 8.22 (b).The other eight are:

The bond between carbons 1 and 2 is represented as a double bond in two of the three resonance structures, but the bond between carbons 2 and 3 is represented as a double bond in only one resonance structure. The C1–C2 bond thus has more double-bond character in the resonance hybrid, and it is shorter than the C2–C3 bond. The C3–C4, C5–C6, and C7–C8 bonds also have more double-bond character than the remaining bonds.

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.

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

ICl can be represented as I^δ+ — Cl^δ− because chlorine is a more electronegative element than iodine. Iodine can act as an electrophile in electrophilic aromatic substitution reactions.

The N,N,N-trimethylammonium group has no electron-withdrawing resonance effect because it has no vacant p orbitals to overlap with the π orbital system of the aromatic ring. The (CH₃)₃N⁺– group is inductively deactivating, however, because it is positively charged. It is meta-directing because the cationic intermediate resulting from meta attack is somewhat more stable than those resulting from ortho or para attack.

Resonance structures show that bromination occurs in the ortho and para positions of the rings. The positively charged intermediate formed from ortho or para attack can be stabilized by resonance contributions from the second ring of biphenyl, but this stabilization is not possible for meta attack.

Protonation of the double bond at carbon 2 of 1-phenylpropene leads to an intermediate that can be stabilized by resonance involving the benzene ring.

Step 1: Formation of primary carbocation.

Step 2: Rearrangement to a secondary carbocation.

Step 3: Attack of ring π electrons on the carbocation.

Step 4: Loss of H⁺.

This reaction takes place despite the fact that an electron-withdrawing group is attached to the ring. Apparently, the cyclization reaction is strongly favored.

Step 1: S_N2 displacement takes place when the negatively charged oxygen of dimethyl sulfoxide attacks the benzylic carbon of benzyl bromide, displacing Br^–.

Step 2: Base removes a benzylic proton, and dimethyl sulfide is eliminated in an E2 reaction.

Only methoxybenzene reacts faster than benzene (See Figure 8.11).

Phenol > Toluene > p-Bromotoluene > Bromobenzene

Aniline and nitrobenzene don’t undergo Friedel–Crafts alkylations.

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.

Thus, the intermediate resulting from addition of HBr to the methoxyl-substituted styrene is more stable, and reaction of p-methoxystyrene is faster.

An electron-withdrawing substituent destabilizes a positively charged intermediate (as in electrophilic aromatic substitution) but stabilizes a negatively charged intermediate. For the dissociation of a phenol, an –NO₂ group stabilizes the phenoxide anion by resonance, thus lowering ΔG° and pK_a. In the starred resonance form for p-nitrophenol, the negative charge has been delocalized onto the oxygens of the nitro group.